I am working on setting up my second tower (40 foot HDBX VHF/UHF tower) with shunt
feeds for 30m, 75m, and 80m.
Last night I worked with the Nec2go model
of the tower (modeled by N0FP) trying to figure out how I wanted to build the
matching networks. I played with the idea of adding a top hat (wire) to the tower
to see what would happen.... not a good option. I then swept the models and put the
data in a spreadsheet so that I could start looking for the components that I will
need to assemble this. After hunting around online I decide that I would make a trip
to the Consignment Center early Saturday morning. Armed with my spreadsheet, MFJ
analyzer, and some alligator clips.
I am glad that I decided to measure the components at frequency. A capacitor could
move 80-100 pf between 3.5mhz and 10.1mhz. I ended up buying some doorknob caps, some
medium sized air variable caps, and three really nice inductors. I should have enough
pieces to match 30m, 75m, and 80m. I am going to make up a little test system so that
I can take it to the base of the tower to tune it up and measure it to figure
out the semi-final alignment. I have the variable caps if needed but I would prefer
to use the doorknob caps if possible.
I spent about an hour at the consignment center and enjoyed the time looking for parts.
Having the computer model, plus the spreadsheet, and the analyzer made it a more rewarding
trip to the metro.