My 70MHz equipment at this moment
I like to continue in QRP philosophy in the new 4 m band too!
So I decided to use low power and further QRPp (very low power!) tests will be conducted in future.
I don't run on the "DXCC race" or any other award, I like to understand the true new band capability under a technical point of view. So I'll work on low power, on antennas and any technical aspect of our Ham activity on the 70 MHz band.
Just for beginning a little amplifier chain was developed in order to achieve at least 28- 30 dBm (about 1W) of RF output.
I found a good result with this little amplifier and added a filtering circuit too.
Schematic is linked here. The 15 pF var/cap will be tuned for better harmonic suppression in the output RF signal.
The transistor choice is due to my junk box content.
The amplifier chain delivers about 30-32 dBm (1/2 W) depending on the input level (no higher than 7 dBm!) and depending on the 15 Ohm emitter resistor (higher value will lead to a lower Pout).
Some pics of my prototype and measurements (click to enlarge).
(I'm sorry for bad quality pics, in few time I'll put better images!)
The prototype ugly construction DDS Pre-driver and driver stage pre-driver tank coil Pre-drive and driver
PA-stage and 50 Ohm load PA-stage Output low-pass filter PA RFC particular SA measure
At the output a filter has been designed using the AADE software "Filter Design".
The filter schematic is here; for the coils I used just a piece of wire inside two toroidal core I had in the junk.
There is non problem to design and to mount this part of circuit, so no more notes.
ANTENNA for the moment is just a 2 elem. Yagi beam. The basic design was taken from the very interesting Martin's DK7ZB website wher I found the strating measures of the 2 element Yagi beam. I used the MMANA GAL software in order to optimize my own beam antenna. Measures are in order to maximize gain and 28 Ohm impedance.
The matching l/4 coax cable is taken from Martin's website as before, its lenght is 87 cm and is connected between coax connector and dipole.
Here are some more pics in order to understand my mounting ideas:
Some measurement equipment will be my next step
See you soon! -:)