[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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