Common mode on coax

Forums discussing Antennas And Towers, Including Homebrew Antennas
User avatar
United States of America WZ7U
Member
Member
Reactions:
Posts: 628
Joined: Tue Sep 25, 2018 3:33 am
Location: rural NW Oregon
Contact:

Common mode on coax

#1

Post by WZ7U »

Common mode on coax; the dreaded situation where your feed line is part of the radiated signal. I wonder if that is my "current" problem? Allow me to entertain you with my latest discovery.....

I currently have up two verticals, raised bases with elevated radials. One "cut" for 20 and the other "cut" for 40. Both "resonate" swr wise lower than intended. Too long.... wish I had that problem elsewhere in life. But I digress.

So anyway, I tire of changing the feedlines when I want to change bands so I had this coax switch left over from Lord knows how long ago. Probably from the 11 meter days. I put it in line and viola, it screws with my "match" bigtime. WTF? I have one of these little gems to have fun with as well as visualize output power making it actually out.


Image

The little meter gizmo is a handy feature indeed. It has shown me how the RF out hasn't always followed conventional expectations. Meaning this...
the 20 meter vertical lets more zap into the ether on 15 & 10, while the 40 meter vertical lets more zap into the ether on 12, 17, 20, 30 & 40. Not exactly how I visualized whirled peas. Boy, put that switch in line and the match gets all effed up. Maybe the switch is to blame, and I wouldn't be surprised if that were the case. It ohms out good, but I spent too much time mapping "resonance" out for the Ducie Island DXpedition so for now I suppose changing bands by coax is how its going to be.

Ideas? :Dontknow:
de Eric, WZ7U, CN86jc
User avatar
AC8UN
Member
Member
Reactions:
Posts: 1357
Joined: Wed Oct 24, 2018 11:10 pm
Contact:

Re: Common mode on coax

#2

Post by AC8UN »

Eric with proper resonant elevated radials there shouldn't be any common mode on the feed line. I would suspect the coax switch, or the jumper to the switch. Could you possibly make your verticals into one fan vertical just like a fan dipole but vertical?
Now civilized for your protection. :Grin:
User avatar
United States of America WZ7U
Member
Member
Reactions:
Posts: 628
Joined: Tue Sep 25, 2018 3:33 am
Location: rural NW Oregon
Contact:

Re: Common mode on coax

#3

Post by WZ7U »

That configuration has crossed my mind. I won't be pulling anything down for the winter though, so I'm just going to have to wing vertically it with what I've got now. I am planning on putting the 75 loop up as soon as all the leaves are down so I can have a better shot into the trees and be able to see where I'm landing. I would've got it up last winter but that pesky double bypass put the kabosh on that.
de Eric, WZ7U, CN86jc
Post Reply Previous topicNext topic

Who is online

Users browsing this forum: No registered users and 0 guests