Wednesday, 27 September 2023

A classic case of Television Interference

Anything which transmits radio frequency signals seems to have the potential to create television interference (TVI). Well, pretty much anything that I can think of, if it is positioned in the wrong place.

There are plenty of things to consider when looking into these cases. Why has the interference started? What has changed? If you can answer those questions you stand a chance of solving the problem.

Fundamentally of course the television has to be capable of dealing with RF signals on frequencies other than the ones it is intended to receive. The manufacturers claim that this is true. These days they are generally pretty good at this, but you cannot rule out this as the root of the problem. And then, even if the TV is OK in this respect, possibly some wideband distribution amplifier is not good a keeping out signals from outside the TV band. I would suggest, look for a wideband distribution amplifier as your first possible culprit.

So, when I got a report of TVI coming from a neighbour I immediately wondered what had changed. Oddly, nothing significant had changed on my side for a few years. True, I had got stuck when my knee failed at a time when I was in the middle of moving antennas about. Now though all those antennas are back where they had always been. Even my 28MHz vertical, which I use on 24MHz as well, is back where it had been several years ago.

If my antennas had not changed, and the neighbour said that her TV setup had not changed for a couple of years, then I was a bit stumped.

This seemed like an odd one, but in the end the same thing solved it as usually solves such things - a braid breaker.

Garex HPF1 HPF and braid breaker similar to the one used

I first used a braid breaker for this purpose within weeks of first being licensed all those years ago. I think I still have that one somewhere ...

Anyway, at first it did not seem too obvious what was wrong and it was too soon to jump to conclusions.

As usual, the neighbour had come to me with certainty about the interference, but no definite dates or times. This is not her fault as it is not her duty to keep a logbook. She had contacted her TV antenna installer who thought it might be due to telephone interference. This type of interference came about when part of the "Freeview" terrestrial TV frequency allocation was given over to mobile phone usage. That sounded a bit unlikely to me.

I had in my mind an incident which happened a couple of years earlier. The same neighbour had come to me as she had knocked the preset channels off the television in her bedroom. She asked me to retune the channels. This proved to be very difficult. When I checked it the signal level at the antenna socket in the bedroom was at a very low level, to the point that tarnishing of the socket was preventing any useful signal reaching the TV. By cleaning the plugs and sockets I managed to get the thing tuned, but it was marginal. At the time I suggested to the neighbour that she call her antenna installer and get him to check out why the signal was so weak.

From this visit I had gained the impression that there were three televisions in the house (bedroom, kitchen and lounge), which implied that there would be a distribution amplifier somewhere. The antenna was probably in the attic space, a common arrangement around here. The main TV was connected to a "BT box", which is an internet connected entertainment console, plus a DVD player. There were plenty of coax and internet leads around to pick up rogue signals, though over the years I have added ferrites anywhere I could, as much to keep them from radiating noise than picking it up.

TV in this area is served by an infill repeater on the nearby headland overlooking Eyemouth. The field strength from the repeater is high, but this repeater offers very few programme options. Several villagers have erected tall masts to point at the main transmitter (Selkirk). Although we are outside the Selkirk service area they still get a reasonable signal, though when I tried this some years ago it could drop out at times. Faced with this problem, most of the other villagers have installed "Freesat" satellite TV, or have subscriptions to Sky TV. We have Freesat.

Unusually, this neighbour uses the local Freeview UHF repeater, which is within sight of her antenna. It seemed odd to me back then that some time ago she was getting such a weak signal on her TVs, and indeed, was only now getting TVI.

I sought advice from Mike, GM3PPE, who kindly offered to help. It is impossible for one amateur to use their equipment and monitor TVI at the same time. Mike also agreed to come over and do some tests. We pondered over what could be happening as we went round with various pieces of test equipment, a jar full of ferrites, and a box of braid breakers. The main TV was getting a strong signal now (something had changed but at first I did not know what). We checked things out and went up to look at the antenna. In the attic space we found a mains powered distribution amplifier with four outputs. The LED light on this was on, but beside it was another Labgear amplifier, also with four outputs, which was disconnected.

TV distribution preamp similar to the one involved

Now we started the tests. I left Mike with a PMR hand portable and I took a second handie back into the shack. I then transmitted on all the frequencies I normally use, beaming in various directions and using different power levels. Mike watched the television in the lounge which had been affected by TV and reported back.

As soon as the tests started the surprises began. Having found the distribution amplifier I had guessed (correctly as it turned out) that the amp was being affected by my signals and not the television directly. I was wrong however in assuming that it was suffering from overload. I had feared that the electromagnetic field I was creating was simply overwhelming the amplifier circuit boards, as sometimes happens. Not in this case.

