[Test-Equipment] Does it really make a difference over the short haul ????

[Steve]

At 05:00 PM 9/1/2020, Doug Hensley wrote:
>This question is for the experts on this list.
>
>The Trompeter Company in California for many years manufactured 
>patching panels that were used by both military & broadcast 
>operations.  Some were 50 ohm but most were 75 ohm for video patching.

I don't recall: did they actually use 75 ohm BNCs? You can easily 
tell them from 50 ohmers: the center pin is much thinner than a 50 
ohm center pin. If you attempt to mate a 50 ohm male to a 75 ohm 
female BNC, you will probably break the female sockets as they are 
too small for the "huge" 50 ohm pins.  Most 75 ohm BNC male 
connectors I've seen do not have a center insulator in the outside 
end of the barrel, like 50 ohmers do, another way to tell them apart.

But if you try to mate a 75 ohm male to a 50 ohm female socket, the 
thinner 75 ohm center pin may not make good, consistent contact with 
the huge 50 ohm female socket pin. This also means that if you were 
to use such mismatched connectors to, say, pass high voltage (don't 
laugh; I've seen some hams use BNC cables as substitutes for the HV 
power cables on such as the Heathkit HA-14 Kompact KW amplifier), 
then a poorly-contacting connector will sometimes arc internally 
inside the female socket pin, making the iffy connection even poorer. 
In a wild case where I saw BNC cables being used at 400 watts on 432 
MHz, one male connector got so hot it actually ruptured... it split. 
You could peel the parts off.

IIRC, Hewlett Packard made their own 75 ohm BNC connectors for their 
equipment; and IIRC, the DEPTH of some of them was also slightly 
different; it seems to me that a center male pin was slightly longer 
than an equivalent 50 ohm male pin. HP would NOT honor their warranty 
if you tried to plug a 50 ohm male BNC connector into a test 
equipment, such as a HP-8590 cable TV analyzer, which was provided 
with a 75 ohm BNC female socket on the front panel.

75 ohm type Ns have the same smaller diameter pins compared to 50 ohm 
type N connectors. You absolutely cannot mate a 50 ohm male N to a 75 
ohm female socket; the socket pin will split and at least one of the 
four pieces short to the outer shell.

 >Question 1:  Given 50 ohm cables are used front & back, does using 
a panel that is rated
 >for 75 ohms really amount to a hill of beans regards ham use?

Assuming 50 ohm connectors were actually used, no, it won't make any 
difference that you can readily measure below 30 MHz. At 144, you 
possibly might measure an impedance bump with a Vector Network 
Analyzer but just a small increase in loss with frequency with a 
Scalar NA. At 432 MHz, however, all bets are off. First off, I 
wouldn't use any Trompeter product at 144 MHz, let alone 432 MHz; 
they just didn't build them for such use. (I suspect Trompeter gets 
nose bleeds when they think above 100 MHz  8-D  )

>Question 2:  If the panel itself has negligible affects, can short 
>75 ohm patch cords be used for RF without degrading the signals?

If they have the correctly-installed 75 ohm BNCs, I wouldn't try to 
mate them with 50 ohm connectors at all, except in a dire emergency, 
such as at a hurricane center where you've been running on emergency 
power for 4 days and there's absolutely no possibility of finding 
another cable anytime within the next week or two (unless one happens 
to fall out of the sky, which I'm certain has happened).

But hams install 50 ohm BNCs on 75 ohm cable all the time; I've done 
it plenty of times, but it's sure a lot of hassle having to shave 
down the outer covering to make it fit into a standard 50-ohm ferrule.

Sure; you can combine 75 ohm cable with 50 ohm systems; just don't 
expect it to be as low-loss (which is an oxymoron  when it comes to 
BNCs, anyway  8-).

SteveH, K0XP