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Old 30 October 2010, 12:30   #1
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Can I fix this VHF aerial?

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
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Old 30 October 2010, 12:58   #2
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You can but try, worth giving it a go in my humble opinion.
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Old 30 October 2010, 13:17   #3
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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.

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Old 30 October 2010, 13:31   #4
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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.
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Old 30 October 2010, 13:53   #5
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Quote:
Originally Posted by ashbypower View Post
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.
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Old 31 October 2010, 01:50   #6
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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
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Old 31 October 2010, 05:23   #7
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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.
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Old 31 October 2010, 18:14   #8
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If you are changing, have a look at Glomex antennas. I've gad vtronix & glomex & imho Glomex are better quality & cheaper.
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Old 01 November 2010, 03:45   #9
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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
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Old 01 November 2010, 05:40   #10
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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.
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