On 1/12/2014 12:29 AM, Jeff Liebermann wrote:
On Fri, 10 Jan 2014 22:55:41 -0500, Jerry Stuckle
wrote:
That's theoretical. Reality is much different.
I think I'll print that on a large piece of paper and have it framed
for my office wall.
Have you ever worked
with a TDR? It's one of the tools we use regularly (and an expensive
one, also).
Sure. Also OTDR for fiber. I don't use the expensive stuff, unless
you include the Acterna HST3000 tester that was stolen from my office
a few years ago. Mostly, I built my own using a pulse generator and
an oscilloscope.
I'm not talking about fiber, and I'm not talking about some half-assed
gadget cooked up in your basement. I'm talking about a real,
professional-grade TDR.
We use expensive stuff because it gives accurate results.
You stated that the losses come primarily from the "impedance bump"
presumably produced by the non-50 ohm connector or adapter in the
line. Well, time to put that to the test.
I didn't say anything about a non-50 ohm connector. The connector can
be 50 ohm and still get an impedance bump. With a TDR you can see every
connector on the cable (as long as they are far enough apart - typically
3-4 feet).
1. Take a length of RG-6/u coax with good connectors. Apply signal
on one end from a generator. Measure the signal level on the other
end with a spectrum analyzer that has a 1dB per division scale. I
want to be able to see small changes.
2. After establishing a reference level, either beat on the coax
cable with a hammer, squash it with a C-clamp, or flatten it with a
bench vise. Squash it just enough to obtain an "impedance bump", but
not a short between the center conductor and shield. Note the change
in level, if you can see it.
3. Now, either un-squash the cable, or find another length of RG-6/u
and attach a TDR. It can be open, shorted, or terminated. Doesn't
matter.
4. Flatten the cable in the same manner as before and note the TDR
display.
What I expect that you'll see is that there will be almost no change
in attenuation, while the TDR display will show a rather radical
"impedance bump". That's because the only thing that the change in
impedance along the cable length can do is create reflections. Those
are a serious problem in a CATV system, but really don't involve
serious signal level losses.
That's theoretical. Reality is much the same.
First of all, we don't crush cables to try to emulate something we can
easily see in real life.
Second of all, you can gimmick up something all you want with signal
generators, spectrum analyzers, crushed cables, all you want.
We SEE the results in real time in real installations with the
appropriate test equipment.
So? Dipoles aren't 50 ohm antennas. They're typically closer to 75 ohm.
I guess you missed my point. If you can tolerate the 1.5:1 VSWR, the
reduced attenuation and cost of 75 ohm coax makes the effort
worthwhile.
No, it's worthwhile because your coax will more closely match the
antenna. And you can easily match the coax to the transmitter with a tuner.
As for handling more power - rubbish. The current in 75 ohm coax is
lower than that in 50 ohm coax, for the same power rating.
Please re-read the article:
www.belden.com/blog/broadcastav/50-Ohms-The-Forgotten-Impedance.cfm
http://www.microwaves101.com/encyclopedia/why50ohms.cfm
I read the article. Unlike you, I understand what it is saying.
We use RG-59 where appropriate, like from an outlet to the set top box.
But our in-wall coax runs are all RG-6 quad-shielded.
RG-59/u doesn't have a foil shield, so it's more flexible. So, I
guess that might work. For short lengths, the increased leakage,
higher ingress, and higher attenuation can probably be tolerated.
However, I use extra long RG-6/u, which somewhat compensates for the
lack of flexibility.
You don't know our installations. We've been in business for several
years, with lots of satisfied customers. Trying to tell a professional
what their job is only makes you look stupid.
But we're also doing less and less coax and more and more Category cable
nowadays.
No MoCA installations?
http://www.mocalliance.org
Nope. It's used by few professionals. Coax ends up with too many
problems. Category cable for runs under 100M or so and fiber for longer
runs.
They're OK for the hobbyist, but I don't know of any professionals who
use them. In fact, checking our main wholesalers, they aren't even
available through them (but other Beldon products are).
Find a different distributor or ask for it by part number.
Thomas and Betts SNS1P6U.
We use several distributors - both U.S. and international companies.
Quick check: Stocked by Allied, Newark, Tessco, Farnel(UK), MCM.
Not stocked by Digikey, Mouser, Arrow.
T&B distributor search:
http://www.tnb.com/ps/dls/dls?ca=corp
These are consumer (retail) outlets. They are not used by most
professionals.
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Jerry Stuckle
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