[600MRG] 200 ohms

[Ben Gelb]

Thanks all for the comments!

Amplifier is putting 600 watts into the antenna, so even with low
efficiency, EIRP is around 1 watt. Hooray for EIRP-based regulation! I'm of
half a mind to change to squeeze another few dB out of the amp... though it
does seem kind of obscene already.

I was using the calculator at
http://www.472khz.org/pages/tools/antenna-simulator.php, but I like the
VK1SV one (thanks Roger) since it backsolves all the parameters based on
the measured Z.

Marshall - the tree conductivity result there is interesting. Picking some
assumptions:

- suppose antenna contacts a 3 foot long 2-inch diameter branch (which
later connects to the tree trunk, which for now assume is 0 ohms)
- pick worst case 1.0 mho/meter conductivity from the result Marshall
shared (high end of the range)

So... according to Google:
3 feet / (pi*1 inch)^2 / (.1 mho/meter) = 143.604827 ohms

I don't think putting a 143 ohm R in parallel with the radiation resistance
would move the feedpoint Z that much... (0.2 Rr would win), and that
assumes perfect coupling of a very conductive tree! So I am starting to
think maybe its not the tree that is to blame (at least not much). Or maybe
I'm not thinking about it right?

Guess I just need more radials :(.




On Tue, Nov 7, 2017 at 8:00 AM, Roger Graves <ve7vv at shaw.ca> wrote:

> Ben,
>
> You have a great signal for your situation. What is your power?
>
> Re. your question, get more vertical height, more capacitive top loading,
> longer radials.
> When I raised my vertical from 30 to 40 ft there was a big improvement.
> Try to extend the radials to the ends of your lot (and beyond if the
> neighbors allow). More may not help (unless you can get to use the other
> two 90 degree directions) until you get them longer.
>
> The feedpoint resistance should go down as you do the above as an
> indication of effectiveness.
>
> Are you using the calculator at
> http://people.physics.anu.edu.au/~dxt103/calculators/marconi.php
>
> Thanks for the QSO last night!
>
> 73,
> Roger
>
> On Nov 6, 2017, at 8:24 PM, Ben Gelb <ben at gelbnet.com> wrote:
>
> My small city lot features a single 30 ft tree, which is currently
> supporting my 30' x 30' inverted L (other end of the horizontal segment
> supported by a 20' fishing pole strapped to a fence post ... so maybe 25'
> in the air).
>
> The tree is at the corner of the lot, so the radials are really just over a
> 90 degree sector (rather than a full circle around the vertical). I also
> only have 3 radials probably each about 20 ft long, with a 10' ground rod
> at the end of each one. There is also a 10' ground rod at the base of the
> vertical.
>
> So, it ain't great. But is what it is on a small city lot.
>
> Feedpoint impedance is ~200 ohms (and with 472khz.org antenna calculator
> estimating ~0.2ohm radiation resistance, thats an efficiency of maybe 0.1%,
> give or take).
>
> So naturally I've been thinking about how to improve the setup. One thing
> is that the tree really is supporting the antenna (wire dragged right up
> over it). Its HV-insulated wire (20kV) to prevent any arcing to the tree
> and burning behavior (pre-HV wire version didn't fare too well, but the
> smolders have been successfully eliminated with the HV wire upgrade).
>
> I guess I am wondering if the tree itself may be impacting the performance
> of the antenna (capacitive coupling to the tree?) and contributing to the
> high feedpoint Z, or if its really all just terrible ground that is to
> blame.
>
> Your thoughts appreciated!
>
> 73,
> Ben, N1VF
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