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#1
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Standing Waves (and Impedance)
W. Watson wrote:
Thanks for your reply. I have a few questions. When you say "standing waves", I take it that one can have more than one on the line? Standing waves are created by two coherent traveling waves moving in opposite directions in a transmission line. In a conventional system of source, transmission line, and load, one of the traveling waves moves from the source toward the load and is called the forward wave. The other traveling wave moves from the load toward the source as a reverse or reflected wave. The reflected wave is usually the result of a load being mismatched to a transmission line. If no mismatch exists, no standing waves are created and the system is considered to be "flat", i.e. one forward traveling wave. How does one know they want to improve their impedance match? For a transmitted signal, we establish a Z0-match to our transmitter often at the input of an antenna tuner. When reflected energy is eliminated on the coax between the tuner and transmitter, we know we have a Z0-match by the SWR meter reading of 1:1. We also use our antenna tuners to tune for maximum received signal on our S-meters. At the Z0-match point, maximum available energy is transferred. If you know the input impedance to a receiver, you can match your antenna system to it to achieve maximum available energy transfer from the antenna. -- 73, Cecil http://www.qsl.net/w5dxp |
#2
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Standing Waves (and Impedance)
Cecil Moore wrote:
W. Watson wrote: Thanks for your reply. I have a few questions. When you say "standing waves", I take it that one can have more than one on the line? Standing waves are created by two coherent traveling waves moving in opposite directions in a transmission line. In a conventional .... How does one know they want to improve their impedance match? .... antenna tuners to tune for maximum received signal on our S-meters. At the Z0-match point, maximum available energy is transferred. If you know the input impedance to a receiver, you can match your antenna system to it to achieve maximum available energy transfer from the antenna. Thanks. A standing wave is the sum of an incident added to the reflective wave. Isn't it possible to send two incident waves down an xline with different frequences, and produce two different standing waves by having some multiplicative relationship between the two incident waves and the xline length? Not a bad explanation from Wikipedia: SWR has a number of implications that are directly applicable to radio use. 1. SWR is an indicator of reflected waves bouncing back and forth within the transmission line, and as such, an increase in SWR corresponds to an increase in power in the line beyond the actual transmitted power. This increased power will increase RF losses, as increased voltage increases dielectric losses, and increased current increases resistive losses. 2. Matched impedances give ideal power transfer; mismatched impedances give high SWR and reduced power transfer. 3. Higher power in the transmission line also leaks back into the radio, which causes it to heat up. 4. The higher voltages associated with a sufficiently high SWR could damage the transmitter. Solid state radios which have a lower tolerance for high voltages may automatically reduce output power to prevent damage. Tube radios may arc. The high voltages may also cause transmission line dielectric to break down and/or burn. Abnormally high voltages in the antenna system increase the chance of accidental radiation burn if someone touches the antenna during transmission. |
#3
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Standing Waves (and Impedance)
W. Watson wrote:
A standing wave is the sum of an incident added to the reflective wave. Isn't it possible to send two incident waves down an xline with different frequences, and produce two different standing waves by having some multiplicative relationship between the two incident waves and the xline length? Sure, it's possible but one wonders about the application. 2. Matched impedances give ideal power transfer; mismatched impedances give high SWR and reduced power transfer. A middle ground - Conjugately matched impedances give ideal power transfer in the presence of high SWR. A feedline doesn't have to be flat to be "matched". All that is required is that maximum available power (actually energy) be transferred. -- 73, Cecil http://www.qsl.net/w5dxp |
#4
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Standing Waves (and Impedance)
To anybody who may be reading -
(1) There's far too much importance attached to standing waves on transmission lines. But see (4). (2) There are colossal standing waves on antennas which are seldom taken any notice of. (3) Anyway, of what use does anybody make of standing waves after taking the trouble to measure them. And the measurements themselves are the most inacurate in the field of radio engineering. (4) And to cap it all, the common or garden SWR meter does NOT measure standing waves on the feedline to the antenna where they might conceivably be of interest. It's all a gigantic hoax! ---- Season's Greetings from Reg, G4FGQ. |
#5
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Standing Waves (and Impedance)
.. "Cecil Moore" wrote in message om... W. Watson wrote: A standing wave is the sum of an incident added to the reflective wave. Isn't it possible to send two incident waves down an xline with different frequences, and produce two different standing waves by having some multiplicative relationship between the two incident waves and the xline length? Sure, it's possible but one wonders about the application. Cecil: Think about a 6 MHz wide analog TV channel. Those antennas aren't flat, and there are 2 transmitters, visual and aural. Putting an analog TV station on the air the first time, particularly low VHF, is a real interesting exercise. Or at least it was back in the stone (vacuum tube) age, the last time I did one. 73, George W5VPQ My real address is my ham call atARRL.NET The ATTGlobal is a SPAM trap snip |
#6
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Standing Waves (and Impedance)
Crazy George wrote:
Those antennas aren't flat, and there are 2 transmitters, visual and aural. The audio is not mixed with the main carrier? -- 73, Cecil http://www.qsl.net/w5dxp |
#7
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Standing Waves (and Impedance)
On Sun, 25 Dec 2005 22:31:09 GMT, Cecil Moore wrote:
Crazy George wrote: Those antennas aren't flat, and there are 2 transmitters, visual and aural. The audio is not mixed with the main carrier? It can be either way. If you choose to use separate transmitters the demands on antenna bandwidth are greatly reduced. John Ferrell W8CCW |
#8
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Standing Waves (and Impedance)
John Ferrell wrote:
Cecil Moore wrote: The audio is not mixed with the main carrier? It can be either way. If you choose to use separate transmitters the demands on antenna bandwidth are greatly reduced. John Ferrell W8CCW Thanks John, my IC-706 will receive the TV frequencies, but since I have never heard any audio, I assumed the audio and video were mixed to an IF frequency and then mixed to the TV frequency. (I have a reference book on TV but haven't looked at it in a long time). -- 73, Cecil http://www.qsl.net/w5dxp |
#9
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Standing Waves (and Impedance)
"John Ferrell" wrote
Crazy George wrote: The audio is not mixed with the main carrier? It can be either way. TV aural uses a separate transmitter from TV visual, because the visual amplifier in a TV tx is not linear enough to amplify both the aural and visual waveforms while maintaining r-f intermods sufficiently low (to FCC spec). The aural and visual signals are combined with mutual isolation of the txs, and radiated by a single antenna, typically. In an emergency, TV stations sometimes combine A&V at exciter level and pipe them through the visual PA, which is operated at reduced power to minimize r-f intermods. Typically the TV station is not meeting spec then, however. If you choose to use separate transmitters the demands on antenna bandwidth are greatly reduced. Reducing antenna bandwidth needed also would require separate antenna systems for the aural and visual transmitters. That is done, occasionally - but not often. This doesn't reduce the bandwidth needed by the visual tx by very much, however. Generally it's more cost-effective to use a single antenna to radiate both A&V. RF (RCA Broadcast systems field engineer, 1965-1980) |
#10
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Standing Waves (and Impedance)
"Cecil Moore" wrote in message om... W. Watson wrote: A standing wave is the sum of an incident added to the reflective wave. Isn't it possible to send two incident waves down an xline with different frequences, and produce two different standing waves by having some multiplicative relationship between the two incident waves and the xline length? Sure, it's possible but one wonders about the application. Some repeaters use one antenna for two repeaters on 144 and 440 mhz. Comercial transmitters do this all the time. Usually the transmitters are in the same band. |
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