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Re: Topband: EZNEC 5.0 +

To: <>
Subject: Re: Topband: EZNEC 5.0 +
From: "Tom W8JI" <>
Reply-to: Tom W8JI <>
Date: Sat, 6 Dec 2014 08:47:46 -0500
List-post: <">>

>We can have 67% reflected power and still have nearly 100% of transmitter >power getting into the antenna and being radiated.

Then could someone please explain why the manufacturers of ham, broadcast AM/FM/TV, and other transmitters specify the maximum SWR (e.g., minimum return loss) for the loads they may drive at full, rated output power (no foldback)?

If "nearly 100%" of the r-f power output of such transmitters was radiated by the antenna system regardless its VSWR/return loss, what would be the need for such OEMs to specify a maximum load SWR?

Easy. It has nothing to do with reflected waves reaching the PA device. "Reflected power", or SWR, can actually make a PA run cooler.

The job of a tank circuit or output matching circuit is to establish a LOADLINE for the power amplifier device. This is found through a form of Fourier analysis of the time varying voltage and current at the output device, or through experimentation. This loadline is selected to provide either the best energy transfer from the device, the best linearity, or some other target goal or combination of goals.

When the load mismatches the output impedance transformation design termination impedance, the PA load impedance deviates from optimum.

As far as the PA stage goes, SWR really just indicates an impedance mismatch from ideal. The PA termination impedance change is transformed through the output matching to move the loadline.

If the tank system can be readjusted to a new value, you can have a 50:1 feedline SWR and not hurt the PA system. The only limit is voltage and current rating of components up to the output device end of the matching system.

But again, a mismatch from the impedance the tank was adjusted for simply changes the loadline seen by the PA device. Mismatch can make PA heat decrease, increase, or stay the same. It can make distortion increase, decrease, or stay the same. It is nothing other than a change in loadline to the PA device.

By the way, there is no source impedance to match in a typical PA. The PA device impedance is varying constantly through the RF cycle. With class B or shorter angles, the PA device is switching completely off for a portion of the cycle. The only "impedance" is loss loading of the system. This is why tank voltages on the anode of a tube can go ten or twenty times the DC supply voltages with certain termination impedances.
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