Cutting coax cable

[oldboot]

So when someone reads the thread title and says...gees I better read to see if I can cut my coax and comes across pages upon pages of this...what is he/she supposed to think?
They are not going to read about propagation and the likes to see if they can cut a coax.
I think I made a definitive answer way back...by saying read the instruction sheet. If they say the length can be altered then do so. if not don't...especially if the item in question is under warranty.
Please don't get me wrong...to see blokes having a discussion is good.
But consideration to other people with no background knowledge on the subject but wanted to find a simple answer to a simple question without reading volumes about theories or research or what-ever must be considered.
If you want to know why aerials work or discuss design or theories then start another thread. It's quite simple.

If this was a tech support Q& A page i would agree with you.... but it isnt.
This is not an encyclopedia. It is not a technical journal. It is a discussion forum like many others.
The primary function is discussion.
As I mantioned other threads on this and many other boards get a great deal further off the subject than this one and no body seems to care in fact many enjoy where the thread takes them.

As I said before.. if they want to know if the should cut their coax... they have their answer in the first half dozen posts..........


no on with some further information
The GME site its self is pretty poor as far as information on marine areials.
but some of the marine catalogues are more helpfull.

in 27Mhz gme have three areials

AE364H which has loading coils and a cap in the base... fitted with a 5M cable
AE96 the 2.4M double swivel.... fitted with a 4M cable
AE66 the 1.8M double swivel that has a 2M cable

in VHF they have four areials.... there may be one more

AE31 which is designed to mount up a mast and has a socket on the bottom.
AE30 which has a bas like the AE346H.... there may be stuff in the base fitted with 3M of cable
AE87 2.4 m double swivel has a coil half way up and fitted with 3M of cable
AE346V 1.8m double swivel fitted with 5M of cable.

I note that there are a number of differing cable lengths and none of them are a neat multiple of a quarter wave length.

cheers

[foxx510]

Don't forget velocity factor, quarter wave length will be different in coax to in free air.

[oldboot]

wave length at 160Mhz = 1.875m
velocity factor of RG58CU 66% = 1.24m
still nothing comes up even.......I supose if 3meters was approximate 10 x quareter wave length is 3.090 meters..... and 5 meters is close to 16 quarter wave lenghts.

I can not believe that cable length could be that critical 4 full wave lenghts down a transmission line......

cheers

[foxx510]

Nah I was just putting it out there, easy to forget. I agree that there is a high probability of cable length not being critical. Here's another idea, maybe many many years ago when they first started making the antennas it mattered. Then it was just carried over into newer models because it used to be a rule....

[oldboot]

If there wasn't some sort of design requirement, why would you supply aerials with such a variety of different length cables?

Now here is a good one.......one of the marine catalogues lists a 5m GME extension cord and states that it is only suitable for 27Mhz areials.

What the.......

Baffelin'... just baffelin'...eh jock.

cheers

[TheRealAndy]

wave length at 160Mhz = 1.875m
velocity factor of RG58CU 66% = 1.24m
still nothing comes up even.......I supose if 3meters was approximate 10 x quareter wave length is 3.090 meters..... and 5 meters is close to 16 quarter wave lenghts.

I can not believe that cable length could be that critical 4 full wave lenghts down a transmission line......

cheers

It is well possible, but without knowing the antenna design its impossible to speculate on. I am almost tempted to go buy a GME antenna and strip it down to see how its made. Might do it yet, but I will give GME a few more days to respond to my email first.

[oldboot]

Ah and now the plot thickens........seems the boys at GME use some other sort of measurement other than SWR in assessing these aerials.

I seem to remember something about this from the dim dark past.

Seems to be that these ground independent aerials don't look good with an SWR meter..... never did and never will.... they are suposed to radiate well regardless.

any way my contact is making enquiries of some fomer work mates who will remember the full story.......

cheers

[finga]

I have a navman radio and navman base etc and about 10 metres of coax rolled up under the dash in a 4.3 mtr quintrex. The aerial is about 1 metre from the radio, years ago I would have chopped the cable and shortened it for neatness and to make sure I never caused any probs because of the coil of coax etc and maybe some losses. But now I don't have the gear I will simply coil it up and zippy it up in place.

We are in the elecronic section, with radio/safety questions, ...NOW.......to cut or not to cut, that is the question, lets get the answer.

