The Heath SB-220 used a 1 or 2 millihenry choke. A lot of reactance, even at 3.5 MHz. Basic rule I see is that anything suitable as a plate choke should serve equally well as a safety choke.
Last failure I had in this department was rated 56 uH, but got hot with a few hundred Watts across it in a Palomar 300A. Turns out to have three turns total on a ferrite core. Never did try to look up the self-resonant frequency. For whatever reason the losses were out of hand. Learned this isn't a useful application for that part. Ended up using a choke rated at 22 uH and 1 Amp.
And putting it across a dummy to see what happens to the match could be done with an antenna analyzer or a SWR meter. If it changes the impedance match it doesn't matter why, it's just not suitable for that job.
And that 56 uH that tried to melt its plastic jacket? Works great to filter the grid bias voltage in a Pride DX300. Just goes to show that one size seldom fits all.
73
Last failure I had in this department was rated 56 uH, but got hot with a few hundred Watts across it in a Palomar 300A. Turns out to have three turns total on a ferrite core. Never did try to look up the self-resonant frequency. For whatever reason the losses were out of hand. Learned this isn't a useful application for that part. Ended up using a choke rated at 22 uH and 1 Amp.
And putting it across a dummy to see what happens to the match could be done with an antenna analyzer or a SWR meter. If it changes the impedance match it doesn't matter why, it's just not suitable for that job.
And that 56 uH that tried to melt its plastic jacket? Works great to filter the grid bias voltage in a Pride DX300. Just goes to show that one size seldom fits all.
73
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