[Lowfer] 600MRG> Re: Why put a coil under the top hat ???

[David Stinson]

We had a fellow on 1750 mtrs that believed in "top loading" coils.
Trouble is, he didn't calculate the inductance needed to resonate
the antenna- tophat-ground system as a resonant tank;
he just wound a huge bunch of wire on the coil, thinking
that "the more coil the better," since an unloaded whip would 
have been a quarter of a mile high.  This big lump, which he estimated
at 11 milliHenrys (!), was just as good an RF choke at the top 
of the antenna as it would have been in the B+ lead of a P.A.
The capacity tophat was cut-off from the vertical element 
just like a trap does in  a tri-bander beam; might as well
have saved the wire.  Couldn't tell him any different, either.  
As a result, he had the weakest and least-heard 1750 mtr 
beacon in California.  AFAIK, he never changed it.

There are "no free lunches" in electrically-short antennas.
A coil near the ground is easy to keep tuned, but provides a 
large capacitor plate to ground, so a lot of the RF current 
circulates at a low altitude and thus, adds little to the radiation 
resistance.  A higher coil can be a tuning problem and 
a mechanical problem.  IR losses are a problem anywhere.
I've often thought a good way to do it would be to split the
inductance needed to load the vertical element, putting some
on top of the element and a smaller, tunable coil at the bottom.
I did this once at 189.84 KC, but my antenna was a wire "T"
flattop, and the big coil swinging in the wind in the desert
was a tuning headache, so I didn't go very far with the idea.
A good ground system (big 50-foot chicken-wire "X" under the 
antenna, ground rod and a connection to the site-wide
water system), and a large capactive top-hat assured good
current up the vertical.  I only needed 700 uH to resonate
the antenna.  A rigid vertical might do better with a 
"split coil" system.

All the best for 2010,
D.S.