UFH Ariel lead - cut it or leave it?

[BundyBrumby]

I was hoping that some one could clear this up

I have been told by different people to cut the length of the UHF ariel base lead to suit your car, but others say to leave it the length it came and roll the excess up.

I just bought a new white fark off ariel and dont want to bugger it up

Cheers, Jono

[mickyd555]

if the lenght was that critical it would come pre terminated to the required length. You should be fine to cut it, ive cut mine and it works fine.

[AussieGQ]

What brand and model is it?

GME have a habit of using the coax for tuning purposes. So if its a GME I would suggest you contact them.

[-Scott-]

It's a long time since I studied RF Voodoo, but (IIRC) coax cables are designed to impedance match source and load impedance. If the source (radio) and load (antenna) are the correct impedance, and the cable is rated for the same impedance, the length of the cable is irrelevant.

Even with a mis-matched load, I don't think the system can be tuned by altering the length of the cable. If the cable is an incorrect impedance for source and load, careful tuning of the length may help, but I'm not sure. Without the correct measuring instruments, you'll never know what's right anyway.

I'd say the biggest reason to NOT shorten the cable is re-terminating it correctly. Coax cables MUST be terminated correctly, and if you get it wrong (i.e. short the two conductors) you can blow your radio's RF output amplifier.

If you must shorten your cable, pay a professional installer to do it, and get a receipt stating what was done - if he/she stuffs it up, they can pay to fix your radio!

Good luck,

Scott

[AussieGQ]

The length of the coax can most certainly match the radios back end to the antenna. As I stated above, GME do this on their marine antennas.

[84ZOOKSTA]

CHOP IT.



It is best to have the coax as short as possibe because rg58 coax has massive losses.

If i installed my antenna and left the full 5meters of coax on there and the radio transmitted 5watts i would have a total loss just accross the coax of 1.65Db whic would result in the TX power at the antenna of 3.4watts (1.6watts loss)

If i was to do the same install but with only 2 meters of coax and tx of 5watts, total losses would be .657Db and then the tx power at the antenna would be 4.3watts (.7watts loss)

But all those numbers are referenced to coax only, you have to add losses for connectors and VSWR so it is good to get it installed by someone in the know with all the right gear to test and Tune if required.

Cheers
Simon..

[AussieGQ]

Now work out the loss if the impedance is out and you get high reflected power? Say 4:1.

I agree with making the cable as short as possible if the design of the antenna permits, however there are antennas around that do not permit.

[84ZOOKSTA]

5 Meters- TX 5watts VSWR 4:1 cable loss 1.64Db VSWR loss 1.135Db Power at the antenna 2.637watts.

2 Meters- TX 5watts VSWR 4:1 cable loss .66Db VSWR loss .595Db Power at the antenna 3.747watts.

In my experance in installation of both mobile and base equipment i have never come accross an antenna that requires special coax lengths.

I use RFI, GME, MOONRAKER, Pacific Aerials and even Mobile 1 equipment and none of these antennas that i have worked with require special lengths. but i must say i have never done anywork with marine band so i am unable to comment about the marine antennas.

Simon.

[AussieGQ]

I work in radio comms full time and only work with RFI, Andrews and GME equipment, the rest are rubbish for commercial applications! Saying that, I dont rate GME to high either.
Most manufactures of antennas do use the coax to match the impedance of the radio to the antenna, mostly in ground independent antennas.

You'll also notice in your posted losses that the higher frequency the higher the loss. Step up to above 70cm and with the same reflected power you will have pretty much nothing at the antenna.

[84ZOOKSTA]

the losses i have posted are all at 477Mhz.

[familybus]

simply if you can hide the excess lead! dont chop it because one day you might want to move it or even remove it and sell it! it wont do squat if you cut it shorter providing your only runnin 5watts, if you were runnin a howdy doody setup and wanted to get technical id say yeh but for now dont worry bout it! you wont even notice the dif from cut or not cut!