Shared publicly  - 
 
Added some RF choking to my feedline, finally. I still cause tons of RFI in the 'shack' but at least I'm not resetting my video card or derping up my mouse. Signal reports on 10m local are down by 1 S-unit. I suppose my next step is to add tuned counterpoises instead of the random wires I got on it now.

The antenna is a S9v 18ft fed through a 4:1 unun. 80% efficient at worst, which is an OK compromise considering it works 20m through 6m off one feedline.

#amateurradio #hamradio
2
Jason M's profile photoMichael G (mngrif)'s profile photoJeffrey Phillips Freeman's profile photo
15 comments
Jason M
+
1
2
1
 
I need to do the same thing on my feed lines here ... on occasion I come through the TV on 75m ... sometimes [more commonly] on 10 and 6 [6 being the most common]
 
Oh every band I use is awful for RFI. It's the nature of this kind of antenna system to want to use the feedline as a radiator. I also have an OCFD up in the attic and it's just as bad if not worse (because it has gain on some bands).
Jason M
 
Yea it's my s9 vertical that causes the interference the worst ... on 75 [which has just recently started interferring with a new tv we got [well it's 2nd hand tv]] it's my W%GI Mystery antenna[all band dipole fed w/ladder line]
 
Yeah the only way you'll not have RFI is if you're running a true 1:1 match. The S9 (either model) isn't even rated to work on 75m without a loading coil. Mine will tune up on every single band, but I know better than to bother trying to actually use it.
Jason M
+
1
2
1
 
I made a coil :p AFTER the RFI problem showed up ... but then again, I usually only use the s9 on 40 and 17 not 75
 
1:1 match vs a higher SWR is not what causes RFI or common mode. This happens from a lack of balance between your two poles and is NOT related to SWR.
 
well physics tells you the right answer... Hams are usually idiots on teh technicals. They just rewire things till it works and then assume why.
 
Guilty as charged.

So, why does putting RF chokes there help me?
 
because RF chokes/baluns are for exactly that reason, to block Common Mode. They are not (int he case of 1:1) used to improve SWR
 
Well of course they won't improve SWR. So why was there common mode currents on my feedline?
 
well depends on many factors.. but simply put, one of your poles of your antenna had a different complex impedance than the other. as such it radiated more energy than the other one. The difference between the energy radiated between your two poles is what winds up as Common Mode. This can happen from a few things if your poles are different physical lengths, they have inductors/capacitors attached to them which don't match, if one pole is closer to some object than another pole (like a house or tree), or if your feedline crosses the field lines radiated by your antenna (causing it to become inductively coupled with your antenna).
Add a comment...