NEC-LIST: Site Attenuation Measurement / Antenna Factors ...

-----Original Message-----
From: Michael Foegelle [mailto:Michael.Foegelle_at_emctest.com]
Sent: Friday, June 16, 2000 9:15 AM
To: Tim Harrington (AEI); Kefeng Liu
Cc: 'patentlawyer_at_ieee.org'
Subject: RE: NEC-LIST: Site Attenuation Measurement / Antenna Factors

Gentlemen:

Unfortunately Clifford's response cut off Doug's e-mail address, so
hopefully you can forward this or post it to the discussion group.
Here at ETS we've used vector network analyzers for antenna
calibration and site validation for the past five years. They provide
results that are far superior to that possible from a spectrum
analyzer/tracking generator configuration due to the excellent dynamic
range, linearity, and external signal rejection. If you perform a
full two-port calibration, you can also eliminate the necessity of
pads since the calibration can correct for mismatch. However, there
is one significant limitation and a couple of pitfalls to be aware of.
Clifford is quite correct that the first issue is the height scan
requirement and the lack of max-hold functionality on network
analyzers. I've tried to get HP (Agilent) and others to listen and
convince them that there is an untapped market here if they'd just add
max-hold to their firmware, but so far I've been unsuccessful. Thus,
the only way to use the network analyzer is to perform the max-hold
from test automation on the PC. In order to do broadband scans with a
large number of points, this also requires height stepping of the
tower, since the sweep time becomes too slow to maximize on the fly.

An additional danger to be aware of is a limitation (I consider it a
defect) in the popular HP line of analyzers (HP 8753, 8720, etc.). If
the cable length is too long, the path delay becomes longer than the
step period, thus the transmitted signal does not reach the tuner
before it steps to the next frequency. Thus, the tuner is not
centered on the received signal. In minor cases, this results in a 10
dB error or so, but the inexperienced user is not likely to realize
where the problem comes from or even that there is a problem. (Worst
case, the receiver sees the noise floor because it's stepped so far
that the received signal is outside the bandwidth of the receiver.)
For broad span measurements, this will typically be indicated by
falling steps in the signal as you sweep to higher frequencies. For
narrow span measurements, you'll never notice the effect, since the
receiver bandwidth will typically still cover the incoming signal, but
it will be recorded at a different frequency point on the display.
The result would be that a resonance would appear to be at different
frequencies depending on the bandwidth, frequency span, and number of
points. The only solution is to forcibly slow the sweep speed
(typically by narrowing down to 10-30 Hz bandwidth) which greatly
increases the test time. We've pointed this problem out to HP, but
they do not seem to be interested in fixing it. We've tested the
newest analyzers from Anritsu/Wiltron and they do not seem to suffer
from the same problem. I've been told that the new Rhode & Schwarz
units do not either, but I haven't had the opportunity to test one
yet.

As far as ANSI testing, the next revision of C63.5 will contain more
generalized wording that make it clear that network analyzers and
automated measurements are acceptable for performing the calibrations
specified in the standard. After all, a network analyzer is really
just a high quality signal generator and receiver with a few
additional features over a spectrum analyzer/tracking generator
combination. It's just that in their current incarnation, none of
them have the max-hold automation built in.

All-in-all, vector network analyzers are great pieces of equipment to
have around, and if you have one that's going unused, I suggest you
take a look at it. Chances are, you've only scratched the surface of
its potential! (Ok, so I got my Ph.D. using an 8753B! It allowed us
to perform measurements and see things that had never been seen
before, so I'm definitely a fan! =)

Sincerely,

Dr. Michael D. Foegelle

-------------------------------------------------------
Dr. Michael D. Foegelle | 2205 Kramer Lane
Senior Principal Design Engineer | Austin TX, 78758
EMC Test Systems, L.P. |(512) 835-4684 x650
Received on Mon Jun 19 2000 - 07:27:25 EDT