Common UHF problems

Ive got a few questions to ask the more knowledgable:

1. Can the UHF be damaged by using it without an aerial (coaxial rattled loose). If so, is there anyway to test if this has happened?

2. What is the best db aerial for hilly and densly forested areas? Ive heard 3db is best?

3. What is "ground dependant" and is this design the best for a 4wd?

4. My UHF is wired straight to the battery (positive and negative) as this was suggested in the instructions of the UHF so that the unit can be used without having the ignition on. EDIT: is this an ok set up? Is there any downside to this set up?

5. What is the best MHz aerial? Ive seen listings for 477MHz and 27MHz


I did notice the cable had come out of the back once a while ago but im not sure if i used it like that or not.

I tried to use it last weekend and i could hear and talk if the other car was very close (like 15 metres) but not any further away. The other vehicles say that im very distorted.

In open area i could hear the odd bit from the others up to 100 metres away but they couldnt hear nothing from me.....

27mhz is AM Lucas and 477 is uhf.........

sweet, thats one question down. Cheers:)

Never transmit with no aerial attached as it can damage the transender....

Hilly areas, 3Db is best but not sure about forrestry areas

not sure if number 4 ia question or just a statement but mine is also wired directly to the battery, with an inline fuse on the positive

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Originally Posted by Bloodyaussie

Never transmit with no aerial attached as it can damage the transender....

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Can you elaborate on that? Is there anyway to check if this has happened? Can it be repaired?

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Originally Posted by Clunk

not sure if number 4 ia question or just a statement but mine is also wired directly to the battery, with an inline fuse on the positive

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oh, sorry, i was just wondering if this set up had any negative affect? I gues not. Should i justr make sure it has good connections to both?

Yes it can be repaired but if it is an older system and not worth much then it is not worth doing..

There are specialist shops here that can test it but I dont know what you have down there.... mostly communication specialists.

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Originally Posted by taslucas

oh, sorry, i was just wondering if this set up had any negative affect? I gues not. Should i justr make sure it has good connections to both?

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no negative effect and yes make sure all cables have good connections..

with the antenna cable, try and keep it one length with no extra connections as with each connection you get a slight loss in Db............. now where was that thread that ET responded to, which talked about all this

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Originally Posted by Bloodyaussie

Yes it can be repaired but if it is an older system and not worth much then it is not worth doing..

There are specialist shops here that can test it but I dont know what you have down there.... mostly communication specialists.

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Its a brand new 80 channel Uniden unit. I brought it over a year ago but its only been turned on for about an hour all up so i hope its not fubared. Ill ask at a communications shop, cheers

They might try and up sell you so tell them you have another set up at home you will install if its beyond repair....

bloody oath i hope its not beyond repair. I hope its not broken for that matter! lol, ill start with another aerial

Are your mates also using 80 channel jobbies too? We were on a club trip a little while ago and we had difficulty between 40 channel and 80 channel users hearing each other on certain channels.

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Originally Posted by Clunk

Are your mates also using 80 channel jobbies too? We were on a club trip a little while ago and we had difficulty between 40 channel and 80 channel users hearing each other on certain channels.

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hmmmmm very interesting, no i think everyone i know still has the 40 channel...

Hi Lucas,
If you have transmitted without an antenna you may have damaged the transmitters output amplifier, most modern radios can cope with transmitting without an antenna for short periods without damage. If you have damaged the radio you should still be able to receive ok but only be able to transmit for a short distance, if you are also having trouble receiving that sounds like a faulty antenna or cable.
The way to test it is with a SWR meter, if you were in Adelaide I could check it out for you, not sure what you could test yourself. Who installed your aerial? What sort of antenna do you have?
A "Ground Plain Independent Antenna" is the best to use as this will work more effectively in all directions and I find they work best on a gutter bracket rather than on the bulbar. I use an RFI elevated feed ground plain independent antenna and have two whips, one a 6.5 dB and the other a 1/4 wave and swap them over depending on what I am doing- 6.5 on the highway, 1/4 wave when 4wding. This link shows the difference-http://www.rfi.com.au/consumer/gain.html
Wiring to the battery is best as you have less voltage drop, positive to battery via fuse and negative to a good earth point close to the battery.

Cheers TPC,
Ok so my antenna is just a real cheap cr@ppy one so the first thing ill try is a new, better quality one. I think i will try to keep it on the bullbar as theres always a lot of trees where we drive and it would probably get a caining on top of the vehicle.
So what is the 1/4 wave whip? Ive seen interchangeable whips (ie: 6.5db antenna with a 3 db interchangeable whip) but havent come across the term 1/4 whip. Does it just reduce the 6.5 to a quarter?
Thanks for the link, i have seen that diagram before and so decided that 3db is the best. It will be very rare for me to need long distance and even more rare to have a long line of sight to the next vehicle. 90% of its use is less 500 metres BUT through very densly forested and hilly terrain.
I have started looking at the GME range but theres so many varients! So far i think i need:

3db for the terrain im in (possibly with a 4.5 interchangeable whip).
Around 600-800mm tall to help help clear the roof line from the bullbar mount.
Stainless steel as opposed to fibreglass to help with tree strikes. (perhaps with one of those lock pins in the spring).
A ground plain independant.
Nice thick heavy duty/good quality coaxial cable.
And my fingers crossed that i havent wrecked my UHF unit!
Does that sound like im on the right track?

