UHF Antenna interference, is it a myth?

#1
I was having a read up on antenna placement for UHF radios, and there was a common theme which I'm having a little trouble getting my head around. It seems that every man and his dog claim that by placing two UHF antennas in close proximity, you're going to create interference and cause problems for yourself. However, if you have two antennas, you'd either have two radios or a switch right? Therefore, you would only ever be transmitting on one antenna at a time (unless you want to transmit holding two mics).

Therefore, there will never be interference between the two right? Or can the antennas somehow passivley intefere when recieving?
 

Wang

Well-Known Member
#2
i run 2x UHFs and my antennas are beside each other.
And yes i get interference if both radios are on and i key up on one. eg had one on CH40 listening to traffic or scanning and the other on CH26 talking to the other legends of the NSW 4x4 earth crew in convoy. when you key up on either radio the other wigs out.

i will get around to moving one soon
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boobook

Well-Known Member
#4
It's about changes the performance of the antenna even if one device is off. Some people have 2 UHF's, some have a phone and a UHF, either way it's bad juju. The rule of thumb is antennas should not be closer than 1/2 the wavelength of that of the lowest frequency. So no closer than half of about 68cm for UHF CB. So as a VERY minimum antennas should be no closer than about 14" apart. 28 inches is much better.

When they are close, the antenas become one system. Having 2 close together, reshapes the beams an they will tend to focus to the sides of the vehicle.

Look at your TV antenna, it is highly directional at right angles to the elements. Only the second element from the right is connected by cable to the TV. The others "focus" the beam to the TV station and limit sensitivity to the sides. Same if you have 2 UHF CB antennas next to each other, they will focus the beam to the sides of your car and make the performance to the front and back of the vehicle worse. Add that to the fact that the antenna is already in the wrong position on a bullbar, your 5W CB is probably behaving like a 1 watt one would do on the roof. Or worse.

It may look cool to people who don't know, but it's bloody awful radio practice. Don't do it.




Note: sometimes you do see antennas close together ( like on a wifi router), they are generally set up as MIMO, that works on the same principle, but by design to steer the waves. Different thing.
 

boobook

Well-Known Member
#6
I'm with Wang. I like to have 2 UHF's. Usually because I end up as the one up front or up back in a group. It is good to keep a side channel for that if both have 2 UHF's. Also I recently got a GME XRS (?) which can scan 50 channels per second. I set that to scan every channel except my working 2 channels as a lookout, particularly good on tight roads or on dunes liek the Simpson, etc.
 
#7
It's about changes the performance of the antenna even if one device is off. Some people have 2 UHF's, some have a phone and a UHF, either way it's bad juju. The rule of thumb is antennas should not be closer than 1/2 the wavelength of that of the lowest frequency. So no closer than half of about 68cm for UHF CB. So as a VERY minimum antennas should be no closer than about 14" apart. 28 inches is much better.

When they are close, the antenas become one system. Having 2 close together, reshapes the beams an they will tend to focus to the sides of the vehicle.

Look at your TV antenna, it is highly directional at right angles to the elements. Only the second element from the right is connected by cable to the TV. The others "focus" the beam to the TV station and limit sensitivity to the sides. Same if you have 2 UHF CB antennas next to each other, they will focus the beam to the sides of your car and make the performance to the front and back of the vehicle worse. Add that to the fact that the antenna is already in the wrong position on a bullbar, your 5W CB is probably behaving like a 1 watt one would do on the roof. Or worse.

It may look cool to people who don't know, but it's bloody awful radio practice. Don't do it.




Note: sometimes you do see antennas close together ( like on a wifi router), they are generally set up as MIMO, that works on the same principle, but by design to steer the waves. Different thing.
Good gouge mate, I was wondering if something like this was the case. RF isn't my strong suit.

What's to stop things like body panels or barwork doing a similar thing?
 
#9
Boobook is 100% correct, even if one antenna is not connected to anything it's still likely to degrade the performance of the active one.
 

boobook

Well-Known Member
#10
SNIP

What's to stop things like body panels or barwork doing a similar thing?
Bingo Crashtastic. They do the same thing. That is one of 4 reasons to put the antenna on the roof, not the bullbar.

Cargo barriers, metallic tinting in windows, your body, all affect the radiation, reducing or altering the reception. Any object in the same plane ( height) as the antenna will come into play.

The other 3 reasons to put the antenna on the roof are - height is might, bullbars can wobble an antenna to death, and the wife hates it blocking their view.

A simple knock down bracket will save roof mounted antennas from car park roofs.

 

denmonkey

Well-Known Member
#11
As book said, anything metal will impact the performance of your antenna when near by. High and clear is the best bet.
Granted you're only transmitting 5 Watts and whilst unlikely at the power output is it possible to bugger your radio transmitting so close to a receive antenna.
Antennas is close proximity will result in neither antenna performing well for both transmit and recieve.
As the om advice said, get em up high and get them in the clear.
If it's all you have to work with though, it still works better than none.
 
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