Shortening Antenna Cables

[Daisy Burnett]

Hi Guys
Trying to continue to improve radio workability. I know that years ago when you shortened the antenna cable for a radio you had adjust things using an SWR meter. I have been led to believe that this is no longer the case with advent of better technology. What I would like to do is shorten the cables on both my 27 meg and VHF radio's to get rid of that coiled up coaxial cable. Can I do this without effecting the performance of the radio's?

Thanks
Daisy

[Taylor2]

Yep for sure. Shouldn't be a problem, as long as you are confident reterminating the plug on the back of the radio. Its not the best option to shorten the cable by cutting the coax and then rejoining it half way along the run, you really need to retierminate at the plug. Extra cable is handy if you ever want to move your radios or antenna in the future. Cheers

[TheRealAndy]

Depends on the antenna. With certain GME 27mhz antennas you CANNOT shorten the coax. From memory, with pacific antennas you can shorten the coax. BEst to check with the manufacturer first.

[lethal098]

It is reccomended to not change the length of tha cable as they are a tuned length, however left over cable should not be coiled up, it should be laid out in a straight line where possible, i have run mine down the side under the gunwhale and then back up to the radio.

Cheers Lee

[murf]

instead of coiling would figure eighting work better? I have never delved right into radio gear

cheers Murf

[ifish86]

i didnt think coiling coaxial cable would matter to much as it extremely low v

[Taylor2]

Shortening the cable will not affect the transmitt receive side of your radio. Shortening the cable will actually improve its efficiency, marginally. Changing the length of the antenna WILL affect the tx/rx of your set.

Coiling coaxial cable should not be a problem as long as the bend radius is not small enough to damage the centre conductor. The shielding on the coax is to reduce interference. You shouldn't get inducatance, not on a set like that.

[ShaneJ]

Some poor information here.

GME antennas are not to have the coax shortened at all. The coax is a tuned length to match the impedance of the radio to the antenna itself as these antennas have no loading coil/balun etc

[oldboot]

We had a serious go at this one a little while back.

Some of the ground independent areials......that is an aerial that does not have a metal base and requires a ground connection.......use a specific length of cable as part of the hokus pokus they have to do to make a single stick work ground independent.

I made enquiries of a mate who used to work for GME as a technician, and he pressed one of his mates that used to work for GME as an engineer.

It seems they use something more modern but similar to a grid dip occilator to sweep tune the aerials for an optimum cable length.....this is why different model aerials have different cable lengths.

If you measuer one with an SWR meter it won't look good...if you try to tune it by SWR you will make it worse.

The newer GME areials that come as a base and a seperately selected areial may be different, but anything GME and white and over 12 months old will be cable length critical.

You can probaly get away with chopping of a plug and reterminating...if carefull but that is about it for shortening.

As far as coiling up the excess.....at this frequency, as long as the coils are bigger than 3 or 4 inches diameter it should make very little difference...what you do not want to do is kink the coax.

A mates boat had the excess bound up in a longshank of tied up tight with cable ties and stuffed in somewhere......this ment it had a bout half a dozen tight kinks in the cable.

If it is a GME or any other older style ground independent aerial DO NOT shorten the cable.

cheers

[Daisy Burnett]

Thanks guys. A few mixed signals here so I might give GME a call and see what they have to say. I know in the past there was a problem in changing lengths but I heard someone say that it was no longer a problem. I will post the info after I ahve spoken the GME and let you know the good oil for GME antenna's at least.

Cheers Daisy

[oldboot]

Mate... been there and done that.

Unless the aerial in question was baught in the last 12 months AND it is the new range where the masts are interchangable, ( they screw into the a universal base) you are wasting your time.

YOU can not shorten the cable on any of the older GME white aerials...full stop........end story.

The reason I asked some ex GME mates was because I did not get any explanation as to the "why and how" from anybody at GME.

cheers

[ahamay]

Some poor information here.


LOL... cracks me up...everyone's a friggin expert. Maybe there should have been a poll as well then he could have gone with the majority ;)

[TheRealAndy]

I also had a crack at contacting GME (same thread that oldboot is referring too) to find out why you cant shorten cables, but I never got a technical answer.

[Marlin_Mike]

LOL... cracks me up...everyone's a friggin expert. Maybe there should have been a poll as well then he could have gone with the majority ;)


No expert here ahamay..................LOL ;D

But ask your self one question Daisy Burnett: Is the radio working fine??? Yes??? Then if it aint broke why try to fix it?????????????????????????

Mike

[Spin]

I have an SwR meter here
Who wants to donate there antenna for the test .

