Peculiar. I didn't start seeing 50 commonly until the 70's onward. It
really threw me, because much equipment, and documentation prior to that
(probably going back another thirty years) was 52, and I was sweating it
before I realized, it didn't matter. Radios with 52 Ohms stamped next
to the connector, filters, accessories, et cetera.
Does one suppose it depended on the industry? Doing some research
today, there's no end to the references, and debates of 50/52, so it's
still somewhat of a big deal out there.
I no longer have direct access to JPL, but my library is filled with
obscure documents, and reports collected over the decades. My most
recent acquisitions are, Potential End-To-End Imaging Information Rate
Advantages of Various Alternative Communication Systems, Some Practical
Universal Noiseless Coding Techniques, Reed-Soloman Encoders -
Conventional vs Berlekamp's Architecture, Some Practical Noiseless
Encoding Techniques Part II, and, Channel Coding And Data Compression
System Considerations for Efficient Communication of Planetary Imaging
Data, 1969 Flight Projects... I used to grab everything, but now I
focus on things I'm more likely to play with on my own or may provide
insight to some of the items I have.
Along the lines of antennas/radio, I picked up NASA TN D-5081,
Performance Characteristics Of The Apollo Astronaut Backpack Antenna.
How this wound up in Israel is probably an interesting story I may never
know. No impedance is referenced.
Back to impedance... I don't have a JPL lab here, per se, however, over
the years, I grabbed whatever JPL, CalTech, Lear, McDonnell Douglas,
Boeing, Shiley, places with locations, but no names, from Long Beach to
Tehachapi, et cetera, didn't want. Pallet, after pallet, after pallet
of interesting, and peculiar items, some going back to day-one. One
interesting older seemingly radio related item was a 40kW rotary spark
gap. You'd think radio, but it was part of the oscillator for a carbon
dating machine.
So, I dug out a few "50 Ohm" items. This is particularly interesting:
Kay attenuator, model 300, spec'd at 50 Ohms, measures 52.4
Daven attenuator, RFB-451-50, spec'd at 50 Ohms, measures 52.0
DB Products load, spec'd at 50 Ohms, measures 51.3
DB Products load #2, specs at 50 Ohms, measures 51.4
Unknown model load, something aerospace related, 52.5
Unknown load, spec'd at 50 Ohms, measures 330. Clearly has issues.
Identical model, ~47.5, erratic, hard to get a fix, may also have issues.
Microlab attenuator, spec'd at 50, measures 53.2
Tossing out the two that may have issues, that's 52.13 Ohms average,
which matches the old standard, regardless of labeling.
Now it gets more interesting. I recall in the 70's, the wars over 50/52
Ohms. 52 was the norm, 50 was really coming into play with consumer
level items. Why, exactly, from 52 to 50, I can only speculate. I
heard, ease of manufacturing, quality control, marketing, all sorts of
things, but why an industry, precisely, shaved 2 Ohms after decades of
manufacturing, and millions of products, and accessories, I never heard.
The 50/52 battle was over loss, and "getting out". '52 Ohms resistance
is more than 50, so that means more loss. More loss in the cable means
less power getting to the antenna. NEVER use 52 Ohm cables or antennas
with 50 Ohm radios. You won't be heard well, if at all, 'cuz it messes
with the SWR, and you may even fry your radio!' So, if one has an only
option of what to never use, but common, what becomes the new standard?
50, even if it's really the same old 52 stuff, just re-labeled, and
close enough.
I pulled out three more loads. These are Archer, made in the late 70's,
or early 80's for consumer stuff, specifically, CB. They measure 50.8,
49.2, and 50.4, 50.13 average. 52.13 earlier, and 50.13 now? Thinking
my measuring device is off a little bit... Still two quite distinctive
numbers following two periods/levels of standards. So 50 Ohms doesn't
really enter the picture until we get about as far from Aerospace
quality as we can, 70's-80's relatively cheapo-CB stuff, as now measured
decades after the fact.
Interesting that your JPL items, and my various items are "50 Ohms".
Mine are labeled one way, measure another. Did you measure yours, or
just read the label? If they really are dead-on 50, I wonder what the
application was, or if it just made the math easier to guesstimate.
Fortunately, cable-wise, 50, and 51 Ohm is common, as is 75. They are
either close enough to 50, or 52, or don't matter (75). I've had a
recent project that uses 92 Ohm (someone else's project, no shortcuts
with oddball impedances), and a pile of 125 Ohm in storage. I never
found a use for the 125.
One last test. Signal Corps, UG-529/U impedance matching unit made by
O.E. Szekely. 53.4 Ohms on the BNC end, 122.6 on the twinax. I suspect
this is supposed to be 52, or 53 to 120, or 122, I forget the exact spec
now, and it's not labeled further. Either way, closer to 52, than 50.
There's an example on ebay, with a designation of type 507-A, with
different specs, 53 to 95.
Kurt
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