Topband: EZNEC 5.0 +

[Richard Fry]

Hello Paul,

RE:

> Typically a transmitter will fold-back delivered power when its output Z 
> is fixed (e.g., 50 or 70-ohm) and SWR exceeds some predetermined amount 
> set by the manufacturer. This is typical of broadbanded solid-state 
> amplifiers with a fixed output Z that use no output matching network. 
> These transmitters are designed to work into just one line Z and hence a 
> strict maximum SWR at the transmitter's output terminals.

>From my perspective/research/experience from nearly 40 years in the 
broadcast industry before I retired in 1999 -- including 15 yrs at RCA 
Broadcast, then 19 years at Harris Broadcast Division ...

Normally the impedance at the active output elements (plates/collectors etc) 
of a transmitter is much lower than 50 or 75 ohms, so that it can drive a 50 
or 75 ohm load connected to the transmitter output connector with good 
efficiency.  If the net impedance at the output elements of the transmitter 
exactly matched the load impedance of the antenna system, only 50% of the 
available r-f output power would be dissipated by that antenna system.

Broadcast/other transmitters using vacuum tubes often included adjustable 
matching components in the cabinet so that the transmitter safely could 
produce 100% of its maximum rated output power into a load SWR of 1.7:1, or 
so.  But many of them also include(d) power foldback to protect the 
transmitter if the load SWR exceeded that 1.7:1 nominal value.

Some solid-state broadcast transmitters do not include such matching 
components, and consequently their maximum load SWR for full rated ouput 
power is about 1:3:1 or so, and they fold back if load SWR exceeds that 
value.

Rich
http://rfry.org