On 03/21/2014 08:49 AM, Jeff Liebermann wrote:
On Fri, 21 Mar 2014 07:54:51 -0700, Jon Danniken
wrote:

Speaking of cable impedance, will I see much of a difference using 70
ohm cable on an 50 ohm antenna? Assuming less than 50 feed of distance,
does it really matter that much?

I use 75 ohm coax for most everything and recommend using 75 ohm
cable. It has less loss per length than the equivalent size 50 ohm
coax. It's also cheaper and more available. Like you, I have piles
of the stuff scrounged from CATV "surplus".

Biggest headache are the adapters needed to go from (waterproof)
F-connectors and BNC/UHF/N connectors. Some of my RG-6a/u cables now
have BNC connectors instead of F-connectors, which helps a little. I
still prefer the cheaper F-connectors.

I have a couple dozen BNC ends, but I need to get some female/bulkhead
connectors to go along with them.

Another headache is interfacing with 50 ohm test equipment. I used to
have a pile of elaborate pads, and simple 25 ohm resistor in series
adapters. Unless I'm working with very low losses and measurements to
3 decimal places, or am trying to work with low VSWR systems, mixing
impedances doesn't seem to matter much. I've also measured various
pads from my collection at both 50 and 75 ohms, and found about 0.3dB
difference, which is about the accuracy of my pads. These days, I
just ignore the problem and use the pads interchangeably.

Incidentally, the usually quoted 0.18dB mismatch loss is based on the
assumption that the antenna and the transmitter are both 50 ohms, and
that only the coax cable is 75 ohms. 0.18dB is the loss at one end of
the cable, not both. Worst case is twice the loss, or 0.36dB.
Since the antenna will be closer to 75 ohms than 50 ohms, we can
ignore that end. The receiver input impedance is also not a perfect
50 ohms, so that can also be ignored. Bottom line is to not worry
about the whole mismatch loss question.

More on 50 versus 75 ohms:
http://www.solred.com.ar/lu6etj/tecnicos/En_75.htm
http://www.belden.com/blog/broadcastav/50-Ohms-The-Forgotten-Impedance.cfm
http://www.microwaves101.com/encyclopedia/why50ohms.cfm
http://www.dkdinst.com/articles/50ohmnotes.html

Great links, thanks Jeff.

Jon