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| Terrestrial television antenna at GM4FVM, with bandpassfilter and masthead pre-amp |
The past month brought record February temperatures to Scotland, reaching over 18 degrees when at the same time last year we were cut off for five days by deep snow and had -11 degrees.
The cause for all this was a high pressure which poked its nose out from Continental Europe and settled down over the North Sea for a while. This is the opposite direction to the normal flow and resulted in the High blocking the normal passage of westerly winds from the Atlantic (or northerlies from the Pole).
This High did bring some tropo conditions, but it wasn't a great event. What has caught the attention is that such wide extreme of temperatures, such as the variation from last year, is exactly the progress we would expect to see if current climate change predictions are indeed valid.
Of course I am worried about the climate, but let us look at the radio for now, and then move on to the spellbinding subject of mast head pre-amplifiers.
Things kicked off on 2 metres on 22 February when I worked DL3TW at 13:57. It was slow to build up, but I worked DL6BF to round it off on 2 March. Even for a 28 day month it is unusual for an opening to last here from 22 of one month into the second day of the next month. To be fair, there was a tailing off followed by a brief ridge of high pressure at the start of March, but it still lasted a long time.
I have now reached 12 DXCC and 32 squares on 70cms, and they all feel like a lot greater individual achievements than 30 DXCC and 146 squares on 2m.
The High has gone, the storms have returned, and all that is left for me at the moment is 4m meteor scatter. Moonbounce has produced few results recently, though I have heard Japan on 2m and China on 70cms.
Moving on ...
I said last time that I had thrown together a temporary sequencing jig to use while my Gemini 2 linear amplifier was out of operation. It is back now, but I had promised (threatened?) to write something about sequencing.
Before all the complex stuff I just want to say this. Adding a masthead pre-amplifer on 2m and 70cms absolutely transformed signals received at GM4FVM. Sure it is a fiddle, unless you buy a linear with the sequencing circuitry built in, in which case you still have to build or buy the pre-amp, but the results are excellent.
Where you locate a pre-amp becomes very important when dealing with Earth-Moon-Earth (EME) communication, but the same principles apply in all cases, and especially when it comes to using linear amplifiers. If you cannot hear stations you cannot work them.
In EME stations have to overcome very high levels of path losses, around 250dB. This means that the receive sensitivity of the station needs to be very high. Standard transceivers are not good enough for this, so a pre-amplifier is required. However, it is much more effective to amplify the received signal at the antenna before it passes down the coax cable and into the receiver (a so-called "masthead pre-amplifier"). The best signal-to-noise performance is achieved by locating the pre-amplifier before the signal suffers losses in the coax.
Another case is when you add a linear amplifier to an existing transceiver. By raising your transmitted signal by, say, 3dB, you may well feel that you need to raise the receive performance by a similar amount. This is because the stations who can now hear you may only be running the basic transceiver power you were running previously, and you won't hear them. This is why many common VHF linear amplifiers, e.g. RM Italy and Microset, have pre-amps built in. These may or may not be an advantage to your system, but once again a better place to do pre-amplification is closer to the antenna. Most high-end VHF linears do not have built-in pre-amps for the simple reason the makers expect high-end stations to have the pre-amp near the antenna on the mast head.
In an effort to receive out-of-area television, the terrestrial system at GM4FVM has a masthead pre-amp. Our local TV installer fits them as standard. On receive-only systems this is pretty simple to achieve. To save running a separate DC supply for the pre-amp most systems use a set-up which puts a DC bias voltage on the coax which is peeled off at the masthead and used to power the amplifier. This is done using a "Bias-Tee".
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| Simple masthead pre-amp arrangement for television reception |
It gets harder to apply such a simple set-up to a transmit/receive system like in a radio amateur's station. The pre-amp would be destroyed if driven by RF power during a transmission. Thus amateur pre-amps often have an RF-sensing "VOX", which will bypass the sensitive circuitry using a relay as soon as it senses RF power coming up the coax. The drawback of this system is that by the time the RF is sensed it has already arrived, so the damage may have been done. Also, the relays will take time to operate, increasing the risk of damage before the bypass is in operation.
The simplest way round this is to exploit the circuitry used in almost all pre-amps made for amateur transmitting use. By convention, when the DC power to the pre-amp is cut off, the relay is de-energised and defaults to the by-pass route. This allows just a simple on-off supply to the pre-amp. "Power on" activates both the receive circuitry and the relay, "power off" effectively turns it to transmit by turning everything off and so the de-energised relay switches everything out of the RF path. Note this this is the reverse of the convention for linear amplifiers, where the relays are energised on transmit.
Thus at a basic level all you need to do is to turn the pre-amp supply off at the same time as the push-to-talk line from the transceiver is grounded and you should be safe. However, you cannot use the PTT line as it works in the opposite sense, and using a relay to switch it makes it too slow to avoid damage on the pre-amp.
It can get even more complex than that, at least if you want more certainty at higher powers. You know that if something goes wrong with the switching circuit and the pre-amp remains live for any reason, the VOX should operate. However, most pre-amps set a very low limit for the power the pre-amp can handle when switched by VOX. This is because higher power risks building up before the VOX relay has activated.
