Thread: Antenna mounting

Originally Posted by p38arover

Yes, I know I should (I am a qualified radio tech - I even have a BOCP) but, like most people, mounting take precedence over radiation pattern.

I only run UHF and I'm currently using a ground independent antenna. I've probably got around 5-7 metres of RG58C/U which has a loss of about 2-3 dB at 477 MHz.

I guess by moving the antenna to the rear, I could reduce the loss to about 1.5dB but will the radiation pattern be worse? I don't have the test equipment ot measure it.

Ron

Sorry if I sounded patronising - wasn't trying to be. I didn't know you were a tech. Anyway glad to meet another radio tech on here. I've been "off the tools" for many a year now and stuck behind a desk ( unfortunately ).

As you'd know ( but others reading this may not ) if you have 3db loss in your cable you lose half your transmit power between your radio and the antenna ( ie from 5 watts down to 2.5 watts. ) so if you can get the length down below 3 metres or so, that's gotta be a good thing don't it ?.

now, radiation pattern, as long as the antenna whip is entirely above the roofline, it should be fairly omnidirectional. More importantly, it would be good and high, and as you know UHF is "line of sight" so the higher the better.

With the antenna whip low down behind the back door as in the pic earlier I would say the radiation pattern and antenna efficiency will be c**p - quite frankly.

Above the roofline, there is the problem of it hitting branches etc but IMHO on the bullbar it hits plenty of things too !

I've seen quite a few antennas just "fall off" bullbars, possibly due to the "resonance" effect set up, causing metal fatigue.

Sticking up above the roof may be a problem in carparks etc, but if it is on a spring base you can bend it over and hold it down wih a bungee cord or something.

you're right - with radio as in life, it's always a compromise between performance and convenience !!

Perhaps use a bracket similar to the pic Ron posted, but add telescopic pole that can be lifted when better performance is required, and dropped when going through carparks etc.

Here are some pics of my bracket. Designed for the load from a HF autotune antenna.

My autotune is on the bullbar for the moment. The extra bracket for the reverse light was made to be a spacer so the autotune will clear the 33" spare tyre.

The spacer between the spare wheel mount and the door, is only to compensate for the dent in the door (from reversing misshap with spare tyre).

I'll just put in my oar here. Firstly, for the record, both my Landrovers have the UHF antenna mounted on the bullbar, like almost all work vehicles have round here. For my use, seems to work perfectly well. The advantage of the bullbar is you can see it about to hit something! Not like the antenna on the roof of my tractor, which is now bent to near horizontal after a brush with overhanging shrubbery (out of sight, out of mind). Mind you, it still works!

One of the things you need to consider is that UHF is almost purely line of sight - which means that greater effective radiated power is of little use except that it enables you to talk over the top of others. So radiation pattern is a lot more important than loss in your transmission line. Same sort of thing with the VHF on my yacht - the additional line loss with moving the antenna to the masthead is irrelevant compared to the extra ten metres or so height. Also consider one of the nuisances we get here occasionally is the ultralight aircraft using "our" UHF channel - they come through very well from over a hundred miles at ten thousand feet, using 1w handhelds!

So by far the best place for your UHF is in the middle of the roof, but since your range is mostly going to be limited by the nearest hill, unless it is in a very bad location, you probably won't notice the difference.

John

Here is to resurrecting an old thread...

Mounted to the front of my Disco by way of an ARB B/Bar is an elevated ground independant UHF antenna ( 5dB ) on spring mount and a VHF whip.

I was thinking of making a bracket of the rear wheel mount and having another lot of mounting points so I can move the antennas to the back when on trips and wanting more range hopefully achieved by being higher from the vehicle.

Question is - Does anyone know what should be the minimum distance seperating the two antennas? The UHF is 5W, the VHF 25W. I have been advised that the 25W will interfer with the UHF when transmitting but can't confirm this. Currently they wouldn't be more than 600mm apart I should think...the distance between the two tabs on the ARB b/bar.

The coax run to the rear may be only one metre more than to the front so I'm not too concerned with power loss...or should i?

Thanks in advance.

Just finished a small lathe job with 60mm solid reducing down to the 49mm wheel hole and then further reductions and polish off. Cabling through the mag and into the vehicle. Easily mated with the yaesu 857 located inside right storage bin and led remote press button to activate tune cycle in the headlight adjustment panel which area was vacant (behind cover). Remote head in roof mount above front interior light. Works a hoot for me as a ham. I found an old sell call 8422 box and these easily mate to the radio as well if you can find one via the paket connector for permanent or semi permanent if you need, (just turn radio volume down) as sell call is fixed audio out.

The Yaesu was selected as cheaper than icom, less current draw in receive, also can make extension cables rather than icom only stuff. Wanted and Icom so would have built in either as both reliable.

Yaesu and icom tune interfaces can be found on their groups site and does work very well. I know there are negatives but I just wanted to put the autotune on for a couple of trips a year and remove radio when finished with the harness left in tact. I'm an old believer in stick as well and back with the series one, I just used the fishing reel with copper line which really meant I was in for a roasting by others until I fired it up and before the women used it for clothes line!!!!

The downside with ham in the TD5, depending on your luck is injector noise!!! and I'm not alone here with ebay coming to the rescue for 'ferrite' packs way cheaper than Jaycar. On this subject you need to get mix 31 material but most ferrite's don't tell you what material they are. Another site on this forum shows the areas for install if your having RFI. Hope this assists with a mounting option for larger HF stuff. Geoff