Modifying the Hytek 3.7 - 4.2 GHZ LNA for 3456

This type of LNA is readily available from any number of sources at low cost. Most of these are specified as having a noise figure of around 1 dB. Some are a little higher, other slightly lower. From the ones I have seen they are all specified in terms of degrees Kelvin (K) and not dB. A 74 degree rating, which is common, is just ever so slightly under 1 dB NF.

They are quite easy to convert to Ham use on 3456 MHz. To do the job correctly some tools and/or patience is required. I prefer the power tool route but you may not have the tools I used available to you. What I wanted to do was to convert the input to an SMA connector and not have to use a WR-137 waveguide to coaxial adpater. As it turned out, that was fairly easy to do but I had to be very careful drilling holes. Read on to find out how I converted this from a TVRO LNA to a simple but effective 3456 LNA.

The above picture is the complete unit as I received it but with most of the cover screws already removed. It is made of aluminum and has the typical WR-137 input and a type "F" connector output. This unit is a stand alone LNA and does not have a down converter. Some TVRO products have both but this one fed the down converter through a long coaxial cable run, which was generally RG-8/U. See another of my pages for converting a LNA which is also a down converter.

Here is the business end of the waveguide input that has already been cut down with a Portaband saw. A hack saw will work as well. Notice a nice wide gold plated probe encased in Teflon that originally was inserted into the waveguide. The diameter of it makes it very broadband to cover the entire 3.7 to 4.2 GHz TVRO band.

This picture is of the back of the unmodified unit showing the 4 gain stages. This amplifier is rated at 42 dB of gain which is way too much for most Ham installations. The input is on the top left and the last RF stage is also on the top but to the right of center.

Since 4 gain stages provide more gain then desired, I decided to abandon the 4th stage and take the output after the thrid stage. RF out could also be taken after the second stage if even less gain is require. In this picture the first stage is out of view on the left. The second stage is marked "B" and the thrid stage is marked "A". Notice how the fourth stage has been isolated by cut and removed PC board traces. The blocking capacitor feeding the RF out to the "F" conector has also been removed so as to completly isolate the DC input. I drilled the holes for the SMA connector directly over the loop that fed into the gate of stage four. Mark and drill all three holes with a number 50 drill. Tap the mounting holes for the two hole flange (I recommend 2 hole type) for 2:56 and then redrill the center hole with a number 20 drill which is .161" diameter. Mount the connector first then make the solder connection.

In this picture I have removed the "N" connector used for RF out and partially tapped the original hole for a feedthrough capacitor. Unsolder the Teflon insulated input probe being careful not to damage the PC trace. Mark the holes for the SMA connector and then repeat the drilling and tapping above. Be very careful not to damage the PC board when the drill bits punch through. Install the SMA input connector and then solder the center pin to the PC board.

Make sure all of the solder pieces and drilling residue have been removed. If you have an air compressor, gently blow the filings out of the LNA. Now connect a nominal 12 VDC to the "F" connector. The LNA should draw about 60 milliamps if it is working correctly. Replace the cover and you should have a working LNA ready to go for 3456.