Electronics > Beginners
What's wrong with this RG58 cable?
T3sl4co1l:
It's just lossy as shit.
It's probably still legit "RG-58", it's a terrible spec. RG-8/U is a lot better if you're curious, or RG-316/U if you want to get spendy(?).
Doesn't look to be an impedance mismatch issue. But it would be helpful if the cable were a few times longer, or the pulse a few times sharper. :-//
The output with a higher termination looks cleaner because more high frequency content is getting through, to make up for the attenuation in the forward direction. It doesn't seem to bounce because the subsequent reflections are attenuated twice (or more) over.
This is a neat tip for signal quality on PCBs and cables, by the way: if you expect you'll be dealing with HF losses, a somewhat lower source termination, or somewhat higher load termination, can compensate for cable "drool". :)
Tim
gf:
--- Quote from: T3sl4co1l ---Doesn't look to be an impedance mismatch issue [...] The output with a higher termination looks cleaner because more high frequency content is getting through, to make up for the attenuation in the forward direction. It doesn't seem to bounce because the subsequent reflections are attenuated twice (or more) over.
--- End quote ---
At least the 75 Ohm SAT cable response (which is mis-termianted either, when 50Ohm are attached) looks indeed different and settles clearly after the 2nd or 3rd reflection (particularly visible in the shorted response), while the bad cable response has this long, smeared tail.
Sorry again for the limited equipment. For non-professional use, I don't want to spend so much. In fact my intention was not to do cable testing or RF work @GHz. I just stumbled into this topic when I got the feeling that something is wrong with the new cables - so I had to start some investigations...
gf
tggzzz:
--- Quote from: gf on March 13, 2019, 07:09:03 am ---Sorry again for the limited equipment. For non-professional use, I don't want to spend so much. In fact my intention was not to do cable testing or RF work @GHz. I just stumbled into this topic when I got the feeling that something is wrong with the new cables - so I had to start some investigations...
--- End quote ---
No apologies necessary.
It is always pleasant to see someone checking, thinking, investigating, asking for help, listening, understanding. The most exciting words in science aren't "eureka", but "that's strange...".
T3sl4co1l:
Indeed, doubt everything. Measure everything, the best you can, with what you have.
It's quite convenient really that you have a pulse generator juuust fast enough to resolve that length of cable! :)
Likewise, be careful not to read too far into measurements that you "can't" make -- this is why knowing electronics theory is equally as important as knowing electronics practice.
For sharper pulses, just some jellybean logic will do very nicely actually (74LVC family and such), or an avalanche pulse generator is quite easy to build (there are many threads here). :-+ That will quite easily make your scope the limiting factor (these need upwards of 500MHz bandwidth to resolve well). The instrument is still the limiting budget factor even today, I'd say, although if you don't mind working in the frequency domain, there are some amazingly affordable spec and generator tools out there these days.
Tim
radiolistener:
I have several 1 meter LMR195 pigtails from aliexpress and they all have bad SWR=1.3 at 28 MHz.
One of them has clear marking LMR195, so there is no mistake with cable type.
I tried to heat soldering points on connectors with iron in order to improve soldering quality, but it doesn't help.
Then I tried to measure cable capacitance and inductance.
Capacitance was within LMR195 specification.
But inductance was twice more higher than specification.
So I think this is just defective Chinese coax cable with wrong impedance. :-//
I also bought several 1 meter pigtails with RG58 cable from aliexpress.
But they works ok and have SWR ~ 1 up to 200 MHz.
Just connect 50 Ohm RF dummy load on the end of cable and try to connect it to transceiver output through SWR meter. Try TX with different frequencies at 10-100W and see what is SWR. Also try to sweep it with vector analyzer. In such way you can check if your cable is usable.
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