[Elecraft] OT: Long loop antennas

[Ron D'Eau Claire]

Stuart wrote:

The Army Rhombics were terminated with 600 ohm non inductive resistors as I
recall. Made them unidirectional.

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Yes, a rhombic is a "diamond shaped" loop with the feed at one pointed end
and, normally, a 600 ohm resistor between the wires at the opposite end.
This form is called a "non-resonant" rhombic because it displays almost no
"resonant" effects. Any RF not radiated into space that reaches the far end
of the diamond is absorbed in the resistor so there is no reflected energy
back along the antenna wire. With no RF being reflected to the feed point,
there is very little change in the SWR at the feed point over a very wide
frequency range. They are normally fed with 600 ohm open wire line. It is
directive in the direction opposite the feed point. The rhombic can be run
without the termination resistor, making it bi-directional (since the RF is
reflected from the far end, it produces radiation back in the direction of
the feeder end). The reactance (so SWR) at the feed point of a rhombic
without the terminating resistor changes much more quickly with frequency
and it is often called a "resonant rhombic" compared to the terminated
"non-resonant rhombic". 

But it's real beauty is in the gain and, due to it's sheer size the
resistance to QSB and non-critical dimensions. The performance of the
rhombic has earned it the nick-name "King of Antennas". 

The gain comes, like any antenna, by its being directional, and the gain can
be considerable when the legs are long in terms of wavelengths. 

The fellow asking about loops indicated that he had supports and LOTS of
Real Estate. That's why I suggested looking into the "King of Antennas". The
Real Estate requirements for most of us will be met at our own QTH only in
our dreams <G>. But photos of virtually any MH/HF commercial shore station
will include some shots of their "rhombic farm" usually indicated by a
forest of telephone poles on a big parcel of land... And usually at the
seashore at that! 

Sigh... Dreaming again <G> 

Ron AC7AC