Hi jim,
I knew that If I waited long enough someone would post the truth
about antennas operated near (closer than 1/2 wavelength or so)
earth. Note that this applies to ANY ANTENNA near earth
regardless of its polarization or whether it "connects" to earth
or not.
73, Eric N7CL
>From: "Jim Reid" <jreid@aloha.net>
To: <towertalk@contesting.com>
>Date: Sun, 10 Oct 1999 14:43:19 -1000
>
>Bob wrote, in part:
>
>> The GAP is really a vertically-polarized 1/2-wave antenna with
>> tuning rods to provide the multi-band (and RG8X coax inside the
>> main tubing for 80/75 meter operation) capability. Radials, per se,
>> aren't required because there's no "missing" 1/4-wavelength to be
>> made up for, which is the case in the Marconi.
>
>> Both my GAPs were ground
>> mounted......
>
>> I personally do not consider these antennas dummy loads...
>
>They are not dummy loads; however, electric field lines from
>the high RF voltage near and at the end of the dipole, very
>close to the ground mounting point, do couple to the ground,
>and do induce current into the ground resistance: which current
>should be radiating as displacement current in space!!
>
>Here on Kauai, WWVH uses Rohn 1/2 wave vertical dipole
>towers with BIG center, feedpoint insulators! Upper bands
>use phased vertical dipole towers to generate cardiod pattern
>propagation West. And, there is a HUGE radial field
>installed to prevent exactly the E-field coupling to earth
>as described above ! Estimate about half the radiated power
>(3 dB) is now radiated, and saved from loss because of
>these extensive radial fields.
>
Snip... (discussion of WWVH's supply problems)
>Anyway, it appears from the Gov's experience, that radials under
>1/2 wave vertical antennas, with the lower end near ground, can
>result in a much higher efficiency radiating system as judged
>from field strength measurements way out in the Western Pacific.
>
>More about the WWV / WWVH antennas at:
>
> http://www.boulder.nist.gov/timefreq/wwv/quests/trans.html
>
>73, Jim, KH7M
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