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Old 01 November 2010, 06:28   #11
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Originally Posted by jwalker View Post
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
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Old 01 November 2010, 07:10   #12
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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.
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Old 01 November 2010, 07:37   #13
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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.
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Old 01 November 2010, 08:03   #14
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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.
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...small adjustments to length.. won't fry your radio though.
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Old 01 November 2010, 08:09   #15
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Yeh, there's always someone ready to misquote or unable to read.
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Old 01 November 2010, 08:10   #16
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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.
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Old 01 November 2010, 08:11   #17
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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.
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Old 01 November 2010, 09:37   #18
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I wouldn't bodge what is essentially life support equipment just to save pennies.
Well said.
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Old 01 November 2010, 20:15   #19
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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.
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Old 02 November 2010, 04:18   #20
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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
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