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LF: Re: Impedancebridge

To: [email protected]
Subject: LF: Re: Impedancebridge
From: "Alan Melia" <[email protected]>
Date: Thu, 27 Nov 2003 12:11:04 -0000
References: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
Hi Peter, I do not know that bridge but I have several others that cover LF.
The problem when doing aerial impedance measurements in the presence of a
high field from local sources is the difficulty of getting a true deep
"null" at bridge balance. A good bridge with a clean sinewave drive (no
harmonics) will normally give a null of at least 60dB and often as much as
90dB or more. Any "contamination" of the null will lead to erroneous balance
and incorrect measurements. I find that most bridges will expect to be run
with a drive of around 2 to 5 volts, and will not be damaged (normally) by
about 10volts. If in any doubt this should be filtered to avoid harmonics.
10 volts is about +27dBm in 50ohms , but if Mainflingen is giving -5dBm you
only have a margin of 32dB which is not really enough for accurate
measurements. I do not know the answer except to build a high-power bridge
and use a few watts of drive. A very simple transformer ratio-arm bridge can
be built with toroids and driven from an LF QRP TX. You may have to be a
little more caeful about the higher voltages that the drive will produce.
This way at least you could get a margin of 60 to 80dB and would not need a
sensitive null detector though it should be tuned. It is possible that a
synchronous detector could be an advantage....the Lock-in amplifier effect
giving a very narrow bandwidth. Just a thought that this might be worth
trying with a "normal" bridge where it might give significant advantages
over a normal wide-band not phase-locked detector.

I hope that gives you some ideas
Cheers de Alan G3NYK
[email protected]

----- Original Message -----
From: "dj9dw" <[email protected]>
To: "LF_group" <[email protected]>
Sent: 27 November 2003 10:53
Subject: LF: Impedancebridge


Hello LFers,
is the Gereral Radio 1656 covering our LF-Band?
Can it take 20dBm or a bit more? My friends in Mainflingen deliver -5 dBm
out of the umbrella here. So I have to run rather high levels for
measurement.
Are there alternative meters?
The modern antennaanalyzers as MJF or AEA quit in my qra.
tks 4 reply es 73 de Peter, dj9dw.







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