UHF Aerials

Submitted: Saturday, Feb 02, 2008 at 19:00
ThreadID: 54156 Views:7793 Replies:7 FollowUps:26
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I wish to mount 2 UHF aerials ( for 2 UHF Radios) to the bull bar on my 4x4.
How far apart do I need to place them? or how close can I have them without causing problems?
I also have a HF Tap aerial and mobile phone aerial attached to the front of the bull bar.
Will the UHF aerials interfere with the next G phone aerial?
Thanks Ian
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Reply By: Member - joc45 (WA) - Saturday, Feb 02, 2008 at 19:20

Saturday, Feb 02, 2008 at 19:20
Hi Farrelly,
Mounting any antenna close to another (less than half a wavelength) will affect the radiation pattern.
If you are running two radios on the same band then I'd keep them as far apart as possible, as when you transmit on one, then the other receiver will be drowned out, even if not on the same channel. Too close (within half a wavelength) and you could damage the receiver input circuitry (most have input protection, but still a risk).
Gerry
AnswerID: 285185

Follow Up By: pop2jocem - Saturday, Feb 02, 2008 at 19:47

Saturday, Feb 02, 2008 at 19:47
Please excuse my stupidity, half a wavelength, can you tell me what that equates to in linear measurement i.e how far apart do you need to mount said antenna to avoid interference and damage?

Thanks Pop
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Follow Up By: Member - joc45 (WA) - Sunday, Feb 03, 2008 at 10:29

Sunday, Feb 03, 2008 at 10:29
Sorry!
formula for wavelength (metres) = 300/f
where f is the frequency in MHz.

UHF CB is approximately 470MHz, so 470/300=1.56 metres.
Half a wavelength will therefore be 780mm
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Follow Up By: Mike Harding - Sunday, Feb 03, 2008 at 10:53

Sunday, Feb 03, 2008 at 10:53
Loose 10 points and go back to the start :)

300/477 = 629mm for a full wavelength and 314mm for a half.

Mike Harding
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Follow Up By: Mike Harding - Sunday, Feb 03, 2008 at 11:03

Sunday, Feb 03, 2008 at 11:03
For those of you who are interested; the formula Gerry gives above is shorthand for the full formula which is:

wavelength = c/f

where "wavelength" is in meters - 'c' is the speed of light (3 x 10^8) - and 'f' is the frequency in megahertz.

Transposing the formula means we can also find the frequency of a given wavelength by:

frequency = c/wavelength

Mike Harding
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Follow Up By: Member - joc45 (WA) - Sunday, Feb 03, 2008 at 15:57

Sunday, Feb 03, 2008 at 15:57
Mike,
dunno what happened when I punched those numbers in! LOL! Blame my calculator!
You're right of course!
Gerry
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Follow Up By: Mike Harding - Sunday, Feb 03, 2008 at 16:36

Sunday, Feb 03, 2008 at 16:36
You confused your guzintas.

You forgot if:
x goes into y
or
y goes into x

Mike Harding :)
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Reply By: Member - SNAKE (RAOUL) QLD - Saturday, Feb 02, 2008 at 22:40

Saturday, Feb 02, 2008 at 22:40
Hi, A full wave length on UHF CB is 63cm.Cheers Snake
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Follow Up By: Farrelly - Sunday, Feb 03, 2008 at 13:51

Sunday, Feb 03, 2008 at 13:51
Snake.
If I place the 2 UHF aerials more than 300mm apart they will be OK ???
What about my Mobile phone aerial will it have to be another 300mm away or can it be placed between the 2 UHF aerials.
I just dont want aerials all over the place I would like to keep them as close as possible or else I WILLl be looking though a picket fence
Thanks Ian
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Follow Up By: Member - SNAKE (RAOUL) QLD - Sunday, Feb 03, 2008 at 17:50

Sunday, Feb 03, 2008 at 17:50
Hi Ian,
Mate most mobile antenna installations are a compromise because of lack of room,try to keep the antennas as far apart as you can.If you get a single bar surf board type bar on top of the vehicle roof you can mount a UHF antenna on each side of it and the mobile phone antenna on the bullbar.Also keep the coax cable runs as short as possible.No doubt someone will you various stories regarding the cable lengths,as long as you dont have a very long run you will be ok.Hope this helps Ian,if not ask. Cheers mate Snake & Josie
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Follow Up By: Member - SNAKE (RAOUL) QLD - Sunday, Feb 03, 2008 at 17:54

