Sunday, 23 April 2017

5 element for 50MHz up at last, Aurora missed.

It is hard for me to imagine this, but my 5 element PowAbeam has been lurking in my garage since last November. I had always planned to put a better antenna up on the CUG mast once it was in place. In the event it had to manage with the 3 element section of my old Vine 50MHz/70MHz beam for the past six months.

At last a day arrived when I had the energy and the weather to get the new antenna up. All it should take was an hour or so to assemble it and replace the co-ax plug ... ?

Aurora was predicted, so I was hoping for a quick job. The antenna is aloft now, but it did not go up there easily.
PowAbeam 5 element for 50MHz now installed at GM4FVM
The difference in scale between the two antennas goes beyond the increase in boom length - from 2m to 3m. The Vine is a compromise antenna for two bands, the new PowAbeam is specifically built for 6m. I am hoping for at least a 2dB improvement. And then again the PowAbeam is a lot more substantially made and should resist the winds well. Being larger, it is also a lot heavier. Whilst my old HF minibeam is heavier still, it has a much shorter boom. I suspect that this is the largest antenna I have ever used.

It is hard to represent the difference between the antennas, so I took some photos ...
The two antennas with car for scale

With the old Vine (left) you can see the 4m element remaining as this can be a dual band antenna
Anyway, PowAbeams are easy to build. All you need is the correct Allen Key ("hex key"), and a couple of spanners (10mm and 13mm in this case). They say you can do it in 10 minutes but I try to make sure that the elements are correctly measured in the centre using the Stauff mounting blocks. It was longer than 10 minutes, but still easy to do.

Taking the old antenna off and installing the new one on the mast was simple enough. First snag then was corrosion in the co-ax. I had to take off the PL259 plug used by the Vine and replace it with the "N" type used by the PowAbeam.When I opened the self amalgamating tape which was covering the existing connection the PL259s looked bright and clearly no damp had penetrated the seal. On removing the 259 I noticed that the solder inside seemed to have softened and turned to mush. Time to investigate further.

Cutting back the outer sheath revealed corrosion on the braid.
Corroded braid in the co-ax (note that the short length near the plug was relatively clean)
The black discolouration is familiar to me as I a similar problem with this run of co-ax when I tried to fit the ill-fated (and poorly performing) Mirfield "Quad Band" vertical about a year ago. At that time I cut the cable back for the same reason.

As the plugs seemed clean and there was no damp this time, I think this is another sign of the problem noted last year. Possibly I just happened to cut back to a short clean bit last year. Maybe there was further corrosion then if I had gone far enough back to find it. So this time I cut back in stages and found it stretched quite a way down the braid. I eventually found reliably clean bright braid after removing the last metre or so of cable.
Clean braid after cutting back a metre of cable
It remains possible that the damage to the cable may have gone further along and will cause further problems. For now though it seems only to be the end section, and the antenna seems to be performing well. 

That unexpected work took time, and to add to my frustration my mobile phone was constantly beeping to warn me of an Aurora. I also got a text from a local amateur to warn me. Infuriatingly I had to reply that my mast was tilted over and it was taking a while to get everything back aloft. I could have left 6m and operated on 4m and 2m, but once this job was started I though I had better finish. After all, the good weather might not hold.

When you want to get finished, I find that it is usually the last thing which lets you down. In this case it was the compression "N" type plug. It looked pristine, but when I took it out of the bag it turned out that there was solder on the brass centre pin. I must have used it before. I cleaned off the solder and tried to fit it. As usual, the two major issues I have with these plugs can to hold me up.

First, I cannot understand how you can effectively solder the centre pin. I know the theory, which is that if the pin is hot enough the solder will sink into the pin by capillary action and coat the cable centre. It is just that it never seems to work that way for me. With Mrs FVM diverted from some essential task to hold the co-ax I finally got it on somehow.

Then the second problem arose. As I so often find, it was impossible to start the lock nut threading into the barrel of the plug. As this compresses the rubber seal you have to do it, but in the process it is necessary to push the barrel one way, the nut the other way, and turn one relative to the other at the same time. As Mrs FVM had returned to whatever it is she does, I had to finish it myself. And it just would not go in. I knew that once I got the threads started it would be fine but ... grrr, it just would not start.

Eventually, of course, I got it going, got the mast vertical and the beam is now officially "up". Whether 2dB makes any difference remains to be seen. So does the question of whether it is indeed 2dB or a lot less. We shall see.

But progress has been made, I suppose. It took over four hours to do it.

The Aurora seemed to be over by the time I got back into the shack. Via Aurora I heard various beacons, and GM4VVX on 2m and GM4UYE on 6m. No contact resulted.

More Aurora is promised for later today. Now that I am ready, what chance is there of that happening?

73

Jim

GM4FVM

4 comments:

  1. Jim,

    Glad you have got the new antenna on the mast at last! I confess to having antennas in my loft that I have owned for years that have never been put up at all. I have a pair of crossed Yagis for 144 MHz still in their box...

