Preface

I began working amateur satellites in January 2002. I started with the FM birds and ISS packet (as many do), and had quite a bit of fun. I even had a voice QSO with Valery Korzun (RZ3FK) aboard the International Space Station (ISS)! This was a great experience, and a great building block!

As soon as I found the linear transponder LEO (low-earth-orbit) birds, there was no turning back! In July 2002, I disassembled my satellite antennas (I moved to a new QTH), and shortly afterwards, RS-12/13 (my favorite bird) perished.

I decided to try the most advanced amateur satellite yet - AO-40! This would be a whole new challenge - I had no experience with frequencies over 70cm!

Many hams believe that satellite equipment is expensive. Not true! Satellite operation *does* require use of a full-duplex rig, or two separate rigs. If you own an all-mode radio, you already have the most important part of your downlink system! This does not need to be a 2 meter rig!

Don't worry if your IF radio is not brand-new or top of the line! Don't worry if you only have an HF rig! For example, I have a simple Hamtronics CA-144 receive converter (2 meters to 10 meters), and have used it to hear AO-40 on my Radio Shack HTX-10 (a Bargain 10M rig)! This unit takes the primary downconverter's output and further converts it down to 29 MHz (2401 MHz to 145 MHz to 29 MHz). You can build a kit (such as Hamtronics), or possibly design your own -- these are simple projects! I may use this arrangement for full-time telemetry monitoring, or to allow another operator to tune around the passband during QSOs. In any case, this makes a decent backup receiver for AO-40!

Don't believe it if you hear that satellite operation requires a special $2000.00 rig and lots of technical knowledge. Here is a picture of my radios set up for Field Day 2003. These two rigs make a very capable satellite station, and are able to be run off a single power supply. The only required items not shown in the picture are the dish and converter, and UHF yagi for uplink. The Icom IC-706Mk2G is the uplink radio at 435.xxx MHz, and the Icom IC-245 is the IF radio tuning 145.3xx. BTW, the code key is a special, one-of-a-kind, homebrew item and is not for sale.. hi hi

After you have your downlink working, you can concentrate on building your uplink system. Many options exist here, also! If you do not have a radio capable of 435 MHz SSB/CW uplink, you can use your HF rig, and 'chain' together transmit converters for your 70cm (or even 1269 MHz) uplink!

Even if you do have to buy some equipment, it is possible to build your satellite station inexpensively. If you're willing to spend carefully (buy used equipment, Ebay, etc) I believe that you can have your satellite station going for the cost of a new HT! Which will be more fun to operate?

I was most recently QRV on AO-40 with a 36" x 26" Primestar dish, helix feed, and Transystem AIDC 3731 d/c. My Icom IC-245 serves as the IF radio at 145 MHz, and the IC-706Mk2g is the uplink radio at 435 MHz. This downlink equipment works quite well together, and is capable of detecting the transponder noise floor at reasonable distances and squint angles. It took months of experimentation to discover this combination. The article below details the evolution of my downlink.

This site is mainly geared toward beginners. I have not included complex calculations, etc, just a sample of what equipment and techniques have (or haven't) worked well for me.

Building my AO-40 downlink

I have been receiving AO-40 since November 2002. I began buying surplus satellite TV gear (mostly on Ebay) to modify for AO-40 receive. A lot of surplus equipment is available, and much of it operates around 2.1 - 2.6 ghz, so it is usually pretty easy to modify these units for our 2.4 ghz amateur satellite allocation.

An essential tool is a 2400 MHz signal source. These are usually very simple circuits, which can be built easily, or purchased in kit form.

The signal source is a great help, as it puts out a very weak signal (ideally at 2400.000 MHz). Listening for this signal will help you to find the downconverter's output signals on your IF radio. For example, if your downconverter uses a 2256 MHz local oscillator (LO) and 144 MHz IF, you should hear the 2400.000 MHz signal at about 144.000 MHz. Remember that most downconverters are very temperature sensitive, and may need to be warmed up to avoid frequency drift.

When first listening for the signal source, listen +/- 50 khz or so. You'll find it eventually! After some experimentation with known frequencies (such as AO-40) you may decide to 'fine tune' your IF frequency using the trimmer capacitor on your downconverter (often located next to the crystal) so that tuning your IF rig is more intuitive. E.g - If you have a 144 MHz IF, 2400=144, 2401=145,etc. Any frequency adjustment you make is multiplied by the scheme used in the d/c (usually 256!)

My first attempt to receive AO-40 was with a California Amplifier brand integrated antenna/downconverter, model # 130003. This unit originally operated from 2500 MHz - 2686 with an IF of 222-408. Unmodified, this unit cannot hear 2400 MHz very well, if at all. Details of the modification can be found here . The modification is explained in great detail.

Many thanks to Fernando, CX6DD for all of the great information. I would not have been successful with my 130003 without his help! Performing this mod also helped me to discover how to modify a different downconverter that I purchased.

Results- This unit is capable of receiving AO-40's middle beacon (just above the noise), and possibly some of the strongest CW, but forget about using this for AO-40 (unless you use this as the feed for a dish - more on this below)

This unit uses a linear polarization patch feed (horizontal or vertical depending on orientation) , and is useful for locating the satellite or dish aiming. You can expect some noticeable QSB on AO-40. This unit also hears UO-11's beacon fairly well.

When first listening for AO-40, be sure to pay close attention to the squint angle calculated by your tracking software. Try listening when the squint angle is as low as possible. FYI- My best receive system yet does not function well above 20-25 degrees squint. If your tracking program does not display squint, try using Predict (Freeware) This software is available for Windows and Linux platforms.

From the Amsat web site: "Experience shows that the S-band downlink signal falls off by approximately 1 dB at 10 deg squint, 3.5 dB at 20 deg and 8 dB at 30 deg. Above 30 deg the level drops quite sharply, and severe fading is also experienced."

