Clip-on RF Ammeter?
Mont O'Leary
can be used to measure common-mode current on a coaxial feedline --
something similar in function to the Amprobe used for measuring mains
current.
Something simple that could be used in conjunction with a digital voltmeter
would be nice.
Thanks, Mont - K0YCN
JGBOYLES
>can be used to measure common-mode current on a coaxial feedline --
>something similar in function to the Amprobe used for measuring mains
>current.
Hi, Mont Head for your local Radio Shack and get RS part # 273-105. Snap
together ferrite choke core. Wind about 3 turns of #30 wire through core to
use as a pickup. Down at your DVM make a diode detector with 1N34 diodes and
.01 caps in a voltage doubler circuit. I made mine on a small RS circuit
board with banana plugs that plug directly into the DVM. Very sensitive, I can
get readings when using an MFJ-259 as a signal source.
This core will work with RG-8x coax, but I don't think it will snap around
full size RG-8. Good luck, hope you don't find too much common current.
73 Gary N4AST
OC_CAM's Razor
Let me describe what I use. I have an Amidon FT-114-61 ferrite toroid
with 20 turns of magnet wire. The toroid is slipped over an S0-239
barrel connector and two nuts hold the toroid in place. Coax cables
can screw on each end of the barrel connector. The 20 turns goes to
a small PC board with a load resistor, 1N34, and an RC filter which
then drives my DC digital voltmeter. It gives relative readings. The
PC boards and components are left over from a commercial endeavor I
was involved in years ago. It rectifies RF. If anyone is interested
in acquiring one of these PC boards, please send me an email.
--
cheers, CAM http://www.qsl.net/w5dxp
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K9SQG
also but I don't remember the reference info.
73s,
Evan
Roy Lewallen
equivalent) split ferrite core. Wind 10 turns around one half the core.
(Each pass through the hole when the core is assembled counts as one
turn.) Connect a 50 ohm resistor across the winding. This is very
important -- otherwise the core will choke the current, affecting the
measurement. If you want, you can connect the winding to a piece of 50
ohm coax, and put the 50 ohm resistor on the far end of the coax instead
of directly across the winding. This allows remote measurement. Clamp
the core around the coax you're measuring.
The actual current on the outside of the coax shield = N * V / R, where
V = the RF voltage across the resistor
N = the number of turns on the core
R = the value of the resistance
For R = 50 and N = 10, the wire current in amperes = the resistor
voltage in volts / 5.
The insertion impedance (that is, the impedance presented to the current
by placing the core around the coax) is approximately R / N^2, or 0.5
ohms for the recommended values. This is low enough to not significantly
disturb the current.
To convert the RF voltage into a DC voltage, you can use a diode
detector consisting of a series diode and bypass capacitor of around
0.001 - 0.1 uF or so. A germanium diode will allow accurate measurements
to the lowest voltage, followed by shottky diodes, then silicon. At
moderate signal levels, you'll get the peak voltage, which is about 1.4
times the RMS value.
Roy Lewallen, W7EL
Richard Clark
wrote:
>I'm looking for a design to home-brew a clip-on (or clamp-on) wattmeter that
>can be used to measure common-mode current on a coaxial feedline
Split ferrite as others have mentioned;
One LED with leads shorted inside the center.
Very sensitive up to about 40mA. Or if you want to go Old Tech (and
still Low Tech) simply stick an NE-2 along side the coax.
73's
Richard Clark, KB7QHC
Ian White, G3SEK
>I'm looking for a design to home-brew a clip-on (or clamp-on) wattmeter that
>can be used to measure common-mode current on a coaxial feedline --
>something similar in function to the Amprobe used for measuring mains
>current.
>
www.ifwtech.co.uk/g3sek/in-prac/clip-on.htm
(also available in Japanese!)
This is one of the most useful instruments you can build - and also one
of the simplest. For RFI work it's absolutely indispensable!
The downside is that you'll use it so much, the plastic hinge on the
split core will break (well, it never was meant for repeated use). It's
worth the trouble to build it a little better using a spring clamp.
>Something simple that could be used in conjunction with a digital voltmeter
>would be nice.
For many purposes such as RFI investigations, it's better to have a
small moving-coil meter built into the instrument, to give you
on-the-spot indications. For the occasional accurate measurement,
include a jack where you can plug in the DVM.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
Henrik
This link works for me:
http://www.ifwtech.co.uk/g3sek/clip-on/clip-on.htm
H
"Ian White, G3SEK" <G3...@ifwtech.co.uk> wrote in message
Ian White, G3SEK
Sorry, I posted the wrong URL. Thanks, Henrik!
Wes Stewart
<G3...@ifwtech.co.uk> wrote:
|Mont O'Leary wrote:
|>I'm looking for a design to home-brew a clip-on (or clamp-on) wattmeter that
|>can be used to measure common-mode current on a coaxial feedline --
|>something similar in function to the Amprobe used for measuring mains
|>current.
|>
|As well as all the other good replies, there's a page at:
|www.ifwtech.co.uk/g3sek/in-prac/clip-on.htm
|(also available in Japanese!)
Try:
http://www.ifwtech.co.uk/g3sek/clip-on/clip-on.htm
instead.
Rick Littlefield
can be used to measure common-mode current on a coaxial feedline <
MFJ has two. For experimentation, I recommend the smaller low power one I
designed when I worked for them. It is sensitive enough so you can use it with
your MFJ antenna analyzer or a signal generator as a signal source.
Rick K1BQT
Tarmo Tammaru
"Roy Lewallen" <w7...@eznec.com> wrote in message
news:v2p87m2...@corp.supernews.com...
> Begin with a high-permeability (Fair-Rite/Amidon type 43 or 70 series or
BTW, Surplus Sales of Nebraka has .5 in hinged 43 type beads. There was an
earlier posting on this where somebody was having trouble finding cores big
enough for RG8 type coax.
Tam/WB2TT