Please help, trouble shooting a grounding issue

[cjruger]

When looking at my carrier on a oscilloscope, the carrier with no audio, will change size when i touch the radio. Is this a rf grounding issue, if so what to i ground and to where? Ground to neg on power supply or to actual earth?

 

[Robb]

How are you sampling the RF for the O'scope?

 

[cjruger]

A wire wrapped around the coax with each end to the o scope jack, inductive pickup i believe?

 

[N5IT]

You may have better results using an unbalanced method. Try using a 10X probe with capacitive coupling via a .01 uF ceramic cap. Try with or without probe ground attached to radio chassis ground.

Otherwise take a look at the link below and the description of an inductive RF sampler circuit and sampler unit built into a Hammond box. The ferrite core is type 43 material, size FT 37, and is commonly available.

RF-Workbench-1

In addition to the isolation, you're getting attenuation to boot. 73, and good luck.

Dave

 

[N5IT]

I am going to crawfish just a little here. Before possibly performing any of my previous suggestions, I would insure that all of your test equipment, radio under test and power supplies are all supplied by the same AC outlet. This helps to reduce possibility of ground loops. You might also try grounding the scope input to the radio chassis ground to see if that will resolve it first. Good Luck

 

[Shockwave]

You could have significant common mode currents on your coax. The level of RF being detected on the coax may actually be dropping when you touch the radio because you are absorbing some of that CMC. It's the same principle as having RF feedback in your audio that goes away when you put your hand on top of the transmitter.

 

[N5IT]

......and those CMC's are what is allowing you to sample the rf in the first place. If you're not interested in absolute magnitude of the signal and merely looking for noise or distortion, then I would not worry about it too much. Otherwise try a different sampling method. Good luck.

Dave

 

[Robb]

http://www.worldwidedx.com/home-brew/32110-rf-sampler.html

That . . . and a dummy load . . . yes; make sure the pwr supply is well-grounded . . .

 

[N5IT]

Thanks for the link to the sampler from moleculo. I wonder if he recalls where he picked up the small case. It is a perfect fit for the task at hand. I read through to the link from Captain Kilowatt, regarding Harold Kinley's article that described the modified T connector sampler. You learn something everyday. I have been reading Harold's articles for quite some time, but did not recall that gimmick tool.

I have a couple of MFJ dummy loads that I use at work, so I think I'll install ports on them as well. A great idea.