Bob Wood wrote:
"I want to put up a 40M V Beam antenna to enhance communication in one
direction only.---I am thinking of 3/4 wavelength on a side with a
horizontal separation angle of either 72 degrees or 120 fegrees."

Arnold B. Bailey catalogs a bidirectional horizontal unterminated Vee in
"TV and Other Receiving Antennas" Ends of the Vee must be terminated in
their surge impedance for unidirectional response. Bailey gives 800 ohms
as surge impedance of the wires in the Vee.

All antennas in Bailey`s catalog are "optimized" for a frequency of 200
MHz (1.5 meters).

The horizontal Vee is center-fed at its apex. So, it opens / closes
toward its best directions, since It is unterminated. It is
bidirectional.

Bailey`s Vee is several wavelengths per side. Its drivepoint resistance
is 160 ohms. Gain is 10 dB. 3-dB bandwidth is 3%. It still behaves as a
standing wave antenna without termination.

The Vee is made from two horizontal 35-foot #10 wires for 200 MHz,
forming an included angle of 35-degrees (21-feet between outer ends).

At 40m, the wavelength is about 27 times that at 1.5m. So, that`s the
scale factor. The wires become about 933 feet long.

The same wire formed into a rhombic is unidirectional if terminated. It
only takes one termination resistor, and the rhombic gives 3 or 4 more
dB gain than the same wires in a Vee. The rhombic is a little shorter
overall than the Vee, too. At 40 meters the rhombic requires about 567
feet overall length and is about half as wide as it is long.

If you can accept a total gain of about only 4 dB, you can have an
antenna with much less wire and space. Two parallel center-fed wires,
each about 1/2-wavelength, and 1/4-wave apart, elevated at about
1/2-wavelength, and connected together with parallel open-wire line in
the plane of the wires will give a drivepoint resistance of near 50 ohms
across either antenna.

Which direction is favored depends on which dipole is fed directly and
which acts as the directly connected reflector. 3 dB bandwidth is 60%,
which is much better than the 3% of the unterminated Vee.

Arnold B. Bailey gives data on page 521 of "TV and Other Receiving
Antennas" for the "Half-Wave Antenna and Connected Reflector". He
credits P.S. Carter, Proc. I.R.E.,vol. 20, p1032, June 1932.

Best regards, Richard Harrison, KB5WZI