I should clarify my question; my intent is to do everything practical to
protect the FETs - overtemp, overcurrent, and excessive reflected power. The
usual location for the reflectometer is after the low pass filter - this
provides an accurate indication of the load match. But if the wrong band LPF
is selected, this does not protect the fets.
Logically one could move the reflectometer to between the fet output
transformer and the input of the low pass filter. This would protect the fets,
but would always show about a 2:1 SWR due to the high 3rd harmonic (-10 to -12)
reflecting back from the filter. Perhaps this is the best solution - and maybe
up the reverse power threshold to normalize it.
Or, modify the LPFs with a "diplexer" arm that shunts 3rd harmonic to a small
50 ohm load. This would have the effect of reducing the reflection to the fets
as well ...
Chris Scott
W4NEQ
-----------------------------------------------------
But my question relates to swr mismatch protection. My project is a 2 x
Blf188XR 1200 watt amp, 80/40/20, for ssb service. I've ordered a 4z4rb board,
and got my two Fets for 129 euros each from
www.pcs-electronics.com<http://www.pcs-electronics.com>. I spent $190 on a
0.75" x 6" x 12" copper heat spreader, and it will be mated to a large aluminum
forced air heat sink. As much as I hate to do it, it seems that soldering the
fets in is best to keep case temps reasonable. That makes one doubly worried
about blowing the fets.
They brag about 65:1 swr survivability, but this is pulsed. Clearly a post LPF
reflectometer ref sample is necessary to shut it down with a poor match, say
above 2:1. But I see another pitfall; the switched LPFs being set to the wrong
band.
So, is the best solution adding another reflectometer after the output
transformer and prior to the filter to protect against that? With an expected
third harmonic being 10-12 dB down at this point, the threshold might need to
be high, like 5:1 ...
Chris Scott
W4NEQ
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--
4Z4 RB
Baruch Zilbershatz
Ha-Shibolim 18
Nitzanay-Oz 42836
Israel
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