[Test-Equipment] Does it really make a difference over the short haul ????
David Smith
w6te at msn.com
Fri Sep 4 10:47:22 EDT 2020
Doug,
That article was an amazing find! I've been in communications for over 55 years and never read an explanation like that. It makes very good sense.
Thanks, 73,
Dave W6TE
-----Original Message-----
From: test-equipment-bounces at mailman.qth.net <test-equipment-bounces at mailman.qth.net> On Behalf Of Doug Hensley
Sent: Friday, September 4, 2020 7:27 AM
To: test-equipment at mailman.qth.net
Subject: Re: [Test-Equipment] Does it really make a difference over the short haul ????
Thanks everyone for the input on this. Both the experience reports on HF as well as the explanation of physical differences helps me understand the strategy that I need
to adhere to. Part of my operating is QRP so small differences do matter.
While looking for connector options, I stumbled across this remarkable article by Belden. If you have not read this, I highly recommend you take a coffee break and read it. Wow, it is quite a perspective.
https://eur05.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.belden.com%2Fblog%2Fbroadcast%2F50-ohms-the-forgotten-impedance&data=02%7C01%7C%7C81e116718af142aca6cf08d850dea9a2%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637348264540391413&sdata=I%2BSRaIBnzweiCdbSp1EmckJozhKxFLU5LnlADchalGc%3D&reserved=0
Enjoy and leave some comments if you can.
Doug W5JV
----------------------------------------------------------------------
Message: 1
Date: Tue, 01 Sep 2020 21:44:31 -0700
From: Steve <k0xp at k0xp.com>
To: Discussion of Electronic Test Equipment
<test-equipment at mailman.qth.net>,
Subject: Re: [Test-Equipment] Does it really make a difference over
the short haul ????
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
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