Mike quickly reported that I was indeed causing TVI on some bands (4m and 2m, which surprised me), but lowering the output power by 3dB stopped it entirely. Likewise, moving the beam antennas 10 degrees or so away from the TV antenna also stopped the interference. Rather than totally swamping the amp this looked like there were simply unwanted frequencies getting into it and being amplified, probably arriving down the antenna coax. 

With swamping, where the signal is getting directly onto the boards, quite small signals can cause big problems by desensitising the amplifier, and there tends to be a "cliff edge" effect whereby only by reducing the signal to a very low point does the interference stop. I was still thinking about the time in the past when I found low signal levels, and I though any desensitisation would cause problems. Here, though, the interference weakened very quickly, and seemed to be stopped entirely at about -3dB lower signal at my end. This should be fixed fairly easily, I thought.

Next stage was to try a filter in the antenna lead between the antenna and the amplifier. I had various devices in my box, including 50, 70 and 432MHz notch filers, and several general purpose braid breakers. At this stage Mike's wise approach came into play. He studied the data sheets for the various braid breakers to check which would introduce the minimum attenuation to the wanted TV signal. My approach was a bit gung-ho, so I would have just tried something. Mike rightly wanted to select the best one for the purpose. The best one of the bunch turned out to claim an insertion loss of better than 2dB "at UHF". Other details were rejection on the inner of better than 60dB at 30MHz and below, and better than 25dB at 30MHz and below on the outer. Given that 3dB seemed likely to do the trick we tried this. 

The choice was made on the basis that although the HPF1 filter I brought had greater rejection of the unwanted signal, it did not quote a insertion loss, whereas the HPFS had a quoted figure for loss at TV frequencies. Based on the tests we felt that we did not need more rejection of lower frequencies. Of course, I use frequencies up to 432MHz (I doubt if 1296 is an issue) so the attenuation will be less as the frequency rises, but it seemed like a useful first step before trying the notch filters. In the end we did not need to try anything else.

I fitted the braid breaker (the HPFS, not the similar HPF1 in the photo above) and returned to the shack to do the tests again. Mike reported no TVI. Not on any band, at any power, beaming in any direction. Mike then moved on to the television on the bedroom and achieved a similar result. So what had happened to suddenly cause this TVI?

The homeowner told us that the antenna installer had installed the current distribution amplifier a couple of years ago. The old one, which I guess had failed and was the cause of the weak signals I had noted back then, was a neat Labgear. Like most Labgear amps, no doubt it was well screened. The new one produced a stronger signal for her but was wide open to interference. So why did this not prove to be a problem immediately when the amp was replaced? Well, probably because this was when I had my antennas moved while I was recovering from my knee operation. At that time they were displaced, but lately I have put them all back where they had been originally.

When the TVI was reported I could not work out why, when everything was where it always had been, there was suddenly a problem. But the new preamp had been there for a while and it could not have coped with my original layout as the old preamp could.

Incidentally, we never did see the TV antenna involved. Once we found the preamp there was no need to climb any further. We simply connected the braid breaker into the input of the amplifier. Checking the antenna would have involved a lot of attic crawling which was not needed. Clearly there was enough signal getting down the coax, wherever it was coming from.

Also, there turned out to be a fourth output from the preamp, feeding a TV in another bedroom. We just did not know what we were going to find that day. As it turned out, there was a proper preamp, and it worked quite well once it had a filter added to the input. I attached a label to the filter "Do not remove" - we can but hope.

Initially I had feared that, as in some other houses, we would find a non-amplified "aerial splitter". These "dumb" splitters introduce 3dB loss per output. Some antenna installers fit them to avoid installing mains wiring and possibly later rule out preamp circuit failures. It is true that the earlier preamp had failed, so perhaps the point is arguable, but for me a properly installed preamp should always produce the best results.

So that seems to be that. It is impossible to check every outcome at different power levels and beam directions but several weeks have now passed and no further complaints have been received. I have checked and the neighbour seems quite happy with things as they are now.

I could not have solved this problem without the help of GM3PPE. His advice was very valuable, and he carried out the tests at the other end. He tactfully discussed the issue with the neighbour and provided a very useful expert for her to rely on. Thanks Mike.

Now, what is next?

73 Jim


Thursday, 14 September 2023

A tropo opening at last.

Perhaps the strangest thing about this latest tropo opening is that it is so long since the last one. My most recent posting here on this subject was in early June, 3 months ago.

