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).
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