Friday, August 27, 2010

SWR and Max power out?

Bil Paul, KD6JUI, of Dixon, California, wrote to the ARRL's Doctor, noting that he noticed when he was tuning for optimal background noise while in receive mode, it came close to -- but didn't match -- the dial positions for minimal SWR while transmitting. He asked which of the two antenna tuner dial settings would result in the most transmitted energy while in transmit mode. He also wants to know if minimal SWR always indicates the most transmitted energy going to the antenna system.
Here's what the Doctor had to say:
In answer to the first question, by setting the antenna tuner to an SWR of 1:1, you have transformed the impedance at the bottom of your antenna feed line to 50 ohms, just what your transceiver is designed to deliver its rated power into. If your receiver input impedance were exactly 50 ohms, then that setting would likely also be the position that would yield maximum receiver noise. As it happens, there is no such guarantee that the input impedance of the receiver will be exactly 50 ohms. Although it should be pretty close, a slight change may yield a stronger signal into the receiver

Regarding the second question, the transceiver is rated to provide its design output power into 50 ohms, usually within a specified SWR range. As was pointed out by Eric Nichols, KL7AJ, in a QST article last year ["Keeping Current with Antenna Performance," Feb 2009, pages 34-36], an SWR of 1:1 does not generally result in the maximum power output. By building a transmission line current meter, such as described by Eric, or later by Paul Danzer, N1II ["A Simple Transformer to Measure Your Antenna Current," Sep 2009, page 35], you can actually tune the antenna tuner to get the highest output. This corresponds to the maximum current into the antenna, which may occur at some setting different from either of the above.
Be careful, though: If you exceed the maximum rated SWR, the resulting voltages or currents in the final amplifier or output filter of the transceiver will exceed design specs and damage may result. The likely small increase in transmitted power is likely neither worth the trouble nor the risk, in my opinion.

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