I think of Summer in this part of the UK as not being particularly hot but often having long settled sunny spells associated with high atmospheric pressure which brings tropospheric enhancement. Not this Summer. After a promising start, July and August were grey and miserable with low pressure for weeks on end. Of course the low pressure does not last for ever, just like the high pressure never did.

During the period 4 to 9 September a series of high pressures passed from West to East. For a time there was a low pressure system to the North West of me (as there often is) which brought winds and shut down the propagation, but generally conditions remained up for most of that period.

Things seemed to be best on 144MHz, with 432 not bad and 1296 pretty poor.

144MHz contacts at GM4FVM 4to 9 September 2023

39 QSOs into 30 different squares was a very agreeable outcome.

Those of you who have clicked on the image to give a better view of the map may have noticed that F6ASP appears to have his station located in the middle of the English Channel. If that was true the UK Border Agency would be out to try to send him back to France. In fact what has happened here is that because I only have a four character locator for that station the map assumes that he is in the middle of the JO00 square. This happens a lot, and I have to look up six character locators for some stations who do not list them on, or for which VQlog does not have the full details. In this case F6ASP does not have a more precise locator anywhere that I can find it, so he will have to remain in a watery position for now. 

Queue the tune "Sailing By" written by Ronald Binge, which is played by the BBC before the Shipping Forecast. This is now used by anyone too lazy to find anything more relevant to the sea. I am as lazy as the next man. I know Claude Debussy's La Mer and Benjamin Britten's Sea Interludes might be better but perhaps I will just stick to what I always do. This "don't blame the lazy" theme will return at the end of this posting, but by then it might have some amateur radio relevance [about time Jim, this is not a music blog].

On 5 September I had the chance to operate for a short period during the NAC contest. I switched to using the EU contest mode in WSJT-X and worked SM7VUK, DG1BHA, SB7A, SK7OL and SM7SPG in 33 minutes. I could have worked more but had to go to attend to domestic duties. Contests have their uses but I am not about to become a slave to them.

Things seemed to be going pretty well on 5 September so I decided to try coming on to 2m at 05:00 local time on 6 September. This was to see if the supposed good conditions early in the day would pan out. I was pretty pleased then to work EU3AI in KO22, a distance of 1757km, at 05:40 UTC. Of course I have worked him before (!), but this time it was tropo which is definitely the hardest way to work Belarus. 

Having worked Belarus on 2m both of the easy ways, by meteor scatter and Sporadic E, doing it on tropo seemed to complete the set somehow. Just as people say that if the Jumbo Visma cycling team win the Vuelta a EspaƱa this week, resulting in a clean sweep of winning all three Grand tours this year, somehow doing it three times in different circumstances makes them invincible. It doesn't mean that of course, and nor does me working Belarus three different ways mean anything very significant other than that I was in the right place three times over.

Still, I am claiming that my early arrival on the band that morning was the result of superior planning on my part and shows how knowledgeable, dedicated and devoted I am to my hobby. It doesn't mean that of course. Ever heard on insomnia? What else do you do at that time of the morning other than listen to the radio?

I also heard two Russian stations in the Saint Petersburg region and one Ukrainian station, but did not work any of those. I cannot complain though as 15 countries worked on 2m is five days is still pretty good.

OH1ND in KP00 is 1493km, still a good trip on 2m, and LY2WR in KO24 is 1681km, barely 70km short of EU3AI. So there was plenty of action short of the ODX.

Moving on to 70cm

432MHz contacts at GM4FVM 4 to 9 September 2023

35 QSOs on 70cm is only four less than the 2m total and shows how well we can do on that band during reasonable lift in conditions. SM0DJW in JO88 is a pretty good contact on this band at 1242km. There were more contacts into PA on 70cm and there is no doubt that the path from me into Benelux on 70cm can often be better than 2m. Once again the DXCC total is pretty good - nine countries worked on 70cm would have been a good lifetime haul for me when I started out on this band. 

Meanwhile on 23cm...

1296MHz contacts at GM4FVM 4 to 9 September 2023

On the face of it, 23cm looks like a bit of a washout compared with the other bands. It did not feel like that at all. The QSO with  G4YTL was particularly interesting. We were in contact via KST chat and after setting up the initial details, it took 20 transmissions from me on FT8 for David to decode the two that he needed to complete the contact. That is over a distance of just 427km during elevated conditions. We then switched to Q65 and competed a second QSO in just over a minute. In FT8 David could not decode 18 of my 20 transmissions, while on Q65 he decoded all of them (just three needed to complete).

