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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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18th Feb 2012, 5:13 pm | #61 | |
Octode
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Re: Receiver aerial input impedance: questions.
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John |
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18th Feb 2012, 6:03 pm | #62 |
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Re: Receiver aerial input impedance: questions.
Re: post #60: indeed John, but these terms 'resistance' 'impedance', 'voltage' etc. have been in use for many years. Could it be that since 'd.c.' came along (in the popular consumer sense) prior to 'a.c.' that the terms used within 'd.c. technology' were simply 'transposed' to a.c. circuits? (Although that did not seem apply to power considerations: watts, VAs and VArs). Whatever, the fact remains that we are stuck with them, so a careful and a qualifying description would seem to be needed whenever these terms are used. I shall have to start making an effort to use terms such as 'reactive ohms', 'impedance ohms' and 'real ohms', for example: hopefully I will be understood!
Al. Last edited by Skywave; 18th Feb 2012 at 6:10 pm. Reason: Add previous post reference, line 1. |
18th Feb 2012, 6:56 pm | #63 |
Pentode
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Re: Receiver aerial input impedance: questions.
"If you can't understand the reasoning & logic . . . then it must be intuitively obvious!"
Al, I think the second part of your signature has to be applied for day to day discussion.Only when we get into deep theoretical debate do we have to be explicate in our meaning. I've enjoyed this discussion and have sent links to our 2 trainee engineers.I've asked them to read this thread and highlight the points they don't understand.When we meet, I will try to assess their intuition with regard to this subject.Sometimes you know something is right, even though you can't explain the reason why! Now I'm looking forward to the follow up discussion on low noise receiving antennae. John
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18th Feb 2012, 7:53 pm | #64 |
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Re: Receiver aerial input impedance: questions.
I'm puzzled that people seem puzzled by resistance vs impedance. Impedance is a general term which includes both resistance and reactance. If the reactance happens to be zero, then you can still call the whole thing impedance.
Resistance can be slightly ambigious, because it could mean the resistive part of an impedance or it could signify that this particular impedance had zero reactance - the context should indicate which is meant. Finally, some people get confused by the modulus of impedance |Z|=sqrt(R^2+X^2). 75ohms impedance does not mean |Z|=75, it means Z=75+j0. Finally finally, resistance, reactance and impedance are all measured in ohms. Ohms is a scale, not a direction. A capacitance can have a reactance of -j40 ohms. The '-j' gives the direction, the 40 ohms gives the size. Transmission line characteristic impedance is usually a pure resistance at RF frequencies. It is called characteristic impedance, not characteristic resistance, because at other frequencies (e.g. audio) that same cable will have a characteristic impedance which is not pure resistance. Even at RF it is only approximately a pure resistance, although it is a good approximation unless the cable is very lossy. |
18th Feb 2012, 8:00 pm | #65 | |
Octode
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Re: Receiver aerial input impedance: questions.
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Now I've been compelled to pull out me old A level book (McKenzie) and he rather neatly states "The term (complex impedance equation, i.e. sqrt (R^2+jX^2)), analogous to resistance, is called the impedance of the circuit and is measured in ohms". This is the first impedance definition given in the chapter covering Alternating Current. Electrical terms such as impedance are normally used in the engineering community on the tacit understanding that the recipient is "versed in the art", and similarly we don't normally add all the (required) caveats like "this is the ac impedance but only quoted to an accuracy of X at a frequency of Y and temperature of Z measured with a signal level of W...." John |
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18th Feb 2012, 8:34 pm | #66 | |||
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Re: Receiver aerial input impedance: questions.
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Al. |
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18th Feb 2012, 8:49 pm | #67 | |
Dekatron
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Re: Receiver aerial input impedance: questions.
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Although, an ideal 75Ω coax cable will also be 75Ω at audio frequencies, in fact it will be 75Ω right down to DC. But to do a check measurement on your AVO, you'd need an infinitely long length (or at least a quarter of a million miles of it, to get a steady stable reading for a couple of seconds). |
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18th Feb 2012, 9:20 pm | #68 | |
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Re: Receiver aerial input impedance: questions.
Quote:
The aerial connection of a receiver states "Input impedance 75 ohms". This figure is produced by the dynamic resistance of the aerial input tuned circuit. So we have 'impedance' on one hand and 'resistance' on the other. Replace the phrase 'dynamic resistance' by 'dynamic impedance', (or replace 'aerial impedance' by 'aerial resistance') and the terms then become consistent. I hope that that helps. Yes, you, everyone else and I know what is meant, but that is not the point. Al. |
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18th Feb 2012, 9:27 pm | #69 | |
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Re: Receiver aerial input impedance: questions.
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Al. |
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19th Feb 2012, 2:39 pm | #70 | ||
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Re: Receiver aerial input impedance: questions.
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A-level textbooks can be a good way to start, but not always reliable. I guess the problem the author had is that he can't assume that all A-level students understand complex numbers, as they only appear in Further Maths in most cases. That means he can't use the concept of modulus, as students think of that purely in terms of a sign change for negative numbers rather than its true meaning of magnitude. |
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19th Feb 2012, 4:05 pm | #71 |
Dekatron
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Re: Receiver aerial input impedance: questions.
You can achieve this with a real cable - it doesn't have to be lossless. If the resistance of the (non-ideal) conductor is complemented by an 'insulator' which itself has leakage resistance, you can again get a constant 75Ω right down to DC. (Of course, this cable will be lossy).
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19th Feb 2012, 5:37 pm | #72 |
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Re: Receiver aerial input impedance: questions.
Yes, of course, I forgot that. In practice it rarely happens, though.
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20th Feb 2012, 12:51 am | #73 |
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Re: Receiver aerial input impedance: questions.
Time for a review: I generated this thread with a number of questions to which I now have the answers. So to summarize: when a receiver states an aerial input impedance of 75 ohms (or whatever figure), that figure is determined by the so-called dynamic resistance of the first tuned circuit and any purely resistive components that shunt that dynamic resistance which follow that aerial input connection. Consequently, the aerial input impedance figure is not a pure resistance but an impedance where the reactive part of that impedance is zero. Over a given tuning range, that stated aerial input impedance will deviate from the nominal stated value, but will always remain as an impedance whose reactive component is zero. (This is simply because the input tuned circuit will always be tuned to resonance).
I have reached those conclusions based on my own measurements, (based on one receiver, an Eddystone 888A), calculations and comments received here from other members - and for the latter part, I would like to say "thank you" to all who have contributed to this thread. So, subject to any comments from other members that clearly indicate that my above stated conclusions are in substantial error, there is nothing more I need to add to this thread. Al. |
20th Feb 2012, 12:07 pm | #74 |
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Re: Receiver aerial input impedance: questions.
Minor corrections:
1. Only true if the receiver was designed for input matching rather than best noise figure, although this could be true for the majority of valve-era receivers. 2. Assumes perfect tracking, otherwise you get deviations in both resistance and reactance. |
20th Feb 2012, 12:52 pm | #75 |
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Re: Receiver aerial input impedance: questions.
Yes Dave, I agree with your comment about tracking errors introducing a reactive component. It was sometime after I wrote that comment that I then realised that over-sight. What I overlooked was the simple fact that the receiver I did my measurements on had a relatively narrow bandwidth. I shall have to repeat my measurements on a number of other valve comms. receivers and study the results: should be quite revealing.
Thanks for your comments. Al. |