[Battlemesh] ESD vulnerability with small TP-Link devices modded for ext antenna

[Ben West]

Sorry if this is not directly relevant to Battlemesh.  Sharing, since I
think some list members are playing around with modded TP-Link devices like
the TL-MR3020 and TL-WR703N.

I've found, at least for the MR3020, that the popular instructions for an
external antenna posted online
here<https://apollo.open-resource.org/lab:argus#modifications>seem to
leave the device badly vulnerable to ESD damage during
thunderstorms.  Two MR3020's I'd modded this way (i.e. with a 0ohm resistor
de-popped and the pigtail attached to an exposed pad) appear to lose ~ 50%
of their transmit power after a lighting storm passed over.  That is, two
MR3020's with the same mod and same 7dB dipole, two lightning storms on
different occasions, and both times the node's transmitter power drops
substantially at some point during the storm.  In each instance, the MR3020
was indoors but close to an exterior window.

Since the instructions for modding the
WR703N<http://blagg.tadkom.net/2012/09/15/better-wr703n-antenna-mod/>for
an external antenna are basically the same, I'm guessing this would
also make that device similarly vulnerable.

My initial hunch that the advertised approach of de-popping a 0ohm resistor
for the antenna pigtail is bad, because it's removing a filter capacitor
used for blocking DC (or other unwanted transients) from being coupled from
the antenna back into the Atheros chipset.  I'm going to try severing the
trace leading to the onboard patch antenna on an undamaged WR703N and
solder the pigtail there, as that will at least preserve the filter caps
TP-Link designed in (at the expense of impedance mismatch).

-- 
Ben West
http://gowasabi.net
ben at gowasabi.net
314-246-9434
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