[AMRadio] HV Wire


Jim candela jcandela at prodigy.net
Sat Dec 23 16:44:31 EST 2006



Brian, W5ami said:
>I don't see using coax.  There should be no RF getting back into the
>HV anyway, if there is, you have other problems to address.

>Far as shorts, a HV short is going to find ground regardless if it's
>the shield of the coax or some other very close ground.  Maybe I'm
>missing something, and that is VERY possible since it's 4:00 AM and
>I've been up since 1:30 AM!




Reply by Jim, Wd5JKO

Brian,

    You are right in that straight HV wire is OK, but there are two
downfalls this way.

If the HV DC is to a RF power amplifier, that DC will have less than
infinite attenuation to the RF energy coming from the RF amp, and likely be
pretty 'HOT' with RF where the RF might be several volts RMS when The DC is
several kilovolts, and that RF might be harmonics of the fundimental RF
frequency which might cause interferance to other equipment or
communications. The RF filtering of the B+ can be 60 db down and still be 3
volts AC on a 3000 volts dc level. Also all capacitors go through series
resonance at some point, and above that frequency they look like an
inductor. Take a ceramic .01 uf disc capacitor, and wrap the leads around a
grid dip meter coil (form a shorted loop), and look at the resonant point.
That point will likely be around 7 Mhz. This is a great bypass from 160 to
40 m, but on 20-10 meters the bypassing gets progressively worse. The point
here is all RF amps are to 'some extent' leaving RF on the DC wire feeding
them. Shielding is easy, RG-59 is cheap, and a perfect match to MHV
connectors (look like BNC but 5 KV rated).

A case in point, take an Arc-5 transmitter, and turn on the LV B+ for the
1626 oscillator. That B+ is bypassed in the rig before it goes to the rear
power connector, but it is very 'HOT' with RF for reasons mentioned above.
Bypassing and RF filtering is tricky, and unless done well, we have power
wires that end up radiating RF.

The other reason I like to shield is for safety. Lets say you use 40kv rated
unshielded wire for 3kv. Are you safe? What happens if a mouse bites the
wire, or if the wire gets snagged, and a slice results? Then picture your
self with D-104 in hand, and your big toe touches that wire where the
insulation is compromised. Now your XYL has an estate sale to rid here of
your ham gear at 10 cents on the dollar!! ;-)

The choice is yours,
Jim
JKO
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