[Elecraft] EH Antenna Patent

[John, KI6WX]

After reading all of the posts on the EH Antenna, I went to the patent
office at www.uspto.gov and printed out Patent # 6,486,846 to see what is so
special about this antenna.  The patent makes the following claims.

-An antenna radiating structure that has two coaxially mounted parts (such
as a non-resonant dipole or a bicone)

-A matching network so that the E and H fields are in phase (this is a
complicated way of saying that the voltage and current are in phase, i.e.
the antenna input is resistive, not reactive).

I see nothing unique in the claims about the radiating structure of the
antenna.  The patent was probably issued due to the design of the matching
network.  The detailed description describes a dipole operating at 7 MHz.
Each side of the dipole is a 12" long aluminum cylinder with a diameter of
4.5".  The disclosure states that this antenna plus matching network had a
bandwidth of 500 kHz.

The radiation resistance of this dipole is pretty low, but the large
diameter cylinders will also have a low electrical resistance due to the
large diameter, so the radiation efficiency might be reasonable.  I haven't
tried to analyze the loss in the matching network, but these can be
significant when matching to the low radiation resistance.

The gain of this antenna will be the same as a short dipole, which is a few
tenths of a dB less than a half-wave dipole.  The real key to the
performance is probably the loss in the matching network, which I think will
be greater than the loss in the radiating elements of the antenna.  If you
had a lossless matching network, the performance of the antenna at a single
frequency would be the same whether you used the patented matching network
or any other matching network that provided a resistive match to the
transmitter.

The biggest advantage to the patent is that it allows the antenna to have a
much broader bandwidth than a conventional short antenna.  However, you can
also get a broad bandwidth using an antenna tuner.  If you ignore losses in
the antenna and the matching network, this antenna will perform no better or
worse than any other physically small antenna, and should have virtually
identical performance to a half-wave dipole at the same height and
polarization.  The losses in the antenna and matching network could make the
performance significantly worse than a resonant dipole.  There is no way
that this antenna could have more gain than a resonant dipole.

-John
 KI6WX