Tony:
Let me suggest
you use a Network Analyzer (maybe the NanoVNA?) and take a look
at some of the work by AC6LA.? Start with
which describes how to measure TL characteristics.? Second is to
look at .?
If you determine the TL parameters according to the first link,
you can input that data into the second link and then compare
your info with Dan's coax database.?
I've found the work by AC6LA to be excellent and have
characterized a specific manufacture's 75-ohm coax for use in a
160 meter phased array.
I think this is a much easier approach that trying to worry
about the uncertainties of using 2 TinySAs.
Regards
charlie, k1xx
On 12/14/2020 3:33 PM, Tony Dudieron
via groups.io wrote:
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Hello all, I am wondering whether I could use two TinySAs,
one as a signal generator and the other as a spectrum analyzer,
to verify the amount of signal loss across various lengths of 75
ohm RG-6QS cable.? The frequencies of interest would be 200,
500, 600 and 700 MHz.? The system I am working on would be used
for OTA television distribution.? I have tested the cables for
return loss with a 75 ohm terminator at the far end, and the
return losses are all between 36.8 and 40 dB, which to my
understanding is pretty good.? The cables vary in length from 15
feet to 60 feet.
?
For each frequency, this would be done first by generating a
known signal level with the TinySA that will be the signal
generator, with the signal generator directly connected to the
other TinySA (which will measure the signal) with an
interconnect adapter (for the minimum loss possible).? I would
then compare the directly connected signal level to the signal
level shown on the SA when the cable under test is inserted
between the signal generator and the SA.? I would also like to
perform a similar process to verify losses through some high
quality splitters that are rated to 1GHz (return loss at the
input on these splitters are all between 35 and 40 dB when all
output ports are terminated to 75 ohms).
?
The 75 ohm cable(s) that I am testing have the following loss
specifications per 100 ft from the manufacturer:
200 MHz? 2.0db
400 MHz? 3.0dB
700 MHz? 5.9dB
?
My questions are:
- will the performance of the TinySA, both as a signal
generator at these frequencies and as a SA, be sufficient to
provide me with useful data?? I am concerned whether the
readings I obtain at the SA will be stable and accurate enough
for the data to be of any value in accurately assessing cable
losses on each leg of the system.
- Will the impedance mismatch of the 50 ohm TinySA to the 75
ohm cable be a serious concern to me here, either as a signal
generator or a SA?? I am considering using 50 ohm to 75 ohm
resistive minimum loss pads (5.7dB loss).? If that is
recommended, should I use a pad on both the signal generator
as well as the SA?
- Is there a better way to accomplish what I am trying to do
above that won't be prohibitively expensive?? I have access to
some other test gear, including analog and digital
oscilloscopes, as well as an analog Instek RF signal generator
(can reach 450 MHz via harmonics) and a freq counter.? I tried
to do this using OTA over-the-air signals directly and
measuring them with an ATSC analyzer, however, the OTA signal
levels constantly change and are not steady enough for me to
discern how much of the variation is due to cable losses.
I appreciate any input or advice anyone might be willing to
offer.? Thanks in advance, Tony.
