The mechanical layout of coaxial cable limits the available propagation speed range and impedance range. The logarithmic relation of diameter ratio to the capacitance / length unit and inductance / length unit keeps all practical cables pretty the same.
If you change the dielectric constant, you have to change the diameter ratio also, to preserve the wave impedance, and the speed factor will stay quite near 2/3.
What will it drive? How much attenuation can you stand?
That will present less than 50 ohms to the line, do this:
T-pad:- 500 ohms in, 50 ohms out, -20dB, theoretical values:
Version 4 SHEET 1 880 680 WIRE 192 96 144 96 WIRE 240 96 192 96 WIRE 192 176 192 96 WIRE 192 288 192 256 FLAG 192 288 0 SYMBOL res 160 80 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 478 SYMBOL res 336 80 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 19 SYMBOL res 208 272 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R3 SYMATTR Value 32 TEXT -48 176 Left 2 ;500 ohms in TEXT 376 184 Left 2 ;50 ohms out
If the source R>50, use a parallel resistance to bring the effective source resistance to 50 ohms, You might need to properly pad down to 50, if the source doesn't like 50 ohm loading. For example, you can get a 1000 to 50 ohm pad, accurate in both directions, for a 20dB insertion loss.
If source R
Zo ~ sqrt(L/C) (L and C per "unit length")
TD ~ sqrt(L*C) ...Jim Thompson
Air core coax (with occasional disc spacers) can be bought at 50 ohms and is close to c. GR used to sell true precision air core 50 ohm lines; probably someone still does.
You can buy foam coax that's 0.89 c.
Wouldn't surprise me. Looking through MWJ, I never cease to be amazed at the number of companies still making niche stuff, like interseries adapters at 400+ bucks a pop.
It's like looking at a machine tools catalog: what in the world is that thing for?
That isn't true. The propagation speed goes as 1/sqrt(epsilon_r). It does not depend on the ratio of diameters. Coax in which the dielectric is mostly air will have a speed factor much closer to unity. Look up 'air dielectric coax'.
Jeroen Belleman
(Richard Feynman)
OK, Thanks Fred. I don't really need a new design. I'm just trying to understand what I already did :^) Is the above "tee" circuit any better than 450 followed by 56 ohms to ground? It certainly attenuates more.
George H.
My Dad was at Moto Government Products Division (now General Dynamics) on East McDowell 1957 > 1997. I had some friends whose dads worked at "Research" on 56th Street. I knew a lot of crosseyed women who spent 8 hours a day at SPD working through microscopes.
I like fat cable because it has the least attenuation at UHF.
For Teflon and closed polyethylene, yes, not for polyethylene foam.
It's -30dB (20log(Uo/Ui))! That's a waste!
A minimum loss matching L-pad between 500 and 50 ohms would have 474.3 ohms in series and 52.7 ohms to ground at the 50 Ohm side. That's about 4dB better and the best that can be done with resistors.
Jeroen --Every dB counts-- Belleman
I remember it well. I started at 52nd St and McDowell (SPD, now ON Semi) in '62. For many years we had to trot over to GPD to use their screen-room facilities, before we finally got our own.
Indeed! ...Jim Thompson
speaking of charging up cables...
we had an interesting failure...
we had a small GPS active antenna on the end of about 100 ft of coax... the ant / LNA has a simple circuit powered by DC on the cable and we had bias T to feed +5 volts into the cable at the lab end.
one day, the pig tail / adapter at the LNA end developed an intermittent short... should be no big deal the +5 v supply was current limited to an Amp or so..
but after the short was repaired we found the LNA was fried...
I thought about it and realized that when the cable was shorted about
1 Amp was flowing, and when the short cleared, 1 Amp continued to flow for the duration of the cable prop delay. That was like a current source and the voltage at the LNA end when up to whatever it takes to keep that 1 Amp flowing for the duration...Mark
What the hell are you talking about? Did they only hire people with eye problems? Or are you just being an ass.
Michael, Please stay out of threads you don't understand. Dave and I have run semiconductor production lines... cross-eyed is only half the joke... t*ts moving the microscope stage was a major problem >:-} ...Jim Thompson
That doesn't work. Coaxial cables have basically no parasitic inductance, there's nothing to charge up unless perhaps the cable was grounded at both ends. or some other wierdness with the powersupply.
Not quite. A coaxial cable isn't a pure inductor. It's true that after the short clears, 1A continues to flow into the cable for another cable delay. At the LNA end, neglecting the LNA current, that would translate into a 50V step that propagates back towards the power supply and which persists for twice the cable delay. What happens next depends much on the power supply's behaviour.
But you were right that this can kill an LNA that isn't protected against such events.
In this context, coaxial cables can hold nasty surprises. Just last week I wanted to check the connectors on a piece of cable that had been sleeping in a cupboard for years. It was just a few meters long. I foolishly neglected to discharge it before connecting it to my Tektronix S-6 sampler head. That was the end of that one. What a shame to break such a fine piece of vintage hardware for a trivial measurement attempt. (Snarl!)
Jeroen Belleman
I have. Measuring trr. Switching from an amp forward. Source termination on the pulser, and a 1 ohm load. straight into a 50 ohm, 1GHz vertical amplifier. Took a week to get the source termination good enough to be sure that what we saw was the diode, not the test rig, with some quite expensive resistors.
Datasheets can tell big lies;-)
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