How to measure cable loss with V655

[ai8p]

All,

Well, I got quite a surprise today - I measured the feedline for my DXEngineering multi-band wire antenna. I discovered that it isn't 300 ohm, it is 330 ohm (I can live with this) but I also discovered that VF isn't 0.88, as claimed, but 0.95 (this I am less happy with, as I will need to add to my feedline).

However, I can't figure out how to measure the loss of the feedline with the AIM software V3.5.5. I went back to V 3.5.0 and used Measure Cable to get loss. I made a shorted connection at the end and left it shorted, even when it asked me to make sure that it was Open (I want loss, not Zo, so this shouldn't matter).

I got some interesting results - loss was reported as:

1Mhz - -.249DB
1.8Mhz -.421
2 Mhz -.842
3.5Mhz -.32
4 Mhz - .511
7 Mhz -.465
7.3 Mhz -.481
14 Mhz -.728
14.3 Mhz -.541
21 Mhz -.308
21.45Mhz -.430
28 Mhz -.605
29 Mhz -.341

OK, I'm confused - how do I measure loss with 3.5.5, and what do those crazy loss measurements mean?

de Dennis, AI8P

[Bob]

Hi Dennis,

The original "measure cable" function has been replaced with "Distance to Fault". You can read the cable loss in the data window on the right side of the graph. Move the cursor along to see the loss at any frequency when the cable is open or shorted.

For balanced transmission line, it may be benificial to use a 1:1 balun to convert it to unbalanced at the RF port. Baluns wound with coax will add some length to the overall measured transmission line length.

If you're using a laptop computer, try running it on battery for critical measurements so that it's floating.

-- 73/Bob

[ai8p]

Bob,

OK, I see that now but a couple questions.

Is that Short/Open Cable Loss the loss for the length of cable analyzed or is it converted to per 100 feet?

Any clues on why the cable loss doesn't increase monotonically with frequency as I expected it to?

Thanks

Dennis

May 26, 2008 10:48:06 GMT -5 Bob said:

Hi Dennis,

The original "measure cable" function has been replaced with "Distance to Fault". You can read the cable loss in the data window on the right side of the graph. Move the cursor along to see the loss at any frequency when the cable is open or shorted.

For balanced transmission line, it may be benificial to use a 1:1 balun to convert it to unbalanced at the RF port. Baluns wound with coax will add some length to the overall measured transmission line length.

If you're using a laptop computer, try running it on battery for critical measurements so that it's floating.

-- 73/Bob

[Bob]

Hi Dennis,

The loss is the actual loss of the cable being tested, not the loss per 100 feet.

The line may be interacting with nearby structures in such a way that the loss is a more complex function of frequency. Make sure the value for Zo is set to the Zo of your line.

73/Bob