Message-ID: <565D78F1.5060106@lineone.net>
Date: Tue, 01 Dec 2015 10:39:45 +0000
From: LineOne <dhchurch@lineone.net>
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To: rsgb_lf_group@blacksheep.org
References: <CAA8k23Q8YSBO5NE8Qtyt=9TJ8J18tAjLTYFBRxS+LRp+WOHY0A@mail.gmail.com>
In-Reply-To: <CAA8k23Q8YSBO5NE8Qtyt=9TJ8J18tAjLTYFBRxS+LRp+WOHY0A@mail.gmail.com>
Subject: Re: LF: Beware damage from active antennas
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Status: O

Can you "mute" the receiver, i.e. kill the internal power during TX, 
possibly just to your LT5524 stage, short circuit the input before it 
reaches anything active or kill the power to your active antenna?

I don't rate diodes, even faster ones than the 1N914/4148, for overload 
protection, they do make good mixers to receive broadcast signals off 
Long Wave.

I did once mistakenly DC couple an active antenna using 12V into the 
R5000 but it only destroyed a small inductor.

High, M0DSZ

> 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
>
>