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Author Topic: 10 and 20 meter dipole  (Read 1380 times)
KJ7OT
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Posts: 15




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« on: July 29, 2002, 02:42:45 AM »

Hi everyone

I have a half wavelength dipole cut for 10 meters. Would it also work at a quarter wavelength on 20?
Any info is appreciated.
Thanks,
Ryan KJ7OT
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WB2WIK
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Posts: 20666




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« Reply #1 on: July 29, 2002, 10:37:05 AM »

At 1/4-wavelength overall it's not really a dipole, but it's still a doublet.  It's not efficient, has a low resistance and high capacitive reactance, but can probably be matched using a good tuner, provided you feed it with twin-lead or ladder line.

A 1/4-wavelength overall doublet is very difficult to match using coaxial cable, and the cable itself becomes very lossy in the process (due to the mismatch at the feedpoint).  

A better idea: Make the dipole just a bit longer, and install a pair of 10 meter traps having a high L/C ratio, to inductively load the wire and reduce its physical length for 20 meter resonance.  Tons of information on this in the ARRL Antenna Handbook and many other publications.

WB2WIK/6
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K0BG
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Posts: 9899


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« Reply #2 on: July 29, 2002, 10:38:02 AM »

In a word, no. What you would have is a very low impedance input, and a high capacitive reactance. Yes, if it were fed with ladder line or the coax feed line was short, you could use a tuner and tune it to 20. But I have a better solution.

Chapter 7 of the ARRL antenna book covers several different trapped antennas for the various HF bands. You could use the formulas to design a 10/20 meter trap. It antenna would end up a little longer than your present one, and would appear as a half wave dipole on both bands.

You can't use a 20 meter dipole on 10 meters either, as the feed point would be a very high impedance, and difficult to match with any configuration.

If you don't wish to construct your own traps, B&W sells a 10/20 meter dipole kit, as do several other companies.

Alan, KØBG
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W0LPQ
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Posts: 31




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« Reply #3 on: July 29, 2002, 10:42:50 AM »

Another idea, which is one I use, is add another chunk of wire and make 2 dipoles from the same center support for 20 or 15 or 40 or 80..!

I use a dipole with sections for 10/20/40/75 and it works fine. Low SWR.  This idea is also in several ARRL handbooks and Antenna books.

If you have the room, go this way.  Traps tend to be lossy, but do work if you are short on space.

Steve has the idea..!  I think that is what he has used in the "olde" days...!!

73

Bill, W0LPQ
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