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Re: [TowerTalk] Protection of a preamp

To: normanlizeth@gmail.com, towertalk@contesting.com
Subject: Re: [TowerTalk] Protection of a preamp
From: Chuck Smallhouse <w7cs@theriver.com>
Date: Tue, 21 Jan 2014 14:09:30 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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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