Lenght of UHF Cable

Submitted: Monday, Jul 07, 2008 at 14:55

ThreadID: 59561 Views:2532 Replies:3 FollowUps:3

This Thread has been Archived

Pickles

G'day all
I am currently transferring my UHF between vehicles. i have a GME4705 aerial and a uniden split system 015uhf. when i first insatlled the antenna i stupidly cut the coaxial to the length i required and later was told that this was not a good idea. The UHF still worked well with a range of 10-15km while in the vehicle.
What i would like to know is should i extend or change the coaxial back to its standard length or am i ok to reuse it at the shorter length. (given it reaches to where i want it to)
thanks Nick

Back Expand Un-Read 0 Moderator Share

Reply By: Robin Miller - Monday, Jul 07, 2008 at 15:17

Monday, Jul 07, 2008 at 15:17

Hi Nick

Cutting the cable to length and keeping it short is the right approach and this reduces the losses in the cable which can be significant at UHF.

Even more so for phones , I reduced the length of the antenna cable by half or my phoneand improved its performance.

Of course you need to have the equipment to make a proper co-axial re-connection , its not good enough to cut and just twist some wires to-gether, else you could be worse off

AnswerID: 314245

Follow Up By: Member - Phil G (SA) - Monday, Jul 07, 2008 at 22:29

Monday, Jul 07, 2008 at 22:29

Robin,
Pickles may have been wondering whether shortening the cable would bugger up the antenna's SWR. I gather this is not an issue with UHF. Is that correct?

FollowupID: 580329

Follow Up By: Robin Miller - Tuesday, Jul 08, 2008 at 08:23

Tuesday, Jul 08, 2008 at 08:23

Hi Phil

Shortening the cable won't stuff the Vswr.

Its actually a little more complex than that , as a cable has losses and these make the Vswr look better than it is.

If you had 100 meters of cable at UHF , the losses dominant and you could even disconnect the antenna and still show a good Vswr. Of course you wouldn't here much on your radio.

The theory is the same at HF but because of lower losses and the wavelengths being longer the cable can act less as a transmission line and more like a matching element that can be used to match the antenna's feedpoint impedance.

This oftens causes visible side effects and much myth to be generated.

FollowupID: 580350

Follow Up By: Mike Harding - Tuesday, Jul 08, 2008 at 09:59

Tuesday, Jul 08, 2008 at 09:59

I had an interesting issue in this area a few months ago: I made a new bush antenna (an inverted vee dipole cut for 80m) for my Amateur radio on HF and attached a random length of RG58 co-ax (about 10m). During testing I could not get a decent SWR no matter what I tried, including an antenna tuner.

Finally I attached an RG58 extension cable I had brought with me, I think it was around 7 or 8m long. SWR drops to near zero on 80m and is pretty good on the other bands via the tuner.

I assume the initial length of RG58 simply happened to be the exact length to set up some kind of standing wave and prevent a proper match?

Mike Harding

FollowupID: 580360