[TowerTalk] To get a truly balanced antenna feed

[jimlux]

On 6/21/16 11:21 PM, Jim Brown wrote:
> On Tue,6/21/2016 9:31 PM, Roger (K8RI) on TT wrote:
>> Didn't you once say "there is no such thing as a balanced antenna"?
>
> Yep, but I was speaking more in terms of perfection -- i.e., that in the
> real world, most practical antennas are unbalanced by their
> surroundings, even if we attempt to make them a balanced antenna and
> feed them with 2-wire line and a so-called balance tuner. It is that
> imbalance that causes common mode current. If the imbalance is
> relatively small, and if the antenna is resonant and fed with coax, we
> can choke it effectively.
>

Which brings up an interesting thing..
one reason that folks want to use a balanced transmission line is that 
it's low loss (mostly because it's high Z, so the current is low) 
because the antenna is not well matched.  You can use a tuner in the 
shack (or the plate output tank on a tube rig) to do the matching 
without paying a huge penalty in transmission line loss.

On the other hand, it's hard to make an effective choke for open wire 
line, compared to the simplicity of just putting a bunch of turns of 
coax through a suitable core.

I'd also contend that a lot of the claims of "excessive loss" or "low 
loss" are based on an over simplified model of the system: someone looks 
up the "loss per 100 ft" number and uses that. But that number usually 
assumes an integral number of wavelengths.  In a mismatched line, the 
current and voltage fluctuates along the line, so the loss per unit 
length varies (at HF, the dominant loss is IR loss).