coax length

[coolbreeze]

Time and time again I have made a comment about how coax length does matter.
Booty had very often argued the issue. But in this post he sucked up to the guy...
I disagree with fact 4 though. 1-3 shows why fact 4 is bull shit

this was posted by some other tool bag

The electrical half wave length of LMR-400 at 27.205Mhz (middle of the CB band) is:

15.365 feet (15' 4 5/8")

6 electrical half wave lengths would be 92.2' (92' 2 1/2")
7 electrical half wave lengths would be 107.5' (107' 6")

FACT #1:
This takes into consideration that the LMR-400 has a perfect velocity factor of 85%. It may, or it may not. In reality, the velocity factor of a piece of coax can vary across the same exact reel of cable! Coax is not perfect ! This is why phasing cable manufacturers will use a network analyzer and trim the coax down to the perfect length for that piece of coax.

FACT#2:
That electrical half wave length of coax is tuned specifically for the frequency you choose. It will be too short for channel 1 and too long for channel 40. The length you have will work fine for any frequency just as a 100' piece of coax works fine for any frequency. After all, 100' is an electrical half wave length of some frequency, and people use that all the time, right?!

FACT#3:
The ONLY advantages of using electrical half wavelengths of coax are twofold;

* Your reading at one end of the coax is IDENTICAL as the reading at the other end of the coax, but only on the frequency that cable was cut for. If you build your antenna for channel 20 and have it in the air, its easier to stand on the ground to take your SWR reading then hanging from under the antenna 60' up.
* For phasing two or more antennas.

I have an electrical half wave length of coax I use when building I-10K antennas cut for 27.205MHz. This allows me to stand on the ground, away from the antenna, and test the feedpoint for R-50 and X-0 on 27.205. As the analyzer teaches us, as we move away from that frequency, the readings will change. But it will be dead nuts on 27.205! I've tuned a couple up at 27.025 and 26.915, but only used a very short jumper for those. Again, the reality is that its close enough.

FACT#4:
The correct length of coax you should run from your antenna to you shack is; The shortest length that will reach! Again, look at the two examples in FACT 3 above for using an electrical half wave length of cable. If these are not relevant to your operations, don't worry about them! Why do I say, "the shortest"? The longer the cable the more the loss!

I use electrical half wave lengths for testing purposes and phasing antennas. I use what ever I have on hand for everything else. I hope this helps.