[600MRG] A dilemma....

[William E. Isakson]

Ken, Just figure that the "traps" are low pass filters and calculate each
band as needed.    Also, for the lowest bands, if you put a loading coil to
resonate the band in the center of its path, that will get you the most
effective antenna for that band.   You cannot reach that high to adjust it,
of course, so you under-rate the high up coil and put a smaller one at the
base to resonate that one.   You will want to bypass that to work on the
lower bands.   However, I think that it is better just to pick the few
bands that you actually use and make the antenna for those.  Also, if you
want the high bands, you want something with more gain.
Bill Isakson


--------


Bill Isakson     AC6QV
Roseburg, Oregon USA
bill.isakson at gmail.com



On Sun, Aug 30, 2015 at 8:16 PM, Kenneth G. Gordon <kgordon2006 at frontier.com
> wrote:

> 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
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