[TowerTalk] Protection of a preamp

Dan Schaaf dan-schaaf at att.net
Tue Jan 21 16:35:40 EST 2014

Now, all you need to worry about is a lightning strike. Hopefully your
preamps short the input to ground when powered down. And a fuse resistor of
about 22 ohms 1/8 watts in series between the antenna and first transistor
that will blow if lightning strikes nearby. I learned this lesson the hard

Best Regards
Dan Schaaf
K3ZXL www.k3zxl.com
60 Meters www.60metersonline.net
NOBSKA www.nobska.net
Cape Cod Instruments www.gnm-inc.com
Tel: 941-766-0706
Fax: 941-766-0707

-----Original Message----- 
From: Chuck Smallhouse
Sent: Tuesday, January 21, 2014 4:09 PM
To: normanlizeth at gmail.com ; towertalk at contesting.com
Subject: Re: [TowerTalk] Protection of a preamp

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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