Oh just another thing, what is the difference between dBi, dBd, dB, and 1/4 wave? That RFI site suggests the first three over 1/4 wave.

You'll need to spend $100 on an antenna , I have a tall one mtr 6db for hwy use and a short 450mm one for hilly trips.
I ran an earth from the base to the neg on my battery. Most in my club run a very small 100mm antenna in the middle of
the roof [best spot] but not all wont a hole drilled, If antenna is on FR of 4x4 then signal is going to the RL. Not 100% sure we need 80 channels
as we have plenty ATM with 40, I for one wont be up grading even though I have an 80 channel one sitting in my cupboard.
I did have it in my camper as a home base radio,

I'll get some more info for you today

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Originally Posted by taslucas

Oh just another thing, what is the difference between dBi, dBd, dB, and 1/4 wave? That RFI site suggests the first three over 1/4 wave.

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dB is a measure of gain.
dBi is gain over a theoretical antenna that radiates out in a perfect sphere.
dBd is gain over a dipole antenna, and a dipole antenna has a gain of 2.15dBi.
dBq is gain over a 1/4 wave antenna.
A 1/4 wave antenna has a gain of .15 dBi.

When looking at gain figures look at the dBi or dBq figure.

The antenna cable used only needs to be RG58 or cellfoil if you want lower loss but get some convoluted tubing to run the cable in for protection.

Sounds like you are on the right track with the antenna style, if I get time latter on I will have a look through different antennas available for what would be good if you have not already found something.

As TD said, an antenna in the middle of the roof is best but few people want to drill a hole.
I don't like to see any wiring for the radio going direct to the negative of the battery as if you ever have an earthing issue at starter motor you can end up having the starting current go through your radio, have seen this happen at work with customers vehicles several times.

Thanks for that mate. Plenty if good info there. I will keep looking but Im in no real rush.

You need a 5 watt unit with a good antenna , you can buy those base loaded [silver] antenna that you just change the tip to suit the terrain, Tips include short 4" job a to one about 450mm long or the spiral S/S wire tip. As TPC mentions the less joins in antenna cable the better, mount opposite other antenna ,as it can cause interference in some cases. Swr meters are cheap, but trouble is you only need it once. They were popular when CB or 27mhz radio was in vogue you snipped a little off the tip of wire until correct reading was reached. Your local UHF/communication shop should know whats best for Tassie, but I'm guessing it's not a tall antenna. Also look what others are running

Hey John, thanks for the reply and I know your only trying to help but please read through the thread first. Most of what you said has been covered.

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Originally Posted by taslucas

Its a brand new 80 channel Uniden unit. I brought it over a year ago but its only been turned on for about an hour.......

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Quote:

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Originally Posted by taslucas

................ So far i think i need:

3db for the terrain im in (possibly with a 4.5 interchangeable whip).
Around 600-800mm tall to help help clear the roof line from the bullbar mount.
Stainless steel as opposed to fibreglass to help with tree strikes. (perhaps with one of those lock pins in the spring).
A ground plain independant.
Nice thick heavy duty/good quality coaxial cable.
And my fingers crossed that i havent wrecked my UHF unit!
Does that sound like im on the right track?

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Quote:

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Originally Posted by Clunk

no negative effect and yes make sure all cables have good connections..

with the antenna cable, try and keep it one length with no extra connections as with each connection you get a slight loss in Db............. now where was that thread that ET responded to, which talked about all this

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I remember reading the length of cable needs to be a certain length also.
result is better reception and or output.
sorry, this is something I know nothing about so cant elaborate further.

You know your antenna is too big when you park in underground car parks or drive through at KFC. Mine is powered to the ignition, didn't want to risk a flat battery, it's all personal choice on the power mode! Omni directional is probably fine for convoys mounted on the bulbar.

It's more the trees and over grown tracks that will impact on aerial length for me. I do like KFC though...... Lol

Some clarfiaction:-



1/4 wave length antenna is related to the freq being used. For optimum power transfer the antenna is cut to a length that it is equal to a 1/4 of the freq wave length. 1 wave length at 477 Mhz = 628,93 mm, therefore a 1/4 wave length is 157.2mm (628,93 mm / 4)

dBi is a measurement that compares the gain of an antenna with respect to an isotropic radiator (a theoretical antenna that disperses incoming energy evenly over the surface of an imaginary sphere.)

dBd compares the gain of an antenna to the gain of a reference dipole antenna = Roof mounted 1/4 wave antenna.

Db is a difference eg:- if I have a 3 dbd and a 6 dbd antenna, then there is a difference of 3db.

