Apr 222013
 
Nice, straight and neat

Nice, straight and neat – click for full size image

Finally the antennas are up and working. Make no mistake some members of the club have worked very hard to make this happen, they should be proud. There is a lot more detail here than at first meets the eye. For instance Robert M0CRX took the time to make a very good balun former. This balun works extremely well on all the HF bands including 40m.

I am not an experienced HF operator but I have observed a lot over the years by being around. One thing that I can say for a fact is this: This is the quietest 40m antenna we have ever had up at G4NOK. It really is amazing to hear such a quiet band, the noise floor below signals is below S1 and then boom there is a huge s9+40dB signal there. This is also testimony to the HF rig’s good receiver, I was pleasantly surprised at how good the TS590 is in this respect.

The balun is not a true balun but a choke balun; it’s purpose is to prevent currents from flowing on the outside of the coax, you can see the balun structure in between the two antennas on the stub mast. The more observant will notice that the balun coil is at right angles to the feed. This helps to stop the fields from the antenna being induced on the turns of the coil. You will see this kind of thing all over the place. A standard way of preventing coupling between two inductors in a circuit is to orient them at 90 degrees to each other. The orientation of the balun simply follows this rule. The photos below shows the balun and feed in more detail, just click on the images for more detail.

Further out showing how the balun is clamped to the boom

Balun is clamped to the stub mast perpendicular to the antenna boom – Click for full size

A longer stub mast would allow the balun to come up from the underside of the driven element but this way is much stronger. This is important for us as we have an exposed hill top site. This way will also help rainwater run away from the cable. At the moment there are just ty-wraps holding the balun to the former but we will be coating the balun in a waterproof coating of some type in the near future.

 

Close up of the balun - maybe 'Ugly' but very effective

Close up of the balun – maybe ‘Ugly’ but very effective

Here is a slightly closer view, the inspiration for this implementation of the ugly balun came from Robert and the following site:

Build the ‘Ugly’ Balun

Notice the lack of connectors – cheaper , lower loss and nowhere for water to gather.

Close up of the A3S feed - Extremely waterproof

Close up of the A3S feed – Extremely waterproof

The tails to the feed are not super short like they would be for a VHF antenna but they are still quite low inductance at the frequencies we are concerned with here; namely 40m, 20m, 15m and 10m. There is a little give in case the antenna slips round in the 70 mph winds that we experience quite often. We have a plan to help ensure that there is no slippage of the antennas but you will have to wait to see what that is. I am a believer in letting the stub mast slip in the rotator a little rather than risking shearing the drive shaft of the rotator.

What I don’t like is the antennas slipping and thus damaging the feeds.Mick M6MWP has an idea how to stop this altogether.

Waterproofing 

For the A3S this consists of self amalgamating tape, heatshrink (2 layers), more self amalgamating tape and finally liquid rubber all over the eye terminals where they fasten to the boom. I don’t think that the water can get in unless the coax outer sleeve gets punctured. If you click on the image you will see that the photograph is so fresh that the liquid rubber is still dripping. It’s a bit tricky to get off as well so I recommend wearing some protective gloves.

VHF dual band yagi

As I may have said before the Innovantennas 12 ele dual band antenna is the best constructed antenna I have ever seen, it is extremely robust, chunky and yet reasonably light. You can tell the quality of the construction from how straight we have been able to get it when assembled. This antenna is dual band and so needs a balun that can cover both bands – once again a choke balun was chosen. Due to the higher frequencies involved the balun is quite a lot smaller. Also as we now on VHF it is imperative that there is no unnecessary inductance in the feed, so the coax tails are extremely short. I think that they are as short as possible. The cable passes through the boom and then the balun is formed by winding 6 turns on a furniture polish can. This is then kept in place with a few ty-wraps and the attached to the underside of the boom, once again at 90 degrees to the driven element – only this time in a different plane.

On 6m the antenna seems to be as good as my old M2 6M7 even though it is a little bit shorter. It is definitely a quiet antenna on 6m and GB3MCB in Cornwall was 559. On 4m the antenna is better than the old one but is a little off tune. The antennas have not been tuned at all yet.

Here are a few detailed photographs.

20130421_161333-1

Waterproof and low inductance – as recommended by G0KSC – click for full size

Choke balun under the boom

Choke balun under the boom

 

 

 

 

 

  3 Responses to “Antennas are up and connected”

  1. Thanks Conrad for the detailed and well presented report. They certainly do look good and. Am immensely proud of my own work and that of everyone else. Its a great pleasure to stand back look up and say I was a part of that and most of you can.

    I am going to write a construction guide on the balun so that visitors searching for info on the A3S may do so as we ourselves did.

    It was interesting that Cushcraft depict that the choke turns are not critical.

    Out of all the sites on the web Conrad looked at it happened to be one where my fence mounted balun is shown after I had e Diamond CP6 up in 2010.

    This former was a variation on that and made from scrap with exception of the coach bolts and jubilee clips.

    The bolts are heavy duty galvanised M10 7 150mm bolts so should be weather resistant and very strong.

    The supports for connecting HF to VHF beams that Mick suggested is by using fibreglass poles. These do need to be quite strong.

    I am going to look at a trial with clansman mast sections and if they work buy either two sections or A whole new mast kit for £35.

    It should be remembered that it is not an exact science and user needs vary considerably between station and although not seen anywhere else it may be the solution for us.

    Once everything is complete i will prepare a PDF report detailing full installation, advice received, equipment sourced, coaxial lengths and types. This will form part of the clubs maintenance record and be a guide for those following procedure in the future. By publishing the document online it will also raise our profile .

    More to follow post equipment trials.

    Robert
    M0RCX

  2. Might be worth buying some black UV resistant cable ties to replace those white ones with.
    Other than that.. well done to all involved.

    • Yes Robin, probably would have used black UV resistant cable ties if we’d had any. Not to worry we can change them later. Other than that – thanks

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