"harry" wrote in
ups.com:

....
I believe you are going to be more concerned with the transmission
line loss and the additional loss incurred due to the mismatch if the
mismatch is severe.

Harry,

At the risk of reawakening the source end re-reflection noise, this is a
receiving application, if the receiver is 50 ohms (as the OP stated) and
50 ohm coax is used to connect to a short monopole over ideal ground, the
VSWR on the line is 1:1 (or near enough), and loss is the matched line
loss.

The implications of that are that the short monopole, which has a source
impedance of very small radiation resistance and loss resistance, and a
huge capacitive reactance is loaded by the 50 ohm impedance looking into
the coax.

You can relatively easily work out the o/c voltage in the whip as a
result of an incident E field (it will be ~E*length/2), but you must
estimate the capacitive reactance as it is the main determinant of the
voltage divider formed by the antenna source impedance and the 50 ohm
load.


To obtain a desktop estimate of the VSWR, download W7EL's EZNEC demo
version (www.eznec.com) and put in the antenna dimensions, frequency
of interest etc. It will calculate the impedance and VSWR for you.

The impedance from NEC (of any flavour) can be used to workout the
voltage divider I mentioned above.

Alternatively, use a variety of NEC where you can excite the antenna with
an incident wave and observe the voltage developed on a load inserted in
the model. The model described
http://www.vk1od.net/SmallUntunedSquareLoop/ssulNEC.htm at does such a
thing for a loop, you could do something similar for a short monopole.

My thinking is that a short monopole is less attractive at these
frequencies than using a dipole of known gain and impedance, a dipole is
convenient (probably more than a short monopole with a large ground
plane) and more sensitive (ie lower Antenna Factor).


Take those values and go to http://www.vk1od.net/tl/tllc.php and plug
in your values along with the cable you plan on using.

You can also pull the ARRL Antenna book and look for the formula to
calculate the additional transmission line loss due to a load
mismatch.

The formula in this case for the monopole termination loss is 20*log(50/
(Zs+50)) and must be factored into the ratio of E field to o/c voltage to
get AF.

Owen