[BC] Long wire antennas

[Cowboy]

On Tuesday 15 January 2008 11:27 pm, Dana Puopolo wrote:
>  Many shortwave stations use(d) Rhombics...These are a variation of the long
>  wire antenna.

 Some say, but in my opinion not really.
 Some like to call any wire more than 75 or 100 feet a "long wire" but a wire
 isn't long until its length is described in numbers of full wavelengths, not
 fractional parts.

 As Mr. Johnson correctly points out, a true long wire is a single wire, end fed
 against ground, unbalanced, non-resonant, and directional along the general
 direction of the wire. Less than about 1 wavelength, is not a "long wire" at all,
 but just a random end fed. If the wire is fairly long, but shorter than required
 to radiate all of the energy in one trip down its length, it becomes
 bidirectional along the wire as minor lobes develop off the back end.
 As these lobes develop, it becomes debatable as to whether the wire
 is truly "long" even at several wavelengths. 
 Terminating the wire at the far end will eliminate these back lobes, forming
 a pattern resembling that of a long wire, but is not a long wire antenna.
 The shorter it becomes, the broader these lobes become, until the antenna
 becomes a broadside horizontally polarized end fed,
 similar to the top hat portion of an L.

 A rhombic is more correctly related to a V antenna, itself a modified
 dipole, directional along a line bisecting the V horizontally polarized,
 balanced and center fed. Stack two of them by running the V in the
 opposite direction and connecting the far ends, front to front rather
 than back to back, and you have a rhombus where both V's share
 the same bisecting line.
 An unterminated rhombic is bidirectional along this line. The construction
 resembling a large delta loop, but the mode is very different.
 A terminated rhombic, where the corner opposite the feed is terminated
 in a resistor equaling the characteristic Z and capable of dissipating
 1/2 the transmitter power, is unidirectional towards the termination.
 Forward gain is not lost, but merely the radiation towards the back.
 If the total circumference is small, less than a couple wavelengths, it's not
 a rhombic but is a horizontal delta loop with very different directional
 characteristics. A large rhombic is a very high gain antenna.

 Back in my telegraph days, the 8 wave rhombic on 15 mHz had something
 like 27 db gain off the front of that puppy. With the rigs running the full
 15 kilowatts into it, trans-global communication was almost a certainty
 any day at all.

-- 
Cowboy