And they say there's no such thing as a stupid question! UHF's...

Submitted: Thursday, Apr 27, 2006 at 08:34
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Hi all,

I had me a ponder on the way to work when a bloke passed me with what "appeared" to be two UHF aerials on his car. Now I use the aerm "appeared" loosely because I know next to nothing about UHF's. I know that depending on the gain of your aerial (eg, 4.5db, 6db 9db) you will get a different coverage (is that the right word?) each with it's own pros and cons obviously. My thought was this: Can you run two aerials off one UHF and get the best of both worlds? and if so why dont' more people do it? Or is it a lot of hassle for little added benefit??

Cheers for your thoughts!
Scoey!
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Reply By: Member - JohnR (Vic)&Moses - Thursday, Apr 27, 2006 at 08:45

Thursday, Apr 27, 2006 at 08:45
Scoey, when you are travelling in places like the desert over sand hills it is always wise to know there isn't another group over your sand dune who is travelling in the opposite direction. Some scan as they go listening, but also need to be tuned to the convoy channel to be able to transmit.

Aside from that it is wise to carry a spare antenna and if you have been scanning, you will also have a spare radio. When we crossed the Simpson two years back Moses UHF antenna failed and I have heard of others having similar problems.

Some radios can scan but still have the priority channel to transmit to but it is still wise to have that spare antenna.
Cheers,
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John

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Reply By: Member - Paul P (Bris) - Thursday, Apr 27, 2006 at 08:59

Thursday, Apr 27, 2006 at 08:59
G'day

Yes you can run 2 UHF antenna's. Heavey vehicles do it all the time (one on each mirror) to give good coverage around the large vehicle. Same antenna type is used on both sides.

What you might see on a 4x4 is a high gain GSM/CDMA mobile phone antenna (usually a black or white fibre glass antenna) and a UHF antenna (can be a white fibre glass antenna) similiar in look to the mobile phone mounted on either side of the bull bar.

Regards

Paul
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Follow Up By: Scoey (QLD) - Thursday, Apr 27, 2006 at 09:04

Thursday, Apr 27, 2006 at 09:04
But can you run 2 antenna's with different gains from the one unit? This car had the wire type aerial on both sides that looked like the GME job with interchangable whips. One side had one phasing coil and the other side had two phasing coils.

It could very well be that he's running two radios as John said above as well - i just found it interesting.

Cheers
Scoey
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Follow Up By: Member - Beatit (QLD) - Thursday, Apr 27, 2006 at 11:36

Thursday, Apr 27, 2006 at 11:36
G'day Scoey,

I have 3. One for the CDMA and 2 UHF. Have the cables hanging in the firewall and added a short extension to the radio end - can screw in either earial. Use a 9db and a 3db but not sure it is worth the hassle. Handy though if you lose one. They use different bases so I can use most aerials now.

Kind regards
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Follow Up By: Mad Dog (Australia) - Thursday, Apr 27, 2006 at 11:51

Thursday, Apr 27, 2006 at 11:51
Probably end up with a pattern between the two, not worth the hassle of making up the harness and bleep sin' around tuning the system.
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Follow Up By: joc45 - Thursday, Apr 27, 2006 at 16:08

Thursday, Apr 27, 2006 at 16:08
I agree with MadDog, you will get areas where the signal from one antenna is out of phase with the other resulting in cancellations of the signal (unless one is directly above the other!). Also the two antennas will present an impedance matching problem to the set unless you use a proper power splitter.
cheers,
Gerry
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Reply By: Member - Wim (Qld) - Thursday, Apr 27, 2006 at 10:27

Thursday, Apr 27, 2006 at 10:27
Scoey.

I should think you could run the two different aerials through a switch (would need to be made for the task) and just switch from low to high gain. The only problem I can see is the additional loses through the switch. You may lose more than you gain.
excuse the pun.

I run two radios for the reasons John mentioned.

Regards
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Follow Up By: Member - JohnR (Vic)&Moses - Thursday, Apr 27, 2006 at 10:46

Thursday, Apr 27, 2006 at 10:46
Have a great trip mate, I know it can't be long before you are off to the desert. Hope Judy is well too.

Yours was the example I was thinking of actually with the scanning radio, I know others do it too though. I occasionally have the handheld on scan. With the 4400 I can use the priority scan and it has a bit more functionality even though I guess I don't know about it all.

I can't really think why people use two antennas at once these days as I thought that all went out with the 27mhz sets.

Cheers,
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Follow Up By: Mad Dog (Australia) - Thursday, Apr 27, 2006 at 11:52

Thursday, Apr 27, 2006 at 11:52
John, using an antenna on each side of a large vehicle such as a truck helps the signal get around the vehicle to the back and sides. Also the pattern can be made very directional by a ¼ or ½ wave phasing harness. Hams don't seem to bother with dual antennas when mobile so I can't see a great deal of advantage, we are usually interested in good all round mobile coverage.
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Follow Up By: Member - JohnR (Vic)&Moses - Thursday, Apr 27, 2006 at 13:17

Thursday, Apr 27, 2006 at 13:17
Ray, I was going to make a point about the directional tuning but you have put it together better with your understanding. I would be in agreement with the hams as you really then need to have your vehicle in the direction where you knew the distant signal was going to come from.

It really sonund pretentious to have predicted the new signal, utter cr@p in other words.
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Reply By: Scoey (QLD) - Thursday, Apr 27, 2006 at 11:34

Thursday, Apr 27, 2006 at 11:34
Thanks for the responses everyone! It was interesting to think about anyway!

Cheers
Scoey!
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Reply By: Wizard1 - Thursday, Apr 27, 2006 at 11:42

Thursday, Apr 27, 2006 at 11:42
Scoey,
why not contact the experts in this field rather than get lay opinion...Call a supplier of such communications equipment and they will give you an "expert answer".

