Topband: Measuring field strength

[Rick Karlquist]

Tom Rauch wrote:
>> At any rate, it seems like I can do faster A/B testing of
>> added
>> radials with a reciprocal method rather than having to key
>> the station
>> transmitter and run out to the field strength meter each
>> time.  Are
>> there any caveats to this method other than making sure
>> that the RX is
>> terminated properly?
>
> I actually use a "pinger" to measure my receiving antennas.
> It is a battery powered oscillator using a cheap 1843 kHz
> time base crystal that drives a six-foot whip.  A 555 timer
> pulses the source off and on to increase battery life.

I also have one of these oscillators.  It works well for many
purposes, but since you asked the question:  there are two
reasons why you would want to transmit on the antenna under
test instead of receiving.  One reason (probably not applicable
at 160 meters) is that you might not want the receiver and feed
line to distort the pattern of the antenna.  With a battery operated
transmitter, you could feed the antenna anywhere you want with
a floating source.  The other reason is that if the antenna under
test is not 50 ohms, you will be measuring field strength with
the antenna mismatched.  Then when you change the radials, you
don't know how much of the field strength is related to the impedance
change vs true gain change.  Admittedly, this is often a small
error.  If you drive the antenna under test with a transmitter,
you can adjust for constant net power (forward minus reflected power).
You can also measure the antenna impedance and mathematically
correct for mismatch loss.  This would avoid having to shuttle
back and forth.

I played around with the design of a signal generator that would
automatically control net power as the match changed, but never got
around to actually building it.  It would not be difficult to do
that, but I don't need another project.

Rick N6RK