Vehicle antennas for TX and scanning

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Vehicle antennas for TX and scanning

Postby G4RMT » Mon May 08, 2017 7:20 pm

I've got a survey job coming up so have been fitting extra aerials to my van, and have been experimenting with various types and after fitting the 4th have come to some conclusions I figured I'd share.

For years I have often used VHF 5/8th types because I believed the claims of gain (usually 3dB) and my somewhat random experience gave me nothing to that contradicted it. I fitted standard mounts to the van roof (A Vivaro) and experimented with different types, and these were VHF ¼ wave trimmed whips for 145MHz, marine band, high band VHF, 433MHz, 441MHz and 460MHz - plus 5/8th whips for 145MHZ, 160MHz and 455MHz.

There's a lot of info now that states that 5/8th designs actually have less gain than ¼ waves unless they are on a very decent ground plane (like the sea!)

I think they tweaked my 30 mile away UHF ham repeater in Norwich, because it's now listenable to on the radio in the van - never broke squelch before. Now, parked outside my house on the ¼ wave I can access the repeater and it's noisy but stable. Swapping the ¼ wave for the ⅝ results in no signal. Swapping the ¼ wave for the VHF ¼ whip makes it function as a ¾ wave on the UHF repeater, and I get into it slightly better and get the next bar on the meter to light, and a noticeable reduction in noise. This lack of performance on the 5/8th wave made me try something similar on VHF. Marine band on channel 12, swapping the ¼ wave for the 5/8th in this case made such a small difference I couldn't tell. It certainly was not better with the 5/8th. The other thing I noted was that the 5/8th whips were much worse for bandwidth. The analyser tells me the best VSWR on the VHF ¼ wave is 162MHz, and it's pretty flat until 141MHZ at one end and 175MHz at the other - transmit on 145 and 167MHz not problem at all - so as a general antenna it's wideband, and setting the analyser to 440MHz and testing it reveals that again, 430 through to 461 has a low enough VSWR to not worry the transmitter. The ¼ wave for UHF has a similar wide range, but is much less good on VHF, so as an aerial on the van, far less useful for me. It's enough for me to carry on with my VHF ¼ wave on everything. I shan't be wasting money on the clever ones from now on - it would appear that all the posts on how 5/8ths are somehow NOT a bit magic are dead on.
G4RMT
 
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Re: Vehicle antennas for TX and scanning

Postby Micky » Tue May 16, 2017 7:50 pm

Thank you for the report most interesting, I was in Bungay for a while and found the same results until I changed antennas on my handheld, I was 3 floors up in Buckhurst Hill.
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Re: Vehicle antennas for TX and scanning

Postby G4RMT » Wed May 17, 2017 9:56 pm

Bungay - ah. Lovely place. The lighthouse is stunning.
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Re: Vehicle antennas for TX and scanning

Postby Micky » Wed Jun 07, 2017 6:38 am

Re Bungay, I wonder if you worked me, I was M3EMB otherwise known as Eat More Bacon o Empty Milk Bottles
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Re: Vehicle antennas for TX and scanning

Postby G4RMT » Wed Jun 07, 2017 5:25 pm

Sadly Micky, I don't think I ever spoke to any ham from Bungay. I did fix a radio in an ambulance there once - they told me the 'nah' was broken. I was confused. "You know", he said and switched on the two tone which had one horn missing so it just went ni ni ni - no nah!
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