[Elecraft] Capicator Size for filtering across battery power

[Joe Subich, W4TV]

 > That will SOUND like RF feedback, but it is NOT.

True - it's good old amplitude modulation.  Put two signals though
a common impedance and each will modulate the other.  This is the
principle behind Heising (choke) modulation.

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

Kenwood have documented this for "data mode controllers" in their
manuals for 10+ years although they do not explain the reasons
properly.

> 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.

Or use the "Accessory" power output if the rig provides one with enough
current capacity.  microHAM have been designing their interface cables
for this solution since the first microKEYER (more than 8 years).

Unfortunately, Yaesu and Elecraft limit the available current from the
accessory power output which limits the utility of the Accessory output.

> 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.

Better yet, bond the transceiver chassis to the power supply with
large, *bolted* connections rather than rely on small friction fit
connectors (e.g., "Molex" pins, low current "PowerPoles", etc).  Keep
the resistance in the power supply return much less (0.01 x) than the
effective resistance of all the other parallel paths.  The current
will go where it should and any residual current will be small enough
to not cause problems because it can't generate a significant voltage.

> When I say monster cap, I'm talking at least 10,000 uF. That's big,
> and it ain't cheap.

10,000 uF is marginal with rigs drawing upward of 20A on voice peaks.
10 F would not be too small!  However, it must be *at the rig* to be
effective.  The batteries are already the best filter capacitors one
can have adding a few thousand uF across the battery is like putting
an inflatable boom on the seashore and expecting it to stop the tide.

73,

    ... Joe, W4TV


On 10/8/2012 3:52 PM, Jim Brown wrote:
> 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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