As an example when I first fitted my UHF a bloke told me he obtained this amazing feat of UHF communication over vast distances based on this enormous surf rod size fibreglass antenna. So I thought I'd follow his lead.

So I visited the supplier and fitter of my unit. Now you'd think a retailer would be more than willing to relieve me of my money and supply a similar antenna I may not need. To my surprise they actually explained all the theory of UHF communications and why such a setup was of little advantage to what I already had.

The moral is always speak to an expert in that field.
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Follow Up By: Member - JohnR (Vic)&Moses - Thursday, Apr 27, 2006 at 13:21

Thursday, Apr 27, 2006 at 13:21
yes, why not get a salesman?
Cheers,
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John

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Follow Up By: Wisey (NSW) - Thursday, Apr 27, 2006 at 18:39

Thursday, Apr 27, 2006 at 18:39
Hi Wizard.
Scoey didn't ask for expert advice, he was just pondering . And who's to say there are no experts or quailified people lurking. Good on you for getting the right advice/expert answer when you required it.
Just a note from here and certainly not a slant on you Wizard. VKCAT(below) seems (to me) to have a qualified reply, but I'm far from expeirenced. Learning as I go. Ask, listen, learn.

Regards
Andy
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Follow Up By: Member - JohnR (Vic)&Moses - Thursday, Apr 27, 2006 at 21:48

Thursday, Apr 27, 2006 at 21:48
I asked the local servicing specialist the other day about tuning Moses donk but he as good as told me all you could do was pump in more fuel. I don't totally accept his "expert" prescription as I find that all specialists had an angle they can understand and push. I like to get a range of views to ponder as Wisey (NSW) suggests we can but back my decision with FACTS. Even specialist doctors have opinions they push.
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Follow Up By: Bonz (Vic) - Friday, Apr 28, 2006 at 18:38

Friday, Apr 28, 2006 at 18:38
When did Moses get a DONK?
.
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Follow Up By: Member - JohnR (Vic)&Moses - Friday, Apr 28, 2006 at 18:56

Friday, Apr 28, 2006 at 18:56
Bonz, as far as I know he was born with it. It was the way he came to me anyway.
Cheers,
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John

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Reply By: Mad Dog (Australia) - Thursday, Apr 27, 2006 at 11:46

Thursday, Apr 27, 2006 at 11:46
John, using an antenna on each side of a large vehicle such as a truck helps the signal get around the vehicle to the back and sides. Also the pattern can be made very directional by a ¼ or ½ wave phasing harness. Hams don't seem to bother with dual antennas when mobile so I can't see a great deal of advantage, we are usually interested in good all round mobile coverage.
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Follow Up By: Mad Dog (Australia) - Thursday, Apr 27, 2006 at 11:53

Thursday, Apr 27, 2006 at 11:53
Sorry Scoey I'm getting old. This was supposed to be a folloup to John R above
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Reply By: Member - Oldplodder (QLD) - Thursday, Apr 27, 2006 at 16:25

Thursday, Apr 27, 2006 at 16:25
I have a UHF and an older 27mhz CB. Both have a laser s/steel whip aerial of about the same length, one each side fo the car. That car could have been the same.
Also have a s/steel whip aerial for the FM/AM radio. Got fed up branches taking out the extending aerial that came with the car. So that's three :-).
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Reply By: Member - Mike DID - Thursday, Apr 27, 2006 at 16:45

Thursday, Apr 27, 2006 at 16:45
So to summarise
- you can't connect two aerials to one CB (unless you have a special phasing harness).
- you can switch two aerials to one CB using a Coax switch rated for UHF.
- there are lots of aerials for other radios/phones that look like CB antennas.

Mike
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Reply By: Truckster (Vic) - Thursday, Apr 27, 2006 at 16:51

Thursday, Apr 27, 2006 at 16:51
Woop from GQ forum runs several radios in his GQ.. VHF, UHFx2, AM, HF, and a few other NFI freq's.
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Reply By: VK3CAT - Thursday, Apr 27, 2006 at 17:13

Thursday, Apr 27, 2006 at 17:13
Scoey. Yes, you can run two UHF antennas off the one radio.
No, you really can't get the best of both worlds by having a high gain & low gain antenna - unless it is possible & practical to have each antenna well outside the capture area of the other antenna.
An impedance matching phasing harness will be required to couple the two antennas to the one feedline (coax cable).
Is it worthwhile doing it? Probably not!
Repeater sites often used multiple folded dipole antennas to obtain both gain and directivity.

Some more technical stuff that may be of interest.

Using multiple antennas is called stacking. It is normally done to either have maximum gain or to have a very narrow capture area. It is used on repeater sites (above) and also for very high gain directoinal yagi arrays.
Stacking uses identical antennas connected via phasing harnesses. Connecting cables in the phasing harness must be of identical impedance and of the same length.
Stacking can be done vertically and horrizontally or a combination of both.
You will get nearly 3dB gain (double EIRP out or received signal in).
Gain measured in decibels is logarithmic in nature, the laws of diminishing return applies.
Example. 2 stacked antennas gives nearly 3dB gain over a single antenna.
4 stacked antennas will give nearly 6dB gain over a single antenna.
8 stacked antennas will give nearly 9dB gain over a single antenna

If you want to see some really serious stackes arrays go to RN6BN www.73.ru/ a href="http://www.73.ru/">RN6BN

Cheers, Tony
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Follow Up By: VK3CAT - Thursday, Apr 27, 2006 at 17:17

Thursday, Apr 27, 2006 at 17:17
ooooops, stuffed up the hyperlink text but it will still get you to the right site!
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