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.

Cheers,

Keith
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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.

Cheers,

Keith
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VLF Hum Canceller Schematic 



The schematic in my last post looked crappy, so here's a better one. It's a jpeg, so right click on the image to bring up the larger full resolution version.

Cheers,

Keith
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Hum Cancellation for VLF Receivers 

I mentioned my trick for mitigating mains noise pickup in VLF antenna/amplifier setups at the SARA Western Conference over the weekend, and many people asked for more details. Here are a couple of pictures and a quick schematic.

The basic idea is to cancel out some of the local mains electric field right at the antenna by "transmitting" a small field that is out of phase. I have only tried this trick with electric field antennas. It might be interesting to try it with an extra winding around a loop too. First, locate a wall-wart that can be opened up easily. I found this one in my junk box, and it was held together with screws instead of glue. The label is gone, but I think this one was rated at 5VDC. The current doesn't matter since we will be drawing almost no power from it. DC outputs are preferred, since they will have some extra room we can use.





Once you have gotten it open, remove the rectifier/capacitor circuit. Be careful with the transformer secondary leads, that's the part you really need. You might want to use the original output cable, or change to a connector like I did. Figure out how you will access the trim-pot. I found a neat trimmer with a panel mount bushing in a bin at HSC. I haven't found these in a catalog, but Mouser carries a Vishay/Spectrol adapter that holds a standard trimmer on a panel.

If you lucked out, the transformer has a center tap that can be tied to earth. If not, you either need to add a couple of resistors as shown in this schematic, or you need a switch to choose the polarity of the cancelling signal.



With a center tap (real or virtual) the trim pot allows us to adjust the cancellation signal from max in-phase through zero to max out of phase.

To set it up, run some wire from the output of the device up near your antenna. My antenna is a piece of 3/4" Copper water pipe, about 8 feet long ( about 19mm by 2.4 meters). I tie-wrapped a couple of nylon standoffs to the side of the pipe and attached a section of 14AWG bare copper wire to them. You will need to experiment with this, so start with about a cubit. That's the length of your forearm, a very handy unit if you ask me. Connect one wire from the hum-bucker to it and the other to a nearby earth ground.

Now, look at the output of your antenna amplifier on an oscilloscope, and play with the adjustment on the hum-bucker to get minimum mains frequency noise. You won't be able to cancel it completely, but you should be able to reduce it considerably.

That's about it. There are plenty of areas for experimentation. For instance, it should be possible to adjust the phase with an RC network of some kind, or you might make the virtual ground network adjustable too.

Have Fun!

Keith
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SARA Western Conference 

I just got around to uploading my pictures from the SARA Western Conference held at Stanford University last weekend. I had a great time, and I'm sure everyone who attended did too. Bill and Melinda Lord did a fantastic job organizing the meeting, and a special thanks goes to Phillip and Deborah Scherrer who were our hosts at Stanford.



This is the famous Stanford Dish from the front side. After spending most of the conference indoors, it was nice to get out and have a look about. Our first stop was the big dish to get a detailed inside tour. The dish is 150 feet in diameter - you get some sense of scale from the cars in front of it. The whole works including the control room rotates around on four rail car wheels riding on a circular track.

It was a beautiful day, and we could see Mount Hamilton, Mount Diablo, and NASA Ames research center:



I think the rectangular structure in the picture is the inlet to the huge wind tunnel at Ames. In the foreground, the interesting shaped white building is the Wilcox Solar Observatory which we toured after visiting Stanford Radio Club facility. All of these facilities are within a huge park-like area open to hikers. We felt bad riding in cars while they trudged up the hills!

That's it for now, I'll comment on some of the presentations next time.

Cheers,

Keith

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