UHF arials

Please explain your rationale why a lower dB antenna is better in hills & forests.

I'd like to know the disadvantages of a high gain antenna in that situation.

Dave

Just looked at the link in the post below made by Paul P; this is basically how I understand it, so it saved me some typing.

If I am wrong in my thinking I will stand corrected, but GME seem to look at it the same.

A high-gain antenna concentrates the signal in a narrow band - a bit like a light-house shining no light above or below the horizon.

This is great when the vehicle is horizontal and the country is flat. But if you are in a deep valley, most of your signal is aimed at the cliffs, with only a little left to go up and over the cliffs.

The same concept applies to received signals.

Mike

Hi Exploder & Mike

I'd not actually given a lot of thought of propagation patterns and their affect in those situations.

Funny really because we use propagation patterns to our advantage when you bend a HF whip back to the roof rack to get better short distance performance from the HF sets.

Usual considerations given to propagation patterns involved what the shape and amount of bodywork was going to do in close proximity to the antenna.

Thanks,

Dave

Greatings

My little metal one ( as stated in this thread) destroyed itself on corrugated roads hence my new(cost effective fibre glass one.

Also the white/black/grey fibre glass ones are also used for GSM/CDMA antenna ( high gain).

I prefer white. It matches my vehicle colour:-)

Regards

Paul

Laura B,

The size of the aerial has nothing to do with ego. It is all about trying to get a signal.

The small wire ones are good for around town and roads that are smooth as they don't like to be shaken around. It is only wire and one way to break wire is to bend it back and forth and that is what happens on a rough track.
The big white ones are used in the desert area where the land is rather flat and the higher the aerial the further the range. The aerial works on line of sight. Two high aerials will "see" each other and commutate.

The shorter aerials are good for the areas where there are a lot of low branches. They are of a db that will send a signal over a large area but not much distance.

The aerials are mounted on a large spring base that will allow the aerial to move with out breaking

Wayne.

One wavelength on UHF CB (477mHz) is approximately 630 millimetres.
The most basic type of antenna is the half wave dipole. With this antenna, 1/4 wave of wire will be connected to the innner wire of the coaxial cable and another 1/4 wave of wire will be connected to the shield. (Note that for simplicity no baluns have been used)
This dipole antenna is used as a common reference antenna - ie gain measured in dbd (decibel gain over a dipole)
The most basic type of mobile antenna will be a 1/4 wave ground plane antenna. This means that the vertical radiating section is approximately 157mm long and the "ground plane" makes up for the missing 1/4 wave.
The radiation pattern of the 1/4 wave antenna will be the highest of most common UHF antennas & makes it perfect for mountain work. (and low trees)

The highest gain antenna commonly used is the 5/8th wave. This concentrates the RF radiation at a lower angle of radiation. Still good in the mountains plus the open country.

The UHF antennas that are longer than a 5/8th wave are known as collinear antennas. These use multiple 1/2 or 5/8 wave sections with 1/4 wave matching sections in order to feed the signal in phase. The advantage of a collinear is higher gain due to a lower angle of radiation. They also are generally ground independant & thus do not require a ground plane.
Also, some of the really long UHF sticks are simply a ground matching section with a 6dbd antenna which provides an elevated feed point. In VHF / UHF, the higher the antenna the better.
UHF communications are not necessarily line of site. Reflection and refraction also play an important part. Thus it can be possible to communicate with a car over the otherside of the hill without being "line of site" This is known as knife edge refraction.
The spring at the base is to avoid the antenna self destructing over corrugations.

Hope that this is of some help.
Cheers Tony.

where i come from its all about ego and nothing else.......dont need to tell me about what antenna's are used for what.....
laura b

Laura,

I don't know where you come from but like I said it is all about getting a signal.
Because of what I do, I can't afford to have a bit of wire brake and have no commutations and I have found what I am using works for me. I don't drive that much on city roads.

Anyhow I need something bigger for my ego than a aerial, that is why I drive a Troopie.

Wayne

Wayne,

a troopie will do it!!!! lol !!!!! maybe you could come and give the ute boys a lesson in my town!!

Laura B

Banjo, I'm curious.....did the antenna that broke have a spring base? I've been using the same UHF antenna (GME ground-plane independant with a black wire top section, coiled in the middle) for bloody years and have travelled over some mongrel corrogations (eg: Cape York twice) and have never broken or shaken-loose an antenna). I do also have a very short rubber antenna which I screw onto the silver base for around town/local convoy use.
Cheers
Roachie

Roachie

The top wire bit had the coil as you describe, that was screwed onto a heavier bulbous metal section with a largish coil spring.

Maybe just a coincidence but I broke 3 in total. One on the Gibb River Rd, another going up to Weipa and a third between Weipa and the Cape.

Replaced again at Weipa with the fibreglass type and that has lasted for 2 years.

Banjo