There is nothing very starling about this. I have worked David on Q65 on 23cm before. What surprised me was seeing the comparison in modes play out. Q65 is simply a superior mode to FT8. The difference may not matter much on HF or during favourable conditions on other bands. However, once we reach marginal conditions or bring into play scatter propagation as with aircraft scatter or EME, then Q65 shows how good it always has been.

This brings me back to the question I often ask myself. Why, when Q65 is better, do VHF and above operators still not use it? It seems to me to be absurd that many EME operators on 2m still use JT65. If we could bring Q65 to bear during ionoscatter or troposcatter it might well make the difference between completing a contact and failing totally.

Sure, if you are on a chat room like KST (argh, how I dislike that but needs must) you can set up a Q65 contact. But what about the generality of contacts which happen randomly? Could VHF operators not decide as a group to use Q65 instead of FT8? This is a bit like asking why some people still use FSK441 or RTTY. They use outdated protocols because they do not want to change. They have every right to use old modes, and I do not deny them that right. But that is like saying that they have the right to miss all the QSOs. Sure they do. Knock yourself out with that idea, mate.

In conclusion ... this opening was a test for me to check out the two band with two feeds DUAL 2m/70cm antenna. I doubt that it can be quite as good as the I0JXX for 2m and the Wimo for 70cm because they are single band antennas and are designed for that one job. However, it went pretty well and I think I should be content enough about that.

Now, the 23cm antenna might now come into the reckoning for change in some way. Elevation? Time to think again about 23cm EME?



Friday, 8 September 2023

A long term milestone reached at last - and multi-hop Es in September?

It is sixteen years now since GM4FVM returned to Scotland and took up this callsign again. After a busy few years of moving from GM to GI to G, finally we came back to Scotland. Finally (I think). Mind you, having done G to GI to GM before that, you never know.

It took a while to get established here, and I had my first 50MHz contact as GM4FVM on 23 February 2010, over thirteen years ago. That was to GM8BDX in Birgham, less than 30km away, but it produced DXCC number one on 50MHz. 

So that was the start. It was three months before S53CC gave me DXCC number two.

By March 2017 (only seven years later!) I had reached 55 DXCC on 6m, over half way towards that magical DXCC itself. DXCC on one band still seemed a bit of a stretch for me. However, by March 2018 the total was 63 and by the end of 2018 it had reached 71.

I think you can see where this is going.

By July this year the total had reached 93, and a month ago 99. Almost there. However, that seemed to be it for 6m Es in 2023. This year's Es season appeared to be over and certainly as far as multi-hop Es was concerned.

However I still keep an eye on the space weather via Solarham (link in the sidebar). There was a disturbance on 3 September. The data on stated "SWX=Moderate Storm" which was enough to get me interested.

I had been checking in at the shack at various times just in case something happened. Otherwise I was watching an excellent episode of that reliable Kiwi detective drama "Brokenwood Mysteries". This one surrounded a murder amongst a female motorcycle gang and I got quite involved in the plot. I do think that the arrival on the scene of Detective Chalmers has definitely brought a slightly more gritty angle to this series - and it needed it. Anyway, I devoured the whole thing.

When I strolled into the shack again at 20:59 UTC I was surprised to see that there was a lot of traffic on 6m from the direction of South America. In fact, I saw Gordon GI6ATZ working a CE station and David GI4SNA working an LU7. Time for action.

By 21:06 I had worked PU4JOE, and then CE3VRT by 21:09. And that was it. CE3VRT was my first 50MHz contact into CE from here, and also CE was my 100th DXCC worked on the band. Job done.

Contacting PY5EW completed activities for the night, at 21:13.

50MHz contacts recorded on DXMaps on 3 September 2023.
DX Maps tells the tale but maybe does not answer all the questions. All of those contacts into South America are shown as Trans Equatorial Propagation (TEP). Yet, when similar contacts were made a week or so earlier they were all recorded at Multi-hop Es. There certainly was Es about as can be seen by the red lines within Europe.

Was this TEP at 21:13 in September, or multi-hop Es, or a combination of the two? I do not know.

Moving on from that question, I had to check Clublog just to see that they had acknowledged my 100 score. You never know you know, someone near to me might have sabotaged me sending Clublog the details. You do not need to be paranoid to write this blog but it makes it a lot easier to fit in if you are.

Clublog personal DXCC chart page for GM4FVM

Well, I can move on from that too. 

