[Elecraft] DC insertion into antenna coax

[Dauer, Edward]

I would be careful about running DC through the coax.  Rapid rise and fall times of the DC voltage can generate RF spikes, and the collapse of the field in a DC relay if there is one at the business end can produce high instantaneous voltages back down to the rig end of the feedline. 

There are diode and capacitor-based circuit protections that can prevent this, but those phenomena were the prime suspects for why a remote antenna switch using DC inserted into the coax blew away the PA transistors and associated circuitry three times in a K2, until I trashed the switch.  I can’t describe exactly what the protection circuits are, though as I recall they are pretty simple.  Should be something in the archives about it from a couple of years ago.	

Ted, KN1CBR
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    Message: 23
    Date: Wed, 15 Nov 2017 10:42:19 -0800
    From: Steve Sergeant <stevesgt at effable.com>
    To: elecraft at mailman.qth.net
    Subject: Re: [Elecraft] Using T1 with KX2 or KX3
    Message-ID: <1c1e1830-5ccc-d81e-5da7-8c25946733c2 at effable.com>
    Content-Type: text/plain; charset=utf-8
    
    Hmmm. That means that it should be trivial to make a DC-blocking circuit
    that would allow you to pull the ring of J3 to DC (and not RF) ground
    over the coax to initiate tuning. Then you would have part of the remote
    control puzzle solved.
    
    That's nearly enough to finally get me to buy a T1.