"-rick-" wrote in
:


"Doug Dotson" wrote ...
I guess I'll pick up the latest Antenna Handbook and start reading.

Doug, k3qt
s/v Callista

The fundamental work on "small" antennas was done by a guy named
Wheeler. After digging in the filing cabinet I found his paper from
the proceedings of the I.R.E. (institute of radio engineers?) that
preceded the IEEE.

"Fundamental Limitations of Small Antennas" by Harold A. Wheeler
fellow, I.R.E. December 1947

One insight is that a small antenna can theoretically be nearly as
efficient as a 1/4 wave element but it is difficult to match to the
small radiation resistance. (actually you match to the sum of the
radiation and loss resistance). The efficiency is simply the ratio of
radiation resistance to the sum of radiation plus loss resistance. A
small loop antenna which looks inductive makes the job easier as you
can build a low resistance loop and use high Q capacitors for
tuning/matching.

regards,
-rick-

In 1947, matching the very low impedance feedpoint of a loaded vertical
antenna was a problem. But, after the invention of the broadband iron
powder toroids that are very efficient, magnetically at high frequencies,
it's not much of a problem at all.

At the base of my monster 1.8-30 Mhz 15' mobile ham antenna (4' ss base, 6"
diameter monster loading coil, 3' mast, 36" capacitor hat and stainless
whip on top cut so that shorting the whole coil resonates it at 14.250 Mhz)
is a T-200-2 powered iron toroid core wrapped with insulating fiberglass
tape and 12 turns of bare #10 copper. The core is mounted in a plastic
construction box between the posts of banana jacks that are soldered to the
outside of each turn so banana plugs can select the turns ratio. One end
of the coil is connected to "ground", the chassis of the car. Any
reasonable RF grounding system would hook there on a boat. The coax from
the transceiver's 650 watt, 12V linear amp is terminated with a banana plug
to select the input tap, and a short length of braided strap goes between a
banana plug and the bottom feed point of the antenna for the output tap.
Best match occurs when the lowest reflected power occurs on the SWR meter
of the linear amp (or transceiver with the linear out of the circuit). On
my antenna, on the 3.5-4 Mhz ham band for instance, the input tap is across
the entire 12 turns and the antenna is tapped 4 turns above the ground
point. SWR at resonance is perfect, 1:1, and large corona arcs occur at
the top of the whip tip and bent around ends of the 8 spokes of the 36"
capacitor hat, made of stainless welding rod welded to two large
flatwashers at the center. Signals are very comparible to any fixed
station here running the same power. Cars passing blow horns and shout,
"Your Antenna Is On Fire!", out their windows. It will light up a
flourescent tube in your hand at 10' away, easily.

The square of the turns ratio is 9:1 so the antenna's impedance is
somewhere around 6 ohms or so at the feedpoint. The 650 W amp melted the
solder joints on the core using #12 wire for the turns, so I went to #10
which is about as thick as I can go with 12 evenly spaced turns without
shorts. #10 wire gets too hot to touch, but doesn't melt solder any
more...(c; A second similar toroid autotransformer is mounted in another
box with 24 turns of #12 next to the first. It is used for the 160 Meter
band (1.8-2 Mhz). A second loading coil on top of the first (3" diam,
200T) adds sufficient inductance to tune the 15' antenna down to 1.8 Mhz,
but at 650 watts there is so much corona arcing it makes the SWR readings
go crazy so power is reduced to 300W or whatever the humidity around the
antenna can stand at that particular moment.

If one were to forego the old untuned wire/tuner marine antenna
configuration and go with a real tuned vertical, this toroid
autotransformer will very efficiently match the very low base impedance to
the 50 ohm transceiver across the 2-30 Mhz HF band.

The car's electrical wiring resonates around 3.9 Mhz, causing all the dash
lights to glow brightly with SSB modulation on their own, to the amazement
of even ham radio passengers. Thank God old Mercedes 220D diesels have no
electronics in them!

Many hams have gone to a remotely-tuned mobile antenna that uses a powered
motor to move the tap on a center loading coil. Here's what it looks like:
http://www.qth.com/n7lyy/about.html
It uses a screwdriver DC motor to move the loading coil up and down against
a large contactor and will tune the entire band. If a boater were to make
the whip longer than the 66" limit for cars, it would be even more
efficient with less coil turns below 12 Mhz. 66" is so it will tune 29.7
Mhz, the highest HF ham band boaters don't need. This antenna system would
easily mount on a stern rail, using the rail as groundplane if it were all
connected together, and would tune from the radio while watching the SWR
meter on the radio (or output power meter which would just tune for maximum
output power).

Larry W4CSC

Some day I might try this antenna using the handrails of the boat as ground
plane. It's gotta work better than the stupid untuned backstay and
inefficient antenna tuner. It certainly results in much better signal
reports.