The achievement moves me to 1523rd place in the Clublog 6m DXCC list. I would not describe that as having achieved greatness. I am 1523 steps down the ladder from the top, but rising to the top has never been my ambition. I only do this to measure how my station is operating.

Who cares about places on that DXCC list? Not me.

These little milestones just serve to motivate us and to check if things are getting better. They might spur us on to improve the antenna or something similar. After that the next thing happens.

I am more interested in the mode of propagation which got my 50MHz signal to CE3VRT than the achievement of working 100 countries. Even more intriguing was reading CE3VRT's QRZ page about his efforts at 23cm moonbounce using a 1.8m dish.

Hmmm. Most interesting.

73 Jim 


Saturday, 26 August 2023

Italy returns to 70MHz, and data mode confusion breaks out

Italian stations were granted access to the 70MHz band again from 6 August until the end of 2023.

Here is the information for the many of you who read this blog who are fluent in Italian.

Italian 4m authorisation 2023.
OK, given that we are reaching the end of the 2023 Es season the timing could prove to be a bit unfortunate, but we might be thankful for what we have got.

Once again the authorisation is for spot frequencies and a band 25kHz around those centre points. As before the spot frequencies are 70.100, 70.200, 70.300 and new for this season is 70.400 - for FM I guess which was on 70.300 under previous allocations. Power is 10W and all normal modes and directional antennas are permitted.

It is a good 10 years since Italian stations could use 4m and a lot has changed in the meantime. For a start, many stations have equipment ready which can operate directly at those frequencies. Also, several more DXCC "entities" have appeared on 70MHz over the years which are new countries for Italians. 

So, as you might expect, there was an avalanche of stations from Italy seeking to work everyone around, including me. On 12 August I worked 20 Italian stations on 4m. Any why not? New bands, new DXCC, new squares, new "slots", all good stuff for the Italian stations. Only one new square for me though. I have seven or eight squares to get in Italy, plus a few more watery ones which look tricky. Mostly I need to work stations in squares in Apulia, Calabria, Campania, and Molise. Northern Sardinia and Eastern Sicily remain to be worked, as does one surprising square in Liguria and Piedmont right up on the French border. 

The reason why I have not worked these squares is no doubt simple. These are areas of lower population density, though the missing one in Sicily is a bit odd. Still, plenty of work to do there Jim.

I presume based on earlier authorities that San Marino, Vatican City and the Sovereign Military Order of Malta will also follow suit. Once again the timing is not great as there have recently been activations from all three of these DXCC and I worked them all on 6m. So far I have worked none of them on 4m. Ah well...

So what do I mean about confusion? Well, in those ten years of no activity from Italy, FT8 and MKS144 have appeared on 4m. As soon as 6 August came Italian stations and those wishing to work them had to decide on a common frequency. The usual ones, decided upon by German amateurs when they came on the band permanently, were 70.154 and 70.174. Neither of the existing frequencies fall within the Italian allocations, so they used 70.190 for FT8 and 70.210 for MSK144. Fair enough.

Nobody publicised the change of frequency required to work Italians, or not where I could have seen it. I just used common sense, looked at PSK reporter, and changed to 70.190. Since 6 August all my FT8 QSOs on 4m have been on 70.190. Once again, why not? Well, both frequencies are bit close to the SSB centre of activity which is on 70.200, but we can manage that with a bit of give and take. When a UK contest appeared some SSB stations stuck to "their" contest frequency. The UK regulator, Ofcom, says we are "frequency agile". We can move, and that applies equally to data mode users. We do not own the frequencies we happen to regard at some stage as fixed points.

As things stand today, 26 August, a majority of FT8 operators seem to have moved to 70.190. A sizeable number are still on 70.154. I often see UK stations calling CQ on 70.154 and working nobody, while there are many stations in various countries to be heard on 70.190.

If anyone does not want to work Italians they can stick on 70.154, but that frequency is now falling into disuse. 70.154 is also not good for Portuguese stations. Myself, I can listen on both. My preference is for 70.190, at least for as long as Italians are to be found there. I have worked about a dozen countries now on 70.190 so a lot of other operators take the same view.

What frequency you use is up to you folks, so long as it is legal.

Over the past 45 years plus since I started on 4m, the frequency allocation in the UK changed, and  various countries have slotted in slices and spot frequencies all over the band (and slightly outside it). Radio amateurs are ingenious people and we have got over all these things in the past. This is just another one.

I have a big knob on my radio marked VFO. I can change frequency, and I will if that makes sense and does not ruin everything for everybody else. Nothing stays the same for long in my world, and I would not try to keep them the same anyway.