CUT!!
A direct cut and paste from an email direct from Navman
My question...
Message: Hello, Can the coax cable on a Navman VHF aerial be cut to a
more suitable length?
Thanks Scott

The answer....
Hi Scott,

Shortening the cable should not cause any problem.

Regards

Paul Godfrey
Product Support Technician
Navico Asia Pacific

His e-mail address.... Paul.Godfrey@navico.com
Seems pretty open and shut case to me

Ah and now the plot thickens........seems the boys at GME use some other sort of measurement other than SWR in assessing these aerials.

I seem to remember something about this from the dim dark past.

Seems to be that these ground independent aerials don't look good with an SWR meter..... never did and never will.... they are suposed to radiate well regardless.

any way my contact is making enquiries of some fomer work mates who will remember the full story.......

cheers

Just another reason not to touch the GME stuff in my books.

[Travesso]

Yep you can stick a piece of coax up and make it into an antenna , the coax dipole has been my go to problem solver for a quick and dirty antenna for years.

As you will find in any " large book on antenna theory" there are a number of different reasons to need either a specific or minimum length feeder....

The reasons in the 27Mhz aerals may be different from the reasons in the VHF areials... they will be two very different designs.

As far as being rough design......hmm....... remember there are also effective radiated power restrictions....so they problay have to design arround unity gain......at VHF a co-linear would problay be the best and easiest design... but that would give a gain above unity...........

If anyone has a donged or broken example of any of these antennae....I would realy like to pull one appart to see what sort of design they are.......I'm sure andy would like a look too

The 27Mhz ones with the single pivot base seem to be a straight helical whip with a coil and cap base tune arrangement in the base.

cheers

I have broken my VHF aerial... not on purpose! I just forgot to lay it down when I left Yamba to head back up to the Gold Coast... By Broadwater I realised that I had left it up and had a look at it... the fibreglass tube was about a foot shorter than it used to be and there was just two stiff (~1.5mm dia) single core wires sticking out of the snapped tube. It still works though, I just tucked the wires back into the tube and siliconed the end up again...

I tested it the other day up the Terranora Inlet on the Tweed and the VMR said they could hear me loud and clear but it was very staticy when they replied... Maybe I should bite the bullet and get a new aerial?? Off shore (being that theres no structures in the way) it is clear as anything and I can hear the GC seaway as well so I figure its all good

[oldboot]

Some information has finaly come back to me from ex GME employees.

Aparantly the GME ground independent areials are tuned using a sweep generator and a spectrum analiser......using this equipment they are tuned for best resonance with the measuerd coax attached......."the coax can be extended but not shortened".

I have no detail as to the areials being tuned "by design", by batch or individulally.

No replies from GME themselves yet.

cheers

[foxx510]

"the coax can be extended but not shortened".

cheers

Does that mean if you put an extension on you are never allowed to take it off again?

[finga]

Sounds like a lot of pelican poo to me and just another reason not to touch GME stuff

[struktcha_man]

Cutting excess coax off does it affect the tuning of the aerial for a vhf radio. I have heaps of excess and I hate the look of it bundled up and i'd like to tidy it up. Any help would be appreciated

Cheers Phil

If In doubt, just leave it as is and hide it out of sight with a few plastic cable ties
Especially if it is only for looks, I am sure you could tuck it away and cable tie it somwhere out of sight.

Always better to have too much than not enough, if you cut it you can not reverse that.



cheers

[Fatenhappy]

Aparantly the GME ground independent areials are tuned using a sweep generator and a spectrum analiser......using this equipment they are tuned for best resonance with the measuerd coax attached......."the coax can be extended but not shortened".

Now that's surely got to be codswollop from your source !!!!

A tuned length is a tuned length so if you lengthen the antenna you have to change the impedance either inductively or capacitively but I'm not going into that again!

Cheers

[TheRealAndy]

Aparantly the GME ground independent areials are tuned using a sweep generator and a spectrum analiser......using this equipment they are tuned for best resonance with the measuerd coax attached......."the coax can be extended but not shortened".

Now that's surely got to be codswollop from your source !!!!

A tuned length is a tuned length so if you lengthen the antenna you have to change the impedance either inductively or capacitively but I'm not going into that again!

Cheers

I reckon so too.