Cutting coax cable

[Mad-One]

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

[Fish'n Junky]

Mate, it shouldn't affect a VHF radio, providing it is terminated correctly.

[TheRealAndy]

As fish'n junky says. Cut it where you want, but ensure that it is terminated correctly with a new plug.

[foxx510]

I swear I've seen some electrically minded folk on here say it was a tuned length and should stay uncut. I've always thought you could cut it where ever you liked, though, and that shorter was always better.

[SatNav]

1. NO not necessarily so and will depend on the Make and Model of antenna.

[Heath]

Kerry...I mean SatNav is correct, however GME antenna's are tuned to be used with the pre-determined length of cable supplied with the antenna.... don't know about other brands.

[TheRealAndy]

For the techno lads. Characteristic impedance of coax is determined by cross sectional design and dialetric material, not lenght. Therefore if your antenna is matched then lenght has no bearing on the impedance what so ever.

However, I see that some GME antenna's (thanks heath) are not mathced and they claim they cannot be cut. There are three GME models that suffer from this problem, all are fibreglass models with 2 way swivel base.

[Mad-One]

Thanks guy's for your replies.
Think I might just leave it as is

Cheers Phil

[Fatenhappy]

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

G'day Phil ...

Without getting too much into antenna propogation and transmission line theory, the short answer is no, cut the coax off to your hearts content, but as has already been intimated make sure that it is reterminated correctly.

The antenna itself is a completely other story. It is a specific length for a frequency or more commonly, set up for the "best comprimise" for a "band" of frequencies. It has nothing to do with as previously been mentioned, the characteristic impedance.

Logically then, to counter previous descriptions, if length has nothing to do with anything and the so called characterisitc impedance was the be all end all, all we would need to do to transmit and recieve would be to just have any length of coax just stuck up in the air.

The higher the frequency the shorter the antenna length etc etc ....

Cheers
Greg

[foxx510]

If you look at this page http://www.radiospecialists.com.au/m...20antennas.htm you will see that the GME 27mhz antennas have a note saying not to cut the cable, it's pre tuned. Doesn't mention that for VHF. Confusing.

[oldboot]

Just to clarify things here.

All of the ground independent GME antennae ( that is the ones with an insulated plastic base) come with a length of coax cut to a specific length....you will degrade the performance and the SWR of the antenna by cutting any significant length off the cable.

you can get away with cutting a few mm of for retermination of a new plug but trimming a foot or mor off is definetely not advised.

The GME aerials come with instructions specificlay to that effect.
I have checked with GME tech support on the matter
I have two friends who are ex GME employees one a technician one an engineer.
The statement is correct....DO NOT trim the coax on a GME areial that does not require a metal ground plane.

WHY
because the basic areial otherwise known as a dipole has two sticks either side of the feed point.......most single stick areials require a metal groundplane....the ground plane acts as a mirror to electricaly reflect a theoretical substitute for the missing other stick.

once we get into single stick end fed antennae that have no ground plane they have to do all sorts of weird stuff to get the antenna to perform and tune up properly.....having either a specific length of cable or a minimum length of cable are both used as part of this jiggery pokery.

It is quite likley the same is true of other brands.

cheers

[TheRealAndy]

Well sticking a piece of coax up into the air is a valid antenna, so long as its a tuned length. You can also use a piece of coax to tune an antenna, its bad design in my opinion but still a valid method. It seems that GME uses this method of tuning. I have a huge book on antenna design if anyone is remotely interested.

[foxx510]

Just spoke to my mate who is a radio tech, said they see tuned lengths of coax all the time. Reckons the tuning probably drifts too far when people cut the coax hence the do not cut recommendation.

[Fatenhappy]

Hey Old Boot,

Must admit, I have been around Avionics and antennas in aircraft both on the line and now for many years as an instructor for the past 38 Years and have never heard of what you are talking about.

Please don't take that the wrong way, as I am merely saying I have never heard of it .... you can always learn something new in life, no matter how old and that includes me! Bearing in mind, traditionally I have always worked with antennas with a ground plane readily available.

My immediate reaction would always be to go with the OEM (original equipment manufacturers) instructions. If they specifically include the coax as a tuned length, then go for it and do not cut it. If not specified and you are still unsure, query them if you like first, but also traditionally in a conventional installation the coax is normally not an issue with a ground plane antenna installation.

Cheers
Greg

[oldboot]

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