Shortening Antenna Cables

[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