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.
One thought on “Does it matter how long my coax line is?”
Awesome info! Many thanks to whoever it is that composed and posted this. I’m sure many readers (especially newer hams) will get a lot of benefit from this post and others like it. It’s a very concise and understandable piece. Bravo!
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