[oldboot]

The problem is that an SWR meter will not test for resonance or radiated output only forward and reflected power.....the reflected power is merely a bi-product.

just as a bit of background.

there are two similar but slightly diffferent issues.

firstly, and this is an issue more for 27mHz, a quarter wave length is about 9 feet or 2.7 meters........all of the "white stick" aerials for general sale are arround or shorter than that.....so to achive an antenna that will work ground independently at all effectivly, they have to do some hokus pokus that does not follow basic conventional rules of aerial design.......the choice in this case is to use a section of the feed coax as a matching segment of sorts......it seems to work but the SWR will always read poorly in comparison to a 1/4 wave on a groundplane.......back in the day...CBists would try to achive the magic 1.1:1 SWR and could get pretty damn close....

secondly, on both marine bands no aerial gain is permitted. this is more an issue on the VHF band.
It would be relativly easy to design a colinear aerial that resonated well, gave a good SWR reading and would fit in the expected length of a white stick.
BUT it would have gain higher than unity.

So again the manufactures have had to employ hokus pokus to achieve some sort of elivated feed antenna that does not provide any gain.

Again the choice was made to use a section of the feeder as a matching device.

NOW
Not having managed to get all that much serious information ( probaly considered an "industrial secret") about the detail, I can not say for certain what is going on.
Except to say that at VHF you could get several full wave lengths in the feeder cables fitted and you have a better chance of not doing as much damage trimmin a few inches off a VHF aerial than you would doing the same to a 27Mhz aerial.

These designs have been arround for a very long time.....GME were marketing a almost identical aerial way back in the hey day of 27Mhz CB which was late seventies to mid eighties........it is possible the bloke who designed all these aerials is now dead and the reasons for his engineering chioces are lost for ever.

cheers

[oldboot]

Just for those who are interested, here is an example that you can try if you like, that illustrates the issues of resonance, feedline impedance matching and SWR.

If you make up a simple straight single wire dipole...3 insulators and 2 bits of wire.

Connect a standard 50 ohm coax feeder to the centre of it.

Now get out the books and calculate the length with the standard formulars.

Do this correctly and the dipole WILL be resonant.

If you string this dipole up in a straight line and do an SWR run on it it will look pretty poor.

because the dipole will present a feed point impedance of arround 70 to75 ohms, the coax and the output of the radio present an impedance of 50 ohms so there will be a mismatch.

if you shorten or lengthen the dipole at the ends, you will be able to get the SWR to change and maybee to look a bit better...... but indoing so you will change the resonance.

If however you bend the dipole ( still at calculated length) at the feed point till the angle is arround 90 to 100 deg you get a prrtty good SWR reading......becuase you have changed the radiation impedance and this is refleceted back to the feed point.

I had one of these strung up at home as a kid and it went like a ripper and cost me diddly squat.

I used a tandy free battery club card drilled for a PL plug as a center insulator.

This is not the same as the ground independednt areials we are discussing, but it illustrates that SWR is only a bi-product of other things happening.

Above all in transmitting aerials we are looking for resonance, if this means we have to use hokus pokus to get a match or even put up with a poor match (SWR reading) that is better than trying to transmitt on an aerial that is not resonant.


cheers

[ShaneJ]

LOL... cracks me up...everyone's a friggin expert. Maybe there should have been a poll as well then he could have gone with the majority ;)

Just so happens that I am an expert. I'm a radio comms tech and have worked with GME::)

[TheRealAndy]

Just so happens that I am an expert. I'm a radio comms tech and have worked with GME::)

I am an expert too, I know everything about everything. Perhaps you can give us engineer types a formal explanation on the reasoning behind not cutting the coax.

[ShaneJ]

I am an expert too, I know everything about everything. Perhaps you can give us engineer types a formal explanation on the reasoning behind not cutting the coax.


I already did. GME antennas have no balun or impedance matching so they use a tuned length of coax to do the job.

[ShaneJ]

For the people who aren't know it alls please feel free to call GME and ask for Peter Cooke for the official word. I cant remember if he is in Sydney or Brisbane.

[nigelr]

So what should the standard cable length be for a GME 27Mz?
Never been able to get sense out of mine, thank God for the EPIRB!
Cheers.

[oldboot]

The lenght is different for each model of aerial.

I don't know if they just cut all the cables to a nominal lenght bassed on tests or if the tune each one......I'd say at the price it would be a nominal length..

the best bet is to find an unmolested example of your areial and measure the length of that cable.

cheers

[Daisy Burnett]

Guys
Thanks for all the great feedback, I love lively discussion it brings out the best in people I think. I think the wisest statement here was 'IF IT AIN T BROKE DONT FIX IT'. As my radio's now working much better than they were (VHF 100%, 27 Meg about 50%) I think I will combine the advise of both Lethal098 and Marlin Mike and leave the length as it is and run the cables out away from the power leads along the length of the boat and back (without kinking) and that will do me for an answer. Thanks again guys

Daisy