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Proposed circuit:

Attachment:

Tank_Antenna_Coupling_100mW.jpg [ 302.3 KiB | Viewed 1787 times ]
Example:

Given:

Class C operation

1400 kHz

100 mW dc power input to tube plate

70% efficiency dc plate input to RF output

Tank (C2, L2) loaded Q = 12

3 meter vertical antenna, radiation resistance (Rr) 0.08 ohm,

capacitive reactance 4600 ohms.

Antenna coupling transformer T1: 50 ohm Z primary, 0.08 ohm Z secondary,

25 to 1 primary to secondary turns ratio, impedance ratio 0.0016.

Calculations:

100 mW x 70% = 70 mW into tank, with zero loss = 70 mW into antenna.

Sqrt(70 mW / 0.08 Rr) = 0.935 amperes antenna base current.

0.935 x 0.08 = 75 mV RMS antenna base voltage.

70 mW x 12 Q = 0.84 VA tank reactive power

Sqrt(70 mW antenna coupling transformer primary power/50 ohms) = 37 mA tank circulating current.

0.84 VA / 37 mA circulating current = 22.7 V RMS across tank.

22.7 V RMS / 37 mA = 613 ohms X.

613 ohms @ 1400 kHz = L2 70 uH, C2 185 pF.

70 mW / 22.7 V RMS = 3.08 mA RMS current into tank.

22.7 V RMS / 3.08 mA = 5973 ohms tank input Z

(22.7 V RMS x 1.414) + tube saturation drop, around 10 V = 42 V dc plate supply (adjust B+ for 100 mW dc plate input)

100 mW / 42 V = 2.4 mA dc plate current

4600 ohms L3 @ 1400 kHz = 523 uH

Ref. B. W. Griffith,

*Radio-Electronic Transmission Fundamentals*, 1962.

With an ideal ground the Q of the antenna may be excessive, but ground loss should reduce the Q.

Completion of the design of T1 is left for other forum participants.

Operating frequency 1000 kHz to 1600 kHz:

Toroidal core of appropriate material and physical size.

20 gauge wire should be okay for secondary.

Each winding to occupy full circumference.

Primary reactance 200 ohms @ 1000 kHz

Hopefully nothing is too goofed up in assumptions, method or math.

I would like to breadboard this but not able to at this time.

Eric LaGess

WB5HDF