Some manufacture like to quote their antennas in dbi as for the unwary it looks better. A 3 dbd and a 5.1 dbi antenna have the same gain - what is more sellable.

Keep the cable runs as short as possible, and with out connectors.

Stainless / fibre glass antenna - I have seen some stainless whips fail due to road vibration - usually due to a lot of driving on ruts, I have seen fibre glass antenna destroyed by bird strikes.

If you park your truck for long term - the standby current draw from your radio can flatten the battery.

This is ok"-

3db for the terrain im in (possibly with a 4.5 interchangeable whip).
Around 600-800mm tall to help help clear the roof line from the bullbar mount.
Stainless steel as opposed to fibreglass to help with tree strikes. (perhaps with one of those lock pins in the spring).
A ground plain independent.
Nice thick heavy duty/good quality coaxial cable. - Mobile Antenna cables normally come with fitted with rg58 - thin cable. Thicker cable = different connectors to suit

Only use Omni antennas for vehicles - directional antennas are used for base sites ( when you need extra coverage or reduced coverage in a particular area)

Edit, sorry just saw your second post:)

Thanks

Would RG6 quad shield coax used for TV installs be suitable for use?

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Originally Posted by GQ TANK

This is ok"-

3db for the terrain im in (possibly with a 4.5 interchangeable whip). So look for 3dbd
Around 600-800mm tall to help help clear the roof line from the bullbar mount.
Stainless steel as opposed to fibreglass to help with tree strikes. (perhaps with one of those lock pins in the spring).
A ground plain independent.
Nice thick heavy duty/good quality coaxial cable. - Mobile Antenna cables normally come with fitted with rg58 - thin cable. Thicker cable = different connectors to suit

Only use Omni antennas for vehicles - directional antennas are used for base sites ( when you need extra coverage or reduced coverage in a particular area)

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Im looking at something in the GME range, do you have any suggestions?

I think the aerial i got the first time may be a directional one. Would that create the problems im having?

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Originally Posted by Winnie

Would RG6 quad shield coax used for TV installs be suitable for use?

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TV coax is 75 ohms and not suitable for CB's, you will be ok with good quality RG58, the loss will be minimal over a short length.

Quote:

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Originally Posted by taslucas

Im looking at something in the GME range, do you have any suggestions?

I think the aerial i got the first time may be a directional one. Would that create the problems im having?

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I would be surprised if the antenna you had was directional, mobile whip antennas are usually omnidirectional.

Agreed don't use the 75 ohm cable. Mobile antennas are mainly designed for use with rg58. If you where running 15 or more metres of rg58 then it would be an issue.

All the new gme radios have the same spec - it depends on where and how you want to install it.

Im still running an old tx3400 40 channel unit - and don't plan on upgrading at the moment.

Im not sure if you have mentioned what your problem is.

You have asked if transmitting without a antenna will cause a problem (it can blow the RF power amp)

we use to sell mobile yagis that were connected via RG6 but this was to receive RF signals, so probably a different kettle of fish.

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Originally Posted by GQ TANK

Im not sure if you have mentioned what your problem is.

You have asked if transmitting without a antenna will cause a problem (it can blow the RF power amp)

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Yeah I was wondering how you would tell if there was damaged caused by using my uhf with the aerial disconnected.
Also had these problems with reception:

Quote:

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Originally Posted by taslucas

I did notice the cable had come out of the back once a while ago but im not sure if i used it like that or not.

I tried to use it last weekend and i could hear and talk if the other car was very close (like 15 metres) but not any further away. The other vehicles say that im very distorted.

In open area i could hear the odd bit from the others up to 100 metres away but they couldnt hear nothing from me.....

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If you have no antenna it will affect the receive and transmit.

The only way to tell if the rig is ok is to test it with a service monitor.

You could try it with another antenna and see if you have range - and can be heard from a distance.

Mobile yagi - a yagi is a bit like a spot light - you put it in the direction you want to like up. QA mobile mounted on the vehicle will give better gain in one direction - but behind it there will be a null. I use yagis with my coms trailers when Im running low power and linking to another site. I have also used a handheld yagi for dfing (RF direction finding)

Quote:

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Originally Posted by TPC

Hi Lucas,
If you have transmitted without an antenna you may have damaged the transmitters output amplifier, most modern radios can cope with transmitting without an antenna for short periods without damage. If you have damaged the radio you should still be able to receive ok but only be able to transmit for a short distance, if you are also having trouble receiving that sounds like a faulty antenna or cable.

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Well after having some issues with people hearing me on the uhf radio last trip, I think I've nailed down the issue based on the above description.

My antenna is not normally installed, and likely kids have been playing and pretending to use it and possibly " fried it as a result [emoji45]
@taslucus what was the outcome with your radio, did you have any luck getting it repaired?

Otherwise I maybe up for a replacement radio, good excuse to upgrade to 80 ch I guess, any recommendations on what to buy? Would be good to stick with gme as I should be able to reuse existing wiring



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