[Milsurplus] mf antennas etc.

[David Stinson]

The last time I tried to contribute to the discussion of 
electrically-short antennas at MF, I got called names 
and cussed, and I was setting records on LF
before most of those troglodytes had their tickets.
So I'm not too keen on getting into another one.
Let those guys spin their wheels.  But....
( I can see some people cringing now ;-)

First- I don't know everything.
I'm lucky if I remember how to tie my shoes.
And, unlike those mentioned above,
I know that I can be wrong.
But I did spend a lot of time with some good people
a lot smarter than me who were working ernestly
on getting antennas to perform at LF.
And I can't stand watching people I like and respect 
waste time and money on things that have been long
settled, so I'll throw this out and try to ignore the bores.

I watched a really nice guy in California stubbornly
stick to his 10 mH "top loading coil" for years and
years, and none of us could convince him that 10mH
was nearly as good an RF choke at 180 KC 
as it was at 18 MC.  
And the IR losses? "Fuhgettaboudit." 
Hearing his beacon even a short distance 
was very difficult.
Inductively top-loading an electrically-short vertical antenna
 to resonance does not work. 
Even if you add capacitive loading after the coil, 
the IR losses from the amount of coil needed at the low-Irf end 
of the antenna (because of the low C to ground at that end)  
swamp-out any radiation resistance gains 
from additional Irf at the top of the antenna.

But then, everything that makes-up an electrically-
short vertical antenna is a trade-off.  
You can get some improvement if you put a small fraction
of the loading inductance at the top, 
followed by good capacitive loading, but then you have 
wind loading problems.  Same with inductance at 
the middle; you get away from circulating currents from
the bottom-loading coil to ground, but then the antenna
can become unstable when the wind blows, you have
to figure how to do it mechanically, tweaking the coil
inductance to tune is impractical and, worse,
coupling losses to near-field objects like towers, 
buildings and trees go up.   Putting all the inductance at
ground level increases circulating-current losses to
ground because of the larger capacitive element to 
ground at that level,  but makes the antenna easy to tune 
and removes the wind-loading and other problems.
Spreading the inductance over bottom, middle and 
top loading works, but has nightmare mechanical problems
of its own.  7FS did some work on helical antennas
and he had a respectable signal, but he had the 
clear near-field and tall, insulated pole to do it.
And he still needed some bottom L.
"Pick ya poison." ;-)

On the whole, all the work over years with many
smart and driven Lowfers out west lead me to stick 
with inductive loading at the bottom using the best big
coil form and transmitting Litz wire I could find and
large capacitive loading at the top. 
My peanut-whistle signal was heard all over the west 
and as far east as the Mississippi.

(Just for fun:  Clint Turner, KA7OEI,
one of the good guys from those by-gone days,
posted one of my QSLs 
http://www.ka7oei.com/nts_qsl_l.JPG)

The ground in Nevada is pretty much like that
in a sack of pre-mixed concrete.  Even a jackhammer
could not drive a ground rod more than 3-4 feet,
so a grounded counterpoise of chiken wire in a 50-foot "X" 
under the "T" with ground rods at the middle
and at each end of the chicken wire worked excellent.

Now- all of that being said, there's a gentleman
in central Louisiana who's one of the 500 KC guys.
His antenna breaks all the rules-  It's a wire, going 
out the side of a building, low to the ground, 
running over to a tree.  The math says it should 
have the radiation resistance of a sack of potatos
(or, given where he is, maybe a sack of crawdads).
Yet, he is consistantly one of the loudest signals
on 500 KC.  No idea how that can be.
Go figure..... 
Maybe it's that Louisiana Bi-You voodoo magic
(I get to pick since I was borned-n-raised there).

Radio- just when you think the smartest people
have it all cyphered-out, it surprises ya again.
Gotta love it....

73 DE Dave AB5S

P.S. If you wind loading coils on PVC, use white.
Black PVC has something in it-
probably carbon black- that introduces losses.
And at high power, it heats up, too.
Don't know about the gray stuff.
Unless you want low-Q (for greater bandwidth),
can make-up the losses and you're not going 
to melt the stuff, stick with white.