RadioBanter - 50 Ohm Vs 75 Ohm BNCs

In article .com,
wrote:

Telamon wrote (in part):

You need more bandwidth for digital signals because of the faster rise
and fall times. For data to be valid you need a large eye opening in
amplitude and time for margin.

There are two things of significance:
1. The amplitude of the discontinuity.
2. The length of time of the discontinuity.

#1 determines the amplitude of the reflection.
#2 determined the frequency it becomes significant.
--------------------------------------
I started this thread because starting now and increasingly
in the near future there will be "lots" of analog "75 Ohm" video
patch bays, and patch hairpins, and patch cables dumped as the
older ones degrade digital signals. At one local station they found
out they could go through one set normally normaled analog patch.
Inserting eithr a hairpin, short patch used to connect adjacent non
normally connected patches, or any patch cable "killed" the digital
data stream.

I was given a~25 year old 48 bay patch with hairpins and cables.
And while my current antenna system is "50 ohms", this bay passes
signals in the .1`~30MHz range with "no" attenuation. By "no", I can
dirrectly feed the attenuated output from my ancient HP test generator,
adjust it for a just audible signal, then run it through the patchbay,
and it
is still there.

Beats the heck out of my homebuilt BNC bay, and I can now seperate
my receivers from my transceivers.

I did a quick search and found Prompeter 75 ohm patch bays on Ebay
for less then $20. For anyone with a mix of receivers, converters,
filters,
and antennas, this could be a very cost effective routing solution.

And for those who think that their SW receiver antenna input,
rated at 50 ohms, is really 50 Ohms, all I can say is "not very
likely".

As to the mismatch of using a 75 Ohm patch in a 50 ohm system,
I can run 146MHz through a patch set, terminate it in a Narda 50
Ohm load, and I can not measure any increase in VSWR. This is at
low power and one of my main reasons for wanting to not use my BNC
bay is to avoid, or at least decrease the chance of feeding the output
of my IC28A, or HTX100 directly into a receiver. While I haven't
managed it, I know one friend who did.

You mentioned a TDR earlier in the thread. You would certainly see the
difference in cables impedance will that instrument.

I would not buy 75 ohm cable but if you have it for free or very low
cost I would use it.

You will be losing some signal with the 75 ohm cable but not a lot and
maybe not enough for you to notice under most circumstances. The
exception would be a very weak signal on the bottom end of what your
radio can manage to pick up.

Keep in mind that the meter on the radio covers its dynamic range and
that you need a significant change in signal level to see it in the
meters 120 dB range depending on the radio. The range is something like
..1uV to a few hundreds of mV.

The impedance of the radios input will change depending on frequency.
The radios input usually has some type of transformer/Balun on the input
so as you go from low to high frequency you will see the impedance swing
from much lower to much higher then 50 ohms. This low to high swing will
repeat over and over again as you go from 3 to 30MHz. This low to high
swing will keep crossing through 50 ohms though, which is its intrinsic
impedance. The intrinsic non-reactive part of the measurement will be
close to 50 ohms.

This is one reason that you need to check a number of frequencies when
comparing radio sensitivity due to this changing input impedance
changing over the band. A receivers input sensitivity will bobble up and
down over the band.

Your test with the handy talky is seeing the nice 50 ohm load on the end
of the cable. You should be able to see the difference better with an
antenna analyzer that can measure the resistance and reactance
separately. Another possibility is that the length of cable you used in
the test was not significant verses the frequency. Since the line is not
flat you will get a different response depending on the test frequency
or the length of the cable.

As an experiment maybe connect a section of 50 ohm cable to a piece of
75 ohm cable and then the 50 ohm load will allow the meter in the handy
talky to show the reflection. A low power test is just as valid as a
high power test for VSWR as long as it is enough power for the meter
circuit to operate accurately, which should be the situation.

***********************************
Well anyway you are trying to discern if using the 75 ohm cable is going
to make a difference in what you can hear and most of the time it
probably won't make enough of a difference for you to notice if you are
just using the radio. Use an antenna analyzer or TDR or any other
instrument that can measure voltage and phase and you will be able to
see a difference though.

--
Telamon
Ventura, California