Saturday, 6 May 2017

Short Antenna for 40m: Reload!

Late last year, I threw up a shortened antenna for 40m using a coil in the lower 1/3 of a 5m-long elevated vertical. This allowed my 7m fishing pole, which is much more manageable in our regular, strong winds than a 10m version, to be used for all bands.

The idea is to make a multiband antenna that is easily carried down the beach and doesn't cost much.  I also would like to use it at home on lower bands, where it can tolerate very high winds.  Being able to get away from any home QTH is always a good idea to keep fresh, because you never know when someone will start spoiling your potentially huge investment with RFI.

The first coil worked pretty well, but I couldn't get the best match.  I think that was because I used short radials, rather than tuned, 1/4 wave units for a given band.  Even so, the match was reasonable, at about 1.7:1.

With better weather of late, I turned again to having a go at making a coil to allow a much better match on 40m, using tuned radials.


Even in summer, our winds can be strong!  Initial coil testing at 40m.
 
Initially, I had started making a copy of the PAC12-type antenna, made of alumimium rods.  I found this very slow going, not least because I had to cut my own threads and make connectors for the various components, made worse by Imperial measurements!

It also became clear, as I put the first bits together, that an aluminium rod antenna with a long telescopic whip on top was simply not going to survive on a North Wales beach, where we regularly suffer some of the strongest winds on the planet.

So, back to good old-fashioned, wind-resistant fishing poles!

I wound a coil of about 20 turns on a 43mm PVC pipe, with a split pipe of the same diameter slipped over the first.  This creates a slightly wider former, with a gap down the length, allowing a crocodile clip or similar to be easily connected at various coil positions.

The split second tube slipped over the first creates a gap for a matching clip. This is the second experimental coil.


The coil, being very light, simply attaches with a re-openable cable tie to the pole. It looks much less neat than a coil incorporated into the body of a PAC12-type antenna, but that's just unimportant!

My loosely wound coil was disappointing at first, as it showed a rather poor match.  I decided to push to turns much closer together, such that each was spaced by only about 4-5mm.  Immediately, this brought a perfect match with the whole length of the coil used.

The next step will be to make a set of tuned radials for more bands.  The photo above shows my new coil drying in the sun.  This has about 8m of wire - at least double the turns of the first, closely-spaced, so that I can hopefully reach at least 60m. Some hard-setting mastic was used to initially keep the coil spacing in place (few adhesives will stick for long to the PVC tube itself, especially if left outdoors).

I'll be running the antenna on 40m with the first coil from tonight onwards on WSPR, using just 200mW.  A few hours last night showed the system to be working very well.

UPDATE:

Here are the initial WSPR results at 200mW.  The following plot is on the first overnight run, where the Kp index was slightly high, at around 4.  The comparison station is my usual reference station, a full wave horizontal loop in England:

Doing will, compared with a full loop.

In particular, I was very pleased to be one of only two or three UK stations being heard in ZS that evening:



On the second evening, with quieter geomagnetic conditions, the results were even better, again showing the benefit of a clear environment and sloping ground to the sea:

 
Second 12h run.  Nothing wrong with this antenna!


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