[600MRG] Estimating Rr for non-confirming verticals

[Ben Gelb]

Thanks all for replies.

A couple of clarifications that might not have been clear from my first e-mail.

- Though the dipole is fed with ladder line, it is being fed as a
T-top vertical (i.e. ladder line conductors shorted together at the
base) when used on 630m.
- I've had no issue feeding the antenna and getting it on the air.
Have been QRV for the last week or so.
- My question is about estimating Rr (in order to estimate EIRP).

Thanks Roger for the calculator link and validating that my thought
process seemed reasonable at least (if it makes sense to at least 1
other person maybe not hopefully lost). :)

Interestingly, I use this calculator:
http://www.472khz.org/pages/tools/antenna-simulator.php

Result for 12m height, 28m top load length T-top do not give same Rr
from the two calculators.

VK1SV: 0.236 ohm
472khz: 0.385 ohm

That is a pretty significant difference! Any idea which one is right
(or what I did wrong)?

73,
Ben

On Fri, Jul 17, 2020 at 3:28 PM Roger Graves <ve7vv at shaw.ca> wrote:
>
> Hi Ben,
>
> Your idea to just use the vertical portion of the ladder line, the height of the top hat, for the vertical section length sounds good to me. That and the length of the top hat should give a good approximation.
>
> FWIW, I use the calculator at http://people.physics.anu.edu.au/~dxt103/calculators/marconi.php
> It has been quite useful for me to get predictions for the dB improvement expected from various changes (height, top hat length, ground R) to see what might be worth doing and not worth doing and how much power might be needed to get to 5W EIRP.
>
> Once you have your new antenna resonated with a loading coil, you can measure the inductance of the coil and compare that to what the calculator came up with for the L. You could then adjust the size of the top hat in the calculator to get it to show your actual inductance and then see how much the Rr and EIRP changed. That would, theoretically, give a better estimate. But there are so many complicating factors that the estimate is just a “ball park” estimate I would think.
>
> Looking forward to hearing the signal on 630m.
>
> 73,
> Roger
>
>
> On Jul 17, 2020, at 2:12 PM, 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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