I have been building solid state RF amplifiers for the ham bands since I have been a ham. For some reason I like
the challenge of building power amplifiers with LDMOS. I am most active on the VHF/UHF bands so most of my
projects are for that frequency range. Take a look at some of these amplifiers and maybe you can get some ideas to
help with your next amplifier project.
This was my first amplifier project. I is composed of 4, AR313 type amplifier pallets in parallel. A simple cascaded wilkinson
splitter/combiner is used to sum the amps. The devices use are phillips blf248. A 7-element chebyshev filter keeps the harmonics
to a minimum. This amplifier was first built for 144mhz. I decided to do something else for 144mhz and convert this one to
6meters. The amp pallets were not kits they were built from scratch so they ar not as neat looking as the CCI boards. All controls
had to be removed from the front of the main amplifier chassis, as it was on 2meters, to make room for the 50mhz splitter and low pass filter.
A control panel was built using a Crouzet smart relay. When the key line goes low from the radio the plc engages the antenna
relays waits a few miliseconds then turns on the bias. The fans run the entire times the amp is keyed
and for 3 min after. When the radio unkeys the bias is turned off first then the relays disengage. Some other feature include SWR and
over temp shutdown. A simple bargraph for power output confirmation. LCD window with status. Also the bias can be left in the off
position for class c service ie. FM or jt65. Every aspect of the control is software defined. Very easy to change the operation.
This amplifier was built from a module that was removed from an MRI machine. I saw the
unit on ebay for a reasonable price. A listing photo showed that one section had two ar305 style MRF151g
circuits in parallel. I bought the item right away. After completely disassembling the module I cut out a
portion of the heatsink and circuit board. MRI amplifier systems are designed for pulse
service and have very low duty cycles. The FETs were mounted to the aluminun heatsink directly.
A channel 1/8inch deep and 1 inch wide was milled in the aluminum heatsink below where the MRF151g's
would mount. A copper bar of the same deminsions was milled/drilled and tapped to go between the FET's
and the heatsink. The bar is screwed to the heatsink and the FET's are mounted to the bar. This lowers
the thermal resistance enough to allow class AB service. The newly completed RF module was reasembled and
optimized for 144mhz operation. The module, relays and simple control system were installed in a readymade box.
The amplifier has been in service for about 1 year with no problems. Typical output is 400-500 watts with about
45volts on the drains and 15 watts drive. With 48 volts on the drains and 20 watts drive the output will go to about 620watts
but gain starts to compress above 550 watts.
222mhz 500watt amplifier
This was my first original design. It is built around a device from SemeLAB the D1028UK. The circuit uses
hairpin style transmission line transformers for input and output matching. A simple Wilkinson splitter/combiner
is used to sum two 250watt amplifiers. As with all push-pull amplifiers the second harmonic is greatly supressed
by the circuit itself, and the combiner greatly attenuates the third and higher harmonic. The amplifier has been in service for
about 5 years with no problems. The amp runs very cool and and takes only about 15 watts drive for full output.
See the original article.
224mhz 150watt repeater amplifier
The circuit uses a D1020UK device from Semelab to produce 150 watts on 224mhz.
I had the device on hand and wanted to do something with it so I built up this circuit.
The finished amp was going to be used for a reapeter project but was not. I will
probably add a low pass filter and some momentary relas. It could then be used for
weak signal work as a spare or for portable use.
1296mhz 110watt amplifier
This amplifier makes use of two mrf286 ldmos fets to make 110watts on 1.2ghz.
Gain is about 10db and it runs from a 26 volt 10 amp power supply.
The matching line deminsions came from a article posted by F5EFD although they need
to be adjusted slightly for 1296mhz operation. A fair amount of trimming was required for
tune up. I will corect this and test on a new batch of boards soon. I will post the artwork
when its finished. The two fets are combined using Sage 3db wireline.
One wavelength jumpers are used to provide the crossover at the input.
The devices were desoldered from a Spectrain amp board and 1/8 by 3/8 copper bar was
used to make screw down flanges for the devices. An old AMPS
basestation amp was cleaned out to make the chassis.
Some projects under development
A pair of MRF151g 300 watt pallets on a common heat spreader for 144mhz.
More to come on this project including a construction article on the circuit