Hi Group,
I have been asked to perform field strength measurements using standard
radiated methods as well as substitution methods on the same sample(an
intentional radiator). While I have plenty of experience making
radiated measurements, I have never made a substitution measurement. I
thought I have read enough about it to at least give it a go, but I am
getting some results that I didn't expect. I assumed that both
measurement methods should produce similar results within reason(+-5dB
or so or perhaps even closer). The results I am getting in some cases
have deltas of up to 10dB.
The procedure I am employing basically consists of:
1) Making the radiated field strength measurement on an OATS and
recording the raw result at the receiver
2) Substitute the EUT with a signal generator and an antenna(I am using
a Bi-log).
3) I adjust the signal generator level to produce the same level at the
receiver
4) I take the SG level and subtract the losses and add the gains.
The calculated result I figure should be close to the measured value
obtained in step 1, but it's not. I've tried applying the correction
factors in several ways to "Back into" the proper equation, but the
results are all over the place.
One question I have is should I use the AF's of both antennas or just
one? And if just one, should I consider the gain of the other antenna?
My logic is as follows: I presume the AF takes into account the 3 meter
"Path Loss" in some fashion. Therefore, following my logic, if I use
both AF's, the 3 meter "Path Loss" is applied twice. Using both AF's
produces a result nowhere close to the measured value.
I then used the AF of only one antenna and the gain of the other
antenna. Logic is that the 3 meter path loss and one antenna is
considered by the AF of one antenna, and the other antenna is considered
by applying it's gain. Using this method, I can get with 4dB of the
measured result at the fundamental, but I am still out by 10dB at the
3rd harmonic.
Also, should I be using tuned dipoles? If so, why would that make a
difference.
Any publications I can reference?
Any help is appreciated.
Kind Regards,
Sam Wismer
Engineering Manager
ACS, Inc.
Phone: (770) 831-8048
Fax: (770) 831-8598
Web: www.acstestlab.com
