----- Original Message ----- From: "Gary Memory" gmemory@tks-net.com To: "'Edward Cole'" kl7uw@acsalaska.net; "'i8cvs'" domenico.i8cvs@tin.it; "'Amsat-BB'" amsat-bb@amsat.org Sent: Wednesday, September 19, 2007 7:12 PM Subject: RE: [amsat-bb] Re: New Preamp
Adding a preamp in my case is a total disaster. I could probably put a diode in my coax line and then charge batteries with the resulting DC.
73!
Gary, N7BRJ/DA1BRJ
Hi Gary, N7BRJ/DA1BRJ
Your case is similar to my situation in the bay of Naples and our problem cannot be solved using commercially made GaAsFET preamplifiers because they have a very low 1 dB compression point and a very low IP3 intercept point.
Buy the way very rapid advances have been made in the last twenty years in the development of semiconductors.It seemed that at first the field effect transistor (FET) and then GaAsFET were getting more and more popular over bipolar transistor, especially with respect to the noise figure from VHF to UHF
However the modern bipolar transistor is still of importance in receive technology,especially since a well know type of "Noisless Feedback Circuit" developed by Dr. David Norton allow them to be made more linear and exhibits less noise than a FET and greater IP and dynamic range than a GaAsFET
Since desensitization and overload are becoming a serious problem particularly in 2 meters as more and more amateurs started using high power linears and large directive antennas, the Norton circuit seems to be a modern solid state solution to the designe goals in those case like our case when, for 2 meters satellite communications, an extremely low noise figure less than 1.5 dB is not strictly necessary but, a very high IP and compression performance may be more useful.
On today crowed 2 meters amateur band and the high incidence of very near kilowatt transmitters, strong signal performance (1 dB compression and IP3) in preamplifiers is usually much more important than sensitivity.
For the above mentione reason I have homebrewed a Norton preamplifier for 2 meters using a BFG-195 + BFG-134 that exibit the following measured performance: Noise Figure NF= 1.4 dB Gain G= 20.5 dB 1 dB compression point = + 28 dBm = 631 mW of output power and since +28 dBm - 20.5 dB = +7.5 dBm = 5.6 mW it means that 5.6 mW applied to the input of this preamplifier are necessary to compress the gain of only 1 dB Third Order Intercept Point IP3 = +22.5 dBm input
For 70 cm I have homebrewed a Norton preamplifier with a AT-42085 + AT42085 that exibit the following measured performance: Noise Figure NF= 1.3 dB Gain G=19 dB 1 dB compression point = + 26 dBm output power IP3 = +22 dBm input
For those who are interested to duplicate the above Norton preamplifiers as well some special type of power GaAsFET preamplifiers having low NF with high intercept point and dinamic range here are some useful references:
1) High Dinamic Range Transistor Amplifier using Lossless Feedback by David Norton Microwave Journal May 1976
2) A new type of Preamplifier for 145 and 435 MHz Receivers by M. Martin DJ7VY VHF Communications 1/1978
3) High IP - LNA for 432 MHz by R. Bertelsmeier DJ9BV DUBUS N° 2 /1992
4) Proceedings of the 24 th Conference of the Central State VHF Society ARRL Catalog Nr 3266 "Power GaAsFET Preamps, by Ken Britain WA5VJB pages 6-8
5) Proceedings of the 25 th Conference of the Central States VHF Society, ARRL Catalog Nr 3614 144 MHz GaAsFET Preamplifier by Tommy Anderson WD5AGO, page 35-36
NOTE: Only a simple noise generator made with a diode and described in any Handbook is necessary to adjust the above preamplifiers for the lowest NF
If the ENR of the noise generator is not calibrated the value of the obtained noise figure will be unknow but it will be the minimum possible obtainable for that device in that preamplifier.
73" de
i8CVS Domenico