[Boatanchors] AM linear question

[Jim Wiley]

Actually, I am not so sure that this is the case anymore.   Many, if not 
most, AM transmitters these days are "digital", in that they consist of 
a number (hundreds?) of building blocks that are switched on and off at 
an audio rate by a controller.  Each block is part of a large combined 
network that is switched in steps of 0.25 - 0.5 - 1 - 2 - 4 - 8 - 16 - 
32 - 64 - 128 watts then multiples of some fixed level, say for example 
250 watts or even 1000 watts (whatever is needed) to get to the desired 
output, all the way up to 200 KW (requiring hundreds of blocks).  
Keeping in mind that 50 KW X 4 (200 KW) is peak output for "100 %" 
modulated peak AM for a 50 KW transmitter .

The reason this is done is that it makes for peak efficiency (in the 
range of 90% from the commercial AC line to RF out to the antenna)!  
Such transmitters save so much on electricity consumed that they can 
literally pay for themselves in power saved (over conventional 
techniques) in very short times.

An example is KICY, Nome, Alaska,  that paid for its new transmitter (10 
KW, 50 KW? ) in the form of electricity saved over only a few months.    
The sole disadvantage of these transmitters is that they are not 
frequency agile - the combining network is pretty much "stuck" on one 
frequency, but for a standard AM broadcast transmitter that is not a 
problem, as they operate on what amounts to fixed frequency anyway.

The engineering is such that the smallest step is "sized" so that the 
output gives the desired modulation percentage when compared to maximum 
output.  I am probably doing a poor job of explaining this, but the 
basic idea is that the digital "modulator"  has no linear stages at all, 
but the end result is that the RF varies at an audio rate using steps 
that are fine enough so that it sounds like ordinary AM with full 
carrier to the listener but in fact the RF is actually "stepped" in 
discrete amounts as needed to produce AM with full carrier equivalent.  
Think of it as "carrier on demand" modulation.   I am sure a proper 
broadcast engineer can explain this better.

If anyone wants more detail I will see if I can get my BC engineer 
friend (Terry, AL7CE) who is the person that built up KICY in Nome  to 
elaborate more fully.

- Jim, KL7CC






On 12/9/2015 5:56 PM, Bry Carling wrote:
> Yes, and in fact the majority of commercial broadcast transmitters nowadays use low level modulation fed to a linear amplifier.
>
> Best regards - Brian Carling
>   AF4K Crystal Co.
> http://af4k.com
>
> Tel: 321-262-5471
>
>
>
>
>> On Dec 9, 2015, at 7:23 PM, WA5VGO <hbrnut at suddenlink.net> wrote:
>>
>> I'm not saying it can't work. It can work well. The problem is, most commercial amplifiers don't have the cooling and power supply necessary to do the job, and there is a large group of guys out there that have no idea of what it takes to tune a linear amplifier for AM. The guy running the 811A's was driving it with a B&W 5100B running full tilt. These guys get a 50-100 watt AM rig, hang an SB-200 on the tail end of it, tune for maximum output and let it rip.
>>
>> Darrell
>>
>>> On Dec 9, 2015, at 6:13 PM, Paul Baldock <paul at paulbaldock.com> wrote:
>>>
>>> Correctly adjusted linear amplifiers and AM are an excellent combination and should be encouraged. Most of the modern 100W SSB rigs when set to AM will automatically limit the carrier to 25 watts, drive your typical well adjusted 10dB linear amplifier such as a pair of 3-500Zs and you have 250 watts of carrier with a potential for 1000+ watts PEP, and a nice clean signal.
>>>
>>> With this combination we have 1000s of amateurs that can contribute and extend the life of Amplitude Modulation on the bands. If you are trying to limit AM to Vacuum Tube Rigs and High Level Modulation then AM operation will dwindle to even less than it is now.
>>>
>>> - Paul  KW7Y
>>>
>>> at 03:48 PM 12/9/2015, WA5VGO wrote:
>>>> Generally speaking, linear amplifiers and AM are a bad combination.
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