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Author Topic: Antenna questions  (Read 460 times)
AB7JK
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« on: February 07, 2005, 05:06:33 PM »

I am returning to ham radio after being QRT a few years. Question: Does the length of coax feeding a half wave dipole affect the performance of the dipole? Regarding a dipole fed by twin lead: I've read that this type of antenna should have a coax stub connected between the twin lead and the transciever (plus a balun). How do you determine the length of coax or the length of twinlead if the dipole is to be used on multiple bands? Thanks for your help. Kim
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AA4PB
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« Reply #1 on: February 08, 2005, 09:57:07 AM »

If your dipole is presenting a relatively good load (reasonable SWR) to the coax then it doesn't matter what length of feed line you use. Of course the longer the feed line the more overall loss it will have.

If you use some type of parallel line (like twin lead) then you have a balanced connection. Since most transmitters have an unbalanced, coax connection you have to have some way to match it to a balanced feed line. A balun is one way to do it. Using a balanced antenna tuner is another way.

Generally, parallel line is used when you want to feed a doublet (a minimum of 1/2 wavelength on the lowest operating frequency)on multiple bands. This is because on some bands the SWR will be very high and the parallel line will have less loss than coax cable in this application. 300 Ohm TV twinlead is not a very good selection (too much loss). Open wire feeders are much better. Amateur type ladder line (like TV twinlead but wider spacing and cut-outs in the insulation) falls somewhere in between.

If the dipole is 1/2 wavelength on the band you want to work on then you are better off to feed it with coax all the way.
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AB7JK
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« Reply #2 on: February 19, 2005, 05:11:03 PM »

Thanks for the info. Kim
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