Topband: Verifying integrity of 75 ohm coax.

[Kenneth Grimm]

Gary,

A good reminder for me as I plan to walk my beverages today checking for
problems.  I'll take along my F connector wrench and make sure all are
tight.

73,


On Mon, Nov 18, 2013 at 2:00 AM, Gary Smith <Gary at ka1j.com> wrote:

> 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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> _________________
> Topband Reflector
>



-- 
Ken - K4XL
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