Sunday, 16 August 2015

Antenna Gains and Losses, And Finningley ARS Rally

Today, at 06:00, I loaded my car with a fine selection of vintage and surplus components and modules (or junk, as Julie describes it) and headed off to Sandtoft for the FARS Rally. Turnout today was good, a few more traders than last year it seems, and more visitors. And, unlike last year, the sun shone all day!

Sadly, neither of my two assistants could make it. This meant that apart from before the door was open to the punters at 10:00, and after everyone had gone and most of us were packed up (around 14:00), I couldnt leave my pitch, and therefore had very limited spending opportunity! In the end, not counting the cost of a cheeseburger (no hot drinks as i'd taken a flask of coffee!) I only spent a fiver! That got me a vintage roll of solder (but nice and hefty, for a quid!), three antenna whips, two 16x2 LCD modules (the 'valuable' parts, according to the organizing genius Kev) and a small plastic box with three BNC connectors - more of that later!

I wont talk profit on here, but suffice to say I made enough to cover costs, and to consider purchasing now a dew heater for my camera lens!

But the rally hasn't been the only thing going on. Bob M1BBV asked me if I would make him an antenna for 6m FM for SOTA use, in exchange for some camping equipment. A bit of a think and I settled on an end fed half wave design. Using one of my 5m telescopic fishing pole blanks, and some RG-58, I knocked this up in a couple of hours. A piece of PVC pipe with 13t of coax on forms the balun, which is slipped over the pole, and the end of the antenna has a string which is tied to the poles end piece eyelet. This means it stays vertical when extended

A bit of steady trimming and the match was brought to close to 1:1 at the FM calling channel.

 Of course, such an antenna for 6m is pretty long! It doesnt all fit in the photo above! But at least this antenna project was a success. My attempts to build a 20m and up mobile antenna of the 'Bug Catcher' design, unfortunately ran foul of my personal muppetry, when I attempted to remove excess enamel using a wire brush wheel

and thats the end of that coil! On a plus note, i've got the balun for my Cobweb housed into a nice IP rated box. Some silicone is still needed, but theres no rush

So, what about that box from the rally? Well, when opened up, I found this

So out that bit of coax came! In its place, went a VHF Low Pass Filter, and a UHF High Pass Filter, collectively, known as a Diplexor. Although not intended to handle more than a few watts, this means that I could use my tape measure satellite antenna with a dual band radio

With the glues now set, it was today time to work out how to attach the two parts of the tape measure antenna. Really, this is three antennas in one - a VHF/UHF satellite dual band antenna, a 2m SOTA beam, and a 2m ARDF antenna. This means that the UHF section should be a snug fit, but easily removable when just the 2m antenna is needed. The solution to the problem of mounting the UHF antenna onto the VHF boom was solved with another bit of the smaller tube, some more glue, and two holes

Each boom end has a hole drilled of 12mm diameter. An inch length of the thin tube inserted into the UHF boom hole, and secured with glue. The hole in the VHF boom carefully sized to be a snug but not tight fit for this, turning it into a locating lug. A reusable cable tie attached to the UHF boom at the point between the two coax baluns allows the two booms to be secured together.
And the final photo shows my assistant Sam holding the completed antenna

Next step then is to test it in combination, and then have a go at working through SO-50!

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