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That is how characteristic impedance is classically defined, and I'm asking him if he believes that characteristic impedance is dependent on length?
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No, but why this question?
Best regards,
Wim PA3DJS
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But you talk about dependency on length and "infinite" length in the same paragraph. You can't have both variable length (which would include short) and a dependency on being "infinite".
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OK - Maybe I could have stated that a little better.... (?)
My point: Does the length of the coax (or connector - since that's the topic of this post) have anything to do with characteristic impedance? And NOTE THAT THE DEFINITION OF CHARACTERISTIC IMPEDANCE ITSELF INCLUDES AN INFINITE LENGTH. (Which a connector DOES NOT have.) If I ask that question without mentioning that the various equations for characteristic impedance are derived from terms that include inductance per unit length (even if they later cancel out), then the question is absolutely meaningless.
So, I am PURPOSELY avoiding any possibility of a circular definition when I ask this question. Do you understand now?
Once you understand the question, we will look at whether or not the traditional equations for characteristic impedance are even valid (hint: I don't think they are!), given that they do not agree when applied to antennas, (which certainly have a length component). And, since they depend on antenna length, it contradicts the fundamental definitions. Even more interesting, the various calculations do not yield identical results, particularly if you consider an antenna to be a transmission line immersed in three-dimensional space.
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I've never played in that power league, always under 2kW. Once I blew up the top part of a ground plane. The loading coil melted and the top rod with some coil remnants came tumbling down the roof. The other was the balun in the middle of a long dipole wire antenna. A muffled boom outside ... *TWACK* ... coax and part of the antenna smacked onto the ground.
That's true :-)
Over here I might have gone overboard a bit but there's three fire extinguishers. Just in case.
That sounds like the conditions on some islands :-)
Most of my experience is from Europe where UV exposure is a lot less than it is over here. Surprisingly the quad-shield RG6 I have here is faring quite well since 5-6 years. All those rain protector boots, those have hardened, cracked and fallen off by now. Same for the PVC pipe irrigation pipe I laid at around the same time. Even the stuff I painted is beginning to blister up.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
mpm Inscribed thus:
For co-axial cables its a function of the ratio of the diameters of the conductors modified by the dielectric constant of the insulator, or for twin conductor the spacing between them and the dielectric constant of the insulator. ie (impedance = (138 / e^(1/2)) * log (D/d))
--
Best Regards:
Baron.
mpm Inscribed thus:
Considering that an antenna can be looked at from the point of view that it is a transformer coupling energy to or from the ether that is frequency dependant I might agree.
--
Best Regards:
Baron.
It also assumes a match at both ends of the line.
Tom
tm Inscribed thus:
???
--
Best Regards:
Baron.
Sure. Take a piece of coax, any impedance, 1/4 wavelength long, and short one end. What do you see at the other end?
Now match both ends. What impedance do you see at any point of the line?
Tom
That's the simplified lossless mid-frequency model. At low frequencies, Zo goes up because of resistive losses. At very high frequencies, you get moding and complex whoopie-doos. None of that matters if the coax or connectors are a tiny fraction of a wavelength, as in the OP's case; they devolve to a small lumped capacitance.
How he can get 6 dB of loss at low MHz is a mystery to me.
John
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So you're asking if the characteristic impedance of a piece of coax varies with length, when in the same question you define "characteristic impedance" to be impedance (V/I) of an infinite length coax. Kinda silly to ask a question when your question defines the answer, no? Are blue birds blue?
No, one assumes that the term "characteristic impedance" means something in a group with the name sci.electronics.design.
No avoidance at all. Your question was self-circular.
Why you asked the question the way you did? No, I have no idea.
Perhaps the (simplified, btw) equations don't have enough terms to describe every possibility that you think they should cover.
Send in a kilowatt and see what starts to smoke?
I know, I know, that was facetious. Profound apologies :-)
-- SCNR, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Smoke the "phut" out of it?
Yes, with that kind of lump in the line, the transmitter would likely go "phut" phirst.
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Honestly, I don't have the desire to get into a discussion/argument over semantics. The question is phrased correctly. The statement which follows it is also phrased correctly.
I am traveling for a few days, so further thoughts will have to wait.... This may be too complicated a discussion for SED anyway.
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What is the problem with the environment? Temperature? Corrosives? Vacuum? Vibration / shock? other? Combination?
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How did you measure the mismatch and losses. Did you really measure these for a single connector (with a matched load on the other side) or did you measured it with a string of connectors on a bus ?
While the OP did not specify so, this sounds very much like the problems with RS-485 multidrop networks on a single trunk line (such as Profibus-DP).
At speeds up to 20 Mbit/s, it is essential that the branches (drop line/stubs) to the slave stations from the trunk line is only a few millimeters to avoid any extra capacitive loading.
On a typical Profibus-DP slave, the RS-485 transceiver is very close to the connector, thus the stub length is the connector internal length plus a 2-5 mm, provided that the incoming trunk wire and the outgoing wire to the next slave is soldered directly to the same connector pin.
This is quite acceptable at 1.5 Mbit/s, since connectors and cables of varying quality will work OK. However, the stub stray capacitance will cause problems at 12 Mbit/s and series inductances are used in the plug on the trunk line (wire from previous node, series inductance, connector terminal solder point, series inductance, wire to the next slave).
With several long uncompensated branch lines hanging from the trunk line, the measurements from one end of the trunk line, would behave just as the OP described.
Regarding connectors for harsh environment, look for instance at some variants of the M12 (IEC 60947-5-2) connector (up to IP68), which is used e.g. in Profibus-DP or CAN networks in dirty industrial environments.
I do not quite understand the point of making 1/2, 1/4 etc. unit load RS-485 transceivers that will allow 64, 128 slaves on the single line. At least at higher speeds, the connector losses and reliability issues with several dozens of connectors can easily make such systems quite unreliable. It is often better to divide the systems in several segments and use some RS-485 repeaters to drive the segments, even if a single master is used.
tm Inscribed thus:
That has absolutely nothing to do with characteristic impedance !
--
Best Regards:
Baron.
John Larkin Inscribed thus:
I concur ! 6db is +/-75%.
--
Best Regards:
Baron.
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