[TowerTalk] GAP Titan

Eric Gustafson n7cl@mmsi.com
Mon, 11 Oct 1999 13:11:54 -0700



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>
>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

--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm