Topband: Verifying integrity of 75 ohm coax.

[Gary Smith]

Gentleman, and I say that honestly... :)

I have a resolution to the issue and along with it my thanks for 
those who offered really solid ideas to track down. & more to the 
opportunity to learn, I have much apparently to learn about my 259B 
which is more than just a pretty face.

However, the cause of the majority of my problems were the F 
connectors being somewhat loose in the RX array. They were all 
connected to the coax properly, it was the connections to the many 
different chassis' involved; It's beyond me how so many of them were 
not tight and I have more than my share of OCD and leaving so many 
loose makes me wonder if I did close them tightly in the early winter 
last year but not tightly enough and perhaps with cooling/heating 
they loosened some more. 

Yeah, that seems like a stretch but whatever it is, there has to be a 
reason they were all loose when I always crank down on outdoor 
connectors to keep water out. That looseness was the cause of most of 
my issues. I wonder if a diode/rectifier effect might have taken 
place and was creating a signal due to the proximity of Amtrak with 
it's catenary suspended 15KV power supply. I'm not enough of an 
engineer to figure that one out but regardless, tightening solidly 
all the F connectors and cleaning the grounds to each antenna in the 
array made all the difference.

Thanks again to everyone who offered helpful suggestions, it's all a 
learning experience to pass on to the next guy. Or hopefully YL.

73,

Gary
KA1J
> > 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 
> 
> 




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