"Roy Lewallen" wrote in message
...
The answer is 3 mW.

Any version of EZNEC can be used to do this calculation. The demo program
will yield slightly less accurate results because of the limited number of
segments(*).

I modeled two vertical wires, 1 meter high and 1 mm diameter, spaced 1 km
apart, at 20 MHz, over perfect ground. The reported feedpoint impedance
varies with segmentation, from 1.988 - j952.3 ohms at 10 segments/wire to
1.72 - j882 ohms at 100 segments/wire. Accuracy is likely to degrade with
a larger number of segments, since even 100 results in segment
length/diameter ratio less than NEC recommendations. I used 100
segments/wire for the test.

One of the choices in EZNEC of far field strength reporting is in V/m at 1
kW input and 1 km distance. For this antenna, EZNEC reports 300.8 V/m
(RMS) at ground level.

EZNEC also permits setting a fixed power input, so this was set to 1 kW.
The resulting source voltage and current are 21270 V. and 24.12 A.
respectively.

A load of 1.72 + j882 ohms was placed at the base of the second vertical.
EZNEC reports a power of 3.234 mW being dissipated in this load.

Care has to be used when analyzing the current induced in one antenna by
another which is distant using numerical calculations. Errors can occur
due to truncation and other causes when the ratio of distances between the
two antennas is great relative to the segment lengths or to segment
distances within one of the antennas. However, EZNEC gets virtually
identical results when using mixed and double precision NEC-2 calculating
engines, which indicates that the limit hasn't been reached and that
numerical problems aren't occurring. (Another check which can be done is
to reduce the distance between antennas by a factor of two. The power in
the load resistance should increase by a factor of four.)

Another critical matter is the setting of the load reactance. The
reactance is many times larger than the resistance, so a slight error in
setting its value will result in a large difference in load current and
therefore load dissipation. For example, if the segmentation is changed
from 100 to 50 segments/wire and no other change is made to the model, the
reported load power becomes 0.3917 watts. The reason is that the reported
source impedance is now 1.756 - j891.4 ohms, while the load is still 1.72
+ j882 ohms. Changing the load to the proper conjugately matched value of
1.756 + 891.4 ohms returns the load power to the correct value of 3.24 mW.

All given, I'd trust the reported load power to be easily within 10% of
the theoretically correct value.

(*) Results for 10 segments/wire are 1.988 - j952.3 ohms for the source
impedance, 300.71 V/m field strength at 1 km for 1 kW, and 3.24 mW in a
conjugately matched load impedance in the distant vertical.

Roy Lewallen, W7EL

I agree with the E field computation at 1km. NEC2 calculates the normalized
peak E field as 425.452 V/m, which gives 300 mV/m at 1 km. The input
impedance, with 50 segments (#14 AWG, perfect conductor) is 1.747 - j823.798
ohms. The NEC output files shows the TRP at 1 kW.

For some reason I seem to get a different received power. If I model a 1
meter monopole, above a perfectly conducting ground, loaded at the base
segment with the complex conjugate of 1.747 + j823.798, and an incident peak
E field of of 1V/m. NEC computes the peak base current as 0.28636 A.
Dividing by 3.3333, for the equivalent RMS current from 300 mV/m RMS gives:
0.08591 A RMS. power in the load then equals 12.9 mW, which seems to agree
with Reg's figure.

Frank