HFU HF Underground
Technical Topics => Equipment => Topic started by: NJQA on May 12, 2020, 1353 UTC
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Good info:
http://www.na0tc.org/lib/exe/fetch.php?media=technical:rx_protect.pdf
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Very nice. On the home brew jobs I think 1n4001s are going to be better than 1n4148s or other small signal diodes because they're horrific imd generators in certain instances such as nearby ambc stas, and they might not live after a pop or two from a tstorm. The 1n4001s are similar to a PIN diode in their junction, thus almost impervious to generating imd, and they're rated to handle much greater power levels than small sig diodes.
In some of my hf rigs that don't feature built in gdt protection, I place a gdt across the uhf socket(s) and a 20v mov across the dc input.
I suppose one could get away with a simple ne2 bulb across a decent inductance to ground, mounted inside a metal box and call it good. Pretty much what the polyphaser protector inline with the rig in front of me has.
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While the author in his article specifically mentions NOT to use PIN diodes (without saying why) I've got some 6V zeners that I'll make a BTB connection with for this purpose. I've got an amp worth 1300 watts out, and I'd like to not zorch my SDRs when I forget to disconnect 'em.
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While the author in his article specifically mentions NOT to use PIN diodes (without saying why)
I assume because at high frequency, PIN diodes really aren't good diodes as we would understand them at DC. They behave more like a resistor than a hard on/off switch. You really need a hard clamp in this sort of protection application. I think that your zener diodes should be OK, though a TVS would handle more energy.
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In the presentation. the author mentioned "light bulbs as an RF component?"
Light bulbs were sometimes used as dummy loads (crude ones and not impedance controlled) when valve/vacuum tube ham rigs were more prevalent. Maybe some people still do this?
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The recommendation against using PIN diodes has to do with their nonlinear nature and tendency to produce IMD. This is one reason they are usually not used in pre-selector circuits, as in the presence of high RF fields can produce IMD products....but then again so can conventional diodes.
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