[600MRG] Estimating Rr for non-confirming verticals

[J Mcvey]

 I'm no fan of ladder line, because there are all kinds of extra variables it introduces into the picture.( most people who have antenna problems are using ladder line)
The actual feed point ( at the antenna dipole )  is sure to be very capacitive and the Rr is probably less than 1 ohm on 630 meters.

Now you have a massive mismatch at the feed point, so the length of the transmission line makes a difference as to what impedance is presented at the shack end.
The efficiency of such a system is just about nil.
However , they did have the " lightbulb dx challenge" where the participants used lightbulbs as their antennas -and they made contacts! Anything is possible...

    On Friday, July 17, 2020, 5:37:38 PM EDT, Ben Gelb <ben at gelbnet.com> wrote:  
 
 Hi all -

I decided to try feeding my HF dipole (ladder-line fed) as a vertical
on 630m. I did so, and it works. But I'm wondering a bit about how
best to estimate Rr, given that the ladder line feed is not actually
vertical. The first 20 ft or so are close to vertical, followed by a
roughly 40ft slanted section (45 degrees-ish, though not a straight
line since it is not held taught - so it follows a catenary curve).

Top load is 93.8'.

That is what I mean by "non-conforming".

So the question is how to reason about this antenna in service of Rr estimation.

Since the whole antenna is pretty small relative to a wavelength,
perhaps I can get pretty close by decomposing the antenna into its
vertical and horizontal components? The vertical component (at least
ignoring that the 45 degress section actually has a nonlinear shape)
would basically be the height of the dipole feedpoint.

The horizontal component of the ladderline section I imagine would add
to the effective capacitance of the top loading from the dipole
(though its more like "mid load" since its not at the top). Perhaps I
can estimate the increase in effective *top* loading length by
measuring apparent C of the antenna at the feedpoint - and backsolve
the equivalent *conforming* T-top antenna (w/ save vertical component)
that would yield that capacitance. Then use the Rr result for that
antenna.

Other thoughts?

I could also learn how to use antenna modeling software. But sort of
fun to try to think about how you might get there intuitively.

73,
Ben N1VF
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