[Lowfer] HIFER POWER?

Roger Thompson [email protected]
Mon, 08 Mar 2004 09:23:02 -0600


Ed,=20

I made up a quick model and don't get quite the same results.  I find
max gain of 1.78 dBi away from the tower at 45 degrees and a F/B of 1.76
dB.  Gain is down almost 8 dB at 15 degrees elevation.  This is using
the EZNEC real high accuracy ground model with medium conductivity.  I
remember MININEC having some limitations regards ground, but my
documentation is in storage.  I ran the model with MININEC ground
assumptions and had higher gain, but pretty much the same elevated
pattern.  Even with perfect ground, I still get a gain peak near 45
degrees.

I added 4 slightly elevated radials to the slant wire and assumed a feed
point near the bottom end of the wire and get 37.9 + j119.3 as the feed
point impedance.  The real part seems pretty much in the correct range
for the length of radiator.  The unconnected tower will reflect an
unknown reactance back into the fed antenna and I'm not completely
confident of the overall impedance results as I don't know how well the
program handles close coupled elements.  If I were building the antenna,
I'd just measure the actual impedance and build a matching network based
on the results.

Max gain on the horizon will only occur with perfectly conducting
infinite ground and certain types of antennas, so that MININEC result is
suspect for application to any real antennas.

In summary, I'd say the overall antenna system is still dominated by the
tower, therefore the higher angle of radiation still exists.  As to the
gain of 7.9 dB, even if it were dBi, that value would be in the range of
a fairly well optimized 3 element yagi, which I just can't see happening
with this particular arrangement.  I conclude you might as well feed the
tower.

Roger
AD5T
------------------------------------------------------------------


Roger:

	Thanks for the note and seems to confirm what I thought.  I'm
particularly interested in your EZNEC simulation, as the high-angle
prediction agrees almost exactly with what I get here using MacMININEC,
a Mac-based program I gut from Chris Smolinsky (sp?) a long time ago.  I
modelled the tower as a bottom section 25' long and 8" diameter with a
top section 25' long and 2" in diameter, which I think is a reasonable
approximation.  Usable, but not the greatest.  Here's the part I'd like
a comment on:

	In playing around I tried ungrounding the tower and instead
using an ~
20' length of wire grounded at the base of the tower and leaning away
from the tower at the top.  Actual metric parameters:

TOWER (NEITHER END CONNECTED):
1. x=3D0, y=3D0, z=3D0 at bottom, x=3D0, y=3D0, z=3D7.5 at top, 0.2 =
meter dia
2. x=3D0, y=3D0, z=3D7.5 at bottom, x=3D0, y=3D0, z=3D15 at top, 0.025 =
meter dia

RADIATOR (FED AT GROUND END):

	The impedance comes out to about 12.8 + j1.7 ohms. The vertical
pattern
shows a maximum at the horizon, down to about 0.8 power at 15=B0 and =
0.58
power at 30=B0.  Just over a dB horizontal F/B directed, (IF I =
understand
the geometric conventions), AWAY from the tower (as if it is acting as a
reflector).  "Gain" (whatever that means) is 7.9 dB. =20

	Does any of that sound right to you?  It would sure be simple to
implement.

Thanks,

Ed
x=3D0.2, y=3D0, z=3D0 at bottom, x=3D0, y=3D2, z=3D5.9 at top, 0.005 =
meter dia
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