Re: Half wave antennas

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From: Daniel C Winkler (dwink@juno.com)
Date: Wed Dec 04 1996 - 02:36:23 EST


On Mon, 2 Dec 1996 20:02:46 -0600 launerb@crl.com (William H. Launer)
writes:
>qrp gang,
> Odd multiples of a 1/4 wave (i.e. 1/4, 3/4, etc) will
>exhibit impedance near 50 Ohms, especially when played against a like
>counterpoise. A 65 ft. end-fed wire is great for 80 meters. . .

No. Impedance goes up as wire gets (electrically) longer.
Example, 68.4 feet of #16 copper wire in free space is resonant at and
has impedances of:
  7.0 MHz 73.9 ohms
21.3316 MHz 109.6 ohms
35.685 MHz 128.8 ohms
A "65 ft. end-fed wire" is one leg of a resonant dipole on 80 meters.
Your "like counterpoise" is the other leg. It is a resonant dipole.
EZNEC says 58.5 ohms at 3.66 MHz, with counterpoise running 5ft above
ground and the end of the other leg 35 ft up.

By the way. Re: slinkys. Steel has 7 to 12 times the resistance of
copper. It is also magnetic. Would you use a steel core or steel wire
for your final tank? Use copper wire for antennas, or find someone to
copper plate one of those plastic slinkys for you. Mind you, any
conductor will radiate if you squirt enough rf into it. Dummy loads,
trees, shopping carts (read Sturba's "Aerials II".) But for the QRP
crowd, why not put your rf into a resonant dipole , turn your rig down
to 10 mw and get credit for your achievement!

                ; D Dan Winkler N7IVR DWink@Juno.com


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