[TowerTalk] Need Quality RG6 cable

[K9MA]

It makes sense that they wouldn't waste expensive copper on the RG6. 
center conductor.

I built a shielded loop out of RG6, and completely forgot that the 
shield braid was aluminum, until I tried to solder it. I ended up just 
crimping a terminal onto it.

FWIW, I recall that 75 Ohms minimizes loss, while 50 Ohms maximizes 
power capacity. No doubt that was for a solid dielectric, and may be 
different for foam.

73,
Scott K9MA


n 3/11/2021 3:04 PM, Richard (Rick) Karlquist wrote:
>
>
> On 3/11/2021 5:57 AM, Dave Sublette wrote:
>
>> recommendation for good quality 72 ohm coax with foam dielectric for 
>> a V/F
>> of 78% or so.  It has to be foam in order that the feedline be long 
>> enough
>> to reach the center of the array where the control box is located. The
>> connectors on the box are SO-239.  The PL259 can be soldered or 
>> crimped, I
>> don't care. But if soldered, shielding must be copper.
>>
>> So 72 ohms, 78% v/f, good quality.  Where to get it?
>
>> Dave, K4TO
>
> 75 ohms is what is generally available, not 72 ohms.
>
> Everyone likes to talk about insertion loss.  This is actually
> specified.  Some people, like you, like to talk about velocity factor.
> That may or may not be specified.  What seems to be overlooked
> is characteristic impedance, listed as "nominal characteristic
> impedance."  What out for that word "nominal".
>
> I recently built an antenna project using RG/6 from a big
> box store because I was in a hurry.  I got strange results
> and eventually I tested a 100 ft piece of it with a quality
> 75 ohm termination on the end.  Over frequency, the VSWR
> oscillated between 1:1 and 1.3:1.  I tested other pieces of
> it, and they were all very repeatable, but not 75 ohms.
> This coax was proudly marked "3 GHz, for satellite TV".
>
> I then tested some coax that I got VERY cheaply on ebay.
> It had less than 1/2 the mismatch.  Who knew?
>
> Although the original question was about an 80 meter antenna,
> my work involved 160 meters.  When the frequency got down to
> the 160 meter band, both samples of coax exhibited an almost
> doubling of the mismatch, which I suspect is related to the CCS
> center conductor.  I believe what is going on is that the
> Zo is going complex, and exascerbating the problem.  Remember
> that steel is magnetic!
>
> This problem is still under investigation at this time.
> Complex Zo is usually an indication of dispersion.  When
> making phasing lines, many people including me have been
> guilty of measuring the frequency where the coax is a 1/4
> wavelength, and then extrapolating that data point to
> determine what the delay is at, say 1830.  The trouble is
> that if the delay line is NOT 90 degrees, and has dispersion,
> then you will get the wrong answer.  You MUST measure the
> delay at the operating frequency to get it correct, when
> using dispersive line.  Fortunately, with today's VNA's,
> this is now trivial.
>
> Especially for 160 meters, it is probably better to get
> all copper line like RG-59 if you really want to get good
> results.
>
> Has anyone else measured characteristic impedance of RG/6?
>
> 73
> Rick N6RK
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-- 
Scott  K9MA

k9ma at sdellington.us