[Lowfer] Antenna questions

Thu, 12 Dec 2002 21:27:03 -0600

Hey, I missed seeing that mast. Now I have to stop in again to see that.

Mike>WE0H
http://www.we0h.us/lf.html
185.3026kc QRSS-30 & CW@15wpm
ID is "WE"

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Lyle Koehler
Sent: Thursday, December 12, 2002 9:07 PM
To: [email protected]
Subject: Re: [Lowfer] Antenna questions

An interesting antenna proposal, and one of the most "creative"
interpretations of the Part 15 rules I've seen :-) This re-opens the
legality discussions that have gone on for years. The only way to resolve
them would be to ask the FCC for a formal opinion, which nobody wants to do,
because many of us would almost certainly not like the answer.

Part 15 says that the LENGTH of the antenna, plus the transmission line and
ground wire, cannot exceed 15 meters (about 49.2 feet). Historically, most
LowFERs have considered a vertical to be legal if the height of the vertical
section plus the radius of the top hat does not exceed 15 meters. Most of
the LowFER verticals on the air this season comply with that interpretation,
although some people have argued that since the top hat doesn't radiate, it
can be any size. And somewhere back in antiquity, an unnamed FCC
representative gave a verbal opinion that the antenna would be legal if it
fit within a cylinder that is 15 meters in diameter by 15 meters high. That
is the interpretation used by the loopers, and my personal opinion (which
carries absolutely no weight with the FCC) is that anything outside that
envelope would _really_ be pushing the definition of 15 meter "length". A 15
meter cube standing on one vertex? Hmmm.

Actually if you want to go with the 15 meter cylinder rule, a 15 meter high
vertical with a flat top that's as wide as possible and with several wires,
might be almost as good as an 86 foot vertical with three drooping radials.
There are some "canned" antenna models in my web article at
http://www.computerpro.com/~lyle/models/lftxant.htm that will work with the
AO (Antenna Optimizer) modeling software or its freeware lookalike called NF
(Near Field). Probably one of the models can be adapted to represent the
"cube" antenna. If not, it's easy to create a new model.

The most efficient LowFER antennas I am aware of were the verticals used by
TEXAS and OK. Both were approximately 40 feet high with "skeleton" type top
hats; that is, 6 or more radials with one or more skirt wires. Their ground
systems consisted of 100 or so 120 foot radials. TEXAS had a chicken wire
ground screen which was rolled up as soon as the grass started growing
again. Bill Cantrell observed that the antenna current did not change with
the chicken wire in place, but it seemed to make a noticeable difference in
the far-field signal. With high-Q Litz wire loading coils and their
extensive ground systems, both TEXAS and OK managed to get the total system
losses down to the vicinity of 10 ohms, and achieved an effective radiated
power of perhaps 5 milliwatts. For comparison, I estimate the ERP of the
"WA" loop to be about 1.25 milliwatts in its most favored directions. My own
LEK antenna might get up to 2 milliwatts on a cold day, when the losses are
low. Using the 15 meter cylinder interpretation of the rules, it should be
possible for a vertical to beat the TEXAS and OK efficiencies by at least a
couple of dB.

Ground rods at the ends of the radials can't hurt, although the one time I
tried it, there was no change in antenna current while the ground rod was
connected and disconnected.

Erecting a skinny mast of 50 feet or more in height can be a real challenge.
Anything over 30 feet, and the mast turns into a wet noodle that is bent
upon self-destruction (pun intended). I have a Rohn 5-section telescoping
mast that starts out at about 2.5 inches diameter for the bottom section,
and can be extended to about 46 feet. With the bottom section very securely
guyed or, in my case, anchored to the side of a building, it isn't too scary
to extend the mast to its full height with a homebrew 20/15/10 meter trap
dipole (small-diameter aluminum tubing, about 30 feet overall length) and a
TV rotator at the top. I just have to hurry and secure the guy ropes before
a breeze comes up. With three or more people, it's possible to extend the
telescoping mast (with nothing on top) while it is lying on the ground, and
then walk it up to a vertical position. It requires temporary side guys that
form a fixed triangle while the mast is being walked up; these can later be
moved to serve as two of the three "permanent" guys. I'm not sure I would
try that with a 20 foot diameter hat on the top!

With something like 4" diameter PVC pipe, the mast _and_ its top hat can be
rigid enough to erect with a "gin pole" such as a 20-foot extension ladder
guyed to a vertical position, so that there is an elevated point from which
to pull up the mast. Bill Bowers used a system like that for his "OK"
antenna.

Depending upon the rigidity of the mast, two sets of guys may be adequate
for 50 feet. In the case of my telescoping mast with the HF dipole, it is
anchored to a shed at the 10-foot level and has only one set of guy ropes
near the top. It has survived a couple of years that way -- knock on wood!
The somewhat more rigid tower that serves as one support for my LEK flat top
has only two sets of guys, and has been up for about 8 years. I prefer
Dacron UV-resistant "antenna rope" to metal guys, except that the stuff
stretches a lot initially. Metal guy wires are OK if well insulated. I'd
suggest going to the farm supply store and buying a couple of bags of
electric fence corner insulators, either porcelain or plastic, and breaking
each guy into several sections. Getting back to antenna rope, the 3/16 inch
stuff is good for about 700 pounds test, and costs 11 cents per foot from
places like Davis RF (www.davisRF.com). It's somewhat cheaper in 1000-foot
rolls...

Good luck -- we'll all be watching for that signal from Nevada!

Lyle, K0LR

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