Hi Gents,
I got my BLF278 going 300W on 144 Mhz with 4.2W drive, but I made a few changes in the design, which I have documented in previous postings.
This is because I don't want to double up postings and create confusion. (I will write on a posting, which date it was updated...)
I drove the amp with 4.2W because that was full power from my little Icom hand held radio.
Half power gave from my HT is 1.8W. That was good for 240W out. The input VSWR is excellent, but I made some changes to get it down further.
The trimmer PA0V specified was to big. I found that the optimum was with the plates fully open (minimum capacity).
As such I changed the trimmer for a smaller one from only 20pF. This trimmer is now nearly at minimum capcity.
My guesstimate is that this capacity of is now in the order of 5 to 7pF. As such the 20pF trimmer is a better choice.
I also noticed that PA0V had a small capacitor in series with the coax. I think that was done to improve the VSWR, but there was no need for it in my case.
Perhaps because I used different and ATC caps on the input? perhaps because this is a different PCB?
When I drove the amp with 4.2W I could only just see the meter moving when I measured the reverse power.
As such reverse power is 30mw ? 50mW? too small to measure with a 10W bird slug.
This means that the return loss is about 20dB
I did not even bother to further improve this.
Output circuit...
I had only a 4-50pF compression trimmer, so I decided to use it and have a 47pF ATC parallel.
The power was less at minimum capacity so I changed the 47pF ATC for a 68pF ATC and started tuning the compression trimmer again.
I could not find a sharp dip of peak... so I increased it to 100pF parallel on my compression trimmer.
That gave slightly less output, so I went back to just one ATC of 82pF, also tested 100pF, and even 200pF.
Best results: with 2 x 39pF ATC on the output.
One ATC can only just handle the 300W. But the 2 x 39pF is a better choice as it will also be capable of driving a VSWR of 2:1 load.
Efficiency....
I got 9.8 Amp x 48V = 470W input, output was 300W on the bird.
300/470= 64% efficiency (bias was set to 0.2Amp each fet).
I think that is spot of what can be expected for a BLF278 on 144 Mhz.
My bird meter should be spot on, and my amp meter is a digital meter from Fluke.
Bias current
Firstly make sure the trimmer resistor is not set on maximum voltage to the BLF278...... that will give you 10Amp idle curerent or more...
I accidentally did this
It is suggested to avoid this. I recommend to do first bias alignment with 3Amp fast fuse for this reason....
Class AB is recommended for SSB use:
For 144 Mhz you need to measure 0.9A. That means each fet gets 450mA (2 fets in one transistor). I suggest to setup your bias on a warm heat sink.
If you going to use it at WSJT/FM just set it at 0.4 Amp total (2 x 0.2A). that is enough.
It is clear that the lower the bias current, the higher the efficiency.
It is also clear that the higher the bias, the more linear the amp....
In SSB the amp needs to be linear so that is why the idle current is higher.
On the input I made a few minor changes, which I have noted under the material list in bold letters.
You can try both, and choose what is best for you. Lets us know what you did and what the results are.
(I am very happy with this input VSWR).
Stability...
The amp appeared to be nice and stable, no funny things. Very good indeed.
Bias resistor...
I used a 510 Ohm/5W resistor and connected it to the 48V.
If you use 24V or 12V to switch your bias, just change the value.
But at the end of the day, I am a very happy chappy.
The BLF278 works like a rocket
300W out, for very little money, that is excellent.
More goodies to come... I will make a posting on how to make an Harmonic filter, how to combine two amps.
I will also be happy to answer specific questions. Please feel free to email me these.
73 Ron
VK4DD
See the action pictures...