Well, so far, people may have noticed I’ve been rather quiet on the Mt. Glorious VHF repeater, much to my frustration. Google Maps in the Terrain view, shows why…
In red, are the mountain ranges in my way. In blue, the attempted path of transmission. The green arrow shows my transmission location.
(Map Source: Google. Map data ©2008 MapData Sciences Pty Ltd, PSMA)
I can get into VK4RBC without any difficulty whatsoever. It’s a UHF repeater, and as it turns out, the rubber ducky antenna on my handheld, is just 10mm shy of 1/4 wavelength at the required transmit frequency (433.525MHz, the wavelength is approximately 692mm). So 0.5W gets in without any trouble. But… the antenna is not efficient at VHF — it’s a lot shorter than the 500mm required for efficiency, and I’m also far too low for my already weak signal to make it.
Now… if I take a bus over the hill to Keperra, I can get into VK4RBN with only 0.5W of power. No problem. However, here in The Gap, the only way I’ve been able to do so, is using my transceiver at full power (5W; 2W higher than permitted under a Foundation license) and I have to practically sit on the roof to get my signal over the range.
Using a 1/4 wavelength dipole, strapped to an old broomstick (pictured right), I was able to get in to VK4RBN. But only just … the signal was weak, and apparently, very crackly. Aiken (regular in #hamradio on Freenode) confirmed this — we could make contact, but only just. Thus it was necessary to construct a better matched antenna, and mount it up high, preferably on the TV antenna mast.
Whilst talking with Grant on VK4RBC yesterday, he mentioned an antenna design commonly called, the “Slim Jim” which can be constructed from a strip of 300? flat antenna cable, often used to connect TV antennas. They are fed with 50? coaxial cable — very convenient for my set — and can be rolled up for storage. So for a portable antenna, they’re ideal.
Asking around in #hamradio, Magne pointed to this site, which explains the theory behind the “Slim Jim”. Apparently it’s one of many forms of J-Pole antenna, which is itself, related to the Zeppelin antenna (used with the aircraft of the same name). They perform pretty much identical to the end-fed folded dipole antenna — in theory.
This afternoon, I wandered into the electronics store, and bought a soldered BNC socket, and 5m of outdoor-type 300? twin-lead cable (Dick Smith Electronics are selling it for about 80c/m, other places may have it cheaper). About 10 mins of work, and I soon had my antenna constructed. I soldered it to the BNC plug I had purchased. The constructed antenna is shown to the left (click for larger image).
For the feedline… we used to run a Ethernet local area network on coax cable, and still had literally oodles of this BNC-terminated RG-58 cable lying around doing nothing. A small SMA->BNC adaptor on the top of my handheld, and I was soon reaching VK4RBN on the 2m band.
Performance was significantly better than the rubber ducky I had been using. I still had to use 5W of power, but it got through! I had some interference trouble at points, the cause is unknown at this stage, certainly my questionable antenna building skills can’t be ruled out, but despite this, I had a around a one hour QSO with Ken on the VK4RBN repeater (a regular on this repeater). This means I should finally be able to join in with the BARC net on Wednesday nights, instead of just siting back as a spectator.
My one observation — I had planned to tape the antenna up to a fiberglass rod (originally from a tent — the rods themselves have some cracks in them, thus need replacing) and use that as a mast — however I soon discovered that this particular antenna did not like this kind of treatment. Doing so yielded very poor signal strength, and seemed to cause the receive frequency to shift — picking up some foreign radio station (not amateur).
Whether the two metal cylinders that link the segments of the rod together are upsetting the EM fields, not sure. I’ll have to experiment further, but if I let the antenna dangle in the open air, with the matching end up the top — it works fine.
Certainly in future, I’ll have to look at something more substantial, but this at least gets me to the local repeater. 😉