Antenna Gain

Submitted: Saturday, Sep 06, 2003 at 21:35
ThreadID: 7071 Views:1609 Replies:1 FollowUps:9
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A little while ago the subject of (UHF) antenna patterns came up.

Found this page which explains it all rather well.

Antenna theory explained

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Reply By: Eric - Saturday, Sep 06, 2003 at 22:07

Saturday, Sep 06, 2003 at 22:07

Antenna gain is useful if you have a weak signal and your receiver is not sensitive enough to recieve it. With uhf CBs this is usualy not the case, If you have your sqealch control set anything above the lowest setting you are recieving background niose higher than the minimum the radio is capable of recieving, therefore increasing antenna gian only means you turn up the sqealch a little to cope with the higher background niose because the antenna can not descriminate the signal from the niose, The most important factor in cb instalation on a moving vehicle is to place the antenna on the vehicle in the best spot as far as your engine niose is concerned, this is usualy the centre of the roof but the rear of the roof is ok. The way to find the best spot is to walk around the vehicle with a hand held cb and listen to the noise, have the engine running at the normal revs for criusing and have your lights and air con running. You will notice that vehicles like ambulances, where the best coms are vital will have a short antenna in the centre of the roof. Eric.
AnswerID: 30286

Follow Up By: ExplorOz Team - David - Saturday, Sep 06, 2003 at 22:22

Saturday, Sep 06, 2003 at 22:22
I thought it had more to do with the ground plain then the engine noise! The fact the the centre of the roof has the largest solid ground plain was what the antenna theory I did told me.Regards
ExplorOz Team - David
Always working, not enough travelling ;-)
FollowupID: 21323

Follow Up By: Mad Dog Morgan (Vic) - Saturday, Sep 06, 2003 at 23:04

Saturday, Sep 06, 2003 at 23:04
Same here DavidMy Best Mates
FollowupID: 21325

Follow Up By: joc45 - Saturday, Sep 06, 2003 at 23:39

Saturday, Sep 06, 2003 at 23:39
At UHF, electrical noise from your engine does not significantly come into the equation, nor does the general background noise received, unlike HF; at UHF, the thermal noise of your receiver front-end is the limiting factor. Raising the gain of your antenna will increase the signal by that amount above your noise floor of the receiver.
In other words, antenna gain is good.
Placing the antenna in the middle of the roof will ensure the antenna has a consistent radiation pattern in all directions, but really, as long as you have more than about 1/2 a wavelength of metal roof around the base of your antenna (ground-independent antennas excepted) you should be fine.
FollowupID: 21328

Follow Up By: ExplorOz Team - Michelle - Sunday, Sep 07, 2003 at 00:51

Sunday, Sep 07, 2003 at 00:51
Joc45 or Others,

Can you give me some insight into the operation of ground independant antennas. I do understand basic antenna theory but I cannot understand how you can acheive correct ouput vertical polarised radiation without the use of a ground plan. This point of ground independance has always interested me. Mind you I have used a lot of ground independant antenna with radios - I have just never understood how they work.

Regards Michelle,
Spread the word -
FollowupID: 21330

Follow Up By: joc45 - Monday, Sep 08, 2003 at 09:06

Monday, Sep 08, 2003 at 09:06
Michelle, I'll try without diagrams;
A GI, in its basic form, is a vertical dipole; ie, a 1/4 wavelength element above and below the coaxial feed point. The upper element is a whip, the lower element is usually a tube. The coax is fed up the centre of the lower element. A dipole needs no ground plane; it is a symmetrical radiator.
But the problem arises where the coax cable feeding the antenna itself becomes a radiator of the rf, and being of indeterminate length, upsets the radiation pattern and also the matching, upsetting vswr.
This is usually fixed by having the coax cable form an inductor or coil just at the base of the lower element. This coil is in the form of several turns of the coax, and is usually concealed within the base. This prevents the rf being radiated beyond the coil. Larger GIs; ie in the vhf band, often have the coil visible under heat-shrink tube.
At UHF, because the antenna is usually fairly short, the upper element can be extended like ordinary whips with the usual phasing coil(s) to improve the gain, but the lower element always remains at 1/4 wavelength.
Hope this helps, rgds, Gerry
FollowupID: 21457

Follow Up By: Eric - Monday, Sep 08, 2003 at 21:47

Monday, Sep 08, 2003 at 21:47
Joc 45.
You obviosly have a good grasp of theory, but if your statement was correct then you would not have to readjust your squelch from engine off to engine running. True some of the early diesels were electricaly quiet once the battery was fully charged but not many 4x4s dont have some accesory that loads the alternator. The statement that the roof centre provides a ground plane is true, that is why the roof centre is better than the back of the roof, the roof also provides an RF screen from the internal wiring. That is why a short antenna will give a beter s/n ratio than a long one because the long one can "see" over the edge of the screen. Eric.
FollowupID: 21531

Follow Up By: joc45 - Wednesday, Sep 10, 2003 at 23:07

Wednesday, Sep 10, 2003 at 23:07
Great thing, theory - in theory.
I can accept that some more recent diesels with electronic injection control, with processor clock speeds approaching 100Mhz, are capable of emitting harmonics up into the hundreds of Mhz, thus affecting UHF (in theory). I guess I will have to accept that you have experienced interference at the UHF level (see comment further below). It is a fact, by inherent design (usually in the IF), that some receivers are more susceptible to noise interference than others, even with a good noise floor.

But in regard to your mention of interference from charging batteries, the frequency components are much lower in frequency; this should be a relatively simple matter to sort out, and there is already a plethora of info written about how to reduce this.

In regard to the position on the roof being a factor in reducing interference, then it follows that the closed bonnet should be even more effective in stopping the engine interference than the shadow of the roof (in theory). In practice, there will be diffraction of the signal around the edge of the roof, just like you see the glow of city lights over the hill.

The main advantage of a roof mount is sheer height - UHF propogation is primarily line-of-sight (with some diffration effects as mentioned), and raising the antenna to a point where obstructions are eliminated or reduced, improves the situation.
The main disadvantage of a roof mount is the likelihood of getting the antenna knocked off in the bush. If you don't travel where overhanging shrubbery is present, then this is the way to go. Otherwise, a GI elsewhere on the vehicle where it won't get knocked off is the next best compromise. The disadvantage is that the upper body of the vehicle may obstruct the path.

Just as an aside, I have been using a rented 3.0L GU Patrol (with the offending electronic injection) over the last week or so with a mid-band VHF mobile (in the 80MHz band) and two UHF mobiles fitted in the vehicle, and interference from the diesel (or the alternator, for that matter) was not evident in any of the units.
FollowupID: 21743

Follow Up By: Eric - Thursday, Sep 11, 2003 at 00:20

Thursday, Sep 11, 2003 at 00:20
When you say that the engine interferance is not evident, are you saying that the is no niose diference with the motor on or off? The usual problem with uhf cb when people say "I did not hear you" is because the squelch is set to high, my discusion is focused on keeping the squelch as low as possible. I would like you to do a simple test, just set the squelch at the triger piont with the motor of, then start the motor, if the radio does not make a sound I will be very surprised. when I do an instalation I do this test and judge the succes by the amount the squelch has to be riased, the answer has never been zero even on army vehicles with shielded looms. If the niose floor was the limiting factor the result with a 50 ohm resister across antenna terminal compared to the antenna would be the same. Eric.
FollowupID: 21747

Follow Up By: joc45 - Thursday, Sep 11, 2003 at 00:31

Thursday, Sep 11, 2003 at 00:31
The vehicle's gone back to Thrify. Sorry
FollowupID: 21748

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