[600MRG] Resonant frequencies and loaded antennas

[Kenneth G. Gordon]

On 13 Sep 2015 at 22:29, Frank Lotito wrote:

> 
> Ken - One more tid-bit:  Over the past 15 +/- years I've been tinkering 
> with numerous versions of the purchased low-bux ELNEC and then 
> EZNEC antenna modeling software.

You might also consider MMANA-GAL, which I have used a little, and have 
so far found it easier to use than EZNEC and at least as accurate.

>  Over these 15 years a few 
> times I've e-mailed W7EL regarding modeling electrically short, low 
> to the ground antennas w.r.t. wave length.  Roy's reply was almost 
> always the same, and very thorough without burying me in theory I 
> could not begin to appreciate: Paraphrasing from memory (God only 
> knows where I tucked Roy's replies on this topic, HI)- "its extremely 
> difficult even for the top $$$ antenna modeling programs to 
> accurately model these types of antennas.   An very difficult area to 
> model the true nature of the ground beneath the antenna.  The best 
> you can hope for is insight on which direction you should be moving 
> in."

Well, that is basically all I am looking for: insight on which way to move. I 
think at this point, I have arrived at that point.
   
> 
> I am convinced for small / short / low to the ground antennas my 
> EZNEC estimates are just that, estimates.  However, for a few other 
> full sized antenna models where I actually constructed the antenna, 
> the EZNEC model was dead nuts!  In particular, my 5 element 
> 6-meter yagi with a gamma match feed!  Another was an elevated 
> radial 15 meter ground plane antenna with a series capacitor base 
> match and radiator length around 108 degrees (or what ever it was. 
>  That antenna is history.)

I used MMANA-GAL to model an antenna I suggested that AL7N use in 
place of the worm-warmer he had been using. Results turned out to be 
surprisingly accurate. All involved were very pleased with the results.

> 
> I think we should all ask ourselves what are the sources of errors 
> before we take a tool and blindly run with it.

Oh, Indeed!!!!!

>  That includes circuit 
> modeling software, antenna modeling software, and most definitely, 
> test equipment!  Its relatively easy to determine the accuracy of our 
> "lived a charmed life / never been dropped Bakelite case Simpson 
> 260 VOM."

Ha! I have one of those...

> Most of us, me included, can't even begin to characterize 
> the accuracy of devices such as network analyzers, spectrum 
> analyzers, yet alone plow through the mathematics of the more 
> esoteric stuff like antenna modeling software.   

Me either. I am a simple fellow and really appreciate simple answers.

At this point, the only thing I am still unsure about concerning my proposed 
multi-band antenna is how one calculates the length of the wire needed 
between the traps.

At this point, I am simply going to go with what I have, and try it out.

FYI, this is a 140 foot long, overall, sloper. Slope angle about 67 degrees 
(Brunton compass). Height at the top end, about 110 feet give or take the 
hawk's nest. Height at the bottom end, close to 12 feet above the ground.

I'll be using this antenna:

https://www.arrl.org/files/file/Technology/tis/info/pdf/9207035.pdf

adding a trap for 160 meters at its end, then continuing with wire to a 3357 
uH loading coil for 630 meters, then an addition 12 feet of wire.

Modeling the 630 meter antenna only shows resonance at 515 kHz.

I also intend to install a remotely tunable base-loading coil to enable 
"tweaking" on 630 meters, switching it out for HF.

I'll add tuned counterpoises for HF, and an extensive ground system for 630 
meters, then will attempt to feed the entire thing with LMR-400 and an 
antenna coupler.

All this remains to be seen. It will certainly be very interesting if it works. 
Maybe I'll publish an article on it so folks will be more inclined to think that 
630 meter operation is possible from a normal small city lot.

Thanks for all the help, folks.

Ken W7EKB