UHF Antenna cable shortening

Submitted: Tuesday, Jul 20, 2004 at 12:05
ThreadID: 14841 Views:4803 Replies:5 FollowUps:4
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I have like most a UHF antenna mounted on the bull bar. I want to remove the spring base and 250mm of base anetenna when in the city. Is it possibe to regain antenna performance after putting in a joiner at the front of the vehicle when done properly.
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Reply By: Member - Jeff M (WA) - Tuesday, Jul 20, 2004 at 12:18

Tuesday, Jul 20, 2004 at 12:18
If you are going to chop into the cable, you might as well replace the whole lot (other than the little bit left going to the atenna's) with a thicker "low loss" cable. That way any loss in the joins will hopefully be made back up by the better cable. You can shorten it a little, but no too much as you don't want to throw your SWR out too far. But I find that GME and the like seem to have a little more cable on their than what is required and taking 1 meter or so off it can increase perfomance quite a bit.
AnswerID: 68626

Follow Up By: V8troopie - Wednesday, Jul 21, 2004 at 01:18

Wednesday, Jul 21, 2004 at 01:18
Thicker Coax cable also have a larger bending radius, bend it too tightly and the cables impedance goes astray ( never, never kink it).
Thicker cables are harder to pull through small orifices in fire walls, etc.

If your antenna relays on a 'tuned' coaxial cable length then it is a poor design for its intended use. To improve the SWR, at a particular frequency, one alters the antenna length, NOT the cable to it???

Coaxial losses do not really warrant the thicker cable for the short distances involved. Now, if the antenna was mounted at the back of a 30' caravan and the radio was in the car then, perhaps, its worthwhile.

But a better QUALITY cable is always a good idea, it should be a well known brand (Belden, perhaps), have a tightly knit weave , tinned, shield rather than the crappy alfoil or loose weave plain copper stuff one sometimes sees and also have a tinned, stranded centre conductor - not a solid one.

The cable can be joined by using male/female coaxial connectors like the BNC type ( these are almost water proof) but their expense (and fiddly exercise to fit if you havn't done it before) would make a new cable a cheaper option. I take it you are using an antenna base where the cable plugs into and not an antenna with a fixed cable?
Klaus
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FollowupID: 329260

Follow Up By: Member - Jeff M (WA) - Wednesday, Jul 21, 2004 at 13:17

Wednesday, Jul 21, 2004 at 13:17
The amount of cable changes the amount of resistance, ergo changing the length of cable can alter the amount of load the antenna has, therefore changing it's SWR.
Most cars have a rather large gromit on both passenger and driver sides of the firewall that you can easily fit 25mm conduit through it you had to. All conections BNC or otherwise will cause signal loss to a certain degree, you can get both crimp and solder type BNC connectors, solder is more fidly, but you don't require a $60-$70 tool to crimp them.
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FollowupID: 329306

Follow Up By: V8troopie - Wednesday, Jul 21, 2004 at 18:30

Wednesday, Jul 21, 2004 at 18:30
Jeff, radio frequency is an interesting topic, where many misconceptions, like the one you mention in your first sentence, that are passed amongst laymen.
FYI, resistance has little to do in that context, the correct word is impedance. (Look up the definition of 'impedance')
The antenna cable, having the correct impedance to propagate the RF signal, is just a means of getting it to the antenna. IF the antenna is tuned properly the RF signal will radiate. If its not tuned properly, some of the outgoing signal will be reflected back, setting up a standing wave and it is the ratio between the outgoing and reflected power that the SWR refers to.

So you see that the antenna, being properly tuned, will radiate the outgoing signal without having to worry about critical cable length - AS LONG AS THE CABLE & PLUGS HAVE THE CORRECT IMPEDANCE.
You do not want the coax cable to radiate the signal as well, as that what is happening if altering its length alters the SWR, indicating a problem elsewhere.

Yes, there is some attenuation along the cable, nothing runs loss free, so, the shorter the better, especially for higher frequencieslike UHF. But, this loss can easily be exceeded by connector mismatches, poor connections at the antenna, water in the coax jacket, etc.
BNC connectors are a much better choice since they DO match the cable's impedance (if they are 50 Ohm impedance). There are better connectors around, but at a price (N type). The cheap 239 style connectors are a waste of time with UHF frequencies, IMO.
I have used BNC connectors extensively in both the solder and crimp versions, I find the solder style more reliable where there could be a strain on the cable due the gland and clamp nut. The last two items also make the connector just about waterproof.
Klaus
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FollowupID: 329341

Reply By: jackablue - Tuesday, Jul 20, 2004 at 13:31

Tuesday, Jul 20, 2004 at 13:31
Can't you unscrew the base antenna & pull the cable through? I've moved my position a couple of times. Mine is a GME antenna.

Cheers

Jackablue
AnswerID: 68641

Reply By: The Banjo - Tuesday, Jul 20, 2004 at 13:50

Tuesday, Jul 20, 2004 at 13:50
Not sure I'm exactly with you re the question but re antenna and cables etc...... a installation technician did tell me a a couple of years back that a coax on the GME UHF's can range from quite short to a max of 6m. It also seems that all of the bases have a means of getting the cable out and soldering a new one in, even if they appear to not be built that way. Something will screw off or pop out to reveal the connection probably.
AnswerID: 68642

Follow Up By: jackablue - Tuesday, Jul 20, 2004 at 14:05

Tuesday, Jul 20, 2004 at 14:05
If you take the top nut off the extension base, then unscrew the black part you can then see the end of the cable. Be careful not to lose the little nylon insulator. Pull the cable down through the middle. Obviously mine is not soldered & have never a problem with reception. You can then also unscrew the antenna base & just leave the spring part bolted on.

Cheers

Jackablue
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FollowupID: 329168

Reply By: Brian - Tuesday, Jul 20, 2004 at 16:20

Tuesday, Jul 20, 2004 at 16:20
Is it possible to cut a "key way" into the mount so that you can slip the cable out, this should not effect the tightening of the base?????

Just a thought

Brian
AnswerID: 68664

Reply By: Errol Flynn - Tuesday, Jul 20, 2004 at 16:28

Tuesday, Jul 20, 2004 at 16:28
I think everyone thinks you are talking about the cable, rather than the antenna.
You will either wreck the antenna or cut 10 - 30 dB off your gain/range, depending on the type of antanna it is. Some are designed for this purpose and have screw mounts for this.

If you are unsure, why don't you mount it on a hinge bracket and tilt it back for city driving/carparks?

Regards,

Scott.
AnswerID: 68666

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