Radio-SkyPipe Data on-line 

I have some rudimentary plots available from Radio-SkyPipe on-line now. You can see the data here. The prominent red track is from the 24.8KHz Gyrator II receiver monitoring NLK at Jim Creek, Wa. The flat orange track near the bottom is from the 20.1MHz receiver. I'm still working on that one. I hope to have something on the other channel soon too.

I've also taken info from some of my posts here and started to create some "permanent" pages on my solar monitor project. This Linkwill take you there. So far, it just covers the system mainframe and bar graph modules. Over the next few weeks I'll add pages for each receiver/sensor too.


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Orion XT8 Classic - New Base 

I've had my Orion XT8 Classic Dobsonian Telescope for years and I love it. Last year at the Golden State Star Party we were "treated" to a thunderstorm the first night. I managed to stay pretty dry, but the base of my telescope got wet. Particle Board does not like to get wet.

About a week later, it was starting to come apart. Luckily, I had a half sheet of 3/4" hardwood plywood left over from a kitchen remodel we did a couple of years ago. I took the original base apart and used the pieces as templates to cut new ones from the plywood. I reused as much of the original hardware as possible. I changed the connections between the pieces to some steel knock-down furniture fasteners. These are the kind where the screw that holds two boards together threads into the side a special pin that goes in a second hole drilled at right angles to the screw hole. They work really well and don't rely on screw threads biting into the edge of the plywood. Here's a picture of the complete base:

One problem I had was that the azimuth movement was sticky. It was just teflon pads riding on the multiple coats of polyurethane. The original was high pressure laminate on the friction surface. Telescope Builder folklore held that "Ebony Star" laminate in combination with the teflon pads gave the best results. Luckily, Scope Stuff sells pre-cut rings of the stuff complete with a set of pads. The prices look a little high at first, but they do include shipping, which can be expensive on an awkward part like this.

Laminate is generally installed with contact cement. My quandary was how to apply the cement to the base without making a mess. Here's how I did it.

First, I centered the ring on the bottom of the rocker box with a couple of pieces of masking tape. Then, I started covering the laminate with tape, making sure to leave plenty of tape off the sides, and not worrying about completely covering the laminate itself.

When I had it all taped, I made a registration mark on the tape and then took an X-Acto knife to carefully cut around both edges of the laminate leaving a perfectly masked ring.

The contact cement goes on with a little brush in the bottle, so I didn't need to mask any more like I would with a spray adhesive. I put two coats on the base because of the shiny surface, and one coat on the back side of the laminate. When they were ready to go (Read the instructions) I put a bunch of cable ties laying flat across the glued area on the base to keep the ring from sticking all at once. When I had the ring lined up nicely, I pulled the cable ties out one-by-one and pressed the ring into place.

I also replaced the original teflon pads on the ground board with the new ones that came with the laminate ring. Now my telescope swings around nicely. Just the right combination of drag and ease of movement.

I'm planning to give it a really good workout this coming weekend (24-Apr-2010) for Astronomy Day at the Robert Ferguson Observatory.

Clear Dark Skies to All!


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Decametric Antenna Part 2 

I finally got a relatively rain-free Saturday to install the T2FD antenna up on the roof of the garage. To prep as much as possible, I finished up the bases for the three vertical supports. Here's a picture of one of them:

These started out as standard 2" PVC "T" fittings which I cut to fit the ridge of the garage roof. You can see the template I used to mark the key points on the fittings before cutting. I went a little steep on the angle so that they would tend to rest on the ends and not rock in the middle. I just clamped them in a woodworking vise and cut with a hacksaw.

I also pre-cut the ropes to make the installation go faster since the forecast was for some showers randomly through the day.

Here's the antenna installed on the roof. You can just about make out the wires in the photo. The ropes at each end are tied to eye bolts that are screwed into the trim boards on the gable ends of the roof. The eye bolts are screwed in at a slight angle so the ropes won't rub on the edge of the roof. I'll probably need to adjust the ropes once everything settles in.

The finished antenna seems to be working great. I have the 20.1 MHz receiver hooked to it and collecting data on Radio Sky-Pipe. I need to work through the image save/FTP upload process so I can get some recordings posted on the site.

Then I'll be working on finalizing the 111 MHz receiver and modifying my WWVB receiver to give me signal strength measurements.


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Decametric Antenna, Part 1 

Now that I have a working receiver, I need a good antenna. The classic Radio Jove antenna is a dual dipole. I actually considered trying to fit one of these on the top of my garage, but I figured my wife and neighbors would not be pleased... Also, it is fairly narrow band, and won't work very far from it's design frequency of 20.1MHz. I want to be able to explore more of the band without adding more antennas.

After some searching on the web, I came across the T2FD, short for Tilted, Terminated, Folded Dipole. The pdf at this link has a good article called "A Total Flux HF Radiometer" by Colin Clements that discusses the T2FD, and is where I first heard about it. There's also a good article here.

