[TowerTalk] 1/4 wave lines. What frequency should they be cut to?

[Tim Duffy K3LR]

The discussion about cutting "perfect length" 75 ohm 1/4 wave lines reminded me
about the fact that in many cases you really want to cut your lines higher in
frequency than where you ultimately want them to be. Why? Any lead length to
antenna elements or internal switch (relays) etc. is in series with your phasing
lines and is additive and lengthens the "real" line length. This "extra length"
lowers the frequency at which your line is 1/4 wavelength (or 1/2, 3/4, etc.).

For every "inch" of "extra" lead length, PC board trace or relay RF wiring, here
is what happens:
On 160 meters the shift is 1 KHz lower in frequency.
On 80 meters the shift is 4 KHz
On 40 meters the shift is 17 KHz
On 20 meters the shift is 65 KHz
On 15 meters the shift is 150 KHz
On 10 meters the shift is 270 KHz

I have about 2" of lead length on most of my custom made RF switches.
I looked at the the effect of the RF switch wiring on phasing line length with a
scope and came up with the following correction chart
That I use with my "hot target" frequencies:
On 40 meters that I want to be perfect at 7.050 MHz, I cut them for 7.084 MHz
On 20 meters for 14.100 MHz my lines are cut to 14.250 MHz
On 15 meters for 21.125 MHz my lines are cut to 21.450 MHz
On 10 meters for 28.250 MHz my lines are cut to 28.790 MHz

I hope this helps!

SUMMERTIME IS FOR SAFE ANTENNA BUILDING

73!
Tim K3LR

http://www.k3lr.com