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Re: Topband: Modeling close to earth ( was Odd-ball question)

To: topband@contesting.com
Subject: Re: Topband: Modeling close to earth ( was Odd-ball question)
From: Artek Manuals <Manuals@ArtekManuals.com>
Date: Sun, 25 Aug 2019 10:34:03 -0400
List-post: <mailto:topband@contesting.com>
Not enough coffee earlier a better description of the radials: three 90ft radials radials  7'ft high spaced in 120 degree increments  (0, 120and 240 degrees)

Another interesting side notes discovered while doing the modeling
1) A three radial version in a 180 degree half circle (0, 90 and 180) or what I like to call the "up against the fence", resulted in very much the same modeled impedance and only slight pattern distortion ( less than 1db) , Predicted radiated field strength  by adding the forth radial at 270 degrees only resulted in an increase in signal in that direction a few tenths of a dB

2) Inverted L's   pattern distortion can be non trivial depending on the length of the top section. The signal decreases in the direction the top section "points". In the past I had a preference for having very long top sections ( Total length: vertical + horizontal on the order of 150')  so as to increase the real portion of feed point impedance to 50ohms and then tune out the increased reactive portion with a series cap. Depending on how long the top section is signals in the direction of the pointing top of the L can be down by as much as 3 to 6dB !!. At the new QTH the best tree supports resulted in a 6db pattern decrease directly pointed a Europe 8^(... Which is why I ended up with a "T" instead of an "L".)

Dave
NR1DX

On 8/25/2019 8:25 AM, Artek Manuals wrote:
Jerry et all

My personal antenna FOR 160 was/is as follows

160M T:
  60' Tall with with a 78' flattop. The "bottom"  is at 7' ( so the actual vertical element is 53') with three 90' radials at 7' spaced pretty nearly in 60 degree increments ( 60, 120, 240). There is a tapped inductor in series with the ground side (NOT the vertical side) of the feed point used to resonate the whole thing . For the general approach to loading and using non-resonant radials I was strongly influenced by K5IU's work in Spring 1997 Communications Quarterly  (google should  find you reprints of that) . The vertical is #14 bare stranded copper and the radials are #10 Bare solid copper ( no magic on sizes etc just what I happened to have on hand.  Ezenec  says this should be around 13 ohms at 1840 resonance ( with a 23uh series inductor) . In practice it is closer to 25ohms at resonance  with a about 17uh of inductance.

It is then matched with a 2:1 UNUN to 50 ohms. This antenna REALLY NEEDS and uses a common mode choke at the feed point . This turned out to 10 to be ten FT-240-43 torroid's with 4 turns of coax through the batch, lesser amounts of ferrite became quite hot.

After the smoke clears it all appears to work very well I have worked 24 countries on 5 continents ( still need Antarctica and Asia...tough on any band from FL) in just 4 SUMMER months and I am sure that total will climb quickly once the winter season starts. For the FT8 doom and gloomers half of these were on CW....8^)

Happy to share details of the 80 and 40 M antennas off list, not relevant  to the topband list !

Dave
NR1Dx
manuals@artekmanuals.com




On 8/25/2019 1:33 AM, K4SAV wrote:
Dave

Just a clarification.  I didn't want your actual data.  All I wanted was the length of the wires and the frequency.

Jerry

On 8/24/2019 9:30 PM, K4SAV wrote:
Hey Dave

I'm interested in your data.  How long were the radials and what frequency were you using for the measurements.

I suspect that NEC2 may be close enough to be generally useful (accuracy is questionable) for a BOG up to 250 ft on 160.  My measurements (several of them) say that NEC isn't close for a 350 ft BOG.  Usually my BOGs are 1 to 2 inches above the dirt because they sit on dead grass.

I seriously doubt that NEC4 will be accurate for a 350 ft BOG either.  I have seen a 450 ft BOG pattern generated by NEC4 and I can duplicate it with NEC2, (with only minor insignificant squiggle differences) and I know that NEC2 is wrong.

I suspect that as frequency decreases or the wire becomes shorter, NEC answers will improve.

A data point from someone else would be nice to know.

Jerry, K4SAV


On 8/24/2019 7:53 PM, Artek Manuals wrote:
Chuck et all

It is well documented that the the NEC-2 based programs leave something to be desired� with wires on or very near the ground, This includes most of the EZENEC� family and MMANA-G� Purportedly NEC-4 ( there is a Ezenec version which runs with NEC 4 engine ... not a cheap date) does deal with the near earth problem .

How close is "Close" is a matter of conjecture. A friend of mine and I have been working on building and modeling vertical antennas ( Verticals, Inv-L and T's) for 160/80/40 with ELEVATED NON-RESONANT radials at 3' and 6' (google "K5IU Elevated Radials") . The good news is at 3' and above both NEC 2 and NEC 4 models agree within 5% or better.� We have not done any comparisons below 3'

Dave
NR1DX


On 8/24/2019 1:41 PM, Chuck Dietz wrote:
I seem to remember someone saying the modelling programs are unreliable when a wire is close to the ground. Also, there is really no way to model the properties of "ground." It can vary in just a few feet and the moisture content varies from day to day. I think this is a "try it" kind of antenna.
Read other's reported results.

Chuck W5PR








--
Dave
Manuals@ArtekManuals.com
www.ArtekManuals.com


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