Hi Rick,
What you say below is absolutely true, but it might get you strung up and
lynched! This is a lot like insulting one's dear old favorite grandfather.
> I was just doing some rhombic modeling yesterday. I'm not
> sure what "350m in overall length" means, whether tip to tip
> or length of wire. If tip to tip, you are looking at a leg
> length of 200m or so, depending on angle. This is over 6 wavelengths.
> When you get to this size, you have a 3 db beamwidth of less than 10
> degrees in both azimuth and elevation. The gain, if the station you
> are working is lucky enough to be on target, is indeed around 20 dBi.
> This is 10 dB better than a tribander, but hardly the difference between
> inaudibility and 599.
It's just like the guy near Youngstown and his CTVR antenna, with
efficiency by Eimac. When someone is exceptionally loud in every direction
without turning the antenna, the power meter is likely out of calibration.
>The reason why the rhombic has such a baseball
> bat pattern is that it wastes a lot of energy with sidelobes. It is
> entirely possible to work DX off of one of these sizable lobes, again
> if the DX station happens to get hit by one.
> Having heard a lot of hype such as this and reading W6AM's blurb sheet
> on a web site dedicated to rhombics, I was all set to put one up, but
> after modeling, it looks like side by side and/or stacked Yagi's would
> get the same gain over a larger angle, plus be rotatable (try that
> with a rhombic). In my modeling I tried a bunch of different angles
> lengths and heights, so I don't think I missed anything.
You didn't. The actual forward gain of a Rhombic or V is very poor compared
to other antennas with the same main lobe beamwidth, for exactly the
reasons you outlined. There are also two additional reasons efficiency is
poor.
1.) radiation along the wire causes current to taper off, and that reduces
the ultimate gain as the antenna is made longer and longer
2.) ground losses below the antenna are compounded as the antenna is made
longer
Both of these things combine, along with the multiple lobes that waste
power, to limit gain.
This topic is covered very well in MFJ's book Antenna's and Transmission
Lines by John Kuecken. That book contains actual gain measurements of
Rhombics or V's. 4 wl per leg is about the gain limit.
A stack of Yagis absolutely is a better antenna, and the large Rhombics at
VOA sites are the lower gain antennas they use. The primary advantage of a
Rhombic is wide bandwidth. They are always much less than 40% efficient.
73 Tom
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