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Date: Tue, 1 Dec 2015 11:23:09 +0000
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From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Beware damage from active antennas
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Status: O

Yes, there are plenty of options for normal Tx/Rx.  In fact the
existing C/O relay in the 137 transmitter provides more than enough
isolation for that

But I was on a separate active whip at the time, a quite independent
conenction and irrelevent to real operation at the time.  It was the
fact that the active whip could shove out such an alarmingly high
level that concerned me - 13V very sharp spikes.  And I forgot to
mention in the previous post, those 13V edge spikes went to both +13V
and -13V, so technically 26Vp-p

Perhaps the homebrew designs others use aren't so robust in their output stages?

Andy  G4JNT


On 1 December 2015 at 10:39, LineOne <dhchurch@lineone.net> wrote:
> 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
>>
>>
>
>