[RFI] impedance mismatching in coax

Jim Brown jim at audiosystemsgroup.com
Wed Dec 28 13:31:50 EST 2016

On Wed,12/28/2016 10:02 AM, Aaron Kreider wrote:
> 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.

It is a MAJOR mistake to think of power lines as the only source of RF 
noise. Study http://audiosystemsgroup.com/KillingReceiveNoise.pdf   and 
use its guidelines to chase down and eliminate as many of the sources of 
noise as possible. As the tutorial notes, the average home has several 
dozen nasty noise sources.

> 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).

This may be as much the result of the spectra of the noise source(s) 
than the choke(s). Switching power supplies and other similar noise 
sources have spectra that is strongest at LF and below, but still quite 
strong well into the HF range.

BUT -- #43 material is a very poor choice for that frequency range. #31 
is far better, and a choke will need a LOT of turns on a choke for it to 
be effective below 160M. I would start with 20 turns for 160M, 30 turns 
for the AM broadcast band, and 40 turns to go lower. A choke wound on 
#31 is effective over a frequency range of about 8:1. You need to decide 
what frequency range you care most about and use one or more chokes 
tuned to different parts of that range if it's greater than that.

73, Jim K9YC

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