Thread: Does a UHF antenna base need to be earthed?

Originally Posted by UBZ

A ground plane independent antenna still needs its base earthed properly .
but Dosent require a bonnet or roof to act as a ground reference plane for the RF signal .

I was having range issues with my AM CB (ground plane independant) and UHF PRS ( ground plane dependent ) due to the poor electrical conductivity of the Aluminium body panels .
I ran an separate earth from the battery and earthed both the head units and antennas to it , which resulted in a massive increase in range.

This would have shown up immediately at the time of install,if you had used a VSWR bridge,as you would not have been able to tune the the antenna and feedline,the VSWR would have been very high

Originally Posted by LowRanger

This would have shown up immediately at the time of install,if you had used a VSWR bridge,as you would not have been able to tune the the antenna and feedline,the VSWR would have been very high

You must use however a VSWR bridge that is built for the frequency band that you are testing.

SWR8 UHF/VHF SWR - POWER METER | eBay
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Originally Posted by slug_burner

post a picture of your antenna and base or get a manufacturer and part number.

UHF is easier to get ground independent antennas.

Having look around on the net my antenna is ground independent. Looks like I will run a ground wire to the antenna base.

Solid state equipment is especially sensitive to ground problems. Each piece of equipment in figure 1 is interconnected by two ground paths, a ground strap and the coaxial cable that interconnects the equipment. The two paths form a ground loop, as shown in figure 1. Since there is high system gain involved from the millivolts of the transceiver's input circuits to the kilovolts of the linear's output circuit, ground loops can be a serious problem. It's even worse if the ground system is ineffective and the entire station is 'floating' above ground. Breaking the ground loops can lead to the solution to long unsolved RFI problems.

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Well I have to say I have never come across that one and from the little I know being non technical, this only happens with a high resistance between the two earth points .
I was CB radio product manager for Sanyo when 40channel VHF CB was introduced into Australia, and I have fitted many CB installations, and used the old SWR meter to trim antennas. Thank goodness you don't have to stuff around like that with UHF.
There is an easy way to check of course. Try it with the both ends earthed and if there is RF, try it without. It's pretty simple. Of couse if there is still RF , then you have zee problem and have to look at chokes in the positive.
Regards Philip A

Originally Posted by PhilipA

Thank goodness you don't have to stuff around like that with UHF.

RFI is immediately noticeable in reception using AM or SSB as audible noise, whereas noise received by a FM transceiver isn't heard but is still there and manifests itself by desensitising the receiver.

A test for a suspected noise in a FM receiver ( UHF CB ) can be found by using the squelch control and finding where the threshold is, with the suspected noise source operating and with the noise source removed. If there is a marked difference in position of the squelch knob, then that noise can be dealt with.

Something that is over looked when discussing groundings, is that a radio ground of the antenna does not always need a DC ( direct current ) electrical connection to the ground of the power supply .

In the case of running a wire from the base ( where the antenna is bolted onto a bullbar for example ) of a independent ground antenna to the negative of a battery, that is a low impedance RF path to a good radio ground ( the battery ). Now something for the Technocrats, that ground wire should also be DC decoupled by a capacitor to the battery.

As a rule of thumb when I am setting up any antennas and their feeds is I make sure that the high RF current point of the feeder is close to the transceiver and any high RF voltage points ( such as the tip of the antenna ) are well away from the signal input to the transceiver.
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Originally Posted by wrinklearthur

You must use however a VSWR bridge that is built for the frequency band that you are testing.

SWR8 UHF/VHF SWR - POWER METER | eBay
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Not exactly a good example of a piece of equipment made for the operating frequency.The frequency listed on the front of the bridge is 420 - 460 Mhz,and then the add states"including the 477Mhz" Chicken band.Just a little out of range

Originally Posted by PhilipA

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Well I have to say I have never come across that one and from the little I know being non technical, this only happens with a high resistance between the two earth points .
I was CB radio product manager for Sanyo when 40channel VHF CB was introduced into Australia, and I have fitted many CB installations, and used the old SWR meter to trim antennas. Thank goodness you don't have to stuff around like that with UHF.
There is an easy way to check of course. Try it with the both ends earthed and if there is RF, try it without. It's pretty simple. Of couse if there is still RF , then you have zee problem and have to look at chokes in the positive.
Regards Philip A

Depending on if you want the best performance from your antenna or not as whether you trim or not.Don't forget,that the feedline should be in multiples of 1/4 wavelengths or as close as possible.To adjust the VSWR,you can trim the feedline.This can be important,if you want to optimise the performance at a particular point in the operating band.
I am also guessing that it should read either HF or UHF as we never had VHF good buddy radios here