[Elecraft] USB Adapters and RF

[Joe Subich, W4TV]

I see my cut/paste from the other reply left out some important
information.  The following is edited to add the missing references.

*Any time* one feeds an unbalanced antenna with coax and does not have
a current "balun" with a high choking impedance, the coax will act
like a random length "radial" or half of the driven element. When that
"radial" is an odd multiple of 1/4 wave long, the "end" of the radial
will be at a high RF voltage. Old timers will recognize this at the
"hot mic" (RF bites the lips!) scenario. When the RF voltage is high
enough, it can upset electronics ... a 5V P-P voltage at the USB port
of a computer/peripheral can "cancel" the 5V (or 3.3V in some systems)
USB power and shut down the port. RF voltages higher than 5V P-P can
result in *reverse bias* and permanently damage some components.

There are two good tutorials on common mode chokes on-line - one by
K9YC: http://www.audiosystemsgroup.com/RFI-Ham.pdf and one by W1HIS:
http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf
I strongly recommend studying both.

Bottom line, be *absolutely sure* to use high choking impedance common
mode chokes, properly installed, at the appropriate places in your
antenna system. Proper points are at the antenna feed point and at
the "entry window" to the building (outboard of the point at which the
cable shields are grounded so the choke is working against a low impedance).

If you can't do that, only purchase interface products that include a
proper over voltage suppressor on the USB power line (to clamp any
voltage to 0/+5.5V). Before anyone asks - I do not know which low
cost USB to serial converters include voltage suppressors.

73,

    ... Joe, W4TV


On 11/2/2011 3:59 PM, Joe Subich, W4TV wrote:
>
>
>> I run legal limit on some contests. I seem to burn up USB to Serial
>> devices. I am grounded properly. I am suspect of these things in high
>> RF fields.
>
> It is odd that I just answered this same question in a different form
> on the N1MM Logger list ...
>
>> *Any time* one feeds an unbalanced antenna with coax and does not have
>> a current "balun" with a high choking impedance, the coax will act
>> like a random length "radial" or half of the driven element.  When that
>> "radial" is an odd multiple of 1/4 wave long, the "end" of the radial
>> will be at a high RF voltage.  Old timers will recognize this at the
>> "hot mic" (RF bites the lips!) scenario.  When the RF voltage is high
>> enough, it can upset electronics ... a 5V P-P voltage at the USB port
>> of a computer/peripheral can "cancel" the 5V (or 3.3V in some systems)
>> USB power and shut down the port.  RF voltages higher than 5V P-P can
>> result in *reverse bias* permanently damage some components.
>
> Bottom line, be *absolutely sure* to use choking impedance common mode
> chokes, properly installed, at the appropriate places in your antenna
> system.  Proper points are at the antenna feed point and at the "entry
> window" to the building (outboard of the point at which the cable
> shields are grounded so the choke is working against a low impedance).
>
> If you can't do that, only purchase interface products that include a
> proper over voltage suppressor on the USB power line (to clamp any
> voltage to 0/+5.5V).  Before anyone asks - I do not know which low
> cost USB to serial converters include voltage suppressors.
>
> 73,
>
>      ... Joe, W4TV
>
>
> On 11/2/2011 2:34 PM, Lee Buller wrote:
>>
>> I run legal limit on some contests.  I seem to burn up USB to Serial devices.  I
>> am grounded properly.  I am suspect of these things in high RF fields.
>> But...what is a guy gonna do when RS232 Serial ports are totally discontinued.
>> Turn down or turn off the amp.  Hmmmmmm!
>>
>> Lee - K0WA
>>
>>
>>    In our day and age it seems that Common Sense is in short supply.  If you don't
>> have any Common Sense - get some Common Sense and use it.  If you can't find any
>> Common Sense, ask for help from somebody who has some Common Sense.  Is Common
>> Sense divine?
>>
>> Common Sense is the image of the Creator expressing revealed truth in my mind.
>> -  John W. (Kansas)
>>
>> Never interfere with anything that isn't bothering you.
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