To be safe, if the pre-amp should accidentally remain energised, it is best to take some steps to prevent full power reaching the pre-amp before the VOX relays have fully switched to by-pass. VOX might protect the circuitry at lower power levels, but it cannot guarantee to do this at high power. You can only really do this by not activating the linear amplifier until after a time lapse. OK, if some power does come through from the radio during that time it will reach the pre-amp at lower power and thus be safe for the VOX to cope with. Once the VOX has switched the relay for lower power then full power should be safe.
This should give you two elements in your system - one turns the power off to the pre-amp straight away, and the second delays any high-power reaching the pre-amp to begin with to allow time for the relays to work. In fact, either should work if the other does not. You might not think you need both, but you do. Or, at least, you will find that out for sure if you rely on either alone. Pre-amps are expensive, and you have to buy two if you blow the first one up.
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| 2m and 70cms pre-amps on the mast at GM4FVM (I must shorten the 70cms cable) |
A sequencer does everything in a certain order with a time gap in between to give relays time to act. It does this at the start of a transmission, and restores everything in reverse order at the end. If you have added a pre-amp to balance a linear amplifier then it makes sense to control the linear too. At its simplest level you take the PTT line from the transceiver, which would usually go into the linear amplifier. You take this line instead to the sequencer. The sequencer then takes actions in sequence at the start of a transmission:-
Step 1 - cut off the DC supply to the pre-amp
Pause to allow the relay in the pre-amp to act
Step 2 - activate the PTT to the linear amplifier
This way you delay the possibility of full power from the linear reaching the pre-amp before it has had time to act. In the same process you have also changed the sense of the PTT line to cut off the pre-amp power as the first step to be taken and the last thing to be restored, protecting your expensive components.
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| 2 step sequencing for an amateur station using a transceiver, linear and pre-amp |
This is the system in use at GM4FVM. For sure I could add more layers of complexity, but so far it is working well. The only other step I have taken it to slightly increase the built-in sequencing time in WSJT-X ("TX delay") to 0.3 seconds.
You can, however, take sequencing to several further stages depending on your station.
How quickly you radio activates its PTT line in relation to sending out RF will vary - sometimes they might share the same relay, but generally the PTT will be quicker, which is a form of sequencing in its own way. I am content with this, but otherwise you can work the sequencer with a foot switch and let the sequencer operate the radio PTT after it has switched the pre-amp off and the linear to transmit.
Even more complexity can arise with transverters. You could use the sequencer to separate the transmit actions of the transverter from the transceiver, once again to protect the the transverter receive circuitry from accidentally receiving power from the transceiver.
Most designs for home construction, and commercial sequencers, allow you to program various permutations up to about 4, so that you can tailor their actions to your individual needs.
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| The 4 step Down East Microwave sequencer with 50pence piece for scale |
Recognising that most amateurs will want to add a masthead pre-amp to balance the extra transmit power which comes with a linear amplifier, most makers of high power linears will include a sequencer and circuitry to control a masthead pre-amp with it. Certainly my Linear Amp Gemini 2 provides this, and I believe the OM Power and Beko amplifiers also include it. The Gemini includes a plug which, which correctly wired, applies the DC bias voltage directly to the coax without the need for an outboard Bias-Tee.
If you do not add a linear, or it does not include a sequencer as standard, you will have to incorporate a Bias-Tee to add the DC voltage to supply the pre-amp.
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| SSB Electronic Bias-Tee |
The results at GM4FVM - spectacular. As you might expect with losses in the coax increasing with frequency, higher frequencies show most potential for improvement. I was impressed by improved signals on 2m, but at 70cms the results were dramatic. For example, the 70cm beacon GB3NGI at IO65vb is 272km from me. On 70cms during flat conditions I can barely hear it. With the pre-amp in circuit it rises to a 569 signal. On moonbounce, signals vanish entirely without the pre-amp. With my linear amplifier in circuit I can work almost anyone I hear, including ones running low power. This suggests that the balance between my receive and transmit performance remains good.
From time to time I turn off the pre-amps just to see what happens. Terrible. I doubt if I could do without them now.
Of course the results will depend on what coax you are using, the performance of the pre-amps and the basic ability of the radio you are using. However, for serious DX on 144MHz and above I think masthead pre-amps are a necessity.
If you use a multi-band antenna with a single feed you can get wide band masthead pre-amps, though of course single band versions are likely to perform better. You can buy various makes of pre-amp, ranging from very high performance ones down to fairly modest ones, with prices to suit. Mine are on the modest side, but still worthy of use. It is something worth considering, I think.
73
Jim
GM4FVM
Hi Jim, thanks for the informative post.
ReplyDeleteAs the owner of a shiny new Ic9700 and a Gemini 2-500 I am looking at sequencing options for my MVV 144 VOX masthead preamp.
Given the sequencer in the Gemini are you taking PTT line from the transceiver to the sequencer in the Gemini which cuts off the voltage to the preamplifier and then allows the amplifier to function?
Am in NI at the moment and away from the instruction manual!
Gav
ReplyDeleteSorry I missed that post. I only use the DEMI sequencers on non-Gemini amps. For 2m where I have the Gemini 2, I use the built-in sequencer. As you no doubt know, the 9700 should have sequencing built in but I doubt if the Gemini has a DC path through it (it might have, I must check). Anyway if I had a 9700 I would probably leave the DC supply on the coax turned off and still rely on the Gemini.
The DEMI sequencers I use on the Microset and TE Systems linears and none of them have a DC path through on the RF side (they have preamps built in). Unlike the Gemini they have no built in sequencers, so they need an added unit.
Jim