Checking for suspicious CMC

Use this forum to discuss any radio-related topic outside of asking for advice (check out our Questions and Advice forum for those topics). As with all of our forums, treat others with kindness and respect.
Post Reply
User avatar
Andrew (grayhat)
Posts: 210
Joined: Sat May 28, 2022 5:56 am
Location: JN63pn

Checking for suspicious CMC

Post by Andrew (grayhat) »

Ok this one is more about tx than rx, still this old trick is easier and quicker than assembling an RF ammeter; in short, if one suspects about "RF in the shack" a quick way to check is to find the band where the problem(s) are more evident, at that point, pick a run of insulated wire and cut it to 1/4 wave on that band, insulate well (tape...) one end, peel the other and connect it to the gnd of coax coming down from the antenna (or the gnd of the ATU or rig...); lay the wire down on floor/ground, it doesn't need to be straight, it may run around the perimeter or whatever, just ensure the unconnected end is well insulated, since HV will develop there; anyhow, once done, try transmitting on that band, if the problem disappears or attenuates, the you DO have CMC ... and the related "RF in the shack" issues, and you'd better take care of them, otherwise your issue lies elsewhere

HTH
13dka
Posts: 98
Joined: Sat May 28, 2022 4:27 am
Location: On or near a dike

Re: Checking for suspicious CMC

Post by 13dka »

Great advice! Too bad there are only 2 types of hams, the guys who read stuff like this and the guys who complain about their PA doing strange things (and blame the PA for it) in partially unreadable transmissions because the in-shack RF is destroying their modulation. :mrgreen:

I have a guy who's here on vacation a few times each year and he always has that same old problem, to make things worse he's bringing some cheap Italian "brick" PA, really hard to not know when he's back:

3698_Splatter.jpg
3698_Splatter.jpg (121.06 KiB) Viewed 612 times
User avatar
Andrew (grayhat)
Posts: 210
Joined: Sat May 28, 2022 5:56 am
Location: JN63pn

Re: Checking for suspicious CMC

Post by Andrew (grayhat) »

13dka wrote: Sun May 29, 2022 1:25 pm Great advice! Too bad there are only 2 types of hams, the guys who read stuff like this and the guys who complain about their PA doing strange things (and blame the PA for it) in partially unreadable transmissions because the in-shack RF is destroying their modulation. :mrgreen:

I have a guy who's here on vacation a few times each year and he always has that same old problem, to make things worse he's bringing some cheap Italian "brick" PA, really hard to not know when he's back:


3698_Splatter.jpg
groan, if the PAs you're referring to are "RM Italy" or "ZetaGi" then I feel your pain, those things are just "CB" stuff adapted to work on HF, they have NO filters at all....SIGH
Post Reply