[Elecraft] ridge vent as antenna?

[Ron D'Eau Claire]

Considering "equivalent lengths", bending the sections of a doublet more
than 90 degrees significantly reduces the effect of the additional length of
wire. Folding the wire back on itself a few inches from the original
radiator has the effect of lengthening the radiator only slightly. RF flows
over the wires following the broad outline of the area occupied by the wire
rather than following the wire around sharp bends, so the folded-back wire
tends to look simply like a "fat" wire, rather than a wire making a circuit.


Such "linear loading"  works, but the total length of the wire for a given
effective length of antenna is much, much greater than would be necessary if
the wires didn't fold back. 

Another approach to loading is to make the wire a huge coil with a very
large length/diameter ratio (small diameter coil compared to the length of
the radiator). Doug DeMaw (W1FB,sk) and others documented experiments with
these in several ARRL publications some 30 or 40 years ago. I have done some
tinkering with them too, and found that DeMaw's estimates were quite close:
a continuously-loaded radiator required just about twice the total wire of a
linear radiator. In my case, 260 feet of wire wound in a 3-inch diameter
helix (coil) with a spacing between turns that yielded a 40 foot long
radiator was self-resonant at 3.7 MHz. Of course, stretched out it would
take only about 130 feet of wire to resonate at that frequency.

Still, what's to argue with a 40-foot self-resonant 80 meter antenna?  Many
operators have used or at least heard of "slinky" antennas made from the
child's helical spring toys stretched along a plastic insulating rope that
made use of this property to form a shortened antenna. At least one company
sold them commercially. 

Even so, there's one major consideration. The radiation resistance of such
an antenna is very, very low. That is, the portion of the load resistance
that actually converts RF to electromagnetic waves is miniscule compared to
a full-sized antenna. A dipole, if fed at the center, will show about 50
ohms radiation resistance at typical heights above ground most Hams
encounter (it's closer to 75 ohms in free space). A loaded dipole, no matter
how it's done, may show a radiation resistance of only a couple of ohms at
resonance. Actually, many short loaded antennas show only a small fraction
of an ohm of radiation resistance at resonance. 

That's why such antennas are often a disappointment when used against ground
or a simple counterpoise. The resistance of the ground is often hundreds of
times greater than the radiation resistance of the radiator, so the
efficiency of such an antenna may be much less than 1%: 5 watts in, < 50 mW
radiated. 

Loaded antennas fare much better when center fed since there is no "ground"
connection. However, the resistance of the wire becomes significant.
Remember, the resistance of a conductor is much greater at RF than it is at
DC or low-frequency AC due to the skin effect. If the radiation resistance
is only a fraction of an ohm, the effective resistance of the wire may still
consume more than half the power. Still, there's a huge advantage to be
gained by using a balanced, center-fed loaded radiator arrangement over an
end-fed loaded radiator arrangement. 

Linear loading (wires folded back and forth in a zig-zag) and continuously
loading (wires in a helix for the length of the radiator) became more
popular than simple loading coils because loading coils tended to have
higher ohmic losses, being made of smaller diameter wire with
correspondingly lower surface area. Remember, because of skin effect it's
all about surface area: a paper-thin tube of copper an inch in diameter has
the same low resistance to RF as a solid bar of copper an inch in diameter.
So, in general, a larger diameter conductor bent around showed lower losses
than many loading coils made out of small-diameter wires. 

And, under it all, years of experience by thousands of Hams tends to confirm
the belief that a small-diameter radiator outside 'in the clear' beats just
about anything that can be erected inside a building. That's why I was quick
to endorse Ray's idea of a stealth wire outside. 

The quest for the perfect antenna has continued ever since Marconi figured
out that if he hooked his spark gap to a metal plate suspended over his
laboratory table the signal could be detected farther than ever before. That
quest still goes on. Every antenna, no matter how big or small, high or low,
cheap or expensive, is a huge bundle of compromises. The challenge to the
Ham is figuring out which compromises yield the best results in any given
situation, and we're doomed to doing it largely blind. SWR is no indication
of how well an antenna will get out. It only indicates whether the feed
system is working as designed. On-air checks are dubious, at best, in spite
of great care and efforts. Antennas are reciprocal - they receive like they
transmit in spite of what some claim - but it's as hard to evaluate received
signals by ear as it is when doing on-air transmitting checks. When
listening, changes in signal-to-noise ratio can mask actual changes in
actual antenna losses (or gain), and gain is all we care about when
transmitting. Changes in signal strength of 1 or 2 dB (a small fraction of
one S-unit) are often very difficult to measure under typical band
conditions. Sometimes it's impossible to evaluate the gain within a
relatively huge range of 6 to 10 dB because of QSB. And in such cases we're
evaluating a single path to a single distant station at a time.   

But, hey, if it wasn't so crazy and confusing, would it be half as much fun?


Ron AC7AC