[Boatanchors] Current Production AM broadcast transmitters

[Sheldon Daitch]

I don't think Continental is making any AM/HF transmitters with
Doherty amplifiers anymore.

I don't see any smaller transmitters in their product line, but what
AM/HF products are listed all have the Solid State Modulator system.

http://www.contelec.com/SSM.html

Interesting, but it does appear that Continental has essentially abandoned
the smaller power AM market.  Nothing listed on line in my quick view.

Harris is using their digital modulation concepts even at the 1 kW power
  level.  I've not worked with a DAX series transmitter, so I've not
studied the manuals for how the concept works.  I've worked with
the DX series at the higher power levels, and it is an interesting
system design.  The DX systems are better performing and far more
power line to RF efficient than the older high level modulation schemes,
but are far more parts intensive.  On the other hand, they are quite
reliable, depending on what parts fail, of course, but a typcial
DX series transmitter can have numerous parts failures before
it is off the air completely.

As for the wider bandwidths, oh, yea, back in good old days, AM
broadcast transmission systems were required to be flat within 2 dB
up to 5 kHz.  Proofs were required out to 7.5 kHz, and most AM
transmitters could easily pass a 10 kHz tone but not really flat up to
that frequency.

http://radiomagonline.com/tutorials_tips/engineers_notebook/manuals/AM_Proof_BroadcastEngineering2.pdf 


The real restriction on bandwidth in those days started at 15 kHz,
the limitation being both audio bandwidth and distortion related.

A transmitter passing a 7.5 kHz tone with more than 7.5% system distortion
at that frequency could violate the bandwidth limitation at 15 kHz because
of the second harmonic energy from 7.5 kHz.

73
Sheldon


On 2/3/2011 11:26 AM, Richard Knoppow wrote:
>
>>      
>      Most modern broadcast transmitters use either a
> modification of the Dougherty system (Continental
> Electronics) or pulse-modulation. The latter is capable of
> very high plate efficiency with low envelope distortion.
> Weldon's version of the Dougherty amp has substantially
> lower distortion and much better stability but still
> requires two phase shift networks. Both of these schemes
> have the advantage over plate modulation of eliminating the
> modulation transformer, an expensive device and difficult to
> make if the fidelity is to be good. Both also are capable of
> continuous full modulation, assuming the power supply is
> adequate. The latter is valuable for modern broadcast use
> where extreme amounts of processing are used.
>       BTW, is there anyone else here who remembers when AM
> broadcasting stations actually emitted high fidelity audio?
>
>
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
>
>