CB Aerials

Submitted: Monday, Jun 21, 2004 at 14:45
ThreadID: 13974 Views:3010 Replies:4 FollowUps:7
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Has anybody setup/use twin CB aerials (one front / one rear) on their vehicle? and was there any improvement/s?

Thanks
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Reply By: Member - Jeff M (WA) - Monday, Jun 21, 2004 at 16:34

Monday, Jun 21, 2004 at 16:34
I'm not familar with that kind of setup, what is it some kind of Dipole? If you mean just joining two std ariels together you will totally screw up your SWR and possibly even damage your radio.
The best bet if you want good reception front and rear is just get the base of your antenna above roof higher. ie Gutter/roof rack mount 3db dipole or similar.
AnswerID: 64224

Follow Up By: Rod W - Monday, Jun 21, 2004 at 16:52

Monday, Jun 21, 2004 at 16:52
Why aren't you working. Whats a Dipole??

I realise that the best place is smack bang in the middle of the roof (all metal), but there's a roof rack. Side gutter mounting is the same principals as front mounted.
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Follow Up By: Nudenut - Tuesday, Jun 22, 2004 at 15:34

Tuesday, Jun 22, 2004 at 15:34
Jeff, would a UHF work with a long wire antenae?... just curious.
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Follow Up By: Member - Jeff M (WA) - Tuesday, Jun 22, 2004 at 15:45

Tuesday, Jun 22, 2004 at 15:45
Nudenut, nah mate, well I mean if you had a UHF SWR meter (costly) you could probally cut a small peice of wire to the correct length (approx 70cm) however it would not be a very good antenna.
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Follow Up By: Nudenut - Tuesday, Jun 22, 2004 at 16:28

Tuesday, Jun 22, 2004 at 16:28
Jef ...a long wire is exactly that or so I thought....eg.many multiples of the wave length say 30metres long or there abouts?
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Reply By: Member - Jeff M (WA) - Monday, Jun 21, 2004 at 17:02

Monday, Jun 21, 2004 at 17:02
Right in the middle is only neccessary for a good "ground plain" however with most modern UHF ariels they are "ground independant" so it does'nt require that. All you need is to get more hight. UHF Antennas put most of the power output from the base of the antenna so if it's lower than all the big chunks of toyota metal you've got then it's going to get blocked out in one direction.
Also height in general is the winnder with UHF, the higher the better, even inches can make a big difference. Hence a lot of the antenneas around you'll see are "elevated feed" meaning that the big metal bit on the base has nothing really in it, it's just lifting the real base of the antenna up a few inches.
You've probally seen my embarassment of an arial, but it works as good as if not better than all the bigger ones Ive had in the past. And when out with Peter(York) in Yellowdine I was gratefull of it's floppy tiny size as everything else on the car was getting trashed by those "well marked tracks" we were on, but the little dipole just kept on taking it, it was getting bent over onto it'self but it's so short and soft it was/is still fine!
AnswerID: 64231

Follow Up By: Brew69 - Monday, Jun 21, 2004 at 22:00

Monday, Jun 21, 2004 at 22:00
The height is a biggy. Neighbour and i have identicle set ups. I have gutter mount and he has bull bar mount. I get a lot better range than he does. (a lot better)

The Brewman
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Reply By: Member - KG (QLD) - Monday, Jun 21, 2004 at 19:58

Monday, Jun 21, 2004 at 19:58
used to have this setup on the work trucks (but it was on the side mirrors, not front and back)... only had AM CB back then tho - i would assume the same principal still applies to UHF?

yes they are both wired to the one radio... cant say i understand the science of it but i work on the 2 is better than 1 principle :)

btw, this is where the CB term "got your ears on?" came from...
ears being the aerials on each truck mirror

cheers,

KG
AnswerID: 64266

Follow Up By: Member - Jeff M (WA) - Tuesday, Jun 22, 2004 at 11:01

Tuesday, Jun 22, 2004 at 11:01
No the same principle wouldn't apply. Well technically it kind of could but in practice it would be very difficult.
27mhz AM/SSB CB was more similar to HF that UHF. It had a wavelength of 10m. UHF is around the 70cm park. Hence there is a lot less margin for error on UHF than there is on 27Mhz. ie 27mhz vs 477mhz.
The twin ariel I have never seen but I would assume it was a dipole. We used to make them out of 10m of coax and a film canister in the middle. Hang them between a tree and the house, they were cheap to make and worked a treat! However dipoles for UHF are a hell of a lot smaller, ie I have one on my surf that just looks like a car radio arial about 30cm tall and just black rubber style job.
The other thing was that with 27mhz we were trying to get "skip", that was the whole aim of make and positioning the ariels however "skip" is practically non existant on UHF. Height is the key. UHF does have somthing similar (well to the average joe, very different technically) called "induction". Induction normally happens from environmental conditions and tunnels UHF signals down the coast. EG might be able to talk from bunbury to geraldton. But these are almost impossible to predict and you cannot really set your radio up to take advantage of it as such.
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FollowupID: 325448

Reply By: Large - Monday, Jun 21, 2004 at 20:03

Monday, Jun 21, 2004 at 20:03
I'm assuming that when you say CB you mean UHF. I wouldn't even bother with trying 2 antennas as they are designed to take one only. If you are referring to 27 meg CB, years ago you could by a twin antenna kit "twin trucker" tpe that would mount on each side mirror but I dont think you can even get that anymore. Even with 27 meg units, I use a 5' helical whip and that will get me around OZ and sometimes O.S. (if skip running well). As prevoius posts have mentioned about ideal centrally located roof mounted antennas may not be possible then you will have to settle for bullbar mount etc. etc. Depends what you are trying to achieve from your comms setup. Just curious, what do you hope to gain from a twin antenna set up anyway? If just highway, convoy useage then just a "normal set up" should suffice.

Cheers.
AnswerID: 64268

Follow Up By: Member - Jeff M (WA) - Tuesday, Jun 22, 2004 at 11:08

Tuesday, Jun 22, 2004 at 11:08
I canunderstand the want/need for a slightly better range out of UHF, when it the bush doing tracks in large convoys you can get quite large distances between the leader and the charlie, especially if there is a lot of dense scrub and hills. Obviously you can use others in the group to repeat messages to the back, but it's not ideal.
I think that changing to a lower DB antenna would be part of a solution for this, I mean in reality the higher DB antennas are better, however it's not very often that we are all on the nullabour trying to communicate. Normally there are hills and building, bush etc in the road and the lower DB would probally help. The other option is to have two antennas and a coax switch, one higher DB and one lower DB so you can get the best of both worlds, however these would drain some of your output power. There are obviously antennas out there that you can adjust the DB rating by putting different tops on them, however from memory these are only 6/9/12DB. I would expect you would want to try somthing like 2.5 to 4.5DB.
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FollowupID: 325450

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