[Elecraft] K3/K3S 10W models with KXPA100

[Edward R Cole]

Brendon,

Yes, though you would need to construct it.

A simple RF sense ckt would be 1pF coupling cap from the coax line 
(maybe use a coax tee and connect the RF sense box to the tee).  At 
HF the tee will not show significant impedance bump.

The cap feeds a diode like the BAS-16 followed by bypass caps to 
ground (0.01uF and 10uF ought to suffice).  This feeds thru a 1K 
resistor to control a NPN transistor that will key the amplifier to 
ground.  You might add a 1uH choke in line before the resistor if 
concerned that the RF is not completely smoothed to dc.  The circuit 
I described is used in the W6PQL 2m 80w amplifier for RF sensed PTT 
(except different values for bypassing 2m).
http://www.w6pql.com/2_meter_80w_all_mode_amplifier.htm

Another way is to buy a kit.  W6PQL sells an ALC detector board which 
passes RF thru and detects RF providing a negative voltage.  The kit 
includes 750-ohm resistor which is good for 1-2w.  Probably better to 
use a 2K resistor for 10w.  I am using this board kit with 10K 
resistor to sample 2m 80w RF for a RF output meter.
http://www.w6pql.com/Sampling_RF_Power.htm#alc

Let me also comment about using the KXPA-100 with either K3/10 or 
KX3.  I recently bought a KXPA-100 for just those two radios and its 
a perfect companion.  One plus is heat is kept out of either radio 
which reduces frequency drift when running digital modes.  Of course 
digital modes don't need 100w but you can reduce drive to 20w with 
either radio driving the KXPA-100.  I don't plan to use my K3 outside 
the shack but the KX3 may go mobile with the KXPA-100.  I 
incorporated a PTT switch in my new station control panel to select 
either K3 or KX3 PTT which is provided to several accessories 
including the KXPA-100.

73, Ed - KL7UW

From: Brendon Whateley <brendon at whateley.com>
Cc: Elecraft Reflector Reflector <elecraft at mailman.qth.net>
Subject: Re: [Elecraft] K3/K3S 10W models with KXPA100
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Ron,

Would it be possible to put some sort of RF detector between the rig and
the amp to trigger the KEY line? That would seem to be a relatively simple
DIY project if need be.

- Brendon
KK6AYI



73, Ed - KL7UW
http://www.kl7uw.com
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