[Elecraft] TxMon and K3 (tuner bypassed) SWR missmatch

[Jim Brown]

Yes, and that is exactly the right way to understand it -- when an 
antenna is unbalanced, regardless of the reason for the imbalance, the 
feedline becomes part of the antenna unless it is choked. Indeed, that's 
the major reason to use a good choke -- to prevent noise picked up on 
the outside of the coax from coupling to the antenna and from there to 
the radio (by coming down the inside of the coax). The choke also 
prevents RF current from our transmitter from flowing down the outside 
of the coax to radiate into home entertainment equipment, or even onto 
the power wiring.

Many hams think that using 2-wire line eliminates the possibility of 
imbalance, and even call it "balanced line" and feed it with a 
"balanced" tuner. Nothing could be further from the truth -- many 
(most?) antennas, especially wire antennas, are unbalanced by their 
surroundings, and that imbalance causes current on the two wires to be 
unequal. The difference between the two currents is a "common mode" 
current, it radiates just like the current on the outside of coax, and 
by reciprocity, receives noise and couples it to the radio as a 
differential signal, just like with coax.

A major shortcoming of antennas fed with open wire line is that it is 
not practical to choke them to kill that common mode current.

Another important point. The SWR on a line is determined entirely by the 
match between the line and the load -- when we're transmitting, that's 
the antenna, and when we're receiving, it's the receiver. And yes, it's 
same everywhere along the line as long as the line is uniform -- that 
is, the same impedance, and with no stubs or matching networks.

73, Jim K9YC

On Fri,11/4/2016 11:03 PM, Alan Bloom wrote:
> The SWR is the same anywhere on the feedline, assuming the loss is 
> low. If changing the feedline length a few feet causes a significant 
> change in SWR reading, then either the directional coupler in the SWR 
> meter isn't doing a good job, or (more likely) there is feedline 
> radiation. In the latter case, the feedline is effectively part of the 
> antenna; that's why changing the length changes the SWR.