[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: NO smoke

Mark L. Hammond wrote:
>I did, on more occasions than I care to admit, transmit 100W into it.  It 
>survived every time.

Robert Jewell wrote:
>I was sending 50 watts of RF into my 3733, from my FT-847.
>It doesn't seem to have affected the 3733.

Margaret Leber wrote:
>For people who are keeping score, a single unintentional minimum FT-847 
>power (3W?) 2m FM transmission down 75' of cheap RG-58/U into my 
>Millicom-modified Drake appears to have not caused any damage.

You guys have way too much luck!  The history of my station is littered with dead preamps and convertors, destroyed by my careless press of the PTT at the wrong time.  My local GaAsFET vendor got familiar with my regular orders for replacements.  At one stage I considered buying in bulk.

I exaggerate only slightly.  But this time I'm being careful!  I'm powering my Drake 2880 from the 12V output on the FT-847 antenna socket.  So I have put a little box in the feeder that contains a relay also powered off the 12V on the coax.  When the 12V disappears it switches the feed to a 50 ohm resistor.  Extensive tests (paranoia!) have convinced me that no measurable RF ever emerges from the port the Drake connects to.

For anyone else considering this kind of solution, I measured that on the FT-847 144MHz TX, there is always a 5 ms delay between the 12V dropping to zero and the start of RF power output.  That's plenty of time for the relay to flip.

But beware of hanging too much on that 12V line.  It's fed from a current limiter in the FT-847 that starts to limit at 300 mA.  It puts 400 mA into a direct short (the manual says don't try this, so don't).  My Drake draws 190 mA, this protection relay draws 40 mA, so the pre-amp I have on order from Kuhne had better meet its spec of 60 mA.


Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org