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Scope Pickup - K1JJ




 
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« on: June 12, 2005, 11:57:44 AM »

An Easy and Efficient Broadband R.F. Transmitting Scope Pickup

  Submitted by Tom K1JJ

I wanted to thank Steve, WB3HUZ for his suggestion of using a capacitive divider for a scope R.F. pickup.
There's many ways to do it, but I promised to post my final product, so here it is.

In the past I've used a separate coil pickup loop for each amplifier final (7 different ones) with a rotary switch to select the proper one to the scope. (what a pain in the ass)

160-75M was clean, but on the 40- 10M bands I would get varing degrees of harmonic lines.

This new capacitive pick up works GREAT! On 160-10M the scope trace is perfectly clean, and the output amplitude is almost constant from 160-10M!!! IE, When using the same power level, I can select different bands and antennas and the scope trace stays very close to the same size.

Here's what I finally came up with...simple:

Take a tiny Bud minibox and mount three SO-239 coax female connectors. Solder two of these connector inner terminals together with a heavy straight through wire.

These two SO-239 connectors are put in-line with the main 50 ohm coax - put it right after the power meter or final common point to all amplifier outputs. (inline connection)

Then solder a short, stiff 2" wire to the third conector and route it alongside the first wire. Put it 1/8" away in parallel but not touching the straight through wire. This wire's end is free, no connection.

This third female SO239 connector connects directly to the scope vertical input using a standard shielded RG/58 or RG/59 coax jumper.
(remember that this cable carries low level R.F. up to 29mc)
 
On my particular pick up,  an input scope setting of around  0.2V works fine for 100W. Though, you can add more sampling wire or push this sampling wire closer than 1/8" to get a stronger signal if desired... not critical at all.
 
Do whatever it takes to get a strong, clean full carrier trace on your scope while using middle range scope input sensitivity settings.
 
I.E., if you do extreme QRP, then more coupling may be required to obtain a noise free scope trace.
The aluminum mini box should be closed up, thus shielded.
About 1" of parallel wire travel is good enough to get some decent capacitive pick up. I see a great scope trace at 50 watts and can still handle 1500 watts when lowering the scope input sensitivity. Not a sign of harmonics on any band all the way to 29 mc!
Gonna rip out all that old coil loop and coax crap now...
There's always something new to learn.

Tom, K1JJ
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« Reply #1 on: September 06, 2007, 09:31:17 PM »

 ;DExcellent idea I took one step further. I just purchased a used MFJ-700B (?) multi rig/multi antenna switch box. On the left side I enlarged the existing hole to install a BNC chassic connector. I then ran 2 inched of insulated wire from the center pin of the connector along the common lead to the transmitter side switch. I secured this with zip ties. Now I have a capacitive pickup for whatever rig is switched in line. I use one antenna port for my cantenna, so I always have a safe load for testing. Just thought I'd toss this idea out. This is an AWSOME site! Wink
73! Norm K7NCR
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David, K3TUE
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« Reply #2 on: September 09, 2007, 02:47:17 PM »

I use this and it works great.  On my 25W signal with the pickup wire something like one inch away from the pass-thru wire I get plenty of signal to the scope.  It even seems (strangely) like it's too much signal for my freq meter.
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David, K3TUE
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« Reply #3 on: January 03, 2013, 07:38:27 PM »

Thank You Tom, I just made one and working great. Matthew KR4WI
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« Reply #4 on: January 04, 2013, 08:42:20 AM »

I've modded two of my SWR / Power meters with an extra scope BNC port. It is fairly simple to pick up some passive RF from the circuitry and feed it into the scope. No exact measurement , but great for monitoring modulation.
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