RF Grounding - DC block
 

Here, on page 23...

http://www.icomamerica.com/support/k...?Download=1265

....Icom shows four 0.15 uf capacitors in parallel acting as a DC
block / galvanic isolator in the RF ground circuit of an SSB system.

I assume the caps are in parallel to increase the voltage rating...?

Are these plain old ceramic caps, like an 'orange drop' - say 400 VDC?

Many Thanks.

RF Grounding - DC block
 

wrote:
Here, on page 23...

http://www.icomamerica.com/support/k...?Download=1265

...Icom shows four 0.15 uf capacitors in parallel acting as a DC
block / galvanic isolator in the RF ground circuit of an SSB system.

I assume the caps are in parallel to increase the voltage rating...?

Are these plain old ceramic caps, like an 'orange drop' - say 400 VDC?

Many Thanks.

How many do you need? I went through this and now have a bunch left
over. Give me a shout off group and I'll stick them in snailmail.
Gordon

RF Grounding - DC block
 

wrote:
..
Here, on page 23...

http://www.icomamerica.com/support/k...?Download=1265

...Icom shows four 0.15 uf capacitors in parallel acting as a DC
block / galvanic isolator in the RF ground circuit of an SSB system.
I assume the caps are in parallel to increase the voltage rating...?
Are these plain old ceramic caps, like an 'orange drop' - say 400 VDC?

Many Thanks.

The transmitter wants to pass 150 watts to an antenna via a 50 ohm
coaxial cable. Thats a feed current of about 1.7 amps (I^2.R = 150W)
The transmitter would prefer not to use any of those 150 watts on
heating up a capacitor or wire, so they specify an AC resistance much
less than 50 ohms for the antenna ground path at the lowest frequency of
interest.
Let's say at 3 MHz you want to use a DC blocker that looks like less
than 1% of 50 ohms.
1/100 X 50 ohms = 1 / (2.pi.f.C) so
C = 1 / (2.pi.3X10^6 X 0.5) = 0.1 microFarads or more.

On the face of it, just one cap would do it. But there are practical
issues. There is an inductance associated with each capacitor,
so the AC resistance of a single cap starts rising again at some
frequency. If you put four in parallel, then this inductance is divided
by four, while the capacitance is multiplied by four. Sometimes,
designers put a low value capacitor in parallel with the others, because
this has low series inductance at high frequencies...
They are in parallel, so the voltage rating stays the same.
That's the main reason for paralleling them - to keep the AC resistance
LOW at any frequency you might tune, no matter how high.
Ceramic caps are useful at high-frequency and they are durable - may be
variable with temperature - but that's not critical here.

Brian W
Refs: Electronics G.H Olsen. Butterworths
The Art of Electronics Horowitz & Hill Cambridge UP

RF Grounding - DC block
 

wrote in news:294238b7-5ddf-47d5-b576-
:

Here, on page 23...

http://www.icomamerica.com/support/k...?Download=1265

...Icom shows four 0.15 uf capacitors in parallel acting as a DC
block / galvanic isolator in the RF ground circuit of an SSB system.

I assume the caps are in parallel to increase the voltage rating...?

Are these plain old ceramic caps, like an 'orange drop' - say 400 VDC?

Many Thanks.


Nope. The caps are in parallel to increase the total capacitance. The
DC voltage is less than a volt! The RF voltage is zero because the caps
act like a short to high frequency RF.

It's better to use 4 .15uf caps instead of one .6uf cap, too, by the
way. The more caps you put in parallel, the less the series inductance
(divides by 4) that reduce the RF pass through from the cap wires and
internal inductance caused by the way cheap caps are made. 4 caps will
also handle 4 times the RF current than one cap. High current RF
capacitors look like this, at these frequencies:

Two factors determine how much RF current we can put through them, the
capacitive reactance at this RF frequency and their maximum voltage
rating. Here's a little note if you'd like to read about it:

http://www.johansontechnology.com/im...s/rf/JTI_RF_9-
01.pdf

So, if we have .6uf at 4 Mhz and the cap is rated at 400V peak we can
get Xc = 1/2piFC = .066 ohms...very low. I=Er/Xc = 400/.066 = 6060A.
Sounds good until you look at the wires leading out of it...(c;] Other
factors, obviously, are involved. At 150 watts on 4 Mhz, the capacitor
is a virtual short and you'll see no voltage across it with very short
wires.

The series inductance of the ground wiring is what tears up the
effective RF grounding to the tuner. Boaters LOVE neatness! Every time
you make a neat-looking corner on the ground strap, you add inductance
that reduces the effectiveness of the grounding. Every time you add
length, same results. Ground straps need to be large in cross sectional
area and as short as is possible.....not the neatest installation to
show off...

Here's what REAL RF capacitors used in high powered MF and HF broadcast
transmitters look like and how much current they can withstand:

http://www.surplussales.com/Capacito...ps/MicaTransP-
ECap.html

Obviously, looking at the SURPLUS prices from Surplus Sales, you don't
want...or need...them.

You have to experience one exploding inside the cabinet you're sitting
next to to appreciate the RF power's capabilities....(c;] Broadcast
engineering is like fire fighter....months of utter boredom separated by
milliseconds of heart-stopping sheer terror.

--
-----
Larry
Been there...done that.