The T1 specs say that it should function with input power as low as 0.5
watts. An 8 dB attenuator at the output of your 4-5 watt QRP rig should
still give you close to 0.5 watt into the T1, and the attenuator acts as
a buffer that diminishes the reflected power from the high SWR load.
Tune with the attenuator in place, and then remove it after the match
with the T1 is achieved.
https://www.rfcafe.com/references/electrical/vswr-reduction.htm
https://www.rfcafe.com/references/calculators/vswr-return-loss-conversion-calculator.htm
According to the 2nd link, an 8 dB attenuator would turn a 20:1 SWR at
the load into less than 1.4:1 at the output of the rig.
HUGE CAVEAT: An attenuator designed for 8 dB only gives the design
value reduction in reflected power for input and load impedances that
match the design of the attenuator. An attenuator designed for 8 dB
with 50 ohm input and output won't give 8 dB with radically different
impedances. That being said, it doesn't take much attenuation to help
and I think it would be worth a try. I can't think of any other way to
do what you want unless the T1 can be manually stepped through a match
and you monitor the input with a VNA (NanoVNA, for example).
There are a lot of T-network attenuator calculators out there ... here
is one of them. https://www.pasternack.com/t-calcualtor-tee-attn.aspx
73,
Dave AB7E
On 5/22/2023 11:52 AM, [email protected] wrote:
I have a T1 that I have used for several years. I recently purchased a
QCX-mini, and an SW-3B. I normally use my K2, KX2 or KX3 for portable
operations, but occasionally I like to use something different for QRP CW.
Both the QCX-mini and the SW-3B require a 2.0 or less SWR. The
documentation for SW-3B says that operating at any SWR over 2.0 will damage
the radio. The outputs varies with voltage, but usually runs at 4-5 watts
with a 12V supply. If I attach the T1 to that rig, and perhaps the QCX-mini
as well, it will require the rig attached to present a constant carrier to
tune. This seems to eliminate using an auto tuner with the SW-3B at a
minimum in that configuration.
Can someone tell me how I might get the T1 to tune in the field with
something other than the rig. If I could trigger it to tune, then I could
attach to the radio after the tune cycle. My plan is to use a resonant
antenna, but that may not always be possible, especially with a three band
rig like the SW-3B.
Thanks,
Terry, N7TB
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