[BC] AM metering point

[Donald Chester]

> On Saturday 07 September 2013 08:51:46 am Mike McCarthy wrote:
>> OK...I sit corrected. ?Never the less, generally good engineering
>> practice would want to see the measuring device at the last possible
>> point in the system prior to the radiator so that all losses are
>> accounted for and the maximum licensed power permitted is presented
>> exclusively to the radiator.

OTOH, FWIW, the AMATEUR radio regulations have specified that output power must be measured "at the output terminals of the transmitter", ever since they changed the standard from DC input to rf output power.

Well and  good for those commercially manufactured plug 'n play ham rigs; the definition is pretty clear. But my transmitter is 100% home constructed of my own design, built mostly out of discarded components somebody else didn't want, purchased mostly at flea markets, from classified ads, at estate sales, and a couple of items were even picked off kerbside on trash pick-up day. I use a matching network mounted right next to the transmitter box, feeding 140' of balanced, untuned open wire transmission line @ 440 ohms, which feeds into a second, remote-controlled tuned circuit at the tower. The second tuner feeds a resonant feedline section that runs up the tower to the dipole, and also has a provision for feeding the base of the tower itself as a vertical radiator on some bands. I don't have a sprig of coax in my whole entire antenna system.

But since I designed and built the transmitter myself, no-one was there to tell me where I had to locate the output network of the final amplifier stage, so the components are placed wherever I chose to put them. In my case, I decided upon a three-stage network: the first stage is inside the transmitter box connected to the plates of the PA tubes, link-coupled to the second stage which is just outside the box, feeding the 440-ohm balanced transmission line that runs to the tower where it connects to the third stage in a shelter at the tower base, transforming the 440 ohm balanced impedance to the load as seen at the last convenient point in the system prior to the antenna radiator itself. Therefore the legally defined "output terminals" of my transmitter are mounted on that shelter at the base of the tower, where the tuned feeders running up the tower are connected, as well as the wire that is bonded to the base of the tower when it is used as a vertical. No-one can deny that all three sections are integral parts of my transmitter's output network, since consistent with conjugate match theory, every single L and C component in any one of the three stages of the output network can be clearly demonstrated to interact equally with every other component in the system.

This way, my output power measurement is taken AFTER the majority of transmission line and ATU losses, probably giving me at least a 15% power advantage over locating the measuring point right at the main transmitter box. The same would be true with a broadcast system. Measuring the power level at the transmitter side of the ATU would result in a substantial disadvantage, since no tank circuit or matching network is 100% efficient.

Don