John Langdon brings up a good point about interaction between antenna
elements and the tower. He says the Force 12 C4XL interacts with the Rohn
55G. I know a number of hams that have had this same type of problem. The
problem occurs in both beans and quads.
For example, I built the W5HVV/7 Quad in the 1966 QST (was a big gun for a
short time..). I had two problems..one the driven element was too close to
the top of the telephone pole I used then and the 10 meter mistuned as it
was close to the metal pipe I used for antenna mounting.
I shifted the driven element 2' and the problems went away. I realized the
pipe was almost exactly a 1/4 on 10 meters. Now I knew why the Skylane Quad
was so crazy on 10 and the Skylane owner couldn't figure it out. I reported
this to him and it was in his literature on using the skylane with the pole
mounting system in the ARRL lit of the time.
Another friend had the Gonset 3 el Tribander. He was a contester (in
RI..W1PDL). He and several other Gonset owners decided to modify the beam
for 4 active elements on 10 and 15 on a 23' boom. This modification placed
one of the elements just above his tower. The modified beam worked well on
the test platform but went crazy on 10 and 15 when mounted on the tower.
Just by accident he lifted the beam above the tower on the gin pole and he
decided to look again. The SWR almost returned to the platform values. He
mounted the beam 3.5 feet above his tower and did well in both the CQ WW and
ARRL DX. Of course being in Rhode Island helped (so said W1ONK).
A decade later I put up a 4 el Gem Quad on a 71' crank up. The loops nested
around the tower and I never could get any front to back and the SWR came
down but 200 to 300khz off the manual. The guys at Gem Quad said they have
never had the problem. I finally remembered the W1PDL problem. I cranked
it all down, tilted it over, and added a 10 mast so the quad was above the
tower and the problem went away...15 to 20DB F/B and SWR curves right on.
Someone sells a program to look for interactions now. Just keep in mind
that there are other objects that interact not just the beams or quad
elements/booms.
Dave K4JRB
--
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
|