Re: ladderline and balun question

Posted by Jim Dunstan on Jan 12, 2012; 1:49am
URL: http://elecraft.85.s1.nabble.com/ladderline-and-balun-question-tp7179004p7179305.html

At 04:12 PM 1/11/2012, you wrote:
>Gents,
>
>I have a ladder line and balun question that I hope someone can guide me
>thru.

A 1 wavelength loop represents approximately 100 ohms non reactive
impedance.  Feeding this antenna on its fundamental frequency is
relatively straight forward ... and it can be done with coax ...
using some tricks to get it down very close to 50 ohms.  However if,
as is commonly done the loop is used on harmonics making it multi
band the situation is much more complex and the feed is changed from
coax to parallel feed line as you have done.

The impedance at the feed point increases with harmonics with the
addition of an increasing reactive component.  It is common in such
antenna systems to make the transition from the parallel feed line to
coax as it approaches the shack .... since it is generally easier to
physically route it through walls etc ... and it is thought that it
radiates less than the parallel line (usually not the case).

It is possible to actually measure the complex impedance at the end
of the parallel line using an analyzer .... but inevitably there will
be a mismatch to the coax whether using a 4:1 or 1:1 balun.  which
will cause radiation from the coax line.  It is simply a matter of
determining if the amount of radiation from the coax section is
tolerable or not.  It is possible to bring the parallel feed line
right into the shack and connect it directly to an impedance coupler
.... and in fact this was the usual method for many years ... and I
have done this with no adverse RF effects in the shack.

The easiest method of using a large loop on its resonant harmonics is
to install the antenna coupler right at the feed point and use coax
for the full feed line length.  It is possible to make some guesses
as to the impedance at the end of the feed line at the different
frequencies given the length of the parallel feed line.  The
experience you describe would indicate the impedance is rather low as
the 1:1 balun worked well.

The most common strategy for this arrangement is the cut and
try  method.  Stick with what works .... there are no magic rules of thumb.

Jim, VE3CI

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