Saturday, September 17, 2011

440 MHZ(70cm) Preamp

I completed building my 440mhz preamp.  The main reason I built this was to use for AMSAT reception of the amateur radio satellite downlink signals.

The hardest part with working the satellites is acquiring of the downlink signal from the satellite.  That is much harder than the satellite hearing you.

Using a small pre-amp can make a huge difference in being able to establish communications on the satellites.

I chose to build one using the cheap and easy to build kit available from Ramsey Electronics.  They have cheap pre-amps for 2m, 220mhz, and 440mhz.  Each one is only $14.95.  They use a  2SC2498
transistor and provide high gain and very low noise.  They have a tuned filter and provide amplification over about 24mhz of bandwidth.

They are popular with the people on the AMSAT mailing list, where they are said to compare favoribly to commercial pre-amps 10-20x the cost.  Measurements reported on the mailing list indicate performance of 20db of signal gain, with less than 1db of noise gain.

A pretty cheap kit for the usefulness it provides. Here is the page on their website for more info.

I chose an old tin box I had to house the finished kit.  I save all the tin boxes I can find such as Altoids, and any that come with gifts or whatever.  This particular box was from a cartridge that my kids got for the NintendoDS.  Its roughly twice the size of an Altoids tin.  The size was really needed to house the batteries.  In order to keep things simple I used a pair of 9V batteries to power the device.  They are wired in series and provide about 18V.  Even though the specs say to use 15V, it seems to work just  fine with the 16-18V the two 9v batteries in series provide.  The twin batteries should provide a long battery life between changes as well.

The first thing I did was to start to mount the external connectors.  This consisted of a pair of BNC Jacks, and a SPST switch which will be used for the power ON/OFF.

I have become a fan of using various metal tins for my kits and projects.  I have discovered one invaluable tool to use for the metal work is a metal hole punch.  I bought a cheap one from Amazon which comes with dies for making various size holes.  It works quite well, and makes very neat holes in the tins.  I definitely recommend it if your going to be using Altoids tins and similar boxes for any kit building.

Once the external connections were made, I mounted a couple of 9V battery clips inside the box using a bit of JBWeld.

Then I mounted the finished board, and made all the internal wiring connections.  The kit board looks small in this box big enough for the two 9V batteries.

Added a some labels from the ole label maker on the cover, and its complete and ready to go.  Doing some testing with the amp, its sounds quite good.

I plan on using this with my ARROW Antenna, and maybe velcro mounting somehow to the tripod or something similar.  I am also considering adding a LED to the circuit to show the power is on, in order to help visually provide a assistance to try and keep from forgetting to turn it off.


It is also important to note that since this has no TX/RX switching built in this is for RX use only.  I built this primarily to use for working on amateur satellites.  In order to use the AMSAT's you transmit on 2m and recieve on 70cm.   The Arrow Antenna is actually two separate (2m/70cm) yagi antennas on a single mast, and each antenna has its own feed line connection.

Using a Comet CF-416 diplexer, I connect those antennas to the same radio, or sometimes I actually use two radios and have one for RX and one for TX.. this lets you hear yourself echo on the satellite repeater.  

Here is how it would look in a block diagram.  Even though the arrow is a single antenna mast, I show it broken out into two separate antennas in this drawing.

Just for reference here are pics of the Arrow Antenna and the Comet CF-416.

The OTHER option for working the satellites is to scrap the diplexer and just use 2 HT's or an HT and a scanner.  One HT will be connected to the 2m feed line and used for TX, and the other will be connected to the 70cm feed line and used for RX.  The advantage here is you can monitor your signal and listen to yourself bounce off the satellite.

This is what that setup would look like:


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