The AM Forum

I had one of those Eureka moments last night.  After talking to Tom, I read his paper on the capacitive scope pickup and the word "harmonics" hit me like a nuke.

I wish I could have back all the time I have spent chasing harmonics that seem to have been created by the loop type pickups I have always used.

I built his set up in about ten minutes and of course, it works like a charm and no harmonics.

Thanks Tom,

Jack

Link to Pickup.... http://amfone.net/ECSound/K1JJ8.htm

js

heheheh - Thanks Jack -

Yep, I suffered with that fuzz on the scope for years too and thought it might be problems with the rigs. But that capacitive pickup tells the real story and usually produces a clean pattern. If it has enuff pickup, it will give a similar amplitude on all bands, so there isn't a need to constantly adjust the scope's vertical input sensitivity when hopping bands - another feature.


There's some good articles in "East Coast Sound" on this site.
http://amfone.net/ECSound/K1JJ8.htm
http://amfone.net/ECSound/

Good talking with you the other night, OM.

T

I've done inductive loops but I put 50 ohms in series with the loop and a 50 load at the scope.

I use one of those little non-shielded ferrite chokes on the end of some coax and shove it into the tank area. Seems to pick up nice on 160 - 40M. I figure that the self resonance must help out.

Mike WU2D

I use a capacitive pickoff.  a gimmick cap in my home brew antenna selection switch.  A few turns around the center conductor, and then out to a BNC for the scope.  One box has the switch, antenna relay, and scope pickoff, any rig I'm running can be looked at with the scope.  I also T-off the scope to the frequency counter.  I can run three rigs through it to the tuner. I did have a provision for switching in a dummy load and grounding the antenna, but that's fallen away with the link coupled tuner and the clip lead dummy load connection at the tuner feed coil.

When I built my station control 25 years ago, I needed a takeoff for the mod monitor, and being stupid, just installed a small wide spaced variable capacitor, I think its 50pf or so.
Its inside the box that holds the TR relay, the various muting and keying relays, the transmitter select switch, receiver select switch, antenna select switch, and has an output loop for power meters, tuners, etc.
I have gone back and forth having the loop in the TX only path, and in both TX and RX, but have it in both now as sometimes I use radios into an antenna port that have their own TR relays (ricebox, barefoot flex, etc.).

The variable cap has always worked great, from 70 watts to 700 watts, I adjust the cap to set the mod monitor carrier level.

I don't know what sort of load the mod monitor is, but its not frequency dependent.

Brett

I have been using a resistive bridge to couple my scope. It worked fairly well, but it was not always clean, and would at times show a distorted waveform with more deflection on the bottom of the display than at the top. Inserting a wattmeter or other such device into the mix would also cause changes in the display. I opened my station monitor/scope pickup box and replaced the resistive bridge with a parallel wire pickup and it does indeed result in a very clean waveform without any trace of distortion. I'm going to pull out the resistive bridge used for the diode audio monitor and try the same capacitive pickup to see how that sounds.

Sometimes, simpler is better.

:O)

I have a gimmick cap made of HV wire looped around one of the balanced line outpoots inside the mathbox going out to a BNC on the back.

Now I just have to buy a scope.  ::)