Re Tajfun 1000 on 70cm
Czech
version.
This PA can't be used too much time with output power above temp. limit of
the used LDMOS. Look at my a year old note:
Solid state ITB PA unit for 70cm:
Several years ago were designed more "kilowatt PA" units - in that time it
was very interesting issue, beside other reasons due to lot of 70cm EME
DXing and DX EME trips generally. Many new transistors were developed and
many more were used from analogue TV PAs replaced by digital.
However real kilowatt power PA output on 70cm were rather unique and max
total power of such PAs were rather around 750-800W. Such real power limit
were got in majority due to troubles of good cooling - lack of dense
aluminium radiators profiles, high PA SWR sensitivity, complicated
combination of more TV UHF modules (mostly abt. 150 - 180W each). Several
amateur construction were not very successful, because lack of kW
transistors, were much worse un efficiency compare 144 an 220MHz bands.
Relatively successfull is W6PQL design with combined two 500W PA modules
and use of obsolete Rohde Schwarz TV PAs with water cooling from analogue
TV transmitters. Unfortunately Italian ITB PA unit, utilized MRFE6VP1K25
device never reach satisfactory and reliable kW power. Although such PA is
declared as a kilowatt PA, in reality you can use it as a 600 - 750W
output unit only. Reason is simple: that transistor (check the datasheet
from Freescale issued in 2013) says the transistor has limited total power
dissipation 1333W but such power limit needs to be derated by 6,67W per
degree if the devide temp is higher, than 25°C. And because 432MHz PA has
efficiency about 50% only, during max power has the device min 60°C, limit
of the dissipation shall then be decreased to 1100W and the CW limit will
be (Theoretically, without reserve for reliable operation) 550W only. It
is very far from ITB data (900W)! Datasheet of MRFE6VP1K25 by NXP (2014)
doesn't publish any
sentence about necessity to decrease dissipation due to higher temperature...And
on top: however CW (continuos wave transmit) is not morse telegraphy, but
even if you will summ dots, dashes and spaces within 40sec interval, the
PA output can't be able deliver kilowatt - > the result
will be then destroy of expensive LDMOS.
Due to it I would suggest in case of usage PA with MRFE6VP1K25 transistor
on 70cm NOT TO EXCEED 600 - 700W out in telegrapy operation and 650 - 725W
in case of SSB only. Digital operation (MGM) shal be not use above 600W
out! If you will need more power, look for another producer, who offer
combined two 500W transistors in a PA. Another way would be use
- or a big copper heatsink to decrease
transistor temp below 50°C or a liquid cooling.
However, design is going forward, and what is not reachable
today with the above mentioned transistor, could
be possible soon with some new, up to date device, probably based on GaN
technology, which is more reliable at high temp.
73 Vlada OK1VPZ
www.ok2kkw.com
PS: Compare
http://vhelectronics.sk/index.php/en/ham-radio-vhf-uhf/big-tajfun-1000-432-mhz-detail
and
https://www.italab.it/uploads/prodotti
/pdf_1k_4326.pdf
->
It is obvious that VH Electronic with 600W output datasheet is more
serious than ITB, which declares 900W.
Just don't be fooled by the
title
Tajfun
1000 on the PA panel.
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