Sunday, Feb 03, 2008 at 17:54
That should read(No doubt someone will TELL you etc)
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Reply By: Member - Mal B - Saturday, Feb 02, 2008 at 23:08

Saturday, Feb 02, 2008 at 23:08
dosen't the law say no aerials on bull bars mal b
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Follow Up By: Dave & Shelley (NT) - Sunday, Feb 03, 2008 at 03:25

Sunday, Feb 03, 2008 at 03:25
Hi mate
As far as I can tell it is ok for standard aerials. The NT rules is below and it only covers HF antennas on bullbars, not UHF.

www.nt.gov.au/transport/mvr/vehiclestandards/infobulletins/ibv20.pdf

The VIC GOV web site state the following

http://www.vicroads.vic.gov.au/NR/rdonlyres/DFC160FA-319C-4AE0-8212-C38F81C44007/0/VSI29.pdf

w.nt.gov.au/transport/mvr/vehiclestandards/infobulletins/ibv20.pdf

Hope this helps clear up any problems

Cheers

Dave
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Follow Up By: Hairy (NT) - Sunday, Feb 03, 2008 at 14:08

Sunday, Feb 03, 2008 at 14:08
Dave & Shelley,

BUGGER!!!

Ive been driving around illegaly for the past 5 years!
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Follow Up By: Dave & Shelley (NT) - Sunday, Feb 03, 2008 at 22:50

Sunday, Feb 03, 2008 at 22:50
Hairy,

Great isn't it. I don't think anyone will ever notice until there is an accident then it will be your fault.

Cheers

Dave
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Reply By: Member - Olcoolone (S.A) - Sunday, Feb 03, 2008 at 10:18

Sunday, Feb 03, 2008 at 10:18
Jeess Farrelly it would be like looking through a picket fence, why do you need two UHF antennas or two UHF radios?

Regards Richard
AnswerID: 285270

Follow Up By: Farrelly - Sunday, Feb 03, 2008 at 13:47

Sunday, Feb 03, 2008 at 13:47
I'm a wide load pilot . One uhf is to talk to the wide load behind the other is to scan for others on the road and warn them of the wide load. The HF is not normally attached and only used when I go bush. The phone aerial is to extend phone range out west.
I just dont want to cause problems by having them too close.
Do you know if the UHF transmission would affect the mobile phone if the aerials are too close.
Thanks Ian
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Follow Up By: Member - Olcoolone (S.A) - Sunday, Feb 03, 2008 at 17:40

Sunday, Feb 03, 2008 at 17:40
I would not worry to much about this wave lenght B&LLSH*T stuff, some people get carried away.

Depends what the frequency rejection is like on the radios you are using, you could mount an antenna on the bull bar and another on the rear of the vehicle and some radios will pick up the other radio on a differant channel and the some wont.

It's trial and error.

Try using radios that have not got a large RX frequency range like the ones you can put another 60 recieve channels in.

Some commercial radios with a large frequency range have fantastic frequency rejection and some CB's don't.

The mobile phone you will find will not suffer to much if any, you are talking at least 380 MHz seperation and upto 1700 Mhz on some phones.

Regards Richard

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Follow Up By: Mike Harding - Sunday, Feb 03, 2008 at 18:22

Sunday, Feb 03, 2008 at 18:22
>Depends what the frequency rejection is like on the radios you are using

For commercial radios; all frequency rejection ratios or image rejection ratios or any other ratios are not based upon the DUT having a large amount of watts stuffed up its backside from damn all distance.

>The mobile phone you will find will not suffer to much if any, you
>are talking at least 380 MHz seperation and upto 1700 Mhz on
>some phones.

And your explanation for my 146MHz Amateur set breaking into my 477MHz UHF CB (331MHz frequency separation) is...?

Mike Harding
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Follow Up By: Mike Harding - Sunday, Feb 03, 2008 at 18:26

Sunday, Feb 03, 2008 at 18:26
Should mention my 5W 2m handheld stuffs the UHF CB too, it it's close enough, so it's not power supply coupled.
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Follow Up By: Member - Olcoolone (S.A) - Sunday, Feb 03, 2008 at 21:18

Sunday, Feb 03, 2008 at 21:18
Mike your 2m radio puts out 20 watts, a uhf cb will more then likely do 5 watts and I would expect a mobile phone to have very go rejection seeing it is always in duplex.

