Pi Network Inductor question

[W4RFM]

Same schematic you have seen.  This time I question the Pi network inductance.  The "book" call for L 3 to be 1" diameter, 1 inch long #12 wire coil, L4 should be a 229-201 rotary inductor. To run this circuit at 1800 volts/ .150 amp or the max 2500 volts at .250 mA, the total inductance should be between 21.6 and 26 uHY.  The 229-201 is a 10 uHY inductor, The Jim Hawkins app tells me the 1 inch coil will be less than 1 uHY. and I just did the math using the ARRL Handbook method for figuring inductance and it also says .26 uHY.  What am I missing? Fire away...

[kb3ouk]

I think what you missed is that circuit was originally designed to be multiband, and from playing around with a pi net calculator, the designer seemed to be shooting for a Q in the high teens-low 20s across all bands, probably to be able to use what was on hand, and maybe to help performance on the higher bands? Looks to me like keeping a Q in the 20s clear up to 10 meters would keep the values of plate and load C within the variable cap's range, where a lower Q as far up as 10 would require less C, probably lower than that cap would go. Higher Q is also a narrower bandwidth, could've been a measure towards preventing TVI. A Q around 12 would use about 22 uH on 80 meters like you're calculating, a Q of about 24 would be what the circuit as drawn would give. If you're building it to be a monobander, I'd just use whatever value of coil works with the value of capacitor you have or can find.

[PA0NVD]

Take care with switching caps in parallel to the loading cap. This is absolutely NOT recommended though used quite a lot. The connection via the switch should be EXTREMELY low inductance, which is normally not possible. When you swith a 1000 pF and a 300pF loading cap together using a piece of wire, the resonance will be in the 10 meter band with only 0.12 uH That is a few inches or wire!!!. The result is that there is NO loading and NO suppression for 10 meters when transmitting at low frequencies, so lots of TVI , chance for oscillation etc. There WILL be a resonance in the higher HF frequencies when switching a fixed cap in parallel to the load cap, so you never know where the higher harmonics will be.
When transmitting at 80 meter, a 500 pF cap in parallel with the loading cap will result is a resonance and radiation of the 4th, 5th of 6th harmonics with only approx 0.3 uH wire inductance!!, a piece of wire.
Better use a bigger load cap that needs no fixed caps in parallel.

[W4RFM]

My plan is to use this only on 80 meters at 3,880 & 3,885.  This is good info, thanks Shelby.
Again, I plan to use this transmitter only on 75 meter AM freq's. 3,880 and 3,885. I have a 220 pf C2, and a 2000 pf C3, for the pi network, I just want to get the inductor straight. 26 uHY or the 10uHY the design asked for.