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记得哪里讨论过使用两个扼流线圈的帖子，用处是什么来着？？？
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the alpha pa-77, 77dx/sx, 78, 76, 76a, 374 and 374a should not be used on 12m. the plate choke will overheat and burn on that band.
determining l
at the lowest operating frequency, the hv-rfc should have enough reactance to limit the rf circulating current through the choke to a reasonable amount. allowing a rf current of 1a rms through the choke usually does not create problems for the wire-lead disc-ceramic capacitors that are typically used to bypass rf on the power supply side of the hv-rfc. to minimize rf current through the choke, it would seem that more inductance is the answer. however, more inductance means more choke resonances and a greater likelihood of choke fires. a compromise is indicated.
over the years, various schemes have been used to minimize choke resonances. adding gaps at presumably esoteric positions in the winding was represented as a means of decoupling parts of the choke winding--allegedly ameliorating the self-resonance problem. however, when the resonances of gapped chokes are compared to similar chokes without gaps, no real improvement is observed on a dipmeter. this should not be surprising. optimum decoupling between two coils occurs when they are mounted at a right angle. adding end-to-end spacing with gaps is the least effective decoupling method possible. to minimize resonance problems, instead of using a single large choke, use two smaller chokes mounted at right angles.
the highest-l choke that can built that is free of self-resonances in the hf spectrum is roughly 60µh. at 1.8mhz, 60µh has a reactance of about +j679 ohm.
the rms voltage that appears across an amplifier's hv-rfc is approximately two-thirds of the anode supply voltage. for example, an amplifier that is powered by a 3000v supply subjects its hv-rfc to about 2000v rms. if a 60µh inductor was used in this amplifier, at 1.8mhz the rf current through the choke would be 2000v/679 ohm=2.95a rms. adequately bypassing approx. 3a of current on the power supply side of the choke is difficult. a typical hv disk ceramic bypass capacitor can handle only about 1a. another problem is that at 1.8mhz 130pf [minus j679 ohm] of extra capacitance is required from the tune capacitor to cancel the +679 ohms of reactance in the choke. adequately bypassing 3a at 1.8mhz requires a substantial amount of capacitance. to hold the voltage across the bypass capacitors to less than 10v at 1.8mhz, roughly 0.026µf [minus j3.3 ohm] is indicated. to handle this amount of current, four approx. 0.0075µf hv disc ceramic capacitors would probably be needed. all things considered, using more inductance is indicated. limiting the hv-rfc's rf current to a maximum of !a would make the task of bypassing a lot easier. however, increasing the inductance above 60µh is virtually certain to move choke resonances into the hf range. unless these resonances are prudently parked between operating frequencies, a choke fire may result.
to realistically evaluate the self-resonance situation, hv-rfcs should be checked with a dipmeter after they are installed and wired in the amplifier. if a self-resonance is within about 5% of an operating frequency, there may be a problem. when re-parking resonances, it is usually best to remove turns from the choke. this will move the resonances up in frequency--and only slightly increase the maximum rf current through the choke.
in continuous coverage amplifiers, there are obviously no safe parking places for choke resonances. the only solution is to switch hv-rfcs with one or more hv vacuum relays.
hv-rfcs should be single-layer solenoid wound. to minimize wire vibration during operation, the wire should be under constant tension when winding and soldering the ends to the solder lugs. when silicone varnish insulated wire is used to wind a hv-rfc, the finished winding should be given a coat of gloss urethane varnish to hold the wire in place. since varnish will not adhere to teflon wire, a different method is needed to keep a teflon winding taught. small tensioning springs are soldered to the ends of the wire. the springs provide constant pull to minimize wire vibration during modulation. an s-shaped copper foil jumper should be connected across each tensioning spring.
