On 5/16/11 7:04 PM, Grant Saviers wrote:
> Good question.
>
> I should have said "carbon comp" instead of "non-inductive". Carbon
> film and metal film resistors will degrade/fail with high surge
> currents. Carbon composition resistors have a higher pulse current
> capacity.
>
> An inductive resistor might have an undesired Z at a frequency of
> interest, certainly possible with wire wounds. If you can characterize
> the Z vs frequency, and it doesn't impact the transmission line, then it
> might be ok. Since wire wound resistors are available only to a few
> hundred K ohms, this may be academic.
>
> All resistors have voltage ratings, so that is another factor in
> selection. Perhaps another Tower-Talkian can weigh in on that issue re
> carbon comps.
>
> The value of the resistor should be within its wattage rating for the
> maximum transmitted voltage on the feedline, including whatever
> allowance for SWR. Obviously, a lower value will have a lower voltage
> drop for a given static charge current level.
>
From a static bleed standpoint a few megohms would be be appropriate.
Typical voltage ratings for 1/8W and 1/4W resistors are in the few
hundred volt range, so you might want to be careful if you're running a
kW into a 450 ohm line (670 Vrms).
Likewise, in such a situation, 670V would dissipate about 1/2 watt into
1 Meg, so choosing something like 10 Meg might be a better strategy, or
just go to 3 or 4 1 Meg resistors in series.
Static charging current is in microamps, so even with 10 Meg, you're
looking at a few volts at most.
I wouldn't worry about the pulse current capability, etc. If you take
a direct hit, the resistor is going to disappear, regardless. The
concern might be for parasitic L/C. You'd hate to find that your
resistor has an AC impedance of zero ohms right at your operating point.
Easy to check, though.
While it's true that wirewound resistors are in low values, high value
metal film may have significant inductance (e.g. they get the high
resistance by having a long and torturous path on a substrate.. that
torturous path might be a spiral. The data sheet will tell you.
There's all kinds of hacks for this by the way. A lot of black rubber
hose turns out to be a high value resistance. A hunk of charcoal (maybe
the charred remnants of your last antenna that took a hit?). A lot of
black ABS drain pipe has significant conductivity (don't try to use it
for the insulating column in a Van de Graaff generator). Flat black
paint often uses carbon black as the pigment, and so, forms a nice
semiconductive layer. Wood, depending on moisture content.
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