[TowerTalk] Protection of a preamp

Chuck Smallhouse w7cs at theriver.com
Tue Jan 21 16:09:30 EST 2014

To add to Gerald's very experienced comments, the addition of a 
sequencer is highly recommended, especially if the last 
stage/sequence of it will control the "RF Inhibit" function of your 
transceiver.  Some like the K3 have that capability available at 
their interface connector, or you can implement it with a small 
external relay(coax).  This prevents the inadvertent hitting of the 
key paddles or the PTT switch and emitting RF (maybe into the pre-amp 
front end), before the sequencer has progressed through all it's stages.

Also a series of dropping diodes can be replaced by a single 12 Volt 
three terminal regulator such as 7812.  Reverse diodes across all 
relay coils will usually remove back EMF spikes.  In addition to the 
7812, I often precede it with about a 15 to 18 volt zener(with 
dropping resistor) and use plenty of different size  bypassing 
capacitors on both regulators, as will as a feed through, RF 
bypassing, cap .  These precautions will greatly reduce the 
possibility of any of your induced TX RF getting to the LNA via it's 
control cable.  Usually most current LNAs use an internal voltage 
less than 12 VDC (5VDC?), that is supplied by an internal small 3 
terminal regulator.

Current majority school of thought (at least by VHF/UHF weak signal 
ops) is to turn off the DC to the LNA during TX periods.

Chuck,  W7CS

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