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.
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.
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.
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.
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
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.
But we're also doing less and less coax and more and more Category cable
nowadays.
No MoCA installations?
http://www.mocalliance.org
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.
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
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558