UHF aerials - question for Jarrod

Submitted: Monday, Mar 17, 2003 at 19:07
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Just getting round to buying a UHF in addition to the AM CB that iv'e got but am confused as to which is the best aerial fir the job. I understand that in areas of vast flat space (like you guys have got in Aust) the 6db or 9db gain aerial is the best as it gets distance, but doesn't pick up well in the hollows; whereas the 3 or 4.5db gain doesn't have the distance but can pick up the hollows. In NZ where i go regularly we have lots of hills and dips so distance is not my biggest consideration.
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Reply By: GaryInOz - Monday, Mar 17, 2003 at 21:16

Monday, Mar 17, 2003 at 21:16
The best way to answer that is to explain what one of the characteristics are of high gain and low gain antennas.

Low Gain have a a reception pattern roughly the shape of a doghnut (cross section of an "8" on its side) so they have considerably more vertical reception capacity than a high gain antenna.

A high gain has a reception pattern that is a lot lower and wider (like a "_ _") so will tend to be better for horizon type communications.

The actual "cross sectional" area of both reception patterns is the same, just with different height to width factors.

The mounting position in UHF radios using colinear antennas is not really that critical as long asit isn't too shielded by the bodywork. These colinear antennas do not need a ground plane to work like a 27 MHz antenna does. Having said that, try to seperate the 2 antennas by at least 1 metre so you dont blow the **** out of the front end of the radio recievers.

Short answer to your question would be to use a 3dB, good for about 3-5 km depending on severity of the terrain.
AnswerID: 15410

Reply By: Member - Chris - Tuesday, Mar 18, 2003 at 08:47

Tuesday, Mar 18, 2003 at 08:47
If you have a look at the GME website, there is a nice picture of the radiation patterns of various antennae. The information there will back up the above advice I thinkChris
<- 1996 Troopy, the best!
AnswerID: 15468

Reply By: Member - Darian - Tuesday, Mar 18, 2003 at 11:12

Tuesday, Mar 18, 2003 at 11:12
Amen to the above - I have seen UHF running very well in dune country, using a little 150mm antenna - was based on some sort of ground plain siver tube base - not sure of make - my 9 gain GME ran very poorly in that terrain - that was my learning curve.
AnswerID: 15476

Reply By: Jarrod - Tuesday, Mar 18, 2003 at 17:12

Tuesday, Mar 18, 2003 at 17:12
Jaycee, I agree with all of the above except for one small point. Mounting a colinear does require a ground plane or ground independent mount.

I travel in Outback Oz, and Vic High country, and my set up - An RF industries Ground independent base ( with spring), Bull bar mounted on 100 series std wagon. with interchangeable 4.5 db stainless colinear and 9 db fibreglass colinear. (I carry a spare 4.5 db colinear). My radio - an Phillips Fm900 commercial Tranciever. This has served me very well, Never had probs with the 4.5 in the High country, and th "9er" works well in the desert. I have seen too many bleep tered / costly large fibreglass radome antenna's that do the exact same job as mine but dont stand up to a low slung branch like stainless on a spring.

People often forget about insulating against water in lower antenna connections, then there is heat in engine bays ( run coax opposite side of exhuast manifold) and Good quality power cable run direct (fused of course) from battery. It is amazing how much a small voltage drop over thiner cable will effect Tx power. 0.5 volt may reduce Tx power by nearly half!!! ( if engine off, 12.5 ish volts terminal potential, only 12 volts at the radio, when Txing. - theses things are designed and tuned to run at 13.8 volts.)

Buy a radio with GOOD reciever specification. You want to pay particular attention to"Reciever sensitivity" as expressed in microvolts for 12db SINAD I haven't looked on the box of a radio for years but 0.2 uvolts for 12 db sinad would be typical.(the lower the reading the better, i.e., 0.175 uvolts better than 0.25 uV) Compare brands. If its not on the side of the box, ask Mr salesman the question. " What are the reciever sensitivity specs?" Get him to check the manual if need be.
These are the details that will affect the outcome at the end of a days 4X4 ing. It's the whole system that you need to pay attention to detail to.

At the end of the day when everyone is sitting around the camp fire comparing notes on who could hear who over the air at which times and locations, who was breaking up etc, It's comforting to hear " we could always hear you, and you always answered when we were calling!"

Routine Maintenence on antenna mounts is also important. Remove corrosion, spiderwebs/dirt from threads and connectors.

In all honesty, the difference between 3db to 4.5 db or 4.5db to 6 db is realistically not going to make the Huge difference that the sales person suggests. Is he a radio technician with 15 + years Governmnet feild experience?? - if bigger better antennas were that good, My organization would spend money on antenna's instead of costly Base station sites and repeaters all over the state. - you dont get something for nothing with antenna gain.!!!

Hope there is something here that may benefit.

Jarrod....
AnswerID: 15494

Reply By: jaycee - Tuesday, Mar 18, 2003 at 17:50

Tuesday, Mar 18, 2003 at 17:50
That about covers my question guys. Much appreciated for your input. John
AnswerID: 15495

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