Originally Posted by steedthrust
Is it okay to shorten a VHF aerial cable? Am worried it may 'tuned'
to a certain length.
Also is it okay to shorten a GPS cable?
Well this is an interesting question.....
I have always said that the length of the co-ax doesn’t matter on the type of sets we use on the boats.... (and the Amateur VHF set I used to run). This came from knowledge secured at the back of my mind when I did my Radio Amateurs Examination…. 25 years ago! But when I thought about it I couldn’t remember why… So I dusted off some of my old books…..
The problem comes with all the different ways you can feed a transmitter. I used to have a ‘Short Wave’ HF Transmitter, which fed to a long aerial that went down the garden. This was a centre fed di-pole using a balanced twin-wire feed. This had to be a length that was a multiple of the wavelength I was transmitting on, (although there were ways round that). But basically, yes it had to be tuned. HF transmission lines are fascinating…. Did you know you can touch them in one place and get a shock… but touch them a few feet further along and there will be no voltage!
Yes, its really complicated….. but fortunately not on a boat.
On a boat we use a
Ground Plane Antenna, fed by co-ax… which is an unbalanced feed. So long as the impedance of your TX output is 50 ohms… and the cable is 50 ohms…. And the antenna is 50 ohms….. (which it will be unless you’ve done something dodgy), you have what’s known as a ‘Non-Resonant Line’. This means everything is matched, all the power is absorbed by the aerial, (in theory) and the length of the feed line doesn’t matter. Its only if you use funny aerials, (di-poles, loops etc) or the wrong impedance co-ax that you will need to mess about with matching by length.
That at least is my understanding from my 25 year old books – but if anyone knows better……!
Designing aerials is a bit like designing boat hulls. I can design you an aerial tomorrow that will work… and a boat hull too….. but don’t expect them to win any awards!
Oh, and I would think the GPS would follow the same argument….
Rob… If you’re going to buy a new VHF… take a look at Icom!