Woodhouse Communications Aptcp Omnidirectional Antenna

... and its perfect sidekick, Hamtronics' LNG-137 receiver prearnp.

Larry Antonuk WB9RRT P.O. Box 452 Marlborough NH 03455

Ihad completed construction of my new weather satellite receiver {see Hamtronics RI3M review 7J, November 1996). and had hooked ii up to my disc one antenna. I stumbled through the demodulator instructions, and had actually received and viewed lots of Automatic Picture Transmission (APT) pictures. Overall 1 was pleased with mv success. As I began to decode more and more pictures, however, 1 found thai I was becoming more and more critical of the results. The main problem was thai the overhead passes were loo short. Only rarely did 1 get a full scrccn image; in most cases I was recording a two- or three-inch picture with each pass. Even these images were broken up at the beginning and end of each display and the pictures were cluttered with black and white spots and streaks. Something was obviously amiss, and 1 had a pretty good idea what it was. During my days as a Novice I heard an adage that has staved with me: "A dollar put tow ai ds the antenna is worth twenty put towards the receiver." Adjusted for in nation this might come closer to two hundred dollars towards the receiver, but the message is clear. If you want good reception, you ha\ e to make ^ire that you capture the best signal available at your location.

Choosing an antenna system

WTicn it came to antennas for overhead satellite reception I was somewhat in the darL I quickly learned that there were two main paths to take. The first utilized a circular-polarized, fixed omnidirectional 24 73 Amateur Radio Today * December 1996

antenna with a fairly wide beam width, pointed straight up This antenna made use of two crossed driven elements that sat ibove a pair of crossed reflectors, and was called a *1urnsiile" since it resembled, well, a turnstile. The second method also made use of a circular-polarized antenna, but a higher gain, smaller beam width yagt model This was mounted on a dual rotator arrangement, to provide elevation and azimuth movement It provided a much stronger received signal than the fixed option, hut of course ii needed to be continuously moved to follow the satellite as it traced its iire across the sky. There were even computer programs that would automatically turn the rotators, based on information about the satellite you fed in earlier.

I pondered my choices, and compared them to my current needs and resources. Obviously, the rotating method would pro-\ide the best coverage in borderline circumstances, and allow the most data to be squeezed from each pass. On the other hand, someone needed to make sure the rotators were turning the antenna cor-rectlv—it didn't sound like this was something that would occur automatically while I vsas at work, at least not without a whole lot of set-up work on my part. Resides. 1 really didn't care what the weather looked like when the satellite was down near the horizon—I wanted to see the picture mainly when the satellite was right overhead. which was when the turnstile antenna did its best work. The deciding factor had to do with simplicity and cost. 1 found that t could put up a turnstile antenna for about one-fifth the price o! a vagi/rotor combination, and it looked like 1 could get it assembled and mounted right away -

Once I decided on lhe antenna 1 needed,

Turnstile Antenna

the choice of manufacturer was obvious. I had come across a catalog from Woodhouse Communications, in Plain well* Michigan. Woodhouse has a complete line of antennas, all designed for various types of APT reception, I sent a check off to Woodhouse, requesting their APT 2CP turnstile antenna.

I received the APT-2CP about a week later. As soon as I opened the box I was pleased with the purchase. The Woodhouse folks produce a special line of antennas, and they obviously take great pains to do it right. The vertical boom of Lhe antenna is a piece ol one-inch 6061-T6 aluminum tubing. Holes are drilled through the boom for the elements—no U-bolt arrangements here. The 18-8 stainless steel solid rod elements are pre-tapped for the stainless hardware that will hold them in place—no self-tapping screws or hose clamps The baluns arc Hue coaxial baluns. not fertile head types. The balun blocks are machined from HDPE plastic for LJV stability and strength. Every pan til the antenna. right down to the Ampenol connectors and Belden cable, is real quality stuff.

Setting up the APT-2CT

Assembling the antenna was fairly straightforward, and was done in under a couple of hours, including time for a break. The instructions give some hinLs on how to support the antenna with a couple of sawhorscs; l his made construction quite easy. I had started the project in the early evening, and by the lime 1 had the antenna ready to go up on the roof it was already dark. I decided to put the project off until the next day, but I did hook the APT-2CP to some coax which I tossed through the window into the shack. 1 didn't get around to mounting the iripod on the roof until a couple of days later, but in the meantime I found that my satellite passes were now at least twice as long—with the antenna still sitting on the ground!

I finally got the tripod mounted, and mounted the antenna on a piece of mast with the supplied stainless steel clamps. The only advice given concerning the location of the antenna had to do with maximizing the "horizon," In other words, get

My main concern was that I had been too cheap to go out and buy a quality piece of coax for the turnstile. 1 had used a 75-foot piece of RG-8 that I had previously used on HF, and I worried that there might be too much loss in it. I weighed the op-lions , and figured that the preamp would be the way to go. Since I was using the Hamtronics R139 receiver, I ordered an LNG-137 preamp from Hamtronics.

The LNG-137 is similar to the LNG I44 preamp used in repeaters and for weak signal work, but tuned specifically for the 137 MHz weather satellite band. Ii comes preassembled, and requires a +12 VDC supply, (Hamtronics does sell a phantom power kit to allow you to run the 12 VDC up the coax, but 1 didn't want to mess with building up the isolators on each end. 1

"As I collected more and more images, I found myself getting pickier and pickier"

the antenna as high as possible, away from and above any obstructions such as trees and chimneys. (It is recommended that the turnstile not be side-mounted on your tower, and not below your beam!) Once it was on the roof I found that my passes improved even more, and I was able to get full screen images with little problem, In addition, since the antenna was llxed and omnidirectional I didn't need to mess with it at all—1 just let my R139 receiver switch on the tape recorder when a satellite was going over, and collect the passes for me so I could demodulate them and review them at the end of ihe day.

As I collected more and more images, f found that I got even pickier and pickier I began to notice that there were still a few patches of "snow" at the beginning and end of each pass. The point where the satellite signal was the weakest looked like it could still use some improvement.

Improving the picture

I knew that receiver prcamps were available, and that this might help improve my situation. I also knew that a preamp could actually degrade the system's performance—especially if there were a lot of strong signals around. 1 was somewhat near an airport, so 1 figured that I might be hit with aircraft transmissions, but I didn't have an excessive amount of public safety or commercial two-way traffic around the neighborhood.

simply ran a piece of insulated bell wire up to the antenna, and used the coax ground for -12 VDC.) I mounted the LNG-137 on a small (3" x 3") piece of aluminum stock, and mounted it to the mast below the turnstile. I made up a small jumper with the proper BNC connectors, and taped and caulked the unit with silicone sealant.

Testing with the preamp in-circuit showed improvement at the very beginning and end of each pass in the form of fewer of the black and white "dropouts/' So far I haven't noticed any evidence of any of the problems associated with too much gain—intermod products, oscillation, etc. However, in my case, the use of the APT-2CP antenna (designed specifically for 137 MHz) as opposed to a broadband antenna will limit the strength of the out-of-band signals that arc passed to the preamp. And the preamp, being tuned to 137 MHz, will favor those signals over out-of-band signals, as well. (If intermod signals do become a problem, I know I can add even more selectivity with a helical preselector, tuned to 137 MHz.)

The Hamtronics LNG-137 preamp and the Woodhouse Communications ATP-2CP turnstile antenna have proven to be a very straightforward, low cost approach to weather satellite imaging. The ATP-2CP is available from Hamtronics, Inc. (716-3929420} or directly from Woodhouse Communications (616-226-8873 ). EZj

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