Question for UHF Antenna experts ?

I have a CD900 series elevated feed 6.5db UHF antenna mounted on the bullbar. It`s a black antenna around 3ft high with a metal whip section.
My mate has one of those boofy white fishing poles about an inch thick and around 4ft high.

Question is when we are a max distance for communicating I can hear him clear as day but he says I am breaking up and can`t hear everything I say.

Who has the better antenna ?
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Reply By: Member - Graham H (QLD) - Saturday, Jan 03, 2009 at 21:16

Saturday, Jan 03, 2009 at 21:16
I think you probably know already
Have a guess you have a 50-50 chance of being right.

AnswerID: 342478

Follow Up By: Mrbrush - Saturday, Jan 03, 2009 at 21:43

Saturday, Jan 03, 2009 at 21:43
I`m guessing I have but I know nothing about antenna`s
To me it seems it could be either way.
FollowupID: 610167

Reply By: Robin Miller - Saturday, Jan 03, 2009 at 21:50

Saturday, Jan 03, 2009 at 21:50
Hi Mrbrush

There are other factors than the antenna which are more likely the cause.

The units have both a Tx and RX and either may be off channel slightly , or have different relative sensitivities causing distortion to the audio.

Another effect is that the antennas are rarely a perfect match to the uhf unit and have a high resultant VSWR. The UHF can react to this and not receive/ Transmit as effectively.

Just trying to think of something you can do to narrow down the issue without much test equipment , and all I can think of at moment is to swap aerials over and re-check , using a temporary setup.
Robin Miller

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AnswerID: 342488

Reply By: Atropos - Saturday, Jan 03, 2009 at 21:52

Saturday, Jan 03, 2009 at 21:52
Without measuring each antenna and the sensivity of each radio concearned there is no way of knowing...

His Radio could be "deaf", your radio may be transmitting low power.. or a combination of all and more.. without measurment using calibrated test ewquipment its impossible to say which antenna is better than the other :-)

Amateur radio callsign VK3ARK
AnswerID: 342489

Follow Up By: 96 GXL 80 series - Saturday, Jan 03, 2009 at 23:24

Saturday, Jan 03, 2009 at 23:24
Hi John & Jean, catch you on the group on Monday.

This will keep you thinking hihi
FollowupID: 610190

Reply By: Eric Experience - Saturday, Jan 03, 2009 at 22:32

Saturday, Jan 03, 2009 at 22:32
A big factor with uhf is the squelch control. if it is not set correctly you can get the effect you describe. If your mate has a poorly installed radio his squelch may be set to high. modern units with automatic squelch can respond to a poor installation by being permanently "deaf". The gain of the antennas is added so it makes no difference which vehicle the antennas are on, so if you have a low and he is high the signal strength should be the same both ways. The exception to that is if one radio is overloaded, that is the signal is being swamped by a strong signal other than the one you are sending, this could be from electrics under the bonnet or another radio close by on the next channel. Eric
AnswerID: 342496

Reply By: Grungle - Saturday, Jan 03, 2009 at 22:52

Saturday, Jan 03, 2009 at 22:52
Hi Mrbrush,

You would have an RFI made CD931 6.5db ground independent elevated feed antenna and your mate a GME AE4705 4.5 db ground independent antenna. The RFI antennas are probably the best in Australia with the GME coming in around third or forth.

I also have the GME AE4705 antenna and believe they are pretty good. I have tried many different types with some being better than my current selection but I settled on that particular model for a couple of reasons.

1. They are super strong and can handle decent bird strikes.
2. They can be quickly removed (except for the spring).
3. You can take off the 4.5db antenna and add either the AE409L whip in 6db or 9db form for longer distance communications.
4. 4.5 db is a good compromise between hilly and flat terrain.

No-one can accurately calculate max distance from an antenna as so many factors dictate the transmission pattern of an antenna. Decent manufacturers will have a radiation pattern plotted that is based on an EIRP (Effective Isotropic Radiated Power) but this is usually based on their antenna being mounted in the centre of a roof of a vehicle so is not really practical in real life.

Gain of antenna, mounted height / position, how vertical antenna is, quality of coax (attenuation), quality of connector and fitting, output power of radio, obstacles in path of transmitted signal - even time of day can all affect the transmitted signal.

I can say that the CD931 is a top antenna which should see a good transmission pattern radiated. It uses quality double shielded coax (Cellfoil) and connectors are pre-crimped with adaptors supplied for different radio types. Unless you cut the pre-terminated connector off and put your own on, then I would say that the problem you describe above could be with that. Otherwise you could have had something rub through the coax to create some RF attenuation or your radio is down on power.

Popping into a radio communications shop and asking for a VSWR test will reveal the health of you setup.

To put a rough guesstimate to your original question re distance then I would say around 4km in hilly areas, 8km in moderately flat areas with some small hills to 12km in flat country. However anything is possible and transmission distances of 20-30km are not unheard of if conditions are right (being on top of a hill for instance).

BTW we sell radios and antennas.

AnswerID: 342498

Follow Up By: Mrbrush - Sunday, Jan 04, 2009 at 00:05

Sunday, Jan 04, 2009 at 00:05
That sounds like the problem as I can pick him up further with the 6.5db and he just can`t quite pick me up using a 4.5db.
It certainly makes sense.
Thanks grungle
FollowupID: 610192

Reply By: Member - Mike DID - Saturday, Jan 03, 2009 at 23:41

Saturday, Jan 03, 2009 at 23:41
If an antenna has a gain of 6.5 db it has that gain for transmit and receive. Changing antenna gain will not improve just one direction (provided there isn't an impedance mismatch.)

If he can't hear you clearly while you can hear him, it means your transmitter or his receiver is faulty.
AnswerID: 342503

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