Can I fix this VHF aerial?

Richard Selman · 30 October 2010, 16:30

Bust the VHF Vtronix aerial whilst fitting the A-frame (doh). Stripped it down today and can repair the fibreglass section albeit I have lost 6" or so of the 2m. Had to pull out the coax to do this and in doing so it parted near the top. Arrangement is that the main coax from the set runs up the aerial and then the centre conductor terminates in a short brass like cylinder about 2 inches long. Then there is a run of about 24 inches of inner conductor in the plastic insulator without the outer shield solder onto the shield. It is this section that has broken. If I replace this section with a new length and run it up inside the tube will the aerial work? (lost a few inches on the aerial but gained 5 feet or so from the A-frame).

Cheers

Richard

Jizm · 30 October 2010, 16:58

You can but try, worth giving it a go in my humble opinion.

jyasaki · 30 October 2010, 17:17

If it's just the cable itself, it should be OK. The impedance of the cable shouldn't be affected, and teh length shouldn't matter at all.

If it's the radiating elements that broke, well, they're designed to be tuned somehow, and repair may affect performance.

I'd give it a try, and replace if it doesn't seem up to snuff.

jky

ashbypower · 30 October 2010, 17:31

Beware, the radiating element (if that is the dameged piece) is tuned to the mid-band frequency and unless you have a Voltage Standing Wave Ratio (VSWR) meter or reflected power meter, there is a danger at the very worst of taking out the output stage. At the very least the mis-match would reduce the OP power.
My advice for what is an important part of your emergency system is to bite the bullet and replace it.

GJ0KYZ · 30 October 2010, 17:53

Quote:

Originally Posted by ashbypower

Beware, the radiating element (if that is the dameged piece) is tuned to the mid-band frequency and unless you have a Voltage Standing Wave Ratio (VSWR) meter or reflected power meter, there is a danger at the very worst of taking out the output stage. At the very least the mis-match would reduce the OP power.
My advice for what is an important part of your emergency system is to bite the bullet and replace it.

This is correct; a high VSWR (mis-tune) brought about by arial damage will result in poor performance and a potentially damaged radio. Don't check the VSWR with a CB radio SWR meter; it won't work at VHF. For peace of mind and safety's sake I'd replace it.

Richard Selman · 31 October 2010, 05:50

Cheers - probably going to replace it as it was cheaper than I imagined it - only problem is posting 2.4m long parcel.

Like to understand how this works though. Don't know much about aerials but I thought the general principle was that the you stopped the shield and then exposed the inner conductor for a particular length and then had a reflector at the base to create the effect of a larger aerial.

Whats the purpose of extending the shield/sheath above the exposed section of the inner?

Richard

Channel Ribs · 31 October 2010, 09:23

Quote:

Originally Posted by Richard Selman

Cheers - probably going to replace it as it was cheaper than I imagined it - only problem is posting 2.4m long parcel.

Very wise, delivery on an antenna between 1.6 and 2.6 meters should be 20 quid or less.

Pikey Dave · 31 October 2010, 22:14

If you are changing, have a look at Glomex antennas. I've gad vtronix & glomex & imho Glomex are better quality & cheaper.

Channel Ribs · 01 November 2010, 07:45

Quote:

Originally Posted by Pikey Dave

If you are changing, have a look at Glomex antennas. I've gad vtronix & glomex & imho Glomex are better quality & cheaper.

I would say the same thing but with Pacific in stead

jwalker · 01 November 2010, 09:40

Why are folk so fierd of electrical stuff. Antennas are usually very simple in build. I don't quite underststand your description but that section of the antenna is likely to be an impedance matching section and you rebuilding it with the same dimensions as original will be fine. Shortening the main radiating element will detune it from optimum and small adjustments to length can make quite a difference to performance, it won't fry your radio though. If you had a mind, you could extend the radiator by soldering on and equivalent length to that which is missing and let it protrude out of the top then slide a lengh of rubber tube over the end to cover it. Seal off the top end of tube of course. I realise this won't look cool and may offend the sensitivity of a southern wus.

