Return-Path: Received: (qmail 91982 invoked from network); 20 Feb 2004 09:32:33 -0000 Received: from unknown (HELO ptb-mxscan02.plus.net) (212.159.14.236) by ptb-mailstore04.plus.net with SMTP; 20 Feb 2004 09:32:33 -0000 Received: (qmail 5202 invoked from network); 20 Feb 2004 09:32:33 -0000 X-Filtered-by: Plusnet (hmail v1.01) X-Priority: 3 X-MSMail-Priority: Normal X-Spam-detection-level: 11 Received: from ptb-mxcore02.plus.net (212.159.14.216) by ptb-mxscan02.plus.net with SMTP; 20 Feb 2004 09:32:32 -0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Received: from post.thorcom.com ([193.82.116.20]) by ptb-mxcore02.plus.net with esmtp (Exim) id 1Au71M-00016e-9X for dave@picks.force9.co.uk; Fri, 20 Feb 2004 09:32:32 +0000 X-Fake-Domain: majordom Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1Au6zZ-0007Q1-Qm for rs_out@blacksheep.org; Fri, 20 Feb 2004 09:30:41 +0000 Received: from [213.218.75.234] (helo=smtp05.freeler.nl) by post.thorcom.com with esmtp (Exim 4.14) id 1Au6zU-0007Ps-9v for rsgb_lf_group@blacksheep.org; Fri, 20 Feb 2004 09:30:36 +0000 Received: (qmail 11538 invoked from network); 20 Feb 2004 09:30:35 -0000 X-Fake-Domain: unknown Received: from unknown (HELO w8k3f0.freeler.nl) ([62.21.133.232]) (envelope-sender ) by smtp05.freeler.nl (qmail-ldap-1.03) with SMTP for ; 20 Feb 2004 09:30:33 -0000 Message-ID: <6.0.1.1.2.20040220101033.03444740@POP3.freeler.nl> X-Sender: FRE0000086604@POP3.freeler.nl (Unverified) X-Mailer: QUALCOMM Windows Eudora Version 6.0.1.1 Date: Fri, 20 Feb 2004 10:31:25 +0100 To: rsgb_lf_group@blacksheep.org From: "Dick Rollema" In-reply-to: <000001c3f6d5$8a90acb0$fce8c593@rsch15> References: <000001c3f6d5$8a90acb0$fce8c593@rsch15> MIME-Version: 1.0 Subject: Re: LF: RE: RF Current meter Content-Type: text/html; charset=windows-1252 X-Spam-Checker-Version: SpamAssassin 2.60 (1.212-2003-09-23-exp) on post.thorcom.com X-Spam-Status: No, hits=0.0 required=5.0 tests=HTML_MESSAGE autolearn=no version=2.60 X-SA-Exim-Scanned: Yes Sender: Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-PN-SPAMFiltered: yes X-Spam-Rating: 1 Content-transfer-encoding: 8bit Dear Gary, LF-Group,

The coupling between the primary and secundary winding of the current transformer must be purely inductive. But even with a one-turn primary there is also capacitive coupling. Especially when the voltage on the primary is high, for instance when the meter is clamped over an LF aerial when transmitting, the capacitive coupling causes an extra current through the meter circuit. The circuit for that current is closed via the capacitance of the measuring instrument and its connecting wires to earth.

To avoid this effect in my instrument I placed a small screen inside the toroid that shields the secundary winding from the primary.
("Pliers-type RF current probe", Technical Topics, RadCom, October 1992).

It is easy to check whether capacitive coupling influences the measurement.
1. Take a reading.
2. Turn the clamp over 180 degrees. This reverses the current in the secunday winding by inductive coupling but does not effect the current by capacitive coupling.
3. Read the instrument again.

If readings 1 and 3 differ take the average of the two as the correct value.

When measuring current in earth wires the problem does not arise because voltage on the wire usually is very low.

73, Dick, PA0SE

At 11:45 19-2-04, you wrote:

Dear Gary, LF Group

>"When I have looked at designs in amateur radio magazines or internet
for a
>clip on RF Ammeter the designs seem to be limited to 2 ranges (100mA &
1A
>FSD), is there a limit to the range of this type RF Ammeter or can the
>ranges be extented to say 5 Amps maybe? is there a calculation to
extend >the
>range ?"
>My LF antenna seems to be showing in excess of one Amp so I need an RF
>Ammeter with a greater range than 1A.

For ammeters using current transformers, you can increase the FSD by
INCREASING the number of turns on the secondary of the transformer
(current transformation ratio for a transformer being the inverse of the
voltage ratio). So if you want to increase meter FSD by a factor N, just
increase the number of turns on the secondary (meter) side by N. This
divides the current going through the rectifier and meter by a factor of
N also, achieving the desired result. There are a number of second-order
effects that may alter the calibration somewhat, but experience shows
these are usually pretty small.

Shunting a thermocouple meter can be problematic, partly because the
thermocouple resistance is usually pretty low to start with, and partly
because it changes as the heater element heats up with increasing
current. A more reliable way of extending the range up or down is to use
a current transformer again. So using a 1:N transformer with the "N"
winding connected to the ammeter will increase the FSD by a factor of N.
Provided the impedance of the winding inductance is large compared to
the meter impedance, and primary and secondary are closely coupled, this
is an accurate way of changing the meter range - so don't reject
thermocouple meters just because they don't have the range you require!

Cheers, Jim Moritz
73 de M0BMU