[600MRG] Resonant frequencies and loaded antennas

[Kenneth G. Gordon]

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