Power module found.

I was able to get  a power module for 50 MHz for little money. It can deliver 20 Watt by 200mw input. 
This can restart the 50 MHz transceiver project that is on the shelve for more than a year.

working on breadboard

This pictures shows the current status.
RF pickup on mic input has been solved. A speech compressor has been added. Now it is waiting to be build into a decent box. I am considering to add a synthesizer based on the AD9551 dds chip. Then it can evolve into something more usable.

Working, more or less

It has been a long time after the last entry. Building was going on though.
I was able to get the strength of the harmonics to acceptable values.
All modules are build on the breadboard. I did not want a enclosure because of the price. This radio won't be used a lot due to its limitations (power output frequency range)
I may reconsider this though.

Finally the transceiver worked on the dummy load. I got about 1 watt output. The signal sounded clean. Harmonics suppression was adequate. Then I connected it to the antenna. That caused a lot of problems. At first I thought The PA was oscillating but it wasn't. It turned out had severe RFI. I had to rearrange modules and wiring. Now it seems to be OK. To solve it once and for all I need to build it in a metal case.

I think I will try to find a cheap enclosure to build it in. I may add a S meter and a frequency counter.

1 watt amplifier

Finally I found some time to build the amplifier.
The goal was to get at least 1 Watt output without too much distortion.
I made a push pull amplifier using two 2n3866 transistors. The result was not good.
The maximum output power was 0.5 Watt with 30% efficiency. I blew a few transistors.

The 2n3866 was not a good choice as it is intended for 28 volts. So I got some 2n4427 transistors. Those worked better. I can get >1 watt output. harmonics are -20 dbc down. Efficiency is better than 40%.

At the moment I am building a lowpass fiter to suppress harmonics. Using a 3 element lowpass filter I was able to get 1,5 watt output. This filter also improves the matching. Harmonics are -35 dbc . I am replacing this filter with a 5 element filter to get the required suppression of the harmonics.

So far it seems that the 3th harmonic is still too strong at -40dbc. I may have to use better capacitors. The coils are aircoils so those should be OK.

1 Watt PA

I am considering to build a simple 1 Watt amplifier.
The Power module is quite expensive. It costs 60 dollar + shipment. I may buy it later.In the mean time I could build a simple 1 Watt amplifier. That should be enough for local qso's. Maybe some DX as well when the conditions are right. Later, at sunspot maximum, I could buy the module. Replacing the 1 watt Amp with the module should be easy. I hope the module is still in stock then.

I have a couple of 2N3866 transistors available. Two of them in push pull configuration should do the trick. I may get a bit more out of them. Two watts may be possible. However the inter modulation may be to high. I will have to try as I don't know how to calculate it.

Little progress

Last week I worked on the 144 MHz FM transceiver.
See My 144 MHZ transceiver blog
Not much progress to report on the 50 MHz project this week.

I found out that the 50 MHz power module is available at
rfparts.com
This module is the best option. I have looked at other PA's. Lots of PA's can be found on the internet. Unfortunately a 200 mW in 10 Watt out PA suitable for SSB could not be found.

picture of 200 milli Watt TRX

Here is a picture of the current state of the project.














Top left: Receiver
Bottom left: VXO modulator IF filter
Middle: Preamp
Top right: 200 mW TX driver
Top right: Frequency counter

It has 200 milli Watt output. This should be enough to drive a power module. Those modules are hard to get though so I may have to make a PA myself.
TX and RX work. However more fine tuning is needed.