[RFI] impedance mismatching in coax

JW jwin95 at yahoo.com
Wed Dec 28 17:29:52 EST 2016


Thank you for your comment, but, there is an additional factor not considered: The capacitance of the coax cable's shield to ground (I mean the EARTH over which the coax cables lays.)  This forms a capacitor (capacitance) to earth, and works to 'bypass' some amount of common mode noise that might otherwise have coupled through the small amount of 'parasitic' primary to secondary capacitance. I think my run of coax cable was on the order of 120 feet or so, as the cable exited the rear of the house, went around the side of the house and finally to the half-size K9AY (with NO preamp) to the front yard.

As I wrote, Frank, I saw an enormous difference with and without, YMMV. I will stress again that I was competitive with other WSPR receive operators in the area after adding the 1:1 isolation transformer; the effort of adding the transformer was definitely not a NULL effort. 

I cannot imagine how much ferrite 'core' I would need for common mode chokes at 475 kHz let alone at 60 kHz (and lower!) in order to be as effective.


      From: "donovanf at starpower.net" <donovanf at starpower.net>
 To: rfi at contesting.com 
 Sent: Wednesday, December 28, 2016 2:58 PM
 Subject: Re: [RFI] impedance mismatching in coax
#yiv7720091850 p {margin:0;}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 amodest amount of common mode rejection.   K9YC's commonmode chokes are vastly superior.
Aaron never explicitly mentioned the frequency range overwhich he intends to operate his Beverageor any of the details(length, nature of the ground system, etc) for his temporary/quickBeverage.

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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