[Elecraft] low power on 6 meters

[Edward R. Cole]

Willis writes: "The input circuit is bypassed by the relay so the 
only thing in the?path is the
relay, coax connectors?and the wires.? I think it is the wires.?"

Craig Buck writes: "I notice high swr when my AL572 is in the antenna 
circuit - even when turned
off.? When I hook the coax directly to the K3, the swr is 1:1. Haven't figured
out why except maybe there is a low pass filter built in the amp that 
is in line
all the time.?"

I would ask if the relays inside the amps are coaxial relays or 
open-frame dc relays?  I have seen that very common use in HF 
amps.  So it works up to 30-MHz but SWR is high on 50-MHz.  Well 
perhaps you have heard the VHFers refer to HF as the "dc 
bands"?  Manufacturers try to "cheap-out" by using dc relays that are 
not constant impedance and that works OK on HF, for the most 
part.  But when wavelength becomes shorter so that physical 
dimensions of relays and "wires" become elements of transmission 
lines they are no longer 50-ohm impedance.  This IS why VHFers use 
high-quality coaxial relays in their home-built amplifiers with 
bypass lines of RG-58 or even RG-8.

Look inside and see if the relays are "good ole dc relays"?  I have a 
Mirage A1015G 150w 6m amp and guess what the impedance is when in 
bypass?  SWR=1.4 even when I have a 50-ohm termination connected to 
the output of the amplifier.  Connect the termination directly to the 
coax line that connects K3 to Mirage and SWR=1.0  Fortunately, I have 
an antenna tuner for 160-6m and I tweak out the mismatch when Mirage 
is off-line.  Input to the Mirage when on-line is 1.5 so I tune the 
input using the antenna tuner.  The antenna SWR= 1.06 so the Mirage 
is happy without need of the tuner.

Suspect those relays!  If the bypass is simple buss wire that too 
will not match well at VHF.




73, Ed - KL7UW, WD2XSH/45
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