On Thu, 29 Aug 2013 17:13:39 -0400, "Ralph Mowery"
wrote:
"Jeff Liebermann" wrote in message
.. .
Drivel: All of my 50 ohm antennas on my roof are connected to their
respective radios with 75 ohm RG-6/u coax, F connectors, and various
adapters. Also some RG-6/u with BNC compression connectors. No
problems and very little additional mismatch loss.
Often simple dipoles are closer to 70 ohms than 50. Not enough to make any
differance in most ham instalations.
Actually, the mismatched RG-6/u can be better than the properly
matched RG-58c/u. For a given diameter, 75 ohm coax has less loss
than 50 ohm coax. 50 ohms has the advantage of being able to handle
more power, but at the expense of some additional loss.
http://www.microwaves101.com/encyclopedia/why50ohms.cfm
However, what was referring to was mismatch loss.
Voltage_reflection off of a 75 ohm input:
Vr = (50-75)/(50+75) = 0.2
Voltage transmission into 75 ohm input:
T = 1 - (0.2^2) = 0.96
Converting to decibels, the loss will be:
Mismatch_Loss = 20 Log 0.96 = -0.35 dB
mismatch loss. Note that the mismatch loss is independent of the
length of coax cable.
If you compare various common cables with RG-6/u, the benefits of the
better RG-6/u coax are obvious. 0.35dB of mismatch loss isn't going
to make much difference when there's 2 to 5 dB/100m difference in
attenuation.
http://vk1od.net/calc/tl/tllc.php
RG-8x = -12.6 dB/100m at 150 MHz.
LMR-240 = -9.89 dB/100m at 150 MHz.
RG-6/u = -7.78 dB/100m at 150 Mhz.
I saw on youtube where you could take the rg-6 and after you strip it back ,
wrap about 6 turns of duck tape around it just where the outer jacket stops.
You only need a strip about 1/2 or 1/4 of an inch wide. Then you can fold
the braid back and use crimp connectors designed for rg-8 size. That helps
solve the aluminum jacket problem with the PL259s.
Retch. There's no way to tightly crimp a few layers of tape.
Compressing the tape will cause it to cold flow at the glue junctions,
eventually causing the tape to slither out of the connector. Add a
little hot weather and the connector falls apart as the duct tape wrap
unravels. I know because I've done tricks like that eventually
failed. However, several layers of shrink tube might work because
shrink tube doesn't slide.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558