lord loh.

Although I have use the term "impedence matching" several times in the class as well as on the forum, I regret that I know very littel about it.

What is impedence matching ?
Why do it ?
What if we do not bother about it ?
How do I analyze impedence matching networks ?

Can anyone help me with these questions?
I shall be greatful for any link or explaination or e-text.

Thank You.

__________________
Bharath Bhushan Lohray.
M.Sc. Electronics.

Nigel Goodwin

Impedance matching depends entirely on what you are doing, matching an input and output impedance provides maximum POWER transfer, where 50% gets to the load, and 50% is wasted in the source.

Most modern electronics isn't concerned with POWER transfer, but with VOLTAGE transfer instead, in this case having a low output impedance feeding a high input impedance provides maximum VOLTAGE transfer.

Bear in mind, the output impedance and the input impedance together form a potential divider - it's simple to work out the losses involved in that.

__________________
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http://www.winpicprog.co.uk

stevez

As Nigel stated, it depends on what you are doing. In RF (radio frequency) work it is very important. Many modern transmitters (at least amateur radio stuff) will virtually shut off if the mismatch is too great. The mismatch also results in power loss though as I learn a bit more, sometimes the power loss is quite acceptable and at other times it is not. One indicator of a mismatch is the reflection of power back to the source.

Many amatuer radio references treat this subject at various levels. Lots of misinformation and misunderstanding out there so be sure you are looking at reliable handbooks and journal articles. You might search on transmission lines or RF transmission lines.

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stevez

mstechca

You have an output of one circuit that has a resistance of x, and the input of the other circuit has a resistance of y. x must equal y.

To me, if you are dealing with audio or RF electronics, it should be done to allow the signal to flow from one stage to the next with little or no modification.

then your circuit might not work properly.


Use an ohmmeter.

We just did ;-)


Thank You.[/quote]

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-=: The best low-priced components to troubleshoot with are the speaker and the LED :=-

David Bridgen

An ohmmeter will measure resistance, not impedance.

Impedance matching networks can be analysed mathmatically or with a network analyser.

Sebi

If You thinking about antenne-matching, build a simple SWR-meter like this. (I've build about 20 years ago...)

Attached Images

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	swrre.jpg (439.8 KB, 869 views)

David Bridgen

Although an s.w.r. meter can be very useful they tell you only that there is a mismatch but give no information about it other than its magnitude, i.e. that the termination is so many times higher or lower than the line impedance. No indication of sign or angle.

As an aside - I think it much more useful to calibrate them in power. The sensitivity of the parallel line type, shown in the photograph, falls with lowering frequency. Power calibration would therefore be valid over a narrow range.
With other power sensing schemes the sensitivity can be sensibly constant over a 10:1 frequency range.

lord loh.

What is "s.w.r. meter" ?

Short wave radio ? I think not !


And speaking of network analysers, I so fat thought that it only measured packet losses over computer networks. What is the output of a network analyser ? (I mean what is shown on it's screen ? The x axis and Y axis) ?


Please let me know..

__________________
Bharath Bhushan Lohray.
M.Sc. Electronics.

Nigel Goodwin

I seem to recall it stands for "Standing Wave Ratio". They are more correctly called VSWR (Variable Standing Wave Ratio) meters.

Basically it measures the power going forwards and backwards, obviously for a transmitter you want all the power going to the aerial, and none coming back. The result you get is the ratio of power to and from the aerial, and it needs to be as low as possible - with an aerial it won't ever be perfect.

If you've built a transmitter you can use an SWR meter to align it's output stage. Simply fit a 50 ohm dummy load in place of the aerial, and adjust the output stage for minimum SWR - it should be VERY good into a dummy load. This means the transmitter is giving the correct 50 ohm output, you then connect your aerial and adjust the aerial for best SWR, often by cutting small pieces off of it :lol:

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FRIED

Voltage Standing Wave Ratio

Ravi

Hello Sebi

If I am not asking too much, I would appreciate it very much if you could kindly send me your SWR meter circuit as I am very much interested in building on for me. My address is
ravi@ualink.lk

Regards
Ravi
Colombo
Sri Lanka

__________________
\"Waste your money and you\'re only out of money,but waste your time and you\'re lost part of your life\"

stevez

Until recently my knowledge in this area was very limited. I suppose that it still is however I've been reading, studying and have had the help of some mentors. I'll share a bit of what I think I understand or have been told.

VSWR is Voltage Standing Wave Ratio. It could be calculated with power measurements or impedance measurements.

Some meters measure the power going forward and the reflected power. Bird makes a very high quality meter (expensive too!!) that does this. You can home brew as well. You calculate SWR from the readings.

Some meters work like a bridge with three legs of the bridge being non-inductive resistors that match the impedance of interest. The load is the 4th leg of the bridge. If the load is the same (that's what you'd like) there will be no imbalance and no voltage to be measured across the bridge. A voltage indicates an imbalance. A simple version would allow nulling but no actual measurement. Better versions can be calibrated. The resistors in the bridge do consume power so you don't normally leave this in line. You probably could leave a Bird meter in line without significant losses.

If your load matches the impedance of the transmission line the VSWR will be 1:1. Measuring the VSWR at the load doesn't tell you what the transmitter or source sees - unless the load and line are a good match. Usually they aren't. Measure the VSWR at the transmitter or source.

All transmission lines have some loss. Transmission line loss actually helps keep SWR down - the more power absorbed the less reflected. While this can be of some benefit it can also confuse readings. A low VSWR might indicate a very well tuned antenna system or one that has lots of resistive losses. If you keep adding transmission line the VSWR will get better, all other things being equal.

Reflected power goes somewhere - either out the antenna or absorbed in the transmission line. Just to be clear, "out the antenna" includes the radiation of antenna and the resistive losses (those can be high too). Apparently in TV and other transmissions a high SWR can result in fuzzy images. A pulse that was supposed to leave on the first try is partially reflected back then travels back to the antenna again. The energy arrives at the receive some small but significant time later - enough to cause a problem.

A good way to begin understanding this is to just think about the behavior of a single pulse that originates at the transmitter or source. In a well matched system the pulse energy travels down the transmission line, loosing a small amount to the transmission line and is fully absorbed by the antenna or load. A dummy load is designed to behave in this way. Note that the impedance of the dummy load should match the transmission line within reason. I only mention this because the tranmission lines I play with are 50 ohms (or 52 ohms)(RG-8/9913, RG-8X, RG-58, RG-174) 75 ohms (RG-59, RG-6), 93 ohms (RG-62), 300 ohms (TV twinlead) and 450 ohms (heavier twinlead for transmitters).

Anyway, good references are ARRL or RSGB publications. Antenna handbooks are fairly abundant. Google on Smith Chart - that may yeild some good stuff.

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stevez

David Bridgen

That's been answered.
A network analyser is an item of test equipment which measures and displays complex impedances. The display can be in the form of the Smith chart.

Other tools which one associates with impedance measurement are the vector voltmeter and the vector impedance meter.
A bit of time Googling will give you some good leads to further information.

The s.w.r. meter, or directional power meter, is an even more specialised tool which one would encounter only if you are working with transmission lines and/or antennas.

David Bridgen

Ravi, you might be interested in this http://www.davidbridgen.com/dirpwr.htm

stevez

David - would you explain how the circuit works. This design is similar to others I've seen in ARRL and other publications. I trust that it (and other designs) does what you say it does but I don't understand how you separate the forward power and reflected. I look at other diagrams and just can't make sense of it. With a little help I'll catch on.

Thanks.

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stevez