[600MRG] Resonant frequencies and loaded antennas

[Edward R Cole]

Been reading the mail (and smiling a lot):

Now after have commissioned a "Cray computer to run a ten day 
computation", have you ever built to the spec and found the antenna 
exactly as planned?

Hams have long "cut long and snipped"!

I modeled my base loaded inverted-L with EZNEC5 and built the coil to 
the spec and then I hung the mess and measured resonant frequency --- 
and started moving the coil tap.  I ended up using about 70% of my 
coil for resonance.

Then I started looking for a 50-ohm match with tap near the ground 
and found it about 2-1/2 turns.  Looking down the 100-foot+ RG213 
feed the match in the shack was different (but OK).  Probably 
radiating everything from the shack to the end of the inverted-L 
(which is OK).  My worms are cosy in winter (also OK).

Someday I will look at tuning for 160m and install an additional tap vs. traps.

Probably never worry about using it on 80m as the invert-V works good enough.

I will someday contract clearing the weeds off the land and putting 
in a decent lawn at which buried radials will be attempted (be 
interesting to see how the resistance is affected vs my chicken-wire 
radials on ground (CWROG).

Leaves are falling and decide no more mowing so "CWROG" will be 
rolled out for winter; still have to repair and re-raise the wires (soon).

73, Ed - KL7UW

At 02:29 PM 9/13/2015, 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.  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
>_______________________________________________
>600MRG mailing list
>600MRG at w7ekb.com
>http://w7ekb.com/mailman/listinfo/600mrg_w7ekb.com

73, Ed - KL7UW
http://www.kl7uw.com
     "Kits made by KL7UW"
Dubus Mag business:
     dubususa at gmail.com