[BC] Freq agile AM transmitters

[PeterH5322]

>The other alternative would be a 'Power DAC' like the Harris DX 
>transmitters, 
>running highly oversampled and a good output filter.  Harmonic suppression 
>could be very good with a nice broadband design (the DAC output couldn't 
>have 
>as low of an output impedance as the DX design, which requires an impedance 
>transformer at the DAC summing node); with a 50 ohm output impedance 
>obtained 
>without a transformer the efficiency would be rather poor.

Depends upon how much agility is actually required, and how the filter 
function is to be partitioned.

Take a hypothetical example of LA's two ND-U Class As, which, 
hypothetically, desire to implement a mutual backup plan.

KFI would have one or two DX-50s on 640, and one frequency agile unit 
capable of 640/1070.

Similarly, KNX would have one or two DX-50s on 1070, and one frequency 
agile unit capable of 640/1070.

Were the filter, or a portion thereof, to be partitioned into the 
diplexer, then most, or all of the agile transmitter's power section 
could, theoretically, be common to 640 and 1070.

Because of the significance of the L.A. market, economically and 
strategically, a 2+1 configuration would likely be employed there (two 
Txs on the licensed frequency and one one agile one), but in smaller 
markets a 1+1 configuration could be employed.

This concept also addresses the loss of the radiator, which KNX 
experienced in the 1960s, and KFI experienced in several years ago.

180 degrees on 640 is 300 degrees on 1070, and 300 degrees, in a 180 over 
120 "anti-fading" configuration has been proven successful at WHO.

195 degrees on 1070 is 117 degrees on 640, which, while not ideal, is 
better than KFI's current auxiliary, and would allow full power operation 
rather than 25 kW.

There are numerous options for reliability/cost trade-offs here.

As both stations have auxiliary radiators, the mutual backup strategy 
could be simplified by operating the frequency agile Tx into the 
auxiliary radiator, only. This would obviate sectionalizing 640's main 
radiator.

A double failure, with both station's main radiators being lost, seems 
most unlikely, so operating the agile Tx into the auxiliary, only, seems 
like a good option.