[600MRG] Estimating Rr for non-confirming verticals

[Edward R Cole]

I ran the calculator and got close to what I see 
using average ground condition for 13m high by 
37m long 2-wire top hat with 0.6m separation.

5W EIRP with 1.469A antenna current (I measure 
1.4A), 61.7w drive (I use about 100w), ground 
resistance 25ohm (I measure 20).  Rr=0.525 (EZNec = 0.8)

One difference is I have three parallel vertical 
wires spaced 1-foot apart vs one in the formula 
for lazy-L (two top wires) mine are spaced 2-foot 
(0.6m).  All wires are shorted together an both ends.

The thicker vertical lowers Q a little helping 
get a wider bw (I get 5-KHz SWR<2.0).  I have 
four very short ground radials (avg 21m long).

I use about 2/3 of a 650uH base loading coil.

73, Ed - KL7UW

At 02:50 PM 7/17/2020, Ben Gelb wrote:
>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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73, Ed - KL7UW
   http://www.kl7uw.com
Dubus-NA Business mail:
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