My take on ....
> seperated by a sea water inlet [ ie near perfect ground ] ???
> The computer programs say both antennas would have no gain at this
> elevation. Is this true??
The idea is seriously flawed that the ground path somehow invalidates the
measurements, using the protocol specified in the comparison report. The
root assumptions of the modelling will explain why. Mosely is just going to
have to eat the test results and live (hopefully redesign) with it.
1) There is nothing about ANY of the antennas tested that would increase or
decrease their succeptibility to ground attenuation versus the others
tested. In any yagi, even feebly well designed, the free space MAXIMUM
occurs in the plane of the boom going forward. The ground effect horizon
null is a FAR FIELD effect which will reduce the maximum a fixed amount near
the horizon depending on the ground terrain and electrical characteristics.
It does NOT change the impedance of the antenna. The obvious proof of this
is in the creation of ground baffles to kill ground reflection mentioned in
an earlier post. You can go out and build the baffles at various points
between the two antennas, eliminate the null. The creation of a huge baffle
at half a mile out will not affect the impedance. No impedance change, the
comparison is valid.
2) The modeling presumes the ground is a perfect plane with uniform
electrical characteristics. ANY departure from that and the ground effect
gets ill-defined, losing its sharp horizon null. It is almost never as deep
a null as predicted. You can partly model real ground using the program TA,
which allows the entry of height variations and their distance. With just a
little messing with the ground countour, the ground null can be
significantly reduced (ground sloping down from the antenna for five to ten
times the antenna height, for instance). The program cannot, however, add in
the variation in the electrical characteristics of the ground which would
further difuse the null. In any event it is a FAR FIELD effect, and if the
tested antennas are all set at the same height over the same spot, will NOT
contribute any false divergence for the test results.
At one time, Mosley may have been the best in the field. That was then. Have
they been setting pat on the old designs? Metal covered traps? Perhaps more
durable, but you won't ever catch me winding a coil inside or around a metal
pipe. It's lossy. There is induced circulation current. It's as if I added a
slightly resistive closed loop coupled to the coil.
In any event, the text in the ad was the most whiney, wimpy, sour grapes,
point-the-finger-at-anyone-else-besides-themselves, self-serving poor-me
piece I have seen or heard in a long time. I found it embarrassing to read,
the same sinking feeling one gets at a school play when someone's child
starts messing up his lines. They ought to know that there are many
thousands of hams with modeling software now, any one of which could see
right through the illogic in the ad.
I have no association or connection with any of the parties possibly
involved in this dispute. Those who have followed this reflector for a while
will note I have occasionally been on the other side of design and other
issues with the likes of Force 12, K7LXC, W8JI, or whoever. I'm an
Shame, Shame, Mosley.
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