What happens when someone cross bands or what about repeaters why don't they cause havic specialy on 2m, you can have a weak signal on the input and the output of the repeater doesn't interfere or cut out the weak signal.

More then likely a cheap CB you are using or a badly aligned 2m radio.......I would get someone who knows that they are doing to look at it.

Our Icoms don't have a issue.
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Follow Up By: Member - Olcoolone (S.A) - Sunday, Feb 03, 2008 at 21:30

Sunday, Feb 03, 2008 at 21:30
Just checked our Icom 25 watt UHF against our GME TX6200.

With the Icom transmitting on channel 40 and the GME recieving on channel 1, the Icoms wipes out the GME reception.

With the GME on 5 watts within 1m of the antenna transmitting on channel 1 it will wipe out the Icom upto channel 17 after that it works fine.
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Follow Up By: Mike Harding - Sunday, Feb 03, 2008 at 22:01

Sunday, Feb 03, 2008 at 22:01
>I would get someone who knows that they are doing to look at it.

Good idea - what are your hourly rate for RF consultancy?

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Follow Up By: Member - SNAKE (RAOUL) QLD - Sunday, Feb 03, 2008 at 23:38

Sunday, Feb 03, 2008 at 23:38
Had to reply re Olcoolones>dont worry too much about wavelength/its trial&error/get a radio with a large freq range:Im sure professional radiio installers would be rapt with this simplistic approach to radio,just bung an antenna here and a thousand channel radio there and all will be sweet.Perhaps a section on the basics of radio would be apt on this site.We have a few Amateur Radio Operators and others such as Derek(ABR) who could answer questions and help all as radio is vital not only for safety but just keeping in touch while on the road.
Cheers to All, Snake and Josie
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Follow Up By: Member - Olcoolone (S.A) - Monday, Feb 04, 2008 at 08:47

Monday, Feb 04, 2008 at 08:47
It's good to see you do have a sense of humour Mike...LOL

Snake, there is no sure way of stopping this with out spending big big money, I think if people stopped worrying about things that will not make any differance or that are out of there control it would be such a better world.

Iv'e been playing around with radio gear for nearly 30 years so yes I have got alittle bit of knowledge.

Snake what would your solution to the problem be and maybe you can tell me what wave lenght would have to do with frequency rejection in such a small area?

As I said it is trial and error with this as some radios are alot better then others but I think he is pushing bleep up hill.

Alot of radio installers know very little when it comes technical issues or problems.....

Do you honestly think when they install radios they cut the coax to the nearest mm or check the SWR of the antenna......not for $70.00 they don't....they should but they don't.

Why would Derek know about radio issues....he's not god?

So snake seeing I an totally wrong what would you solution be.


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Follow Up By: Member - SNAKE (RAOUL) QLD - Monday, Feb 04, 2008 at 22:15

Monday, Feb 04, 2008 at 22:15
Hi Olcoolone,
Thanks for the reply.Answers not in order.
(1) I would think Derek with his knowledge or 12volt gear should be of help to anyone installing a radio in a vehicle,not all people have a working knowledge of 12volt wiring,I agree Derek is not god.
(2)The techs I know check VSWR as part of radio installation,I dont know how they check tuning,power out etc without knowing that the antenna is resonant on the freq.
(3)>Snake what would be your solution etc. I was answering the question that asked,Im not aware there was a problem.
(4)I know of a lot of people who havebeen playing around with various things for many years but it does not mean they know anything about whatever it is,be it radio,golf or cars.
(5)I gave what was my solution in my first post.
(6)This is the last I have to say about the subject on this post.
Cheers and keep smiling. Snake and Josie
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Follow Up By: Member - Olcoolone (S.A) - Monday, Feb 04, 2008 at 23:08

Monday, Feb 04, 2008 at 23:08
Thats OK but

"wavelength/its trial&error/get a radio with a large freq range:Im sure professional radiio installers would be rapt with this simplistic approach to radio,just bung an antenna here and a thousand channel radio there and all will be sweet."

Where did I say "get a radio with a large freq range"

Whats this stuff about "just bung an antenna here and a thousand channel radio there and it will be sweet"

Can you tell me where I said that I get very annoyed when people put words into my mouth

I think you need to learn how to read or understand posts correctly!

