Jun 232008

Well, I spent much of my weekend fooling around with antennas in one form or another.

I had taken down my HF rig to bring to the Brisbane Amateur Radio Club social — to sort out why it wouldn’t tune up on 10m… the problem turned out to be my power supply. I was using an old 250W AT computer PSU capable of supplying 9A at 12V. My radio, a Kenwood TS-120S is a 100W radio, and draws 20A when running full power. Now I had assumed since the mic gain was turned down to comply with my 10W limit, so the limited power wouldn’t be a problem… not so… it turns out that although I turned the mic gain down, the radio still wants its 20A for voice peaks. This causes the voltage to drop, and of course, instability.

Well, BuyEquip had some 600W ATX power supplies, that advertised a 52A 12V output rail, brand new for $60, so I snapped one up. A few more bits and pieces, and now my radio is much happier on 10m. Interestingly, the box says 52A, the unit itself says 20A… either way, I’ve met my requirements. 😉

Earlier when I had my HF rig set up, I had taken the balun out since I noticed it seemed to be shorting out the feedline (measured with an Ohmmeter), and I couldn’t even pick up commercial SW stations (I used to hear Radio New Zealand quite strong around 7.145MHz).

I later discovered that it’s quite normal for a 1:1 balun to appear as a DC short… my balun uses ~10 turns of not-very-thin copper enamel wire and I guess I’m used to transformers for other applications where one sees a much higher resistance. Transformer Baluns are typically almost purely reactive — remember the reactance of an inductor is Xl=2(pi)fL — at DC, f=0, thus my ohmmeter sees Xl=0.

However, I knew I had done something wrong when wiring it up as when I disconnected the shield — I received Radio NZ S9+10dB, connecting the shield back dropped that to S2.

In the meantime, I used a 5cm piece of RG58, soldered straight to some 300ohm ladder line (surplus from my sim jim antenna).

I wasn’t sure that I had wired the balun correctly — and had lost the plans, so I set about locating some on the ‘net. A quick search revealed this page from the Antrim & District Amateur Radio Society. Well… what a difference it made… my noise floor on 80m went from S7 down to S3!

I spent the evening chatting on the Australia Wide Night Owl Insomnia Net (Friday evenings after 10:00PM at 3.6MHz LSB) — talking with stations as far away as Coffs Harbour, and even heard a feint amateur contact from New Zealand (ZL1?? callsign).

The other issue, was with my handheld. I’ve got two portable antennas for it, neither of them are particularly efficient on 2m, both are brilliant 70cm band antennas. One is the antenna that Kenwood supplied with the radio, the other was a Comet SMA3 antenna that I had bought at BARCfest. Not bad for portable use — but I wondered if I could do better.

People might remember my old project, the Hat-lamp, where I set out to homebrew a headlamp using a hard hat. Two radio amateurs suggested that I add an antenna mount to that — one suggested I could have a SMA-SMA socket adapter there and use my handheld’s existing antenna, the other suggested a SO239 socket on the top with a mobile antenna.

Well I gave the idea some thought… The big issue with this is two-fold: clearance (the antenna would have to incorporate a spring to absorb being whacked against low objects) and the social aspect (what would people think after seeing it). Neither of the handheld antennas were particularly good on the former part — I managed to bend the newer antenna once just sitting down — it’s mostly bent back into shape now, but I didn’t want to risk it. Both would be rather conspicuous though. A mobile antenna would be a rather heavy thing to have sitting on one’s head, so I gave that idea a miss.

I found some stainless steel fencing wire that was quite stiff, and cut off about 60cm of it. The idea was I’d wind the bottom of it into a spring, and a SMA plug would be soldered to the end (using some copper enamel wire to make the connection). Well, I built that Saturday Night, and using it directly on the handheld, noticed an immediate improvement in performance — I was hitting repeaters I normally don’t hit unless I’m plugged into the roof antenna. It is shown below… click on the photo for a larger view.

Homebrew portable whip

Last night, I set out to attach the antenna mount to the hat. One hole and a bit of elbow grease later, I had screwed the SMA-SMA adapter into the hole. The antenna neatly screws onto the fitting around the back of the hat, and a length of coax screws in underneath running to the radio. I haven’t tried walking around outside with it, but indoor performance is good.  The photos below show the socket views on top and underneath the hat’s brim…

Antenna mount/socket topAntenna mount/socket underside

The plan, the whip is still rather conspicuous — and there’s the risk of doing someone an injury if I’m not careful where I point the whip. I’m now looking around for a souvenir peacock feather that I can stick the antenna up the centre of… the idea being the assembly becomes decorative as well as functional (below is what it looks like now, sans feather). Well kinda… it’ll still look weird, but hopefully people will notice the feather rather than the antenna. 😉

Antenna mounted on hat

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