From: [email protected] <[email protected]> on behalf of Jim VE7RF via groups.io <jim.thom@...>
Sent: Saturday, February 10, 2024 11:09 AM
To: [email protected] <[email protected]>
Subject: [ham-amplifiers] Loaded Plate chokes...again

I'm currently fabricating several versions of ferrite bead loaded plate chokeS. ? I used 1" OD, hollow teflon tubing (.145" wall thickness) (.710" ID).? Each of the .687" OD beads (type 61) was shimmed with tape, so they make a snug fit, when stuffed into the hollow teflon tube.? A 1" electrolytic capacitor bracket is used to hold the base of the choke to the chassis. ?1/2" copper straps wrapped around the hollow teflon tube, and machine screwed with a single brass 8-32 machine screw etc. ? A ?1/4" to 3/8" hole is drilled into the amplifier chassis, to allow compressed air in the plenum, to flow up though the .374" ID of all the beads.? Easy way of air cooling.? Actual top temps will be taken with a fluke 62 IR.

The actual winding is made from 22 ga, silver plated, stranded teflon wire. Insulation is pretty thick, and the OD of the wire is exactly .060". ?( wire inside is only .028" OD)
Upper + lower winding terminated on the upper / lower straps, then soldered.? Lower strap is 1" ?from the top of the electrolytic bracket.? That provides 1" of exposed teflon.? On paper, should be good for 10 kv of...'surface creep'.

After measuring the inductance with my LCR meter, it was time to see where the 2 x series resonance freqs are located. ? I used the S11 output, ?then a single provided coax cable to a (SMA female / BNC female adaptor)....then the BNC plugged into a dual binding post fitting, with it's BNC male connector built into the bottom end. ? ?Then 1 ?SHORT test ?lead to top of choke...and the other test lead to bottom o f choke.? Test leads have an alligator clip at one end...and other end is a banana plug. Did the usual open / short / 50 ohm calibration. ?

Ok, found the 2 x series ?resonance freqs asap...on both HB ?chokes. ( I also compared to a new Ameritron choke, since I know where the 2 x series resonant freqs are on the Ameritron choke). ? I had the chokes being tested, laying on top of my bed mattress...along with vna etc. ? I then placed the choke on Styrofoam, so only the extreme ends of the choke were supported, in the horz plane. ?90% of the choke, and all of the winding was in mid air.

Well that was a surprise. The series resonant freqs ?INCREASED in freq by a bunch.? Instead of 19.002 mhz laying on the bed, it was 19.750 mhz, when in mid air. ? Same deal with the upper series resonant ?freq (aprx 32 mhz...which also increased).? I also stood the chokes vertical on the bed, and supported the very top of the choke with the handle of a wooden spoon...same deal. Resonant freq increased by 748 khz.

I can see getting any plate choke near an aluminum sidewall, or large metal triode tube, can easily lower the resonant freq, but not laying on a bed mattress.? I also held my hand parallel to the choke, and once 2.5" away, resonant freq ?decreased, and decreased more, the closer the palm of my hand got to the choke.

W8JI warned about getting plate chokes too close to any dielectric or insulator.? He is correct...I shoulda listened. ?

Next up, knowing all this, is to replace the 3/4" wide copper straps with 1/2" wide copper straps...and rewind the choke. That will allow for an extra (.5" / .060") ?8.33 turns. ? The idea here is to lower the ?1st series resonance point, (19.750 mhz) so it's midway between ?21.000 mhz....and 14.350 mhz. (midpoint is ?17.675 mhz). ? The new 3x6 linear amplifier is not used on the 17m band ?(18.068 mhz to 18.168 mhz) ?nor is it used on 12 + 10m bands.? We have height constraints, so the completed choke is exactly 8.25" tall. (Top lid is exactly 9.25" above the chassis). ? ?The existing 5" winding will be increased to 5.5"..... by fabricating the new 1/2" narrower straps.? Wish me luck.? Existing choke measures 315 uh....which will increase a bit once the extra 8 turns are added.

On choke #2, it's a full 12" tall, and uses a 8.75" winding. Measures 750 uh.? Resonates at ?9.7 mhz...and also 17.8 mhz. I will try the narrower 1/2" wide straps...so I can get another 8.33 turns on it. ?(Then the new winding will be 9.25" and uh will increase a bit).
The idea here is to lower both series resonant freqs a bit. ? Midpoint between 14.350 mhz and 18.068 mhz is 16.209 mhz. ? Midpoint between 7.3 mhz and 10.1 mhz is ?8.7 mhz.? IF I'm successful on choke #2, on paper it ?should work from 1.8 to 30 mhz....(excluding ?the 2 new series resonant freqs)...and will be used on both my 160-15m hb amps.

Sri for the diatribe, but getting any plate choke, loaded or not, close to dielectrics or metal is bad news, and can skew your intended results badly.? But factored in, that can also work in your favour.??