I Thought I Saw a "Transmission Line" (for the first time :P)

[Mechatrommer]

Fiddling with op-amp earlier and feed them with some MHz signal through BNC cables. I got some nasty signal attenuation, even if i disconnected the cable from the op-amp circuit, particularly at 50MHz sine signal. So i curious, stop my op-amp fiddling and try to figure out whats going on. I thought there is some sort of shorting inside the BNC/coax cable that i DIY'ed earlier, measuring continuty check showing my DIY cable is fine (no short), but still, the shit happened. So just as Dave's video on Tektronix probe shock (that i voted everyday ), i come to the conclusion that my cable is acting as a transmission line (antenna) to my signal generator. Here is the setup:

My BNC/Coax Cable Length = 77.5cm (from end to end or tip to tip of BNC connectors) i have 2 of this DIY cable.
Resonance frequency = 60MHz from FG (wavelength somewhere at 500cm), not the exact resonance i think, but to demonstrate it best here.

1.jpg = using one of the cable to read the signal, i suspect there is already attenuation along the line. (but i cannot prove it)
2.jpg = adding another cable through the T-junction (was meant to go to the op-amp circuit) note how nasty the attenuation on the DSO.
3.jpg = adding a terminator at the end of cable solve the problem (attenuation) adding impedance to it to avoid power loss.
4.jpg = adding more BNC cable instead of a terminator will solve the issue as well.

Calculating it, the resonance occured at 1/3 of wavelength (500cm/3=167cm), maybe including the T-junction length and accessories etc.
So watch out newbies (me included), you could just tossing your FG good bye by just connecting a proper length'ed antenna (cable) to it.

[Rufus]

You saw a transmission line the first time you saw some coax. What you saw today was a standing wave. google it.

Signals propagate in coax about 0.6 or 0.7 times the speed of light, you might find a figure in the coax specifications. Remember than when you work out wavelengths.

[Mechatrommer]

ok. what i saw is a standing wave with near infinite SWR right? but still damaging to the source. but no electromagnetic energy is transmitted whatsoever, right?

[jahonen]

So watch out newbies (me included), you could just tossing your FG good bye by just connecting a proper length'ed antenna (cable) to it.

No, ordinary test equipment is not damaged since they are usually designed (I think that is a reasonable requirement!) to withstand ordinary impedance mismatches at their outputs. Furthermore, the power level is usually low.

Regards,
Janne

[Mechatrommer]

Signals propagate in coax about 0.6 or 0.7 times the speed of light, you might find a figure in the coax specifications. Remember than when you work out wavelengths.

0.66667 x 500cm = 333cm... 155cm ~ wl / 2, sounds more reasonable now.