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Date: Mon, 30 Nov 2015 13:15:56 +0000
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From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org, rsgb_lf_group@yahoogroups.co.uk
Subject: LF: Beware damage from active antennas
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Status: O

Many people use active receive antennas on the LF/MF bands;  I use a
commercial Procomm active whip which sits under my 137kHz Tee.   It is
happy to exist in the high E-fields generated when transmitting, even
though it is overloaded and useless while Tx is under way.  But it
survives (it was probably designed to cope with high power adjacent
operation) ,  I had never bothered to see what it was putting up its
coax when the 137 or 475kHz Tx was on air.

Until today...

My new homebrew LF receiver
http://www.g4jnt.com/Coherent_LF_Receiver.pdf   has  an LT5524
digitally programmable gain stage added at its front end - used mainly
because I have quite a few of these, and it provides up to 20dB of
adjustable decent power handling gain.     A week or so ago, the Rx
failed, after I'd had a session transmitting on 137kHz, and the lowest
power setting of 300 Watts. The Rx had remained connected to the
Procomm antenna during this test and teh LT5524 had died from
overload.   The data sheet  states an absolute maximum of 3V on the
input

I replaced the LTC5524 chip, connected four IN914 as paired
back-to-back protection (allowing 1.3V peak) across the input and
thought nothing more of it.  Yesterday, the same thing happened - the
diodes didn't do their job.  Of course, it doesn't help that I AC
couple into the LT5524, being too lazy to make a second  input
transformer as well as the bifilar output one!

So, decided to see what the Procomm actually was putting out while
transmitting on 137kHz - it was horrifying to look at !    There was a
square wave output of about 4V peak to peak into 50R, BUT,
superimposed on leading and trailing edges were sharp spikes of a
hundred  nano seconds or so with amplitudes approaching 13V - the DC
supply.   They are there, presumably, as that antenna has a
deliberately  enhanced frequency response at the top to extend its
operation into the VHF broadcast band (the manual says it is usable
there)

So the pk-pk square wave alone was enough to damage the LT6624
amplifier chip, but those spikes took it though the roof.  No wonder
two of them popped immediately excess RF appeared.

Ironically, leakage up the Rx feed from a single bog-standard relay
used as Tx/Rx changeover in my 137kHz transmitter was only 80mV pk-pk.
Had I been using  to that antenna, things would have survived

Need a better Rx line up.   Perhaps remove RF amplification from
before the Finningly Dongle receiver and compensate that with a
variable gain audio amp before the input to the 1kHz filter /
digitiser

Andy  G4JNT