On Mar 5, 3:14 pm, (Dave Platt) wrote:
In article ,
Roy Lewallen wrote:

I highly recommend that you decouple the feedline with a current balun
(common mode choke). Better yet would be two, spaced about a quarter
wavelength apart. Small antennas can be made to look a lot better than
they really are if the feedline is allowed to become part of the
radiating antenna system, so unless the feedline is decoupled well, any
measurements you make will be subject to speculation as to whether the
radiation was from the antenna or the feedline.

It would also be interesting to use a simple current probe to sweep
along the feedline, and see what sorts of currents might be flowing
along the outside of the feedline, both with and without a good
common-mode choke or two in the feedline.


I agree. I think it's quite possible that even a decoupled feedline
will add enough radiation to make the antenna noticeably better
than it really is.
I'd prefer to see the source mounted *at* the antenna to make it
a decent comparison.
One problem with the decoupling that I can see, is if it's used on
160m, a quarter wave is about 125-130 feet plus or minus...
So even if you stack two chokes or baluns a quarter wave apart,
I can see the feedline noticeably radiating above the lower choke.
Of course, this condition will exist for all types of antennas,
but on the other hand, most antennas will be radiating quite
a bit more from the antenna, vs the feedline.
So the difference between the two sources are much larger,
and the feedline radiation is more likely to be swamped and
made a non issue as far as the total radiation.
But with such an inefficient small antenna, the feedline
radiation will be a much larger part of the total radiation.
My guess on the performance? Probably about the
same as the usual Isotron antenna used on 160m.
And even in the case of the Isotron, I bet a good portion of
the radiation is actually from the feedline.
I don't recall ever seeing any documentation of the Isotron
that recommended decoupling the feedline.
I don't have to stretch my mind to far to wonder why... :/
To quiet the naysayers, "like me", I'd mount a radio directly
to the base of the antenna with a coax union and
do FS tests at certain distances, and then likewise
with the full size antennas.
That would cure the feedline issue real fast.. lol..
Course, that may be more work than Denny wants to
take on..
You could compare it to a mobile set up though, and
keep things on the ground level for ease of testing.
I bet my mobile antenna would whip that thing.
It should, being it's bigger and has more efficient loading.
MK