[600MRG] Resonant frequencies and loaded antennas

[Frank Lotito]

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.  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."

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

73 Frank K3DZ / WH2XHA



________________________________________
From: Kenneth G. Gordon <kgordon2006 at frontier.com>
Sent: Sunday, September 13, 2015 5:48 PM
To: Frank Lotito
Cc: 600mrg at w7ekb.com
Subject: Re: [600MRG] Resonant frequencies and loaded antennas

On 13 Sep 2015 at 14:35, Frank Lotito wrote:

>
> Ken - I believe this statement of yours in error -
> "For instance, one cannot install a loading coil at the very top or end
> of any antenna, since the theoretically necessary inductance would be
> infinite due to the impedance at that point being zero."

Perhaps I worded it wrong: I took this statement as my understanding of a
formula I discovered during my researches, AND from a very similar
statement in one of those articles I have been reading.

I'll look for that and will post it here.

In any case, where voltage is maximum in an antenna, current is minimum,
as I understand it (and it makes sense to me).

In addition, I used an on-line calculator, which was built on a long formula
which was derived and published in one of the articles I mentioned above, to
calculate the inductance I would need if I placed a loading coil at the very
end of a 140 foot long piece of wire.

That on-line calculator would not accept an input to it of the placement of my
coil at the end of the subject wire of 140 feet: it automatically moved the coil
back from the 140 foot point, to 126 feet from "zero", then gave me a value
of about 3357 uH. At 126 feet, this would leave approximately 12 feet of wire.

Subsequent input of this info to EZNEC by a very, very helpful member of
the ham-antennas forum produced the result that my proposed antenna was
resonant at 515 kHz.

So, yes, the statement I made may be wrong, but possibly because I didn't
state the facts properly.

> At the very end of the antenna, resonant or not, the voltage is a
> maximum, and the current is a minimum.

Yes.

>  In my way of thinking, that
> means very-very high impedance.

Not to my way of thinking: to me that simply means a constant but finite
value of impedance of some particular non-infinite value.

> Also, with very short radiators (w.r.t. wavelength,) as we all know,
> large value loading coils are required to resonate the antenna.  As
> you move the inductor towards the end of the radiator the required
> inductance for resonance increases.

Yes. The reason for this fact was also explained in at least one of the articles
I read. I'll try to quote that here in another e-mail.

>  On the positive side, when the
> inductor is moved away from the feed point the wire between the
> feed point and input to the loading coil carries more current.  Carrying more
> current in the radiator is definitely desirable.

Because more of the RF is radiated and not lost.

>  BUT - the larger inductance
> generally means more losses in the coil.   So, moving the inductor towards
> the radiator's end is a balancing act between various factors.\

Yes. Most obviously. This is why building a loading coil with as high "Q" as
possible is the goal, and is not easily attained.

>  If you are
> pushing more RF Power, the balancing act now includes a whole plethora
> of variables related to the inductor's mechanical / electrical design which
> impacts heavily on the inductor's survivability in an outdoor environment.

Yes. Indeed.

There is another on-line calculator which enables one to choose the best
combination of many factors to build the most efficient possible trap.

I'll use that calculator for my 160 meter trap.

There is also another on-line calculator which enables one to build a coil with
the highest "Q". I have not yet used that one, but will shortly.

In the meantime, I'll dig out the article and will post the exact wording for
what I may have misquoted.

Thank you for your input. I have a LOT yet to learn.

Ken W7EKB