This is a work in progress. I am publishing it since it forces me to think more clearly about the project. I am actively working on this project (in parallel with the EME project at the Delaware Valley Radio Association).
A 500 watt linear amplifier is not very useful without a tuner. So, I am taking a detour into the designing and building a tuner. I am going to follow the general outline of Jeff Anderson's (K6JCA) design:
https://k6jca.blogspot.com/2015/06/antenna-auto-tuner-design-part-1.html
I will start by copying his requirements:
- Frequency range (3.5 - 30 MHz), continuous (and perhaps 54 MHz if comes for free - and my Ten-Tec Omni D is quite limited in the bands that it covers)
- Matching network topology low pass L-C Network
- Load matching range: to better than 1.2:1 with any load up to 10:1 SWR
- Input power: 200 watts average, 800 watts peak ICAS operation (https://www.ab4oj.com/quadra/icas.html)
K6JCA has extensive antenna tuner analytical models, test data, and discussions in his blog. If you are planning to design and build a high power tuner, I recommend reading his material. I have frequently gone back and re-read the material to clarify some points for myself and figure out how to solve my design, build or testing problems.
The power specification requires a bit of discussion. The human voice dynamic range at a distance of 25 cm (about 10") from the microphone varies from 70 dB (relative to 20 micro Pa) at normal level to 88 dB when shouting, or a 18 dB range (https://www.dpamicrophones.com/mic-university/facts-about-speech-intelligibility). I am going to assume that the user will adjust the microphone gain for their normal level of speaking, say raised, which is 76 dB (I am a bit hard of hearing). The same site quotes the peak to average ratio of human voice at 20 - 23 dB. But, this is a high quality microphones business site and they are probably talking about opera singers.
I have seen various places on the web that put this range at 12-14 dB and claim that ALC reduces it to 6 dB. SM5BSZ (http://www.sm5bsz.com/dynrange/alc.htm) quotes this number at 10 dB.
K6JCA specification of 200 watts is to support AM transmission (see his blog). So, I will go with it.
I did not start intending to build an 800 watt power amplifier. But with the combination of the standard power supply voltages, transistor max VDD specification and the required impedance ratio of the transformer, I ended up with a max 800 watt output capability. My power amplifier design with a 48 volt power supply and 9:1 impedance transformer at the output can easily deliver 800 watts of output (50/9 = 5.56) with the allowance of 1 volt for overhead:
\begin{equation}P _{out}=\frac {2V _{dd} ^{2}}{R _{L}} = \frac {2\times47 ^{2}}{5.56} = 795\space watts\end{equation}
800 watts of peak power, depending on 10 or 6 dB of peak to average power ratio assumption implies 80 or 200 watts of average power. It is also important to note that 800 watt output only occurs (if ever) at voice peaks. Certainly, that level of power is unnecessary and not advised for digital or CW communication. Hence, for the 800 watts case, the heating effects (current) can be ignored and only voltage effects (breakdown) taken into consideration.
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