Building a Cavity pre-amp for 2m

To have an optimum reception on 2m I was in need for a good pre-amp. I had some MGF-1801 GaAs fets left and needed a small bandpass pre-amp. After searching the net I came across the site of Kevin VE3KH who I also knew from moonbounce contacts in the past. He had an interesting design (I8CVS/I5TDJ) of a 2m Cavity input pre-amp which was good to re-produce. The disign can also be found in Dubus 3/1994. On the website of PA3CSG is also a copy of the design and the complete component list. I started collecting the needed partsand found out that nearly all components were in my junk-box. Only some copper pipe I had to buy.








Some of the hardware parts. The inner conductor of the cavity is made from the inner conductor of a pice of 7/8 cellflex cable. I stripped the cable and peeled off all dialectric material from the inner conducter. It turnd out to be exact 9mm as I needed to get a 75 ohm impedance of the cavity.

The N connector is made of a flange type used on Bird power meters. I had a bunch of them laying arround. I removed the center pin and the PTFE dialectric. Then I could heat the body of the connector with a flame and remove the square flange of the connector.

I needed to mount he connector into the 35mm outer pipe of the cavity. My neighbour who is a plumber helped me out with some tools to make a flange on the pipe.





Then I could fix the inner conductor to an endstop with silver tin (AgSn) which has a higher melting point then normal solder (PbSn) For easy putting the N connector in place I drilled a 6mm hole in the inner conducter, solders a short pice of 6mm coper pipe in it and drilled a hole of 3mm in wich fitted the inner pin of the "ex" bird connector.

After soldering everything together it looks like this





I did not have the complete article from DUBUS so I asked arround and within 30minutes I got a scanned copy in my mailbox. Thanks to my friends on moon-net.

If you need a copy of the PDF file, please drop me a mail and I will be happy to e-mail it to you.

I build a second cavity with minor diffeneces. This time I drilled a 9mm hole trough the endcap and soldered the inner conductor direct to the endcap using 2% silver solder.

I will also use larger tin plated boxes (Arie PC7M is the house suplier of tin plated boxes) for the electronics.

Now the cavity itself is ready and I was able to check them. I did yhis with my antenna analyzer. Well it's a big word for a MFJ ;-) To check the resonance frequency I mounted a 1K ohm resistor at the top of the cavity (where the gate of the MGF-1801 will be connected) then I could measure SWR at the input. The cavity was realy nice dipping with a 1:1 SWR on 144MHz. I tested both cavities and both give a nice and rather sharp dip.





The Finished Pre-amplifier after a spreay-paint job. The pre-amp Were measured last week, both with very good results, one pre-amp had a noisefigure of 0.18dB the other one 0.22dB



A view inside real plumber work.. deadbug style no PCB needed..



Just another view inside.. Just a 5.1V 1.3W Zener diode as voltage regulator.