Home > Daily News, HF, VHF/UHF > Of Antennae and 4WD’s- Part 2

Of Antennae and 4WD’s- Part 2

Following on from Part 1. Today I began examining all of the feed line issues that presented themselves in my 4WD.

I started with the two UHF CB radios. Both are GME TX4400’s– one uses a single antenna mounted on the bullbar and is used for communication with truckies, the other uses co-phased 5/8 wavelength antennas on the roof rack, and is used for communications with our tour convoy. Both systems had issues of poor transmit performance,

I started with the single antenna a GME AE4705 a robust antenna. Despite it’s tough design, I have seen them fail when subjected to the endless corrugations of Australia’s toughest tracks. I began by inspecting the antenna itself which was quite intact. Next I disconnected the antenna from the feed line and tested the feed line loss using my MFJ analyser. I found a hefty 3dB loss over the less than 3 metres of cable and that was at 120 MHz not 477MHz (my analyser does not include UHF), so I began isolating the problem which turned out to be a failed joiner and a bad joint in a PL259. Once repaired the loss came back to about 0.1dB overall at 120 MHz.

I then started on the co-phased UHF antenna system. This turned out to be a bit more difficult but once again came down to a bad job on the terminations. I repaired those and tested the setup on the local repeater. I’ve made the co-phasing harness using all 50 ohm cable which technically is not ideal but I suspect the antennas are less than 50 ohm impedance (possibly about 30 ohms) so the two amount to about 60 ohms which is as near a match to 50 ohm as I would achieve using a 1/4 wavelength at 75 ohms, and the losses of all the joins of the phasing harness at UHF would probably negate any gains from the matching sections, so I am happy so far with this setup. Tomorrow I’ll check the actual antenna impedances. I can’t test the SWR at UHF but I reckon on about 1.8:1 if my impedance calculations are OK. The two antennas are a little less than 2 metres apart.  I suspect they should be odd multiple of 1/4 wavelength to work best so at 477 MHz, the nearest value would be either 1.88 metres or 2.51 metres. I intend to investigate the effect of spacing later.

I then started on the  HF auto tune antenna where I spent most of the day. This amazing antenna performs far better than it should in theory. HF auto tune antennas such as the Barrett 910 are pretty low efficiency affairs, but this antenna has been fantastic nonetheless. I’ve been very lucky to make several  overseas contacts while mobile, and commonly get great signal reports from this mobile setup, which I put down to good earthing and bonding. But the 910 was not tuning below 3.9 MHz no matter what, and that present problems for me on 80m nets and on 2020kHz RFDS.

I have long suspected a problem with the feed line but what I found was a problem with the connections on the cable that is responsible for tuning the antenna. This connection was attached to the chassis near the rear wheel, and though wrapped in self-amalgamating tape, is in a pretty poor state. I think ultimately a new cable will be required, as the contacts are not bright, despite several efforts at cleaning. I’ll have to test the antenna at frequent intervals over the next few days to see if it continues to work, or if the tuning starts to fail again.

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Categories: Daily News, HF, VHF/UHF
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  1. June 10, 2011 at 1:13 pm

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