Topband: Verifying integrity of 75 ohm coax.

[Tom W8JI]

> I just downloaded the manual on PDF and read the "advanced" section
> dealing with distance to fault. Sorry to say I must not be as sharp
> as I used to be and I can't tell from what I've read if the RG6U
> needs a termination for this distance to fault test or if the coax is
> unterminated. Since it's the weekend I can't call MFJ but perhaps
> someone knows this answer & can let me know so I can have at it
> tomorrow.

They probably would not know the answer to that.

The distance-to-fault algorythm uses the repeating of minimum reactance at 
minimum impedance as frequency is changed to calculate distance to fault. 
The further the distance to the lump, the closer-spaced the ripples become. 
If you pick the closest-spaced dips you will get the most distant "bump". If 
you misterminate the cable, obviously you would see that mistermination 
distance as a fault distance.  It is best to terminate the far end with a 
75-ohm resistor. (I always keep 75-ohm F terminators and a few barrel 
connectors in my "field bag" for testing cables.)

This would also be true if you used a rig or anything else and looked at 
SWR. You would really have to terminate the far end in 75-ohms, or whatever 
the cable is, to not "fool yourself". Otherwise, as you moved along the 
cable or as you changed frequency, you could see "lumps" where there really 
are no "lumps".

You don't want to check a system that is misterminated at the far end from 
the cable's surge impedance, unless you really have a good grasp on how 
these things work or you are using a TDR. I can work around it, but it is a 
lot more painful for me than just using a correct termination. :-)


> Apparently I'll need to find the velocity factor of this particular
> RG6U and then can do the math they describe. I just need to know if I
> test this on an open coax or with a 75 ohm carbon resistor at the
> distal end of this 75 ohm coax (or 50 ohm on RG-8). And, does it
> matter if I use either of the  SO239 or the BNC ?

The MFJ269 allows you to enter velocity factor, but it is simple to hand 
calculate it with the 259. If you are off 10% in the Vf, it isn't a big 
problem on shorter cable runs. Use the resistor. The BNC is only for the 
frequency counter mode.

You said you have birdies. What is the nature of the birdies? Are they 
signals that mix, like BC stations beating together, or is it wide buzzy 
junk from power supplies and stuff??? You might not have a bad connection. 
It might be in the antenna system or receiving electronics hardware.

73 Tom