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antenna theory made easy
On Thu, 23 Jan 2014 21:06:12 -0700, "Irv Finkleman"
wrote: I already have a small 12v motor and have tested it with a number of different capacitors. With a simple 10k wirewound pot I can slow it down to almost nothing, or bring it back up to some value, and that beautiful pot just gets lukewarm. Using a DPDT toggle switch I can control the direction as well. Remote tuning on a shoestring! Loose the potentiometer and replace is with a gearbox reducer. You want to retain all the torque and power from the drive motor. Also, don't forget to hi voltage insulate the motor, so that you don't arc over to the motor in transmit. I plan to cut a big cloth circular case to drop over the loop. It will be some nice flowery design, and a plan to put it over the back of a lawnchair so that anyone who walks by and looks up to my 3rd floor balcony will see what appears to be a nice comfortable highback chair! That's mainly for subterfuge! If the lawn chair has an aluminum frame, it will detune the magnetic loop antenna. All plastic lawn chair is required. Also, don't fire up the transmitter when someone is sitting in the lawn chair. At the same time, having been experimenting with assorted antennas over the years (before I began to deteriorate) I had a big yard full of wire, ground rods, and a tower with all kinds of things hanging off of it. I always kept an eye on the SWR bridge for any variations, so any of that sun heating drift will be watched carefully. Thanks for the tip though! For all my reading that is one thing I hadn't encountered before. It's only a problem with high Q antennas. Figure on a Q of 100 to 200 for the lower bands. On 80 meters, that's a 2:1 VSWR bandwidth of about 25 to 35 KHz. It doesn't take much to drift the antenna off frequency. Most magnetic loop users don't notice because they're constantly tweaking the tuning capacitor. I plan to mount it vertically using a piece of 2X4 sitting in a big pot or tub filled with gravel. So much for the disguise antenna. Don't forget to pound a ground rod into the gravel. It won't do anything useful, but will make an interesting conversation starter as to whether a ground is necessary. Anticipating lots of experimentation once I get on the air, I can drill it, nail it, or whatever for various forms of support, and even clamp a whip on it. I also anticipate that I will eventually turn it into something akin to swiss cheese, and when that comes about, I can replace my 'cheapo balcony tower' with ease. Let me know when you're ready to build my inflatable antenna tower. I will also constuct a small platform to hold the tub/pot and by using some of the small plastic swivel casters I'll make the whole works rotatable. Lazy Susan works well. I use that for my direction finder hacks. Use lots of grease if left outside as the bearings tend to rust. I'm counting on the MFJ Analyzer for lots of help. I'm very away of the losses, and where they occur and will be careful. As far as losses are concerned, when running QRP why worry? If you only have a little power to work with, then losses become even more important than if you had hundreds of watts to waste. Under good band conditions like we had in the good old days (with propagation ever again improve?) you can work the world with only a few watts. some guys do it with milliwatts! I only want 75M for very local work within about 200Km, but I know I'll be quite efficient on 20 which is where I have always had the most fun. They still do it with milliwatts. http://hflink.com/jt65/ I'm not sure what's wrong with propagation these days. Probably global warming, or a government conspiracy. A few of the magloops I've looked at on the net were not all soldered -- many just used nuts, bolts and starwashers. They don't work well. One of those I rescued was initially thrown together but not soldered on the assumption that future modifications would be easier without soldering. It didn't work until after it was soldered. Whenever I ran across them, I'd get back to the builder/operator and ask how they worked, and had they taken the resistance losses into consideration. In all cases they were happy with the way things were and making lots of contacts. You can also