On Tue, 13 Jul 2004 09:02:40 +0100, "Ian White, G3SEK"
wrote:

Sorry if this is untimely. My ISP has been up and down, mostly down,
for the last three days.

[snip]
|
|I don't have any of the classic references for optimum design of V-beams
|or rhombics to hand; but unless Richard and I are both doing it totally
|wrong, the V-beam does seem to show some reluctance to increase in gain
|by a whole 3dB for each doubling of the leg length (and the rhombic
|would do the same).

Why would this not be so? The remote parts of the antenna have less
energy to radiate.

As the antenna becomes longer, it becomes less a standing-wave antenna
and more a traveling-wave antenna. Kraus discusses Vs, terminated in
their characteristic impedance, that offer increased front-to-back
ratios because the reflected wave is suppressed.

It is easily observed that as an unterminated V becomes longer, there
is a modest increase in the front-to-back ratio and the real part of
the feedpoint Z becomes nearly constant. The loss in the wires, both
resistive and radiated, suppresses the reflected wave, in effect,
self-terminating the wires. I believe that if it was not for this
loss, the V would be totally bi-directional and the gain would be
proportional to length.

Kraus offers another V that has conductors of "considerable thickness"
that produce a similar front-to-back improvement.
|
|However, that doesn't detract from the reputation of the rhombic in
|particular as "the king of HF antennas" - if you have the real estate
|and can tolerate the fixed direction, the rhombic can give several dB
|more forward gain than almost any other practical antenna.
|
|
|All of this prompted me to try to model the 50-wavelength-per-side
|rhombic that we used for EME, way back when. My recollection is that the
|included angle was 12deg, but the original notes are long gone.
|
|EZNEC+ v4 predicts the spectacularly narrow main lobe that one would
|expect, and it also confirms the well-known finding that if you
|terminate the rhombic at the far end, the pattern changes from
|bi-directional to unidirectional but the forward gain also drops by
|about 3dB. However, 30-40 years ago it was believed that it is not
|important to terminate an extremely long rhombic "because most of the
|forward-traveling wave has been lost to radiation before it arrives at
|the far end." The model categorically negates that belief - even at
|50wl/side, termination has much the same effect as for shorter rhombics.

I'm surprised that you're seeing this. My modeling of even relatively
short rhombics shows that removing the termination does not make the
pattern symmetrically bi-directional and likewise I don't see anything
near a 3 dB change.

Wes N7WS