"Roy Lewallen" wrote in message
...
Frank wrote:

The following code produces Zin = 0.117 - j2717 ohms at 1.9 MHz. What
did I do wrong? Note the high segmentation to place the feed-point near
the base of the antenna.

Frank

CM 9 ft monopole
CE
GW 1 108 0 0 0 0 0 108 0.25
GS 0 0 0.025400
GE 1
GN 1
EX 0 1 1 00 1 0
LD 5 1 1 108 5.8001E7
FR 0 9 0 0 1.8 0.025
RP 0 181 1 1000 -90 0 1.00000 1.00000
EN

If you follow NEC guidelines on the minimum recommended segment
length/wire radius ratio, you should have no more than 17 segments; the
model has 127. In this case, though, it doesn't seem to be disturbing
NEC-2 very much. Also, the half inch diameter is somewhat greater than
most CB whips. If you drop the diameter to a quarter inch, you'll get a
substantially greater reactance which would require a larger loading
inductor and hence result in lower efficiency. Otherwise it looks ok to
me.

As Wes pointed out, the ground system and loading inductor losses will be
so large as to make the feedpoint resistance (and wire loss)
insignificant. The zero-loss feedpoint resistance is, however, useful in
determining efficiency. If you set the wire loss to zero by removing the
LD "card", you get the more useful value of feedpoint resistance when
losses are zero, 0.098 ohms. Using Wes' value of about 50 ohm feedpoint
resistance when losses are included, you can calculate the efficiency as
0.098/50 = 0.2%, or 2 watts radiated for each kW applied. In American
mobile terms that's 1.5 watts per horsepower. I'm always glad to see more
QRP signals on the band.

Roy Lewallen, W7EL

I should have read the manual before modeling, but I was so concerned that
the feed-point be near the bottom. Changing the segmentation to 17,
removing the LD card, and reducing the radius to 0.125 ", calculates the
feed-point impedance as: 0.123 - j3286. I considered Jim's calculation of
0.37 - j8170 to be far too reactive. The fact is these points are so close,
when observed on the Smith chart, as to be virtually identical.

Analyzing a shunt L/series C matching network, with inductor Q at 300,
indicates a loss of 21.5dB with 30 kV RMS at the base of the antenna (1.5 kW
into the matching network). (In the case of 0.37 - j8170 the loss is 21.8
dB, with 42 kV at the base). These voltages are so large, I am beginning to
wonder if I goofed again! The losses, at least, are the same order of
magnitude as Roy's calculation.

73,

Frank