Does it matter how long my coax line is?
Ideal world: No.
Real world: Very possibly.
Unless you have a near-perfect 1:1 SWR at the antenna or unless you’re intentionally making a feedline-based matching system, you do not want your feedline to be a quarter wavelength long at your operating frequency or any ODD multiple of a quarter wave. It will be more difficult for your tuner or transmitter to match. If you have a perfect 1:1 SWR match at your antenna, the length doesn’t affect tuning. If there is any mismatch at the antenna (not 1:1 SWR), the feedline length does affect tuning. The effect is most pronounced at or near a quarter wavelength of feedline.
How long is a quarter wavelength of coax?
Example: Let’s try 3.8 MHz.
- Calculate the free space quarter wavelength: 246/3.8 = 64.7 feet
- For non-foam coax the Velocity Factor is usually about 0.66. (Foam is 0.88) So 64.74 x 0.66 = 42.7 feet.
So, for operation at 3.8 MHz you want to avoid having your coax feedline being at or near 42.7 feet or 128.1 feet (3 x 42.7) or 213.5 feet (5 x 42.7) and so on.
Then what DO I want?
Generally, the best length is a half-wavelength of coax (or EVEN multiples of a quarter wave).
Back to the example of 3.8 MHz. We know that the quarter wavelength of non-foam coax is 42.7 feet.
A half-wave is double that: 42.7 x 2 = 85.4 feet or 42.7 x 4 or 42.7 x 6. (Getting kinda long at that point!)
Q: What if I only need 40 feet to reach my station? What should I do with the extra?
A: Roll it up.
Q: Won’t 40 feet work?
A: If you can tune it and work stations, yes. That’s what counts. But it will probably tune easier and be less fussy at 85 feet.