Revisiting N6TWW's Calculations..

In a preceding post, Mike, N6TWW, discussed the calculations he made in order to develop a phasing harness for a couple of VHF antennas. I borrowed on his numbers to further ingrain this poor boy's TDR technique.

(1) Speed of light = 186,000 miles per second.

(2) 186,000 x 5,280 = 982,080,000 feet per second.

(3) 982,080,000 / 1,000,000,00 = 0.98208 feet per nanosecond.

(4) 0.98208 feet = 11.78 inches.

(5) Light travels 100% in free space but travels only as fast as the velocity factor along a cable.

(6) The velocity factors of popular cables are:

      CABLE    -  VF
      RG-8     - .66
      LMR-400  - .85
      RG-8X    - .84
      RG-11    - .75
      RG-58    - .66
      LMR-195  - .83
      RG-59    - .82
      RG-62    - .84
      RG-174   - .66
      RG-213   - .66
      RG-214   - .66
      RG-217   - .66
      RG-218   - .66
      RG-316   - .79
      RG-400   - .695
      LMR-500  - .85
      LMR-600  - .86
      1/2 HARD - .81
      7/8 HARD - .81

(7) I got 2 hunks of cable I want to determine the length of. One is a long piece of RG-8X and the other is a shorter piece of RG-58. I have another hunk of coax that I want to verify the velocity factor of.

(8) For the longer hunk of RG-8X, I assume that the velocity factor in 84% so that means that when an electron travels through it it's only going 84% as fast as its cousin in free space. Consequently, it is traveling 84% of 11.78 is 9.98952 inches. So let's say 10 inches for grins.

(9) I lashed up the scope and the function generator like these two lads did and found that the RG-8X cable's out-and-back trip was 2.4 divisions at 100 nanoseconds per division. So 100 x 2.4 = 240 and half of that (only want one-way) is 120 nanoseconds. Thus, the cable is 120 x 10 inches or 1,200 inches long. Obviously, 1,200 inches is 100 feet. (Do I really have to?) And, guess what, that's what the guy at the swap meet sold to me: 100 feet of RG-8X.

(10) The shorter hunk was 1.6 divisions at 100 nanoseconds which works out to 160 nanoseconds. The one-way time, of course, is 80 nanoseconds and, since it's RG-58, the distance traveled in a nanosecond is 11.78 x 0.66 or 7.78 inches. So electrons traveling 80 x 7.78 inches or 51.8 feet. Let me get a tape measure and check that number.

(11) Now for the third hunk of coax, I know it's 22.75 inches long and I know that it "scopes out" to 0.7 divisions and 100 nanoseconds. Half of this (one-way trip) is 0.35 x 100 or 35 nanoseconds and 35 x 11.78 or 412.3 inches or 34.36 feet.

But that's the distance in free space, and I know that the coax is 22.75 feet long. So the velocity factor would be 34.36 / 22.75 or 0.662107 or 66% -- which is what they say RG-58 should be.

Funny thing, that!

-72-