Sunday, 20 August 2023

Progress of a 6m opening to Japan and South Korea

I have referred to this 50MHz opening from Scotland to Japan and South Korea on 23 July 2023
before. You can find it mentioned here.

Since then I have tried to follow the progress of the opening on a map. Here is the list of contacts on the map listed by time of working ...

50MHz stations worked at GM4FVM on 23 July 2023

You will probably need to click on this image to enlarge it to see the detail.

With two outlying contacts, the opening started on Hokkaido at 06:51 and then moved progressively down the East Coast of Japan until 07:52. At that stage I became interested in working South Korea and spent a bit of time trying to work three stations there. Between 08:03 and 08:22 I worked 3 stations in South Korea and three more in Japan, after which the opening faded out. 

There was a fairly steady progression down the East Coast of Japan and only real outliers were two contacts back in Hokkaido at 07:58 and 08:01, which I have marked with grey hatching. There may have been gaps when the propagation did not make landfall and the band was open to areas of sea.

Of course once a station has worked me, they are not likely to work me again. You tend to see the front of the propagation moving, while the band is still open for a while behind the front but those stations have already worked me. I think that the reversion to those two stations in Hokkaido does not necessarily mean that there was a new opening, but rather a couple of new stations have come on the band.

We have seen this type of thing before during single hop openings, for instance as reported here in 2019. The pattern of Es contacts moving in a (more or less) steady line over time seems well established. But it was interesting to see it happen at the end of a multi-hop path as far as into Japan. Is this multi-hop Es? Certainly it looks like Es at the far end. I suspect that a similar analysis viewed from Japan would have seen the propagation open in a path across Europe or the UK.

What we do not know is how the signal gets between Es at my end and Es at the far end, I am having doubts about whether the "bit in between" can really be multi-hop Es. If Es moves the way we can see in single hops or at the end of a long path, then similar moves along the path to Japan would knock the whole thing off after a matter of minutes. If there were several hops all inclined to move along as we know Es does, then the geometry would mean that the overall path would be lost very quickly.

I do not have anything to suggest for mechanism working in the "bit in between". A special type of polar propagation has been suggested and who am I to disagree? It would have to be pretty stable to work for  75 minutes over a path several thousand kilometres long.

Anyway, it was interesting to see the same pattern of contacts moving across the region but this time at the end of an extended path stretching as far as Japan and South Korea.



Monday, 31 July 2023

Be thankful for what you got

Can it be almost 50 years since I was first licensed? I sat my Radio Amateur Examination in 1974.

Ah, 1974. Simpler times. My revision notes consisted of one book someone gave me when I bought a receiver. And that was before they invented propagation, so no need to learn about that. But more on the days before propagation in a later post. Perhaps.

1974 had a warm Summer and "Be Thankful For What You Got" by William DeVaughn. What a great record for a hot Summer. Long and slow, with an insistent bass line and tight percussion. They don't make them like that any more.

"Keepin' up with Mr Jones? You don't need a loan!"

Despite William's advice I was later to borrow the money to buy an FT-101. Not exactly keeping up with Mr Jones, I told myself, just trying to hear and be heard.

Deary me. I still just want to hear and be heard. Yet, I get flustered when my station does not seem to be performing. I seem to have forgotten Billy's mantra about not to pushing on. I should be more thankful for what I have.

I also get frustrated about propagation which seems to have become "a thing" in the intervening five decades.

And so I was complaining about 6m propagation on this blog as recently as 9 days ago here

"That gap into North America looks very glaring"

but I also said 

"I have in the past worked in that direction into mid-August. There is still a chance."

Does complaining work?

Next day there was an opening and I worked 18 stations in the US and Canada over the space of 50 minutes starting at 13:09. I then had to wait four days until the next opening at 12:10 when in 26 minutes I worked 22 stations across the Atlantic. And two days after that I found the band open at 21:07 when I worked 9 stations in 23 minutes, and the activity only stopped when I realised that I was falling asleep.

US and Canadian stations worked on 50MHz at GM4FVM from 24 to 30 July 2023.

Total opening time was 99 minutes for 49 stations, which on FT8 means being pretty busy. All three openings met my standard for a pile-up, in other words I was not calling CQ but people were calling me and a queue formed to work me.

Perhaps I should complain less and be thankful for what I have got.

I heard somewhere that patience is a virtue. Maybe I should become more virtuous. 

Not so sure about that one. 