The next thing to try was a separate downconverter and antenna. I chose the California Amplifier 31732 downconverter, which was readily available in late 2002. These converters are of similar quality to the popular Drake 2880 downconverters, typical noise figure is 1.7 db. I experimented with two of these downconverters, and found that one heard AO-40 well. It did not work very long, however (more on this below). The second unit did not seem to have much gain, and needed a preamp to hear AO-40 period.

Another downconverter that I've experimented with is a Pacific Monolithics model# PM-CTV3301. The company that made these converters is out of business, and I've been able to find very little information about these units, but let me tell you one thing -- These units hear AO-40 very well after a minor mod! The published LO is 2278, and the stock crystal is 11.98947 MHz. I've not converted one of these units to a 144 MHz IF, but otherwise, these units are very capable for AO-40 use! Email me for specific info on the mod! I do not have access to a noise figure meter, so I do not know the NF of this unit.

Next, I needed an antenna. I wanted circular polarization, so I built a simple 12 turn homebrew RHCP helix. The helix is easy to build, and the dimensions are not too critical. The circular polarization should eliminate QSB. This unit was made from scrap materials including a length of PVC pipe and an aluminum plate. The total cost was about $5.00, the cost of the type N connector. I had great results with the 31732 converter and this antenna, far better than with the integrated unit. I was able to hear not only the beacon (a couple of S units over the noise!), but many CW qsos and even some SSB (very weak)! It was now obvious to me - I needed a parabolic dish! I'd never experimented with a dish antenna before, and I was intimidated by all of the new terms and abbreviations. F/d ratio, focal point, focal length, it was all new to me!

Putting the pieces together

I looked around on Ebay, and managed to get a used 18 inch by 21 inch satellite TV dish for about $10.00! The price was certainly right! I had read that a larger dish, one-meter or so is the recommended minimum for AO-40, but I was just getting started, and didn't know quite what to expect. I also thought that this small dish would always work for our higher bands in the event that it didn't work well for AO-40.

Now that I had the dish and converter, I had to build a feed for the dish, and assemble everything. I built a simple 5 and 1/4 turn helix, very similar to my standalone helix, but with one very important difference. The dish feed helix is wound LHCP instead of RHCP. The dish reverses the polarization - this is very, very important! The reflector is a scrap piece of metal, and again, the total cost of the feed was about $5.00, the cost of the type N connector.

Assembling the dish feed helix is easy! Howard, G6LVB has a great web site that details how to build a helix and matching section. The helix was wound on a 40mm PVC pipe. I used #10 A.W.G copper wire, and wound about 6-7 turns. I deviated slightly from Howard's plan, using the 3mm chassis connector shim shown here next to my chassis type N connector. Using this shim helps speed construction, although I'm sure the PCB and copper foil work just great also.

After constructing the dish feed and mounting it to the dish, check your work by listening to the AO-40 middle beacon. You may decide later to optimize the helix for your particular dish. This is often accomplished by cutting off 1/4 turn at a time until you're satisfied. If you're like me, you keep cutting until the signal starts getting worse (you cut too much off). Make these adjustments while listening to a weak signal - not the middle beacon. You may end up building a couple of these before you get it just right!

If you have never aimed a dish before, be patient. One thing to know is whether you have a prime focus or offset-feed dish. Most former-satellite TV dishes are offset by about 20 degrees. This means that you'll need to aim about 20 degrees HIGHER than the scale on your dish. E.g - If AO-40 is predicted to be at 30 degrees elevation, aim your dish to about 50 degrees. Adjust az and el to peak the signal.

My dish came with the original LNB and feed strut, so I mounted my feed in the existing location. It was pretty easy, no calculations necessary. All I needed to do is attach a bracket to support the downconverter, and experiment with moving the feed around a little from there. After finding the best position, I used some electrical tape and cable ties to temporarily mount my feed and converter.

After many evenings of testing outdoors, I was finally hearing the satellite really, really well. This was the strongest I'd heard AO-40 yet! Besides hearing the middle beacon (several S units over the noise), I could copy CW (armchair copy) and entire SSB qsos! I began capturing telemetry, and was rewarded with 'CRC OK' blocks, even out to apogee! I was now ready to try my uplink into the satellite! The journey from integrated unit to standalone helix to parabolic dish took some time, and I've learned quite a bit along the way! I'm excited to have gained some experience with microwave frequencies!

On the bird - at last!

After quite a bit of experimentation, luck, and help from many other amateurs (Special thanks to Jerry Brown, K5OE), I was first active on AO-40 in January 2003.

On February 7, 2003, I had a great QSO with Tom Brown, K5UIC. I was using my 18" dish, helix feed, and Cal Amp 31732 d/c -- squints were in the single-digits! This was perhaps the best signal I ever received with the small dish and 31732.

Note: During times of heavy server load, the audio may not stream correctly. Right-click on link as 'Save Target As'. This will save the file to your local disk and provide better audio playback.

Listen to an AO-40 audio clip (MP3 format) - file size 716,115 bytes

The recording quality isn't great, it was recorded analog onto an old cassette player, digitized, edited, and converted from WAV to MP3. Much of the hiss and noise is due to an impedance mismatch between my rig's output and cassette player's input, and improper audio level adjustment. Look for a 100% digital recording of a QSO soon.

Operating via amateur satellites is by far the most exciting ham adventure I've ever had. Each time I work another station I'm amazed that my simple station is capable of making contacts through a spacecraft 35,000 miles away! I have had many QSOs - the majority are from the U.S.A, but I've worked some DX also! So far, I've worked these countries on AO-40:

 
Argentina           Japan

Australia           Canada

Mexico