[AMRadio] antenna tuners transmission lines and more |
Gary Schafer
garyschafer at comcast.net
Sun Apr 23 23:11:43 EDT 2006
> -----Original Message----- > From: amradio-bounces at mailman.qth.net [mailto:amradio- > bounces at mailman.qth.net] On Behalf Of Brian Carling > > The power may not be wasted very much in the tuner, BUT > REFLECTED power goes back into the RF final and is disippated > in the famil amplifier device(s) - at least many people have > written articles for decades describing that marticular "myth" > or so-called "FALSE STATEMENT." I am not so sure it is false > though! Look at your bird wattmeter, or any other directional wattmeter, in a line that shows reflected power. Note that the forward power reading will be higher than the actual power delivered by the transmitter. Example: If your transmitter puts out 100 watts and it feeds a load that presents 20 watts reflected, your wattmeter will read 120 watts forward and 20 watts reflected. If you look in the bird manual it will tell you to find the amount of power delivered to the load you subtract the reverse power reading from the forward reading. In this case you would subtract the 20 watts reflected from the 120 watt forward reading. That gives you 100 watts delivered to the load. The same amount of power that the transmitter is putting out. There is no reflected power left to get back to the finals! Prove it to yourself: Put a wattmeter at your transmitter. Run some coax to an antenna tuner, then another wattmeter, then a 50 ohm dummy load. Adjust the tuner so the first wattmeter shows 20 watts reflected and 120 watts forward. The wattmeter at the load will read 100 watts. The tuner is simulating a mismatched load while you are able to measure actual power into the load. Or you can use a non 50 ohm load and measure the voltage across that load to find actual power into the load. See my explanation in my other post about reflected power as to how it happens. 73 Gary K4FMX
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