"Be Thankful For What You Got" sleeve. Image: Wikemedia Commons



Friday, 28 July 2023

Honey, I blew up the linear amplifier

Well, the linear amplifier blew up, but it seems it was not necessarily my fault. Nor was it the fault of the linear.

This took months to resolve so this is a long posting.

Of course, I have a long history of blowing up linear amplifiers, going back almost 50 years. I trust linears less than any piece of equipment I have. They are almost always about to let me down. Having said that, this posting is NOT about a linear letting me down. It is about me trusting a radio to transmit a low power output, and me taking too many risks by assuming that it would behave itself.

I have tried to write this piece several times, but it looks as if I am blaming the linear, which I am not. I will keep going on about that, just as I go on about lots of things in this blog.

Icom IC-7100 "It was him what done it, Constable".

There is a risk using modern high gain solid state amplifier devices in amateur service. Due to the enormous gain available the danger is that it will prove impossible to reliably restrict the RF power supplied to the device. The attenuator built in to the linear has to be set at a level compatible with the radio, and thought has to be given to the possibility of a sudden RF spike over-driving the device and its supporting circuitry.

So the spike in this case was generated by the Icom IC-7100 (or possibly the computer too) and that caused damage to the linear. Multiple spikes over a long period, or so it seems. How could I let this happen? Well one factor is that the IC-7100 has been operating into other linears with higher value attenuators for years. These larger attenuators make the chances of overdrive are much less. As a result I had become lulled into thinking this was not a big issue.

I have been thinking about this type of thing for a long time. The same problem arises with transverters as I explained here back in 2019. In fact I was extremely careful with transverters and even then I managed to over-drive the attenuator in one of them, though in that case without any calamitous results. It seems as if I failed to learn that lesson.

When it comes to VHF and above the amplifier has the RF power on hand to destroy any masthead preamp or other device further along the chain, and that also has to be factored into the equation.

Despite all the knowledge I had, I still managed to over-drive my linear despite having set the "RF Power" control to the correct level. The simple cause was that I had relied on the ALC operated "RF Power" control to keep the drive power low at all times.

If the RF driver relies on using the ALC circuitry to limit the output power, the chances of a random power spike are quite high. Connecting this transmitter to a computer which has its audio turned down and expecting that level to be uniform is another risk in the chain of potential for over driving the later stages.

At this point I can hear a voice over my shoulder saying that valve linears are much more reliable. You know who you are. Well, maybe. Not necessarily if you take care and if your linear is carefully designed, as this one was. I have three solid state linears which have given more than five years trouble-free service. And some that didn't.

So here I am talking about my Tajfun 1000 500W 2m/70cm linear. I do not think that it was in any way at fault in what happened. Neither was the excellent SHF-Electronik MMV 432-VOX masthead preamplifier which went arrrggghh at the same time. Nope, it looks like RF spikes from the rig that caused the problem.

I have to say that both the suppliers, VH Electronics for the linear and SHF-Electronik for the preamp, could not have been more helpful in fixing the consequences. Aside from the postage, the charge was €70 for the linear and €7 for the preamp. From this you can conclude that it was not the main RF device which failed in the linear but simply one diode.

Tajfun 1000, the victim of the problem, not a problem itself
This is not a review of the Tajfun, a linear amplifier which I think pretty highly of. It is very difficult to photograph as it is glossy black on the front and the display panel is pretty bright so it presents me with a technical problem when it comes to making an image of it.

Anyway, moving on to fixing things, ...

The fault showed up when suddenly the Tajfun lost output power and the preamp suddenly had more gain when it was out of circuit than when it was in circuit. Something had happened.

At this stage Vlado at VH Electronics, who had supplied the amplifier, went to great lengths to try to diagnose the problem and fix it. Thanks to the nature of the design, a lot of things could be fixed and indeed updated over the internet. The software for the display was sent to me and I was able to re-load it on a new SD card, and then the same was done for the firmware for the control board. These things helped but did not solve the underlying problem. After eliminating the relays and cabling, the overdrive had caused a fault which had to be in the amplifier RF stage.

Another clever thing that Vlado was able to do was to interrogate the records of overdrive and power output warnings which are stored in the control system. From this we discovered that the Tajfun had (successfully) coped with a series of huge overdrive situations. The output produced by the linear was at least 10dB above the power settings I had used and 3dB more than the full peak power the linear was rated at. Running any linear at twice its rated power is asking for trouble.

In each case the Tajfun protection circuit had cut out as it should have done. However, it was clear to me that this was not me deliberately operating at these ridiculously high power levels but probably something which was momentary. I was setting things up for 200W output so it was not that which was the root cause of the trouble.

