Saturday, 20 March 2010

HF mobile

I got my new ATU today and have been testing it at home. This one is fully automatic so just needs to have some RF for it to start tuning. There is a control socket (3.5mm jack) too which can be connected to the Icom so it can be operated by pressing the tune button on the radio. But I'll be using this tuner mainly with the FT-817 so it'll be fully automatic.
There are some SWR LEDs on the front of the tuner, on some bands it would not try and tune for a 1:1 SWR, giving about 2:1 at the first attempt. It will probably learn after a while to get a lower SWR, with a few changes at the start of each transmission. Like the AT-7000 tuner, it won't let me use my wire on 1.8 MHz but I'm not surprised with it being 8 times too short on that band. I'll have to try and get some signal out on 1.8 MHz, maybe just using the centre of the coax. For receiving on that band I've been using the A99 because even though it's so inefficent at 1.8 MHz (it'll be like a short circuit with hardly any coupling to the radiating element either), background noise is still what limits the receiver.

Tried some HF mobile whips on the car, I've had these for a few years but not used them much because I only have a smallish magmount on the car. The roof of the car isn't flat enough to hold the larger magmounts. So there's no way I'm going to drive around with a large HF whip on the car.
I found 4 HF mobile whips. I wasn't sure which top sections went with 2 of the bases but 2 were marked up.
28 MHz? - this one was resonant at 32MHz and didn't have many turns on the coil. Probably 28 MHz with the top section too short.
24 MHz? - this was resonant about 26.5MHz so I'm guessing it's meant to be 12 metres.
18 MHz - here is a graph for the 18 MHz whip -


14 MHz - this one looks OK, even with the small magmount -


I don't know why the 12 and 10 metre antennas were so far off frequency, I must have tuned them some time, probably on my old car but the 14 and 18 MHz ones were also tuned on that car so why was it just those? Unless I've mixed up the top sections but if they were swapped round, they couldn't both be too short?

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