Rap 'n Tap
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From: Charlie Cotterman
Date: 11/13/00
Time: 7:00:08 PM
Remote Name: 131.238.48.96
I'm trying not to sound like a dimbulb here...
Figure the length of coax needed. Unwind the same length from the base of the helical. Connect the center to the helical; let the shield float. Inside, do the same. Now, your wire length is the equivalent of having the set at the base of the antenna.
OR -- two lengths of coax taped together. Connect the shields to ground, also one center conductor. Now run the coaxes inside; you have the equivalent of shieleded open-wire feed (100 or so ohms with 50 ohm coax -- use cable TV coax for a 150 ohm setup). See if anything changes when the shields are connected/disconnected from ground (but not each other) inside and outside alternately.
Various quotes from "Crystal Radio:History, Design and Fundamentals" by P.A. Kinzie (available from the Society!):
"A conventional 356pF variable capacitor with a 250uH coil can be used with a long antenna to tune through the entire broadcast band...Shorter antennas require additional inductance, capacitance, or both, to tune below about 800 to 1000 kHz..."
"A medium to long length antenna requires a variable condenser in series with the antenna and antenna coil. A very short antenna requires that the capacitor be connected in parallel with the coil. In some transitional cases it may be necessary to provide switching to accommodate both positions."
"...a set that is very selective...requires that either the variable capacitor be on the ground side or that it be thoroughly isolated from the front panel controls (due to hand capacity)."
What you need to solve the problem is not necessarily a feedline issue -- it is probably much more of a tuning issue. Remember, us hamature radio ops have been keeping radios happy seeing 50 ohms when that doesn't resemble what's really there in the slightest. When in doubt, just hook up that run of decent-quality 50 ohm stuff, bring it in the house, and start tuning from there. Just remember, it's easier to tune a long antenna shorter than it is to tune a short antenna longer.
Charlie