[600MRG] Resonant frequencies and loaded antennas

[Frank Lotito]

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."

At the very end of the antenna, resonant or not, the voltage is a maximum, and the current is a minimum.  In my way of thinking, that means very-very high impedance.


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.  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.  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.  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.


GL -


73 Frank K3DZ / WH2XHA

________________________________________

From: 600MRG <600mrg-bounces at w7ekb.com> on behalf of Kenneth G. Gordon <kgordon2006 at frontier.com>

Sent: Saturday, September 12, 2015 5:38 PM

To: Pat Hamel

Cc: 600mrg at w7ekb.com

Subject: Re: [600MRG] Resonant frequencies and loaded antennas

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<http://www.500kc.com> there is a copy of part of an old NBS

The 500 KC Amateur Radio Experimental Group
Check out the 600 Meter Band Viewer. This allows you to watch signals on the 600 Meter Band between 495 and 510 KHz in 5 minute segments in near real time
Read more...<http://www.500kc.com/>



> 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

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