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

[Artek Manuals]

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 at 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 at ArtekManuals.com
www.ArtekManuals.com


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