The control circuits cannot generate more power than the transmitter can produce so the basic fault had to be in the transmitter. I will deal with that later.

For now I need to say that once he had eliminated all the other possible causes Vlado asked me to send the Tajfun back, which I did. The Freescale device had survived running at twice rated power, and the power supply had coped with supplying the necessary extra amps. Obviously, this overdrive was indeed  momentary. Vlado quickly identified a diode in the bias circuitry and the linear issue was fixed.

ITB RF board inside the Tajfun 1000.

The Freescale MRFE6VP5600H device is in the centre of the photo under the alloy plate, firmly screwed down. At the time of this photo I had tagged on a direct co-ax feed at the bottom of the board, bypassing the relays to check that they were not at fault. This also bypassed the attenuator which can just be seen below the red wire, bolted to the bottom of the RF enclosure.

Early in our search for the fault Vlado had considered that the attenuator could be at the root of the issue. While he had the machine with him he wisely changed the attenuator for the most recent version, increasing the power reduction from 5dB to 6dB. We could find no evidence that the attenuator was at fault, and anyway it seems to me that it would be unlikely that it would fail momentarily, and then recover, several times.

I have nothing but praise for the Tajfun. Having been all round the inside of it I can say that it is carefully designed and well made.

So the Tajfun returned and quickly resumed producing the power I was looking for.

You cannot overdrive any linear, not even a good one.

Siggi at SHF-Electronik quickly repaired the preamp which had been driven with 6dB more than it was rated for. Once again, not the preamp's fault. The service offered by Siggi remains exemplary. It took me a while to get around to dealing with the preamp, and in the meantime I had replaced it. He fixed it and returned it very quickly, and now I have two 432MHz preamps.

Even I cannot use two 70cm preamps. 

I need to thank Richard GI4DOH for helping with the logistics of getting the linear repaired by Vlado. I will not go into the details, but Richard was key to resolving the whole thing.

The fundamental problem was my Icom IC-7100. Well, that conclusion is inescapable. This was the source of the RF drive which blew both devices. I have had spike problems before. I had grown complacent.

I had connected the IC-7100, capable of delivering 35W on 432MHz, and set the output power to 4W as that was what the linear needed. The possibility of the rig delivering a sudden +10dB spike may sound improbable, but actually the full rated input of the linear at 500W peak would have required just 10W to achieve. Anything over 10W would have pushed it too far. The evidence would suggest that it reached 20W at least.

So what to do next? I do not want to have fixed the Tajfun and preamp just to overdrive them again. 

I could have replaced the internal attenuator in the Tajfun with one of a higher value. That is the solution adopted in my Gemini linears which are set for 25W drive. However, I was reluctant to raise the value of the attenuator too much as the heat dissipated has to go somewhere and the linear generates as much heat as we might expect already. Also, what if that single attenuator fails?

In the end I decided to add a second attenuator in the tx line. The idea was that I could change the value if necessary, and move it outside the linear for cooling if that was needed too. Doing a few sums, it looked as if a 5dB attenuator would involve the drive increasing from about 4W to about 12W for 200W output, and leave 8W to be added to the heat inside the Tajfun. The Tajfun has efficient temperature controlled turbo type fans. This means that any spike would be reduced by 5dB, and the result would not over-drive the linear or the preamp.

Additional attenuator which is now fitted inside the Tajfun 1000

Then I have considered other ways of avoiding the spike in the first place. The big problem here seems to have been me relying on the ALC to limit the power into the amplifier. One of the points made at the launch of the IC-9700 suggested that it has a "true power limiter". This appears to be the "TX PWR LIMIT" control. It is in addition to the "RF POWER" control which seems to be a standard ALC-type limiter. I say "appears" as this is not clear in the manual which states "The Transmit Power Limit function limits the output power to the preset level for each band.".

Anyway, the worry I have with this is that if anything did go wrong with this (for example if I messed up the settings) the IC-9700 can deliver 75W into the linear as opposed to the 7100s max of 35W. And, yes, I can get settings wrong.

I am guided by the idea that if something can go wrong, it will go wrong. Years of being steeped in statistics and probability have taught me the validity of that old saying. However, I can also get too carried away. It is not very likely to go wrong. Adding 5dB attenuation to the linear still gives some protection. If I remain worried I can increase the second attenuator and help cooling by placing it outside the amplifier casing.

In any case, I am losing confidence in the IC-7100. The years roll on and new better designs appear. It now looks pretty dated as a superhet radio in a shack full of SDRs.