The AM Forum

Good Evening All:

I am looking for another way to monitor my T368 with a scope.  Aside from a wattmeter, not looking to have a bunch of stuff hang off of the antenna Jack.

What I am thinking is to wind a small torroid and hang it in proximity to the TR switch in the rear of the transmitter.  I can ground one end of the coil to the chassis, and attach my scope input with the scope ground on the grounded side of the coil and the probe side of the scope input on the other open end of the coil.

Will this work as an RF pickup?

Will I need to put a resistor in series between the coil and the scope input to attenuate the signal?

I appreciate your insight on this.

73--Wally W9BEA

Look here:

http://amfone.net/Amforum/index.php?topic=30071.0

73dg

A pick-up coil might work but not one on a toroid form.

Fred

This is extremely simple and works very well...........maybe add a cap to couple the RF to the scope connector.

hook your scope to an unused rx antenna while you transmit....

Depends on your scope.  Most of the Tektronix, HP, etc. "lab quality" scopes have plenty of vertical sensitivity and a couple of feet of wire into the vertical input is sufficient.  For low sensitivity scopes, grab a defunct watt meter (like a Heathkit HM-102) for cheap at the next hamfest, scrap most of it for another project and bring a cable from the pickup coil out to your scope.

For a spectrum analyzer when looking at harmonics and other spurious response you need a pickup which is amplitude invariant over the range of interest but for a simple scope view of the pattern a wide and flat sensitivity characteristic isn't as important.

I believe that is what you have Fred   ;D

Why not? I've used one for 20 years.

I think what Fred means is a toroid won't respond to an external RF field.  Only one that is introduced through the center hole.

73DG

OK. Thanks.

Nice video on YouTube on building a pickup. http://www.youtube.com/watch?v=y4Zt_LJX1Tc

Ken
w3kmp

Thanks DG,  I knew I would get a rise from folks who fail to correctly read the OP's method he planed to try.  That's why I kept my answer brief.

Fred

Why go through all of that fancy schmantzy stuff?? I just use a small low Pf cap right from the center conductor of the coass to the vertical amp inpoot to the scope.

The input impedance of the scope combined with the reactance from a small cap is so high that the transmitter doesn't even know that it is there. As a matter of fact it doesn't even effect the tuning.

Keep it simple................ (or why make it harder than it has to be)

Rodger had a good one with just hang a few feet of wire off of the scope's vertical inpoot connector, or Tum Tum Steve's just hook the scope to an unused antenna. I have also done both of the above and they both work very well, except for picking up a little tiny bit of the background moise on the baseline line.

K.I.S.S...........................

Wrap 5-10 turns of wire around the coass exiting the transmitter, then connect yer scope lead across the 2 ends of the coil.  Works FB with my T-368.

This always seems to become a mystery, and a very large discussion point.

Kinda like "the old family BBQ Sauce" Many have their own recipe for one.
and mysteriously, they all work (and work well). Henceforth, for those of us with a little technical knowledge (and common sense) it's a "no brainer".

for those that arent, they spend lots of jing buying unnecessary stuff they could concoct in a couple of minutes with minimal skill and minimal parts. For those who can't it opens the door for entrepeneurs and products like "Splatter View".

With that I will simply say: "It's all good".................  ;D  ;D

What I have always done, never with a problem, is to just take a line off an unused coax switch position (you have to have coax switches with lots of positions and not enough feedlines) and feed that over to the scope.  Done and done.  You're basically out nothing because no matter what you do, you have to have a line to the scope input anyway (except for the antenna on the input idea).   You probably have to use good switches that have isolation--this might not be a good idea with some junky hammy switch. 

What?  You say I might transmit into the scope?   I've done some dumb things but trust me, switching the transmitter to the scope hasn't been one of them.  Labels on the switch help.  Not drinking alchohol helps too.   8)

Where's the fun in that?

I do that all of the time, often intentionally. The inpoot Z of the scope is so high that it won't draw any current, so it wont hurt anything in the scope.

Or have the outpoot of the transmitter hooked to a dummy load and clip the scope probe onto the center conductor or outpoot side of the pi-net for bench testing.  :o  :o

The good thing about all this is youzall is using a scope to see what is going on.

Muy Bueno! :D

73DG

Let me point out one item that may be overlooked in the video, particularly regarding AM, unless you click on the "show more" link, where he correctly, but somewhat vaguely, explains. The note included in the actual video may be a little confusing and lacking in clarity to someone not already familiar with using the trapezoid pattern.

The method he suggests for generating a trapezoid pattern WILL NOT indicate linearity of the modulated stage, only the linearity of the diode detector in the pick-up.  To check modulation linearity of the final, one must sample the audio directly from the output of the modulator and use that sample to deflect along the X-axis. With plate modulation, use a resistive voltage divider off the modulated +HV line to the final amplifier, along with a DC blocking capacitor and additional phase shift correction capacitors across the resistors as needed.

This seems to be a common Hammy Hambone misconception regarding monitoring one's signal with an oscilloscope (or if you prefer more "vintage" terminology, an oscillograph).

Hear hear! :)

73DG

At the risk of being redundant...

http://home.comcast.net/~msed01/RFpickup.html

I mounted a BNC jack which fits perfectly in one of those perforations in the rear of the ART-13. Off this I have some mini teflon coax going into the RF section near the tank components. There I mounted a small standoff and a pickup of two turns of # 14 enameled wire 1/2 inch in diameter (it does not take much).

Another pickup that works on RG-8 which is not 100% shielded especially at higher power, is to simply wrap 10 turns of # 14 around the coax at the transmitter output and send that to the scope through coax.

With any pickup - the scope will greatly benefit from a tuned circuit as the second and third harmonic will be present and will make the measurement difficult.