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 25C. And because 432MHz PA has efficiency about 50% only, during max power has the device min 60C, 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 50C 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.