[600MRG] Resonant frequencies and loaded antennas

[Kenneth G. Gordon]

On 12 Sep 2015 at 7:52, Pat Hamel wrote:

> Ken,
> It depends on the length and diameter of the wire, the quality of the end
> insulator, and the surroundings for capacitance.
> In the downloads of the www.500kc.com there is a copy of part of an old NBS
> publication. It  will have more than you need, but I think it will be able to
> get you close. Or you can guess 7 picofarads per foot for thin wire and go from
> there. Insulation on the wire decreases the resonance a small amount. Pat W5THT
> & WD2XSH/6

Well, Pat, doing a lot more "research", I am finding out things I suspected, 
but never really knew before.

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.

Adding capacity hats of any sort reduces that necessary inductance to 
practical levels.

When I tried to calculate the necessary inductance I would need here at the 
"end" of my 140 feet of wire to resonate it at 630 meters (476 kHz), the 
program I found on the web instantly "moved" my coil back to 126 feet, then 
calculated I would need 3357 uH of inductance.

>From an article by Hall in Sept 74 QST, I find I can fairly accurately calculate 
where to put a loading coil in an antenna such as my proposed one....for one 
or two bands...but this does not address traps.

My trouble is that I am trying to optimize a multi-band trap antenna covering 
160 through 10 meters, PLUS 630 Meters.

I have an article on a "shortened" trap-dipole for 160/80/40, only 124 feet 
long, which resonates on all three bands (and possibly higher too, but that 
has not been tested). The fellow uses traps which are resonant at the center 
of the desired bands.

I had thought of regarding the 160/80/40 meter antenna above as a single 
hunk of wire 124 feet long, then by adding a trap tuned to 160 meters, using 
the entire length plus the top-loading coil, on 630 meters.

BUT, the inductance, alone, of the three traps undoubtedly add, electrically, 
to the length of wire, and, in addition, adding the 160 meter trap adds 
additional inductance, all of which must be considered for 630 meters.

I had thought that I may be able to subtract the inductance of the traps from 
the inductance of the loading coil for 630 meters, but I am certain that is not 
accurate either.

But I have no idea of the effect of the capacitive reactances of the traps on 
the electrical length of wire either.

Then there is the matter of the wire between the traps and the effect on 
those of the proposed 160 meter trap.

According to another article I was shown, by Yardley Beers (SK) in Aug 87 
Ham Radio magazine, AND according to yet another source I found on the 
web, building traps which are resonant far outside any ham-band results in 
an antenna which is far more efficient than using traps resonant within a ham 
band.

Yardley used traps which were resonant at the geometric mean of the two 
bands: i.e., about 6.5 MHz for 80 and 40.

The trouble with this is that one must include both the capacitive and 
inductive reactances of the trap components in the calculations, which, it 
turns out, are NOT trivial. (Why am I not surprised?)

Anyway, what I am trying to accomplish may not be possible with the limited 
formulae and understanding I presently have.

And attempting such an antenna by "cut and try" would take forever.

Thanks again for your very helpful suggestions.

Ken W7EKB