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Re: Tesla Receiver Coil ..........success?



Original poster: Ed Phillips <evp@xxxxxxxxxxx>

I have a few questions about your beacons. First is your call sign still IZJ out of California? Also, have you been awarded a license to run a 2200 meter (135.7-137.8 kHz band) station under Part 5 rules or are you considering filing an application for same? Can you tell the list a little bit about your present LF transmitter? Is it crystal controlled or PPL? I assume it's a solid-state transmitter. What solid-state device does it use? Is the output waveform to the antenna a squarewave? If so, do you think your transmitter could be used as a solid-state Tesla coil driver? As for transmitter power, how do you measure the input to the final stage in order to comply with the one-watt restriction in this regard? And finally in regards to your antenna, I notice your article "Lowfer Antenna Introduction" at http://www.fix.net/~jparker/curry/1750mtb.htm has been removed from the web. Is it available elsewhere? Do you prefer the buried ground plane, the fully insulated counterpoise elevated on supports, or something in between? What size wire do you use for your loading coil? and what are the coil form dimensions and material. What do you use to tune your LowFer antenna?


Gary

Gary:

That's awhole bunch of questions! When it's On IZJ is the sign of my 181.167 kHz beacon. It's been off for quite a while because it fouls up all the WWVB clocks in the house and besides I hadn't gotten a report (except from Dave) for years. I have no plans to do anything under Part 5 as I have a lousy location here. The antenna is the same modified HyGain "High Tower" I use for 160, 80, and 40 meters; it stands on 12 Mycalex insulators (3 groups of 4 2" square). It's 50 feet high in total and, because of the taper from bottom to top, has an effective height of less than 25 feet so my signal was never a world beater. I heard it as far as San Diego and Mike Mideke always heard it at his place, but of course he could hear anything! There are at least three problems with the radiating system. First, the tower is over a 10 foot strip of very dry soil between the house and the driveway. Second, it is between two close oak trees which seem to soak up quite a bit of energy although I can't prove it. Third, I have a very high ground resistance, of the order of 20 ohms are more. Taking all that together there's not much point in trying to do anything more and I really have no antenna options.

The transmitter is solid state and was built in 1976 and modified a bit later. Uses a crystal oscillator at 32 x the output frequency, a CD4024 divide by 32 circuit, a CD4007 buffer amplifier driving four Siliconix VN66AF "power" fets. The output stage is driven with as 1/4 cycle rectangular pulse and is closer to Class C than anything else; it is pretty efficient as none of the output transistors has ever been warm to the touch no matter how much power I've run. Originally it use a single Siliconix VMP-1 but I accidently blooied that and went to the VN66's instead, which work almost as well. Power output can be between a watt and over 10 watts, but I've usually run it at a couple, rationalizing with my conscience based on the lousy antenna system.
Cliff Walker and I made several transmitter kits and gave them to guys but I don't think any of them ever got on the air. Jack Althouse of Palomar also kitted a transmitter using the same schematic but packaged in such a way that it generated RFI from DC to light. The output is inductively coupled with the primary coil the tank circuit of the transmitter. It is tuned with 0.08 ufd of polystyrene capacitors and stuck down below the loading coil and arranged so I can move it up and down to change the coupling. I think the antenna current is pretty well a very clean sine wave as I don't see much harmonic radation when I get beyond its immediate vicinity. The power supply to the final is adjustable from about 3 volts to 15 volts and normally runs about 8 volts at two watts input.


The antenna capacitance measures to about 450 uufd and the loading coil is a surplus bank wound-litz coil about 5" in diameter and 6" high. It originally was apparently intended as a dummy load because it came with several pounds of very lossy powdered iron slugs in it. When I took those out I got an inductance of about 2 mH end-to-end; Q measured about 450 at 200 kHz. I picked a tap point which was a bit lower than needed for resonance and have a small variable capacitor across a tap down the coil to tune it; tuning is surprisingly sharp.
Not an efficient way to go but the ground circuit loss dominates so who cares? I have a 500 ma TC ammeter to measure antenna current for tuning. The total measured antenna circuit resistance is about 40 ohms and the antenna current runs between 200 and 250 ma depending on how wet the insulators and the foliage on the oak trees happen to be.


That article was written long, long ago when I used a TRS-80 Model 1 and any copy is left on a 5.25" floppy which I can no longer read.
There is probably a paper copy around somewhere because at one time Mike M published it along with some of his material. Nothing really exciting in there but if I can find a copy I'll scan it and mail it to you.
Basically says that the ideal antenna would be a small diameter vertical with a big flat top, working against a low-resistance ground and with a very high Q loading coil. No surprises there but a lot of guys were trying to put a transmitter on the air with no loading coil or such nonsense! I figure if they didn't know better no magazine article would help them but I still put it together when Mac McIntosh was trying to get editorial material to fill the Lowdown. I did include some copies of diagrams from 1910-1920 "Wireless Manuals" showing different antenna installations which were good then and would be equally good now. Some of those guys in "the good old spark days" really appreciated good antenna design, even going to the point of flat tops strung between 200 foot towers and a giant elevated counterpoise system at roof-top level.
No one (or I should say no one except some of those Part 5 guys) would go to that much trouble. For tuning the works I'd make a variometer as part of the loading coil. That works very well and it's easy to keep the Q high.


As for what I'd favor, that's another matter. I'd like to have a 50 foot array of top-loading wires fed by a vertical wire and using a good high Q loading coil. For ground I'd like a set of radials buried just deep enough that they would get caught in people's feet, or else an elevated counterpoise which would be impossible for most of us to install. To do all that would take a dedicated space of the order of at least 75 feet on the side and I just don't have it. Some of the "big boys back east" do. As I think I mentioned Sunday Dave has an 87 foot tubular crankup tower and dangles a wire from the top for his lowfer antenna. I can't remember but I think he's using a variometer for tuning, in series with a big loading coil. I got him off on variometers when he first got started and he's liked them ever since. They only have to be adjusted occasionally so no fancy shaft arrangement or bearings are needed. Main thing is, if it's mounted outdoors, to keep it completely dry!

If you work out the useful Q for a system with the poor grounds most of us are stuck with you'll find out that it doesn't take an exotic loading coil to give better than 90% efficiency. Building a great big coil is fun but probably a waste of time for most of us. I've wound some 6 mH loading coils for TV pushup masts (about 120 uufd capacitance) on 5.25" cardboard mailing tubes, sealed with shellac, and get Q's of the order of 350 or so, plenty good enough. Somewhere I have some calculations I made for different antenna capacitance and ground circuit resistance which convinced me that a Q much of 300 was a waste of effort, except for those with pride in craftsmanship. The use of ferrite cores is always a possibility but it takes a lot of area to avoid saturation and I've never tried it.

Not sure if I answered all of your questions, and probably none to your satisfaction. I would point out that, at least at this QTH, there has been a very serious compromise between theory and practice, in favor of the latter!

Ed

I'm attaching a picture of the transmitter. Notice that it's sitting inside the house and the antenna lead is the center conductor of some RG-8, run about 4 feet to the base of the antenna. I have a lot of pictures of the antenna somewhere but don't think they'd interest you.

http://hot-streamer.com/temp/WHOLE_STATION.JPG