I think you might be makeing things up.....

A simple opps I made a mistake would be good?

If you don't reply I understand and accept your apology.
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Follow Up By: Member - SNAKE (RAOUL) QLD - Monday, Feb 04, 2008 at 23:38

Monday, Feb 04, 2008 at 23:38
IF you dont reply I understand etc etc.What a copout.Suggest you get a life. Cheers and keep smiling.Snake and Josie
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Reply By: Member - Roachie (SA) - Sunday, Feb 03, 2008 at 14:31

Sunday, Feb 03, 2008 at 14:31
I think the safest bet would be to mount the antenna for the radio that you will be wanting to talk to the driver of the wide load (and others in your convoy), at the back of your vehicle. Then I would mount the other UHF antenna (for scanning other channels and talking to joe public) on the bullbar.

The one at the back of your vehicle is going to be ideally located for talking to drivers who are behind you. The liklihood is that the other radio will be used to talk to oncoming truckies etc to alert them to the size of your load etc and, as such, the best place for that antenna is at the front. Hope that makes sense?

Roachie
AnswerID: 285344

Reply By: John S (NSW) - Sunday, Feb 03, 2008 at 16:26

Sunday, Feb 03, 2008 at 16:26
If one UHF is on a fixed channel (ie 20) and the other is scanning, just remove ch 20 from the scan list - that way neither UHF will ever be on the same channel. Then the aerials can be a foot apart without any problems.

I don't know if it was just my setup/UHF's, but in the past when I have had two UHF's in the car, I never had issues with them on the same channel - maybe signal too strong and the receiver blocked it out, or the aerials too close (less than half wavelength distance apart) so the non transmitting one was within the radiation deadzone and never got a coherent signal.

I know that you could not selcall between two TX4000's if they were parked next to each other. You always needed atleast 5mtrs between vehicles.
AnswerID: 285377

Follow Up By: Mike Harding - Sunday, Feb 03, 2008 at 16:48

Sunday, Feb 03, 2008 at 16:48
Nice idea John but it won't work very well. As joc45, above, says the main issue is that when you transmit on one radio you may damage the electronics in the "front end" of the other if the antennas are too close together.

Tuning radio X to a different frequency from radio Y may appear to solve the problem but because the radios are still close in frequency the transmitted signal will "break through" any tuning devices and _may_ cause damage.

When I transmit on my 2m (146MHz) Amateur Radio and simultaneously have the UHF CB turned on the 146MHz always breaks through onto the UHF CB and the antennas are at opposite ends of the vehicle - to be fair the Amateur radio is a 20W output.

I would put the two antennas as far apart as practically possible.

For simply talking to convoy vehicles consider a "through glass" antenna on the rear/side window?

Mike Harding
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Follow Up By: John S (NSW) - Sunday, Feb 03, 2008 at 21:37

Sunday, Feb 03, 2008 at 21:37
Mike,

I get what your saying. But we are talking about two UHF's, not one UHF and one Amateur. In your situation you will find that the amateur is breaking through to the UHF's amplification circuit - bypassing the tuner all together because of the output power the amatuer has.

When I used to work on Ch 5 repeater towers, we use to shield the transmitters & receivers with foil because they were close to TV & FM towers and used big filters to ony allow the ch 5 frequencies through. Even with 50000watt towers nearby, we rarely had receivers blow up. More often it was the transmitters that needed repairs because someone would leave a UHF transmitting on ch 5 and our transmitters would get too hot and shutdown. Although adding air con the the shed helped relieve that problem especially during summer.

At 5 watts, I would be very surprised if todays UHF's would packed it in becasue two were running side by side on different channels. I never heard of Recar having issues with their support vehicles and some of them had three or more CB's installed.
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Reply By: Wayne (NSW) - Sunday, Feb 03, 2008 at 22:46

Sunday, Feb 03, 2008 at 22:46
Ian,

This how I have got around the two UHF radio problem.



As you can I have a few aerials around the Troopie and putting the two UHF as far apart as possible seem to work.
I run two UHF for much the same reason as you, wanting to talk to the convey and also wanting to know who else is around.

The HF is on the back, that way there is no problem when I go interstate.

The GPS aerial is for a Garmin and having it on top of the snorkel gives it a good view of the sky.
The Sat phone aerial is also positioned to have the best chance to find a satellite.

Wayne
AnswerID: 285456

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