[RFI] impedance mismatching in coax

donovanf at starpower.net donovanf at starpower.net
Wed Dec 28 15:58:22 EST 2016

Jim and Aaron, 

If you measure the shield isolation of a 1:1 transformer you'll 
discover -- contrary to popular belief -- that it provides only a 
modest amount of common mode rejection. K9YC's common 
mode chokes are vastly superior. 

Aaron never explicitly mentioned the frequency range over 
which he intends to operate his Beverageor any of the details 
(length, nature of the ground system, etc) for his temporary/quick 


----- Original Message -----

From: "JW via RFI" <rfi at contesting.com> 
To: "Aaron Kreider" <aaron at campusactivism.org>, rfi at contesting.com 
Sent: Wednesday, December 28, 2016 8:46:10 PM 
Subject: Re: [RFI] impedance mismatching in coax 

I found the better option in my case was a 1:1 flux coupled transformer using a ferrite core placed at the *receiver* end of the coax feedline. 
Background: I had noise up the wazoo before putting the 1:1 xfmr in the shack just before the receiver used on a 1/2 height K9AY loop placed in my city-lot-sized front yard. With the 1:1 xfmr I could actually see 60 kHz WWVB and the WSPR stations the others were seeing on 475 kHz. 
I think I used what I had on hand to wind the xfmer, a couple of large #43 beads and 3 turns primary and also 3 turns secondary. Was it optimum? Probably not, but I was at the point where I had to try something, and I got a good measure of positive results. 
Good luck. 

From: Aaron Kreider <aaron at campusactivism.org> 
To: rfi at contesting.com 
Sent: Wednesday, December 28, 2016 12:02 PM 
Subject: [RFI] impedance mismatching in coax 

Over Christmas I built a temporary/quick beverage antenna in a rural 
area, but it had a very high noise level. I had chokes on everything 
which helped, but as I hadn't a lot of planning - I ended up connecting 
two pieces of coax together with 50 ohm and 75 ohm reactance. 

Would this impedance mismatch cause any problems with the Type 43 toroids? 

If not, then I'm guessing I wasn't far enough away from the house (50-75 
feet away), and/or the house had a very dirty AC power. The location was 
perhaps 20-30 acres - so I could get further away from the house and 
electrical lines. 

The noise level at 500 khz was around 20-30 uV and below 100 khz there 
were spikes up to S9 + 40 db. It was pretty clean above 3-4 mhz (so 
this also points to the toroids working better at higher frequencies). 



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