Message-ID: <001c01c3b4e0$8ff3cf20$6507a8c0@Main>
From: "Alan Melia" <Alan.Melia@btinternet.com>
To: rsgb_lf_group@blacksheep.org
References: <002001c3b4d4$b05039e0$ee9bfea9@oben>
Date: Thu, 27 Nov 2003 12:11:04 -0000
MIME-Version: 1.0
Subject: LF: Re: Impedancebridge
Content-Type: text/plain; charset=Windows-1252; format=flowed
Content-Transfer-Encoding: 8bit
Sender: <owner-rsgb_lf_group@blacksheep.org>
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org

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
alam.melia@btinternet.com

----- Original Message -----
From: "dj9dw" <dj9dw@t-online.de>
To: "LF_group" <rsgb_lf_group@blacksheep.org>
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.
>
>
>
>