Pre tuned UHF antenna!!!! Is coax length important??

Submitted: Saturday, Jun 13, 2009 at 15:11
ThreadID: 69777 Views:14161 Replies:4 FollowUps:7
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Hi all, Just bought a pretuned Uhf antenna. Does cutting a few metres off the coaxial cable and refitting the plug, change the SWR?? It can on 27 meg CB!! Just as a side note, Its funny that the mounting hole for the antenna is 1/2" and the plug on the end is about 5/8" diameter.. You have to cut the plug off to put the antenna on the bull bar!! Michael


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Reply By: Tony MD - Saturday, Jun 13, 2009 at 15:29

Saturday, Jun 13, 2009 at 15:29
No, it shouldn't make a difference with the VSWR but will make an extremely slight improvement in performance due to less attenuation in the feedline (coax).
On HF (& this includes 27mHz) The length of the coax should not affect the VSWR so long as the antenna is set up correctly. If it is affected, that would indicate feedline radiation which can be cured by a choke at the antenna input.

Cheers, Tony.
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Reply By: Member - The Bushwhackers -NSW - Saturday, Jun 13, 2009 at 15:34

Saturday, Jun 13, 2009 at 15:34
Hi Michael, cant offer an opinion on the change of SWR, but as for fitting, I learnt on here that if you can cut a slot in the mounting bracket, from the hole to the outside of the bracket, just wide enough for the coax to fit through, you don't have to cut the plug off. You just have to make the hole in the mounting bracket look like a keyhole shape, only works if your antenna fixing nut is already on the cable... loosen the nut off, slide the cable in through the keyhole slot, and tighten the nut.
Cheers, Dave
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Reply By: olcoolone - Saturday, Jun 13, 2009 at 18:47

Saturday, Jun 13, 2009 at 18:47
Yes it does make a differance, if the antenna relys on the lenght of coax to help tune the antenna you will upset the swr by cutting it.

The most common type of antenna that you can not cut the coax on is the little black rubber ducky 250mm to 300mm type that alot of farmers and 4x4er's use.

Most other antennas like the 6dB thin wire types (RFI), the 4.5 dB black generic or the big whites ones can have any lenght of coax within reason.

It depends on what the antenna design is so check with the manufacture or ask some one who knows.
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Follow Up By: Robin Miller - Saturday, Jun 13, 2009 at 19:06

Saturday, Jun 13, 2009 at 19:06
Hi Olcoolone

These types of antenna's don't rely on the coax length to tune the antenna so cutting the cable to position the antenna plug makes no significant difference.
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Follow Up By: Member - Phil G (SA) - Saturday, Jun 13, 2009 at 20:49

Saturday, Jun 13, 2009 at 20:49
Had a friend who's antenna stuffed up on a trip. So he took his UHF radio out of its cradle, sat it on the dash and plugged his stubbie antenna into the PL-259 socket on the back of the set - eliminating the antenna cable completely! It worked very well.
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Follow Up By: olcoolone - Sunday, Jun 14, 2009 at 01:43

Sunday, Jun 14, 2009 at 01:43
Antennas like the GME AE4005 dipole type are not ment to have there coax cut down, this is according to GME as it forms part of the antenna.

Phil the one you might be thinking of is the 1/4 that has a PL259 plug on it.
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Follow Up By: Member - Phil G (SA) - Sunday, Jun 14, 2009 at 07:57

Sunday, Jun 14, 2009 at 07:57
The antenna was the same as mine and stocked (and highly recommended) at Electric Bug:
Electric Bug 1/4 Wave Flexible Antenna with S0238 (Mopole) Base.

The base has the same thread as the PL259 plug. A fairly indestructible setup which I've used in my last 4 vehicles. I don't believe there is any problem cutting the cable length with this antenna but happy to be corrected.
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Follow Up By: Robin Miller - Sunday, Jun 14, 2009 at 08:35

Sunday, Jun 14, 2009 at 08:35
Hi Olcoolone

Those antenna's like the GME AE4005 dipole can have there cable length changed , shortened on lengthened within reason without any significant effect.

That type of antenna is a half wavelength long and this means that it is very high impedance at the base near the coil and as such anything near the coil will have at effect on its VSWR.
The idea is that the antenna be mounted up and away from anything metallic and the only reason for concern is if the cable is cut so short that it prevents the antenna from being mounted away from other things like bonnet/roof racks.

As a rule of thumb that antenna needs to be at least its own length and preferrably double away from metal.

Some people have been known to mount them on a bonnet/bullbar and then cut of the excess cable - this is what would be wrong.

Phils 1/4 wavelength one is not affected.










tstill don' type are not ment to have there coax cut down, this is according to GME as it forms part of the antenna.
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Follow Up By: olcoolone - Sunday, Jun 14, 2009 at 13:12

Sunday, Jun 14, 2009 at 13:12
It's either yes or no with these antennas.

"shortened on lengthened within reason without any significant effect. "

Whats within reason, every ones with in reason is differant.

If I had one of these antennas mounted on a roof rack or gutter mount with the radio mounted in an overhead consol is it OK to cut the coax down to 1m long.

As said in my original post most UHF antennas can have the coax cable cut or made longer, but there are always some that you can't.


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Follow Up By: Robin Miller - Sunday, Jun 14, 2009 at 14:19

Sunday, Jun 14, 2009 at 14:19
Hi Olcoolone

Everyones reason can't be different unless they ignore the rules and install it incorrectly.

It can be logicaly deduced as what is reasonable by applying the rule that the antenna should be a wavelength or more from metal panels.

So the coax can be cut to 1 meter as wavelength is 600mm approx, but if you did you would only have 400mm left in which to take the lead into the car and connect to the radio.
Most cars would need a bit more than a meter to achieve the above, hence these units usually come with some 3m or so which allows for a typical install without neededing to re-terminate the cable.
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Reply By: Michael ( Moss Vale NSW) - Sunday, Jun 14, 2009 at 10:44

Sunday, Jun 14, 2009 at 10:44
Thanks to all who replied. This is what i bought,Image Could Not Be Found Axis brand about 700mm long so i can still use multi level car parks.. thanks Michael
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