Sunday, 10 May 2015

Pack Antennas

One of the problems that need resolving for the Lyke Wake and GB0MAG, is the antennas. We need something that will give good performance, increased range, but also have no impediment to access to our packs, or any risk of snagging on anything, or causing any obstruction to either us or any other walkers. It also must have the absolute minimu of weight!

As the radio to be used is a handheld, this will be secured on one of the chest straps of my pack. The radios own rubber ducky antenna would be far from ideal. Directly mounted better antennas such as my Super-rod-2, would be wildly inconvenient, in fact, bordering on dangerous! So, what is needed is an antenna solution that mounts to our Bergens, high enough to give a good 'field of view', but effectively out of the way.

What looked promising was this W6JJZ 2m Bicycle Antenna - a variation on the J-pole. So, I set about building one

The first thing I had to do though, was find out what the velocity factor was of the coax I intended to use! Luckily, this is something that can be done with my antenna analyser. I did find it a bit tricky at first, until I remembered to add the series 50 ohm resistor!

With the velocity factor found, about 0.66, the next tricky part was the matching stub. Unlike a standard J-pole or Slim Jim antenna, this design turns the matching section away from the bottom of the antenna, and out to the side, forming the section from a coax stub. Fabricating this proved a bit tricky

The lot once built was taped to a couple of top sections of telescopic fibreglass tube, and tuned up. The short required in the stub was created during testing by pushing a dressmaking pin through it to short the inner and outer. It was found that this short is the primary adjustment, and is quite crucial, a few mm either way equating to a MHz or more! Final fina adjustment was made by trimming the radiating element.

The photo below shows the first build, ANT1, attached as for field use to my Bergen. Tests on this one, which has the tuning stub sticking out at right angles, showed 1.5:1 SWR points at 144.6 and 146.6MHz, with 1:1 and indeed resonance (X=0) occuring once adjusted in the FM portion of the band, just where needed. On-air tests showed a 7- S-point improvement over the radios own rubber duck antenna at the same height.

A second build, ANT2, was tried. This was specifically constructed to remove the various joints in the first version, and to run the stub parallel to the feedline, meaning there would be nothing sticking out (the stub is only an inch and a half long anyway!)

This second build proved more difficult to tame than the first. The parallel stub seems responsible for greater body capacity interactions, probably as a result of common-mode currents. A pair of TDK snap-on ferrites solved this, but made the match more critical. On-air performance was the same as with ANT1.

The bottom of the fibreglass mast has a dowel  inserted and secured with filler. This engages with the eyelet tag at the base of the pack to secure the mast.

It is known that one of the issues of this design (the J-pole) is common-mode, due to the absense of a true ground return. The ferrites used on ANT2 act as a choke, preventing these from flowing. Its likely that the wide bandwidth of ANT1 is a result of the common-mode currents and the aparent lack of interaction due to the longer feedline length acting as a counterpoise. This would be a concern if the antenna was to be used in a more RF intense environment, or where RFI is an issue, such as built up areas. But these antennas are for remote field use. On-air performance, and in particular performance from mountain top and ridge line, if of much greater importance, and will be tested i na  couple of weeks on a SOTA.

Oh, and my LiPo batteries have eventually arrived! Ive also aquired a control box for the rotator, so I can now see if that is workable. If it is, It can go up and have the 6m beam on it, and perhaps also a 4m Moxon

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