Don:
I have little or no problem putting an HP8752B VNA on my antenna
feedline, or my N2PK VNA. Both have narrow band receive (adjustable down
to 10 Hz in the 8752B) and hit the antenna with a reasonably hefty
signal, 0 dBm or so. Consequently, unless you have a very strong signal
inbound, measurements can be made without much trouble.
The subject needs further though and library research, on the theory
that an hour in the library can be worth a month in the lab.
Jack
Don Wilhelm wrote:
Jack,
Yes, connecting a measureing instrument directly to an antenna can be
troublesome if there are strong broadcast stations in the area or other
sources of signals that can fool the instrument.
Zack Lau addressed that situation in a Technical Correspondence article in
QST a while ago - his answer was to measure the antenna indirectly with an L
network - the L network provides selectivity - adjust the L network to
provide a 50 ohm match, then remove it from the antenna, terminate what was
the measurement side with 50 ohms and measure the side previously connected
to the antenna.
Sounds like a sane way to approach it to me.
73,
Don W3FPR
-----Original Message-----
From: Jack Smith [mailto:[EMAIL PROTECTED]
Sent: Wednesday, March 14, 2007 4:38 PM
To: [EMAIL PROTECTED]
Cc: Elecraft reflector
Subject: Re: [Elecraft] Re: MFJ259
After thinking on it more, 30 KHz bandwidth is too wide to be useful for
a noise bridge detector, as it will not provide enough frequency
resolution for the lower bands. 7-7.3 MHz would only have 10 discrete
measurement points, for example.
If the device were to be used only to measure components, and not
connected to an antenna, lots of circuit simplification is possible in a
bridge generator / detector approach, i.e., a knock off of the classic
GR916 RF bridge with an integrated source and detector. Like Boonton did
with the 250A RX Meter, for example.
However, as soon as you connect the unknown port to an antenna, simple
detectors are out as the null will be submerged in trash unless the
detector receiver has excellent selectivity and that means frequency
stability as well.
Jack
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