This is not correct.? The output impedance of the PA does not change
the SWR and resistance/reactance of the tuner/antenna combination.
What one is really doing is tuning out the reactance of the antenna
and setting the load resistance to the design value of the
amplifier.? A reactive load increases the dissipation of the PA
finals.? Tuning at low power within the voltage and dissipation
limits is safe.
David N1HAC
On 8/3/22 3:44 AM, Ashhar Farhan via
groups.io wrote:
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Many recommend tuning at a lower power. This risky
as well. That is because at lower power, the output impedance of
the PA will be different. Once you are tuned at QRP level,
increasing the power will result in a mismatch again!?
- f
Unfortunatel, no. The sbitx has no way to
check the swr (yet).?
Even if it had, as we are dealing with very
high power, fiddling with the tuner for 15-20 seconds with
large mismatch will send all the 40 watts back into the
PA, destroying it.
What will work is a resistive bridge. This
will dissipate most of the power in big resistors until
you are tuned up.?
You can paraellel up ten x 470 ohm resistors
of 1 watt to make each of the three legs of the resistive
divider of the swr bridge. A DPDT switch can bypass it
once you are all tuned up.
On Wed, Aug 3, 2022, 12:21
PM Trevor Stone <
tstone@...>
wrote:
In the
operating manual I see the following warning:
```
# 1. Connect the Antenna first!
The antenna jack on the back must always be connected to a
properly
matched antenna with an SWR of less than 2 on the
frequencies you intend
to transmit on. This is a recommendation for all amateur
wireless radios
but more specifically the sBitx. The sBitx can put out
upto 50 watts of
power and if the antenna does not match, it can damage the
power
amplifier transistors instantaneously.
```
Does the sBitx have the ability to tune or check the SWR
without
damaging itself, or does one need to use an antenna
analyzer before
using the radio?
I ask because most of my operation is portable, so my SWR
changes every
time I set up my station.? SWR can be high even for an
antenna of the
right length if it's deployed in a compromised position or
if I made an
error during setup (like last month when I forgot to
attach
counterpoises to an end-fed antenna :-)
Thanks,
Trevor KF0FTJ
--
=-=-=-= Trevor Stone -=- [Flwyd] -=- <tstone @ >
=-=-=-=
Computer science, eclectic philosophy, games, wits,
esoterics, odd hats
?
? ? ? ? ?Thou brazen boil-brained malt-worm!
{embrace society}? ? ? ? ? ? ? ? ? ? ? ? ? POLAROID does
it in seconds.
Calvin: The world is a complicated place, Hobbes.? Hobbes:
Whenever it
seems that way, I take a nap in a tree and wait for
dinner.
David McGaw