TheRadioBoard

[qrp-gaijin]

I'm very pleased with the noise-canceling ability of a two-turn coax indoor HF loop antenna I built following the instructions at the now-defunct Greertech web page (courtesy of the Internet Archive). Sometimes I have some debilitating broadband local EMI and this antenna is able to greatly, sometimes even completely, null out the noise through careful rotation. The antenna is connected directly to my regen without any tuner.

One thing I noticed recently though is that the ability of the antenna to null the noise seems dependent on the feedline positioning. My feedline is longer than suggested (author suggested about 10 feet of feedline for a 40 meter two-turn loop; I have about 2-3 times this length), and I noticed sometimes I need to bunch up the feedline into a mass, or alternatively string it out across the room, in order to be able to null out the noise satisfactorily.

I do have two nulls 180-degrees apart, which the author suggested is a quick test to make sure that the feedline has not become part of the antenna. Nevertheless the feedline positioning affects the noise level, and sometimes seems to also affect the apparent noise direction also (rotation angle needed to cancel the noise may change depending on feedline positioning).

Is it normal that feedline positioning affects the directionality of a loop antenna, or should the feedline positioning normally have no effect whatsoever on the performance? Could noise be coming in through the feedline itself? Would making the feedline shorter possibly help?

Thanks in advance.

[OErjan]

any conductor near (wavelenght or so) any antenna WILL affect the behaiviour of said antenna, exactly how is even more complicated than antenna theory, i gave up after a few months.
safe to say that as long as you do not get severe distortion (marked difference in null bearings, say more than 5-10*) is can be considered a minor "loss".

[qrp-gaijin]

Yeah, I noticed today that opening and closing a sliding screen door also affected the apparent direction of the noise source and affected the necessary antenna orientation to null the noise. So, whenever the noise starts up in the band, I do a little routine of trying to find the best placement and rotation of the antenna and feedline that minimizes the noise. It's a little cumbersome, but eventually I am usually able to minimize the noise to bearable levels.

[KR1S]

Your antenna is not electrically balanced. This imbalance is reflected in the feed line, which is picking up "common-mode" noise.

http://www.w8ji.com/common-mode_noise.htm

http://en.wikipedia.org/wiki/Common-mode_interference

http://www.maxim-ic.com/app-notes/index.mvp/id/2045

73,
_________________
Jim, KR1S


Just build it!
http://kr1s.kearman.com/
http://qrp.kearman.com/