Roy Lewallen wrote:
This analysis should be done in free space, not over ground.
Propagation to the moon should avoid reflection from the ground, and in
any case EZNEC's flat, infinite-extent ground model isn't
representative of what the signal would encounter in real life.
Very long rhombics and vees (for any frequency) cannot ignore the
existence of ground. On the contrary, they rely on it, so a model
including ground reflection is correct for this antenna. This includes
the use of these antennas for EME in former days.
Apologies to Richard for not responding to the antenna files that he
kindly sent across, a few days ago (other deadlines). The discussion has
moved on in the meantime, so it makes more sense for me to jump in again
here.
One other comment. As a dipole gets longer, the lobes move closer and
closer to the direction of the wire. Bending the dipole into a vee
shape aligns pairs of the lobes so they point in the same direction,
i.e., along the vee axis. That's why the optimum angle becomes less and
less as the wires get longer,
That would have been my first point in response to Richard's findings.
As a single end-fed wire gets longer, its radiation pattern becomes
predominantly X-shaped, with the four main lobes moving closer and
closer to the line of the wire itself. The optimum angle for a V-beam is
chosen to make the main lobes of the two wires overlap exactly, so they
reinforce as strongly as possible in the forward direction. If the V
angle is kept constant and only the leg length is increased, the maximum
obtainable forward gain will not be realized.
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).
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.
Unfortunately the segmentation density in my model (7.5 segs/wl) is too
sparse to be confident about the absolute value of the gain. Normally
one should both increase and decrease the segmentation density to
confirm that the predictions remain stable; but this is not possible
because this enormous antenna has already used 1499 out of the 1500
allowable segments.
Since I can't be confident about the gain predictions, there is no point
in quoting and discussing them here - we already chase enough wild geese
in this ng.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek