Re: [IRCA] Cable connectors

[Russ Edmunds <wb2bjh@xxxxxxxxx>]

So what you're saying Craig, is that you used unshielded CAT-5, and your tests suggest that by running it along the ground ( as would be usual for beverages at the beach ) that tends to mimic the effect of a shield ?

And this to anyone - this discussion has thus far been oriented mainly toward stray RF, and the context has suggested stray broadcast RF. How would the use of CAT-5 as discussed handle electrical noise such as radiated TVI or power transformer emissions, etc. ?


Russ Edmunds
Blue Bell, PA ( 360' ASL )
[15 mi NNW of Philadelphia]
40:08:45N; 75:16:04W, Grid FN20id
<wb2bjh@xxxxxxxxx>
FM: Yamaha T-80 & Onkyo T-450RDS w/ APS9B @15'
AM: Modified Sony ICF 2010 barefoot


--- On Mon, 9/6/10, Craig Healy <bubba@xxxxxxxxxxxxxxxxxxxx> wrote:

> From: Craig Healy <bubba@xxxxxxxxxxxxxxxxxxxx>
> Subject: Re: [IRCA] Cable connectors
> To: "Mailing list for the International Radio Club of America" <irca@xxxxxxxxxxxxxxxx>
> Date: Monday, September 6, 2010, 5:59 PM
> > I'm curious to know about the
> construction of the RG-6 cable that you used for
> > your test?
> 
> Brand new, off the roll Belden RG-6 cable.  Shield as
> you described as well as the foam dielectric.  Junk
> RG-6 wouldn't be worth the effort.  In this case it
> made no difference from the new Radio Shack coax which is
> not top quality, but usually OK.
> 
> The cable was cut with an RG-6 specific cutter and the
> connector crimped on with specific hex crimp pliers. 
> DC ohm readings were quite low.
> 
> Another part of the test was to wind a dozen turns through
> a large ferrite toroid.  No difference at all. 
> Also tried a new "un-un" to do a ground isolation. 
> Again, no change.
> 
> > The problem with any balanced feedline is always
> line-balance. Since the
> > electric and magnetic field of the signals extends
> outside the physical area of
> > the twisted pair, there will always be interaction
> between pairs within a CAT
> > type cable. You would need far more spacing between
> pairs than is available in
> > the CAT cable design to prevent interaction between
> pairs.
> 
> I might agree with that except CAT5 is designed to limit
> the interaction. The individual pairs are tightly twisted,
> but not identical turns per foot for each of them. 
> What that does is cancel the interaction.  The four
> pairs are also twisted inside the outer cover which further
> limits pickup.  In the test, I only used a single pair
> and the others were floating.  I set it to use the
> Blue-Blue/White pair.
> 
> > laying the cable on the ground or near to conductive
> surfaces will also upset
> > the line-balance (for UTP cable.) And this prevents
> the common-mode rejection
> > of signals common to both wires of the twisted pair
> from canceling completely.
> 
> If they weren't twisted, that may well be true.  I
> also have to think that the loss of the cable on the ground
> would more greatly reduce the ingress than increase
> it.  While I didn't add this to the article, I did run
> a further test.  I laid out a 50' length and terminated
> it with a 100 ohm resistor.  That made the whole thing
> balanced when the receive end balun was included. 
> After taking a set of bandwidth readings, I laid a 50'
> length of chicken wire on top of the cable.  The goal
> was to provide some shielding via capacitance to the
> ground.  End result was no change at all.
> 
> > Shielded CAT cable is a different animal, outside
> effects being much less, but
> > the inter-pair coupling is still an issue.
> 
> I haven't found a source for the shielded cable or
> appropriate RJ-45 connectors and the different crimp
> tool.  I have had access to a 25' shielded cable but
> that again made no difference from the common type.
> 
> Bottom line to me is that the CAT5 is better than
> coax.  I have not done an A-B test with CAT5 and
> speaker wire.  I may get to that.
> 
> One of the more important things I've learned in my
> lifetime is to leave my ego outside.  If someone has an
> improvement, I'll grab that in a heartbeat. I do thank you
> for your comments on this.
> 
> Craig Healy
> Providence, RI
> 
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