Can anyone (Larry?, Bruce?) help with a spec for capacitors for RF
ground for a HF radio? I want to keep the DC away from the (aluminum)
boat.Thanks,
Michael Porter
Michael Porter Marine Design
mporter at mp-marine dot com
www.mp-marine.com
Hmm....Let's assume you have a vertical antenna like a Shakespeare or
insulated backstay or just a longwire strung between some masts.
When the antenna is near 1/4 wave resonance, its feedpoint impedance
gets very low, somewhere around 15-20 ohms. Using simple math:
P = I squared R so I, the current in the antenna and ground system,
equals the square root of P, 150 watt marine radio divided by R, the 15
ohms or even lower with your nice metal grounded hull.
This capacitor is in series with this current from the ground on the
tuner to the nicely grounded hull. Doing the math you get a hair over 3
AMPS of RF current going through that cap at 150 watts. This CURRENT is
the problem. Simple cheap capacitors, with their little wire leads just
touching the plates in a cheap cap, simply cannot handle this kind of RF
current flowing in them. The caps get hot from the resistance of the RF
current flowing through their tin foil plates and will eventually fail,
probably shorting out as they melt the dielectric so you won't even
notice it...shorting ground to the hull isn't certain death, won't blow
fuses.
So, we SHOULD, to be fair, use an RF capacitor that can handle at LEAST
4-5 Amps of RF current....a transmitting capacitor.....Strike ONE....
Now, this series capacitance in series with such a low impedance at or
near resonance (worst case scenario for the series cap) requires us to
have a very LOW capacitive reactance so the cap acts like a virtual
short to the RF, while acting like an open to the DC you're rightly
worried over...galvanics. So, capacitive reactance is equal to 1/2piFC
the bigger the C, the lower the capacitive reactance, the better it will
work. We don't want this capacitor to be part of the tuning solution,
we want it to act like an RF short. Your lowest frequency, the F in the
equation is around 2 Mhz, if you use the old marine band (again worst
case scenario). So the Xc (capacitive reactance) should be much less
than (I like less than 10% of) that 15 ohms or 1.5 ohms at 2 Mhz.
Filling in the blanks and solving we get about .05 microfarads. There
won't be any appreciable voltage across this capacitor so almost any
voltage rating will do as they are normally quite high voltage
transmitting caps. .05uF or bigger capacitance and 5 Amps are what
we're looking for....round numbers.
About the cheapest place I know of to get real transmitting caps is
Surplus Sales of Nebraska, who specialized for years in the military's
mistakes and pork barrel purchasing but has become more than just a ham
junk store more recently. They do have lots of great RF stuff real ham
operators love. The caps in question are here:
http://www.surplussales.com/Capacitors/Trans_Coup_Caps/cap_trans.html
The most bang for the buck for what we're looking for is the 2nd cap up
from the bottom of this webpage on the right hand side.
..22 uF at 2500 VDC, easily big enough in capacitance to provide a really
low capacitive reactance.
16 AMPS rated...much more than we really need in this application for
safe operation at 150 watts.
But, alas, at $49, if you'll compare to any other caps you'll find the
most bang for the buck....easily fitting our needs for CURRENT and
CAPACITANCE.
The $18 one just to the left of it would work at .001uF if we never used
a frequency below 8 or 10 Mhz....but that's not realistic. More of them
in parallel would work fine, but the costs are higher than the .22 at
$49.
The .22 looks like it would be very easy to drill a hole into a
convenient frame member and bolt the cap solid to the metal frame, then
use two big flatwashers to the bolt on the other end to hook the tuner
ground strap to for best low resistance connection. Do coat both ends
in heavy grease before you're done to protect these irreplaceable bolts
from the bilge water corrosion in the swamp. Grease 'em good and
they'll be fine for many years.
Oh, and please find a place where you can bolt the cap so that the
mounting bolt that's holding it to the hull is on the BOTTOM, or TOP,
preferably the bottom so the cap is sitting upright on top of its
mounting...NOT hanging out sideways with the weight of the cap always
torquing on the bolt trying to pull it out of the cap....not good....
Biggest bang for the buck.
Now, we'll hear a huge blast of noise from all those using 49 cent
ceramic orange drop ceramics with the tiny wire leads. My reasoning and
logic comes from being around broadcast AM all these years....where some
really HUGE EXPENSIVE capacitors are just waiting for the 50,000 watt
blowtorch transmitter to make them into a BOMB!.....always great fun!
.....60 amps or so into a 15 ohm vertical tower 1/4 wave high.
Those capacitors are simply HUGE!
