Losses thru "incorrect" use of ground plane-dependant aerial on UHF CB?

[skidrov]

Hoping some radio gurus on the forum can help me with what I suspect is a how-long-is-a-piece-of-string question... [biggrin]

I have mounted an Icom IC-400 Pro UHF CB in my 110 Defender, and all is working well - both transmit and receive. However, the aerials I'm using are either a small 2 dBi or a larger 4.5 dBi whip - and, these are ground plane-dependant units. Now the key bit, as I understand it: the mount point I'm using for the aerial is on a bracket trailing back slightly from the roof rack (single bar type), very much to the right of the vehicle, about 10cm above the roof.

My (poor) understanding of ground plane-dependant aerials is that they need to be ON and CENTRAL to a large flat metal surface, away from obstructions such as roof racks, for best performance. So, ideally, right on and in the middle of a clear roof. NOT the way I've mounted mine! :no2:

So, my question is this: given I have mounted my ground plane-dependant aerial in the "wrong" spot, how much transmission power/efficiency am I loosing? As I say, everything is working fine, but how much better could it be working? How much better would it get if I changed to a ground plane-independent unit? Or is this too difficult to answer with any real precision?

Thanks to radio gurus in advance! :TakeABow:

[Tombie]

Assuming the string is otherwise ok.

You’re signal will be strongest “across” the roof, and weakest away from the vehicle...

[rangieman]

The majority of 4WD`s mount there aerial on the bull bar does not mean it is perfect for serious transmitting but the location is convenient [wink11]

[Graeme]

If its working well for you then the small ground plane that it has is obviously doing enough of a job.

I recently had reception difficulties with a VHF antenna mounted 5m up on a pole above the iron roof of my house. The setup had been working well but then degraded, not working well in damp weather then failed totally. One of my actions was to replace a mix of old 75 ohm coax and very old 50 ohm coax with new 50 ohm coax. Once the real causes for the failing (glue in an antenna section causing open circuit when wet) then failed reception (SDR dongle went deaf when next to an amplified wifi signal) were rectified, reception was restored but very noisy. 3 sloping ground plane radials made from tubing from an old analogue TV antenna mounted at the base of the VHF antenna restored the former satisfactory reception of digital signals from 200 km away yet the ground plane at the antenna was not required with the original cocktail of coax.

[rovers4]

I have a similar problem ; need a rack with basket as well as antenna mount.

So I made a bracket to take the antenna and fix it to a cross bar, but made sure that the u-bolts bite in to the bar to get a good connection. Even a braid strap from the antenna base ground where the coax threads in to a screw into the bar. Remember to w/proof the screw.

Would be nice to then ground the bar, but that is possibly not likely to happen due to paint/leg padding at the rails.

So just ensure the body of the radio is well grounded to the body of the vehicle and that the antenna plug/coax is in very good nick.

A ground-plane independant style would be better, but not as easy to source.

[Tombie]

RFI CD34 Series springs to mind.

ZCG and GME also do several units.

[skidrov]

RFI CD34 Series springs to mind.

Yup, the RFI CD34 series was what was on my mind IF I thought that I was getting significant losses compared to my ground plane-dependant units, in their current mounting location. But (and I guess it's the piece-of-string factor [bigsmile1] ) am not seeing replies to this thread making me think that's the case...