BATC Forum

Hi,

Just a few notes about the input stages that you find on the PA's that a lot of people use on 70cm...
There are three input stages which can be cut out and work quite well, it will involve removing the whole PA board though.
If you do this be careful of the 50ohm screw down resistors, they are quite fragile.

The way i did it was to use a band saw (or you could use a dremel with cutting disc?) to cut out the lna sections.
I sanded along the edges of the cut out board to make sure there were no shorts between the 4 layer board.
I removed the mixer, hot air gun required as they are well attached!
I then used a sharp blade to remove the parallel input coupling track, this reduces the input loss and improves noise figure.

SMA sockets were soldered to input and output tracks as shown below.
The power supply is +8v on the top most inductor, most of the top surface is ground.

The layout of the LNA's are different but are effectively the same.

Here's some example photos of before/after.
I've marked the DC power input locations and the track to remove.

Measured Gain was 12.9dB and 1.85dB Noise Figure at 437MHz, seems to cope quite well with strong signals from the limited tests i've made so far.
You will need to adjust the helical filter slightly but it's not critical as it's about 30MHz wide.

Rob

Thanks Rob, all three of my inputs are like the bottom one in your picture. If I remove the components at the left hand end of the coupling track, will that suffice or should I cut the track at each end?
I removed the mixer by its selective dismantling, involving 'crunch' type noises. I was then able to solder in a PCB mount SMA with one side of its ground legs removed, thus allowing me to access the centre pin and solder it to the helical filter output track. As I had left the 8V regulator in, I just had to align it, easily done with a noise source and SA. Pretty much wind all the cores out as far as they would go worked for me.
Handy little filtered gain block
I wonder if daisy-chaining two would help with system sensitivity or just swamp things....
Gareth

Much better to peel the track off the board.
Thinking about it there is probably a high and low band version of these units hence the different layouts, maybe even different filters!
Cascading will work if you need more gain.

Rob

hello, it seems that this item is no more on ebay ....another source ?
tkans
francois