[600MRG] A dilemma....

[Marten T Beels]

Ken,

This may be a slightly different thing then what you're looking for, but 
based on the following idea a 5-band dipole can be built with one set of 
traps.  The idea is not operate the traps near resonance, but far from 
resonance and use the resulting capacitive or inductive reactance to 
resonate the antenna.

The best that I can find is an article by Yardley Beers (W0JF) from 
August 1987 that refers to several equations in the appendices.  I can 
find the original article online, but not the appendices.

http://www.k0to.us/HAM/Dual%20Dipole/Beers%20-%20dual%20band%20dipole.pdf

If you know the inductance of your traps, you could calculate the 
reduced length of the lower frequency antenna with something like this:

http://www.see.ed.ac.uk/~jwp/radio/software/loading.html

or this:

http://www.m0ukd.com/calculators/loaded-quarter-wave-antenna-inductance-calculator/

(same formula).

Marten

On 08/31/2015 01:00 PM, 600mrg-request at w7ekb.com wrote:
> Message: 1
> Date: Mon, 31 Aug 2015 03:16:16 -0000
> From: "Kenneth G. Gordon"<kgordon2006 at frontier.com>
> To:600MRG at w7ekb.com
> Subject: [600MRG] A dilemma....
> Message-ID:<55E3C700.27693.1A7996 at kgordon2006.frontier.com>
> Content-Type: text/plain; charset=US-ASCII
>
> OK, Folks, I need some ideas here.
>
> First, "The Situation".
>
> Due to on going construction of an addition to our home, I had to take down my only antenna.
> That antenna was a trap vertical, containing ONE trap at the 33' level, and a "stinger" 21' long
> above that for an overall length of 55', and it is fed with 50 phm coax.
>
> The antenna exhibits an SWR of under 2:1 over all ham bands, except the high end of 75
> meters, and the lowest end of 10, excluding the WARC bands.
>
> Even so, I used it on all bands 80 - 10 including WARC bands with a home-brew antenna
> coupler. I could add a 3 or 4 turn coil at the bottom and could achieve an SWR of 1:1 through
> the coupler on 160 meters, but only for about 10 kHz, then I would have to adjust the tap on
> the base coil.
>
> Since that antenna amounted to 1/2 of a single-trap-dipole, with the other "half" being the
> "image" antenna below the vertical (helped along by several 66' long radials), I had the idea
> of constructing 1/2 of a 160 - 10 meter trap-dipole and installing it as an "end-fed sloper with
> counter-poises" with the "far" end in the top of a 107+ foot tall Grand Fir tree in our back yard.
>
> Since the overall distance from the lower attachment end of the proposed sloper, to the top of
> the tree amounts to about 137 feet at an angle above the ground of about 65 degrees, and
> the overall length of 1/2 of a 160/80/40 meter trap-dipole I found on the ARRL's website in an
> article is a little more than 62 feet long, I thought that by adding a 160 meter trap, I could
> (maybe) load the full 137 feet on 630 meters too.
>
> The big trouble is that although calculating values of components for the traps is not too
> difficult, I have not yet found any way to calculate the lengths that the wires should be
> between, or AFTER the traps.
>
> The traps act like loading coils on the band below which they are "tuned" for, thus shortening
> the wires required to resonate the antenna at the lower frequencies.
>
> For instance, the 40 meter trap inductance acts as a loading coil to make the following length
> of wire necessary to resonate an 80/40 meter trap dipole or vertical at 80 meters shorter by
> around 30 percent (21 feet vs 33 feet), and the effect appears to be cumulative, since, when
> adding a second trap tuned to 80 meters, the length of wire between the 40 and the 80 meter
> traps falls to 11 feet (from 21 feet) to make the antenna resonate at 80 meters, and the
> length of wire AFTER the 80 meter trap required to resonate the antenna on 160 meters is
> only 18.3 feet.
>
> So, how the heck does one calculate what the length of wire should be between the 80 meter
> trap and the 160 meter trap when one adds that 160 meter trap in order to use the entire
> length on 630 meters?
>
> The inductances of the traps is, or can be, known, so this may help.
>
> But this brings up another point: apparently, it is the RF impedance of the LC combo at the
> operating frequency, NOT the resonance, which makes all this work together, but how does
> one calculate that?
>
> I'm stuck at this point.
>
> I suppose I could figure it out "emprically", but that takes a LOT of effort and time, which I
> don't feel I have at the moment.
>
> I have searched the web for several days, and have found a lot of useful websites with JAVA
> enhanced calculation "engines" but none, so far, provide the info I need.
>
> Anyone?
>
> Ken W7EKB