Another approach? Using Triode as limiter for Ham HF Band SDR.
There have been frequent thunderstorms here in Michigan this year.
There are two single board SDR Txcvr in use, a Hermes14 and a HackRF1
A home brew active Rx antenna tuner here was receiving almost continously for 3 years.
It was connected during most thunderstorms. In 3 years there was one failure during a thunderstorm of the input FET (2n4416)
which has a maximum gate rating Vgss of 30V. There is no limiter in this circuit
Modeling in qucs to try to add shunt TVS to the fet input, led to looking at a grounded grid triode as a way to limit the through energy
without reducing the wanted signal.
A suitable triode still available NOS is 6GK5, being a VHF single triode with grid-plate capacitance of 0.52 pF, 180mA heater current,
and a cathode-heater maximum voltage of 100V. The data sheet is useful, having remote cut-off curves right down to zero plate voltage,
along with gm and Rp curves. The tube is powered from 13.8Vdc
There are some freeware papers (Nasa etc) about lightning in HF spectrum. Thse two were useful:
Info on the RF Spectrum of lightning:
NASA Technical Memorandum 87788 Review of Measurements of the RF Spectrum of Radiation from Lightning David M. L e Vine
Info on field intensity with distance:
V.A. Rakov CHARACTERIZATION OF LIGHTNING ELECTROMAGNETIC FIELDS AND THEIR MODELING
The following circuit:
https://app.box.com/s/p8ndefcgfmtkp8ugaieinjmdnzteflh3 was partially modeled in Qucs to design the LC impedance transformers, one at input for 50 Ohm :Rk,
and one at output for Rp:50 Ohm. As the transient solver shows, the grid and plate coupling capacitors can be reduced in value to act a high pass filters, limiting the long duration voltage of the pulses.
Qucs of the input transformation between input sma and cathode:
https://app.box.com/s/geevbpqqu03gqtveqst87ppqodk0tz98 There are antiparallel clamping diodes on 50 Ohm output.When they start clamping, the output transformation of plate signal becomes a mismatch to the high impedance plate resistance, and the level stays solidly at about 0.65V regardless of input level. This allows use of ordinary 1N4148W with Cd=1.5pF
The prototype build was adjusted for cathode bias to provide +3dB of S21 gain and tested for S11, S21, and S22 using nanoVNA, and then tested for compression.(Gain needs to be approximately unity for accuracy of SDR signal levels.)
With the old S.A. here,
At levels below ~ -30dBm, S21 gain is linear with no noticable 2nd Harmonic increase over the level from generator.
In -10 dBm range the (+) signal peaks approach grid conduction which pushes grid voltage toward cut-off, and S21 compression starts.
At about +5 dBm the output clamping diodes start to conduct, holding level at +5dBm.
S11:
https://app.box.com/s/nl4qhcxy4eljd0z7d4i2hlouxiyhf177S21: Add +30dB to this (below) curve to compensate for the -30dB attenuator used to reduce the level from nanoVNA to below limiting level
Off band attenuation is provided by the input and output LC impedance transformers.
https://app.box.com/s/56se1dkpcox27l0snv4ee3j299psjcz6The prototype was tested to destruction. At approximately +47dBm (50Watt, 50Vrms) the cathode arced and tube failed.
Also the 50V mlcc input capacitors failed, which led to uprating those to 500V NPO MLCC.
Layout: to allow fitting into existing Linear Amplifier compartment, the tube axis was laid parallel to the surface mount board, and short flex wires run up to the 7 pin socket.
https://app.box.com/s/ysreoroiro7i94r6cde0y63n8lzvva32