your 56' tower is resonant at about 4.1 MHz - Give or take a bit. I'm
modeling it with a 1' x 1' four legged tower.
To shunt feed it on 3.5 MHz a #12 wire running 1' from the tower and attached
at the 32' point looks good. It takes an 97 pF series capacitance to tune out
the Gamma inductance.
To shunt feed it on 7 MHz a #12 wire running 1' from the tower and attached
at the 26' point looks good. It takes 24 pF to tune.
Relays can be used to switch the feedline from one GAMMA to the other.
A single Gamma can be used at the 28' point with a motor driven variable
capacitor. It looks like the minimum VSWR will be 2:1 on each band. A tuner at
shack can be used to touch it up.
If you'd like I'll input more accurate dimensions for your tower. And
depending on your ground we can add a bit of base resistance to make the
more true-to-life. Then we can adjust the Gamma-to-tower spacing to find a good
compromise for both bands.
TowerTalk mailing list