[600MRG] Estimating Rr for non-confirming verticals

Sun Jul 19 15:56:19 EDT 2020

Thanks all.

The VK1SV calc appears to use the 5pf/m (top load) and 6pf/m
(vertical) approximation for C (formulae 2b and 3b) in Rik's writeup.
Haven't spent anymore time on the 472khz calc to figure out what it is
doing... but perhaps will get back to it time permitting.

Now, I went and measured the loading coil w/ LCR meter and measured
332uH. Also used an air-coil calculator to compute expected value (10"
dia, 4.25" long, 33 turns) and got 312uH. So fairly close, so pretty
confident within 10% anyway.

This would imply ~340pF of capacitance from the antenna (vs. 272pF
predicted by VK1SV calc). So about 68pF more than predicted. If I
consider that the horizontal component of the ladder line section
traverses approx 13m or so and blindly apply the 5pF/m value, that
comes out to 65pF.. which is pretty close (actually, its a bit too
close, since I probably haven't established antenna height within
better than 20% accuracy...). Perhaps next thing is to measure height
a bit more carefully.

Once the pFs are adding up, I guess next question is to think about
what it means to have "top load" distributed across the diagonal
section of vertical. Presumably this means that the segments of
vertical closest to the ground "see" the greatest amount of top
capacitance (and probably have higher current and therefore an outsize
share of the radiation resistance). Seems like for a true diagonal,
should be fairly straightforward to come up w/ an average and use that
to figure Rr.

On Sat, Jul 18, 2020 at 2:03 AM Rik Strobbe <rik.strobbe at kuleuven.be> wrote:
>
> Neil is right, it is ;-)
>
> I found out that using this formulas results is a relative small error (in most cases < 10%) compared to simulation with MMANA.
>
>
> 73, Rik  ON7YD - OR7T
>
>
> ________________________________
> Van: 600mrg-bounces at mailman.qth.net <600mrg-bounces at mailman.qth.net> namens Roger Graves <ve7vv at shaw.ca>
> Verzonden: zaterdag 18 juli 2020 7:44
> Aan: Neil Klagge
> CC: Posting 600m 600mrg
> Onderwerp: Re: [600MRG] Estimating Rr for non-confirming verticals
>
> Hi Neil! Thanks, those could well be the formulae.
>
>
> On Jul 17, 2020, at 8:34 PM, Neil Klagge <w0yse at msn.com> wrote:
>
> Hey Roger, I think the ON7YD calculations for Rr were based on his Capacitance formulas from this page
>
> http://www.strobbe.eu/on7yd/136ant/#CapTop
>
> Sent from Neil's iPad....
> ________________________________
> From: 600mrg-bounces at mailman.qth.net <600mrg-bounces at mailman.qth.net> on behalf of STEVE MCDONALD <ve7sl at shaw.ca>
> Sent: Friday, July 17, 2020 6:13:18 PM
> To: Roger Graves <ve7vv at shaw.ca>
> Cc: Posting 600m 600mrg <600MRG at mailman.qth.net>
> Subject: Re: [600MRG] Estimating Rr for non-confirming verticals
>
> Neil has a nice methodology page here that can be used to find Rr, or get you in the ballpark:
>
> https://w0yse.webs.com/wg2xsvpage.htm
>
> Steve  VE7SL
>
>
> ----- Original Message -----
> From: Roger Graves <ve7vv at shaw.ca>
> To: Ben Gelb <ben at gelbnet.com>
> Cc: Posting 600m 600mrg <600MRG at mailman.qth.net>
> Sent: Fri, 17 Jul 2020 17:51:44 -0600 (MDT)
> Subject: Re: [600MRG] Estimating Rr for non-confirming verticals
>
> Ben,
>
> Thanks for the link to the other calculator.
> I ran my antenna dimensions in both and got similar values to what you reported.
>
> The 472 kHz calculator is estimating a greater topload capacity (and lower required inductance) for the same antenna dimensions.
> The VK1SV calculator page says it uses formulas from an ON7YD webpage, but my scan of that did not reveal which formulas might be used.
> The 472 kHz calculator says nothing about what formulas it is based on.
> So it is not possible to compare the models.
>
> When you measure the inductance of your loading coil, which calculator’s prediction came closest to what you measured?
>
> I like the VK1SV calculator better b/c it does not try to estimate the coil loss. What I can measure is the total system R, which is the sum of (at least) ground loss, coil loss, and antenna radiation resistance (which can be neglected given the size of the other losses at MF/LF). That is the value that the VK1SV calculator requests. IDK what my ground loss by itself is, which is what the 472 calculator requests.
>
> Roger
>
>
> > On Jul 17, 2020, at 3:50 PM, Ben Gelb <ben at gelbnet.com> 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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