"PaoloC" wrote Reg Edwards wrote:

A TWO-BAND ANTENNA WIRE LOADED WITH AN L & C PARALLEL-TUNED CIRCUIT

First of all, Reg, thank you for another simple and useful piece of
software!

The LC circuit behaves as an ordinary trap at its own resonant
frequency which, by experiment, can sometimes be arranged to fall
into a third amateur band. But to calculate such a fortunate
coincidence is beyond my elderly patience.

This comment and some test run of "Not a Trap" intrigued me. I own a 9m
fiberglass fishing rod, which I can now transform in a two-band no-tune
antenna (ie 40m and 20m). Adding a third band would be really cool.

(I originally thought of using your VERTLOAD program to build a two-band
vertical, where the coil behaves as a RF high-impedance and the lower
"half" of the radiator is 1/4 wavelength at a higher frequency.)

From a software point of view a little auto-optimization is needed,
which I am about to describe. I would be most grateful if you can either
write a notatrap2 or show me the way to produce such an optimization
software (I need formulas for computing L and C :-) ).

The algorithm.

The user enters: maximum available height, wire diameter, minimum and
maximum frequency, as now.
Then, instead of L-C height, the user enters the desired "bonus"
frequency. The order of magnitude of frequencies must obviously be in
increasing order:
- minimum (ie 7.030 MHz)
- maximum (ie 14.060 MHz)
- bonus (ie 28.060 MHz)

Given the 1/4 wavelength at the bonus frequency, the software computes
the L-C height, which is fixed. Then the optimization begins.

Starting from the maximum allowable height the software reduces
step-by-step (user defined?) the lenght of the upper part. At each step
the following are recomputed:
- L for minimum freq
- C for maximum freq
- LC self-resonant freq
If the latter is equal/close enough (user defined?) to the bonus
frequency, optimization stops and results are shown.

Then user can shorten/lengthen the rod in order to fine tune the bonus
point of resonance.

Did I miss something in the algorithm?

===================================
Hello Paolo,

The resonannt frequency of the LC tuned circuit, the trap, is calculated by
the program and displayed on the screen.

The only way of putting the trap's resonant frequency into an amateur band
is to experiment and vary the wire overall length, and/or the location of of
the trap along the wire until you have it where you want it.

I think it is impossible to have a program which allows all three
frequencies to be entered, plus the lengths of the two wires, which then
calculates the values of L and C. There are too many variables. And before
writing the program all the simultaneous equations have to be worked out and
solved using pencil and paper.

The program is really required to simulate the actions of somebody who keeps
pruning both wires, climbs up and down the ladder, and varies the values of
L and C hundreds of times. That is the best optimisation procedure. ;o)

Another disadvantage - the length of wire from the transmitter end of the
wire to the location of the trap will NOT be a 1/4-wavelength at the trap
resonant frequency as is usual for trapped antennas.

You will always need a tuner because the antenna's feedpoint impedance
varies so much between one band and another. It is not sufficient just to
have the antenna at resonance.
----
Reg, G4FGQ