Anyone with an FM receiver at home probably has seen a folded dipole. They are often supplied as the basic antenna, made of 300 ohm twin-lead. I've built these myself when I needed a quick and dirty antenna for 100MHz-ish frequencies.

Folded dipoles in general are fairly broad-band, and terminated ones are even more so. The basic idea is to insert a terminating resistor in the center of the top wire directly opposite the feed point. The resistor is supposed to be just a little more than the feed point impedance. How much more is subject to some debate, but generally it's agreed that with a 300 ohm feed impedance, the terminator should be about 390 ohms. I'm using 75 Ohm coax with a homemade 1:4 balun at the feed point.

The "Tilted" part of the description comes from putting one end up in a tree to improve the directionality of the antenna along the ground. I want the antenna to look up, so I will just string mine horizontally.

The roof of my garage is about 24 feet long, and the ridge is oriented close to North/South. I want to keep the antenna ends in a few feet from the edges to allow for guying. I chose an overall length of 6 meters. This should result in a minimum frequency of 16.666MHz (100/L Meters) and a maximum about 4-5 times that. Based on the literature, the wires should be spaced at 0.18 meters (3/F MHz), or just over 7".

The center of the antenna will be made of 2" PVC pipe to hold the terminator and balun. This is the bottom end with the balun and "F" connector

This is the top end with the terminator installed.

And this one is a side view showing the eye-bolts where the wires will attach. The end spreaders will look the same as this, but with nothing inside. Each one will have wire looping through the two loops on one side, and guy ropes on the other. The top will get a matching PVC Cap, and the bottom of each will sit in a "T" fitting that's been cut to fit the ridge of the roof.

Here's a closeup of the balun in my vice. It's 16 turns bifilar wound 24AWG wire with insulation. It's wound on an Amidon FT-140-67 Core. Lots of super-glue holds it all together.

And here's a closeup of the terminator. I have a junkbox full of 2W resistors, none of them 390 Ohms. I have lots of 680 Ohm resistors and 100 ohm resistors. The bottom layer in the stack consists of 8 x 680 ohm resistor in series/parallel to give 340 ohms. The top layer is a set of 5 x 100 Ohm resistors in parallel. Those are in series with the bottom layer. But, that doesn't add up to 390, does it? Well, the 680's were on the high side of their tolerance, and resulted in about 370 ohms. I added more 100 Ohm resistors in parallel to compensate.

Both terminator and balun are clearly bigger than they need to be for just receiving. It cost me nothing extra to do this and it leaves open the possibility of using this as a low power transmitting antenna in the future.

Next up I'll have some pictures of the actual installation.


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Decametric Receiver Construction 

Here's the inside of the receiver for monitoring the 20.1MHz region ala Radio Jove. Since I always want to do things the hard way, I didn't buy a Radio Jove receiver. This is a Ten-Tec Model 1056 Direct Conversion receiver kit that I built and modified for solar radio astronomy. The folks over at Fringe Dwellers have some information about using this receiver, and I thought it would be a good one to try. While the Radio Jove Receiver is easier to build for a complete newbie, the TT-1056 is much cheaper. It does need a few modifications.

The '1056 comes with parts to cover the 15 Meter Ham band, but that is just a little higher than the 20-21MHz we want to cover. Fringe Dwellers suggest replacing C1 and C3 with 56pF instead of the supplied 47pF parts in the kit. I think you might want to go a little higher. I found that 47pF + 10pF in parallel just gets to 20.1MHz at the absolute low end of the dial on my kit.

I also changed the Local Oscillator output from the 1Mohm series resistor to a simple emitter follower buffer using an MMBT5179 transistor stuck to the bottom of the board. It works great to drive my frequency counter without affecting the tuning. You can see the added coax from the board to the rear panel in the picture above.

Finally, I added a 20 MHz pre-amp and pre-selector to improve the front end performance. Here's a closeup of the pre-amp board:

And here's a link to the schematic of it.

Amplification is provided by a Mini-Circuits ERA-3 Monolithic Amplifier. These little building blocks are very handy for things like this. A few years back I picked up one of their Designer Kits which had ten each of the ERA-1, ERA-2, and ERA-3 amps for about $50. That's less than $2 each.

The front end filter is lifted from the Radio Jove receiver schematic. I didn't have any 0.47uH inductors on-hand, so I wound one after designing it using this nice tool at Missouri University of Science and Technology. It takes a few iterations to find a combination of turns and diameter that gets the right inductance, but that's inductors for you. There are lots of ways to calculate the inductance of a structure, but no tools to tell you how to bend the wire to get a specific inductance. Just remember that the radius in the calculation is to the center of the wire. You need to subtract off the wire radius to figure out the radius of the form. I have one of those big drill bit kits from Harbor Freight, so I have lots of form sizes to choose from.

With everything put together, the receiver can detect a signal down to about -130dBm at the antenna input. Now I just need to build a suitable antenna for it.


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