GaryC · 01 November 2010, 10:28

Quote:

Originally Posted by jwalker

Why are folk so fierd of electrical stuff. Antennas are usually very simple in build. I don't quite underststand your description but that section of the antenna is likely to be an impedance matching section and you rebuilding it with the same dimensions as original will be fine. Shortening the main radiating element will detune it from optimum and small adjustments to length can make quite a difference to performance, it won't fry your radio though. If you had a mind, you could extend the radiator by soldering on and equivalent length to that which is missing and let it protrude out of the top then slide a lengh of rubber tube over the end to cover it. Seal off the top end of tube of course. I realise this won't look cool and may offend the sensitivity of a southern wus.

THat is not quite my understanding. Where as to receive a coat hanger will do (almost)

On transmision, any errors in length can have a significant effect on the SWR. Any signal not passed through the antenae will be reflected back into the tranmitter O/P stage and at least damage it.

The changes may work but I would have said that checking the SWR with a marine SWR metre was essential.

Even a badly located or angled antenae can cause the SWR to be problematic.

Gary

jwalker · 01 November 2010, 11:10

Quote:

Originally Posted by GaryC

THat is not quite my understanding. Where as to receive a coat hanger will do (almost)

That's not my understanding. A antenna is equally good or bad at receiving as transmitting. There will always be some reflected signal within the cable and whilst it is good to have this at a low level the radio output stage will be able to cope with a mismatch, a full short, obviously, would not be a good idea. I'm not suggesting that checking with an SWR meter isn't a good thing but, as you suggest, a number of other external influences are likely to change the SWR so unless one is willing to hack a new antenna and trim it in situe, there is no guarantee that it too will be at it's optimum SWR once fitted to the boat.

ashbypower · 01 November 2010, 11:37

Quote:

Originally Posted by jwalker

it won't fry your radio though.

A Bold statement indeed, after 40 yrs in the communication area's I have seen upteen radios from Thermionic throught to Solid state "Fried" as you put it, becaused of VSWR problems at the Antenna, these have beed radios from 10w right through to the kilowatt O/P output power just alters the "Frying" degree and time taken.

jwalker · 01 November 2010, 12:03

Yeh, there's always someone ready to misquote or unable to read.

Quote:

...you rebuilding it with the same dimensions as original will be fine.

Quote:

...small adjustments to length.. won't fry your radio though.

ashbypower · 01 November 2010, 12:09

Quote:

Originally Posted by jwalker

Yeh, there's always someone ready to misquote or unable to read.

Pikey Dave · 01 November 2010, 12:10

In the big scheme of things, the cost of a good quality antenna is fk all. I wouldn't bodge what is essentially life support equipment just to save pennies. TBH I can't even believe we're having the conversation.

ashbypower · 01 November 2010, 12:11

Quote:

Originally Posted by Pikey Dave

In the big scheme of things, the cost of a good quality antenna is fk all. I wouldn't bodge what is essentially life support equipment just to save pennies. TBH I can't even believe we're having the conversation.

Channel Ribs · 01 November 2010, 13:37

Quote:

Originally Posted by Pikey Dave

I wouldn't bodge what is essentially life support equipment just to save pennies.

Well said.

BogMonster · 02 November 2010, 00:15

Quote:

Originally Posted by Pikey Dave

In the big scheme of things, the cost of a good quality antenna is fk all. I wouldn't bodge what is essentially life support equipment just to save pennies. TBH I can't even believe we're having the conversation.

+1

Cost of half a tank of fuel... maybe less depending on how big the tank is.

Richard Selman · 02 November 2010, 08:18

My drysuit is my main bit of life support equipment.....

Anyway my interest now is to understand how it works - you never know when I might need to do it at sea.

Richard