    A couple of comments on your new installation. I would seriously think about scrapping that old coax, who knows how lossy it is now with braid the way it is? You could measure the loss I suppose, but it rather looks like it has seen better days. Think about replacing it with some Ultraflex 10. I know it is a pain dragging new coax through to the shack and nothing is ever a 10 minute job in my experience. Was the antenna from Roger? For some reason I thought he used round booms. RIchard uses square section certainly. The DE looks the same as the one I used on my dual band Yagi.
    The other suggestion is to tape the coax to the boom to keep any RF off the outer. If you bring the coax down to the underside of the boom before the first director and tape it to the boom it will avoid going near the element mountings on the top. Perhaps when you replace that coax you can try that.

    You should be all set for the Es season now.

    Aurora wasn't great, you missed very little DX. A typical Scottish type Aurora.


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  2. Thanks David.
    You are of course correct. I like Ecoflex as it is easy to work with. I have it on my 2m beam. I guess the price of a run of it to the far mast at £99.60 + carriage puts me off. What cost performance? Plus the connectors of course, though they do have a solderless "N" type - which I would need to examine before being convinced.
    Your comments made me think. There are two runs of 213 out to that mast, and one has been there for at least 6 years, since the 4m vertical first went up on the gable end. I suspect that the moisture got into it then as there was no moisture trap with the vertical. Can water pass right through a PL259 plug? I suspect it can.
    Next question? Why are you not using the newer, better, run of co-ax for that antenna, Jim? Good point. I have no idea. Lost in the mists of time. I had fondly imagined that the connections were reversed. The old stuff should be on the 2m vertical which ... erm ... matters less ... even if it is a higher frequency and would benefit from good co-ax.
    It is a bit of a lash up for now and I will improve on it in future. I can measure the loss as you say, and compare it with the other run of co-ax.
    The beam came from the DXShop. The site says he does some antennas in round and some in square section. That site has errors, but the antennas are well made.
    I really must clear out the garage and sell/ scrap the unused antennas.
    After I came into the shack I ordered some more "N" types to restore the stock to approved levels. I love them and hate them at the same time.
    Jim

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  3. Ah careful Jim, I mentioned Ultraflex 10 which is slightly different to Ecoflex. Ecoflex has recently had a bad reputation for developing loss problems especially when used in situations like loops around rotators or crank up towers. However Ultraflex by M&P and their Hyperflex which is even more flexible for going around rotators is very nice stuff. It is available from ML&S, manufactured in Italy.
    I have an aversion to losing RF as heat on TX, and losing signals on receive, and also having to put masthead preamps up outside, hence my obsession with good quality low loss coax cable that will last. I have never faired well with braided coax actually, it was always the weak point in my EME system, and water in the coax or connectors was a complaint for me. However I can say that used or surplus Heliax type cable was one of the best investments I ever made, after my tower. The Heliax seems to last a lifetime, not so any braided coax I have tried. So it pays for itself, and in fact can be had surplus if you are lucky like I was. Connectors are expensive if bought new, but again it is possible to get surplus ones. You need to cultivate a friend in the cellphone or TV transmitter profession.

    I use Ultraflex 10 on 50/70 MHz. Half inch Heliax and Ultraflex 10 for 432. 7/8" Heliax and some old Westlake 103 on 144 ( but plan to replace that with some new M&P stuff too). I have grown to hate putting on connectors too, the easiest ones were for 7/8" Heliax! The special N types for the M&P are more fiddly but one of the better type. I don't use any 259 connectors at all now, I have adaptors from 259 to either BNC or N on radios, or have changed them to N types.

    All a bit different to Belling Lee TV connectors and 75 ohm TV (brown) UHF coax that I started out with on my first antenna for 144 Mc/s.

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  4. OK David, I misread your comments. Sorry.
    I take your point. Hyperflex 10 is indeed a runner. It actually looks to be cheaper than the Ecoflex for the lengths I need. For a small increase in outlay I can replace the 4m run as well, saving on ultimate carriage costs.
    Helix is beyond my horizon. My two runs of 30m and under 20m have several (relatively) tight radius curves so I look at that parameter of co-ax as well as the loss and the cost.
    I do not run as much power as some, maximum 200W for meteor scatter and less on 4m of course.
    There is time to plan this as I do not have the money right now anyway. Finding the damaged co-ax was one thing. Then it was a matter of rigging it up in the best possible way and starting to think about what to do next. If I had not put up the 5ele I would never have known until it failed entirely.
    N types are taking over here on VHF too. My first transmitter, the EMSAC TX2, had a Belling Lee and I also used old 75 ohm TV co-ax. As I had left my soldering iron at my parent's house I used a plastic BL plug with screw fixings. You do not see many plastic bodied RF plugs now. Of course, I worked far more people with that and its dipole than I could ever work today with such simple equipment. There were lots of contacts, but they were very short range.
    73
    Jim

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