[Elecraft] Capicator Size for filtering across battery power

[KD8NNU]

Gents,

Thanks for all the information regarding my request for how to size a 
capacitor across my batteries to fix my perceived problem of RF.

>From what I understand is that this is just the normal current flow during 
voice peaks or digital peaks when transmitting and not RF.

So my options as I understand them are to put a large cap like the car 
stereo guys do across the batteries.   These are large cap's and since I 
have both the amp and radio on the same batteries I will need two cap's one 
at each device.

If I have this wrong then please correct me.

Again as always thanks for the replies on the topic as there was more behind 
a simple question than I realized.

Thanks
Don


~73
Don
KD8NNU
-----Original Message----- 
From: Mel Farrer
Sent: Monday, October 08, 2012 5:24 PM
To: elecraft at mailman.qth.net ; jim at audiosystemsgroup.com
Subject: Re: [Elecraft] Capicator Size for filtering across battery power

Good points Jim,

I would like to add a clarification and results that I use.

1.  On point 2)  I would rather power the accessories on the battery end of 
the rig power cord as the voltage regulation will not have the rig IR 
losses.

2.  On point 4) I have used a 5 Farad cap with 3 inch leads to the rig.  It 
does the best of trying to cancel out the IR losses on voice pikes and does 
a great job.  This is the one used by the car monster audio amps.  Cost is 
around $50 direct and less on eBay.

Mel, K6KBE

--- On Mon, 10/8/12, Jim Brown <jim at audiosystemsgroup.com> wrote:


From: Jim Brown <jim at audiosystemsgroup.com>
Subject: Re: [Elecraft] Capicator Size for filtering across battery power
To: elecraft at mailman.qth.net
Date: Monday, October 8, 2012, 12:52 PM


On 10/7/2012 7:06 PM, David Moes wrote:
> That is interesting.    I suspect that they are reacting to the
> elecromagnetic field created by rapid current fluctuations in the DC
> lines rather than picking up RF.

NOT electromagnetic fields, just simple Ohm's Law.  There's IR drop on
the power cable between the battery (or power supply) and the rig, and
if the rig is running full power, peaks of at least 20A, with an
envelope that follows SSB modulation.  If you are powering accessories
from that same battery and have audio connections between those
accessories and the rig, the IR drop (modulated SSB envelope) will
appear between the chassis of the accessory and the chassis of the K3,
and will be added to the audio.  That will SOUND like RF feedback, but
it is NOT.  W8JI told me about this several years ago.

There are at least four good fixes for this issue.  I've set up and
tested all of these, and all of them work.

1) Don't power accessories from the same supply that feeds your rig.
Instead use a separate wall wart for them.

2) Power accessories from the same supply, but use a simple power
splitter (Y-cord) at the RIG end of the power cable to get the accessory
power.

3) Power the accessories from the battery or big power supply, but bond
from chassis to chassis of all interconnected equipment with short, fat
copper.

4) Power the accessories from the battery or big supply, but use a
monster capacitor across the terminals right at the rig.  When I say
monster cap, I'm talking at least 10,000 uF. That's big, and it ain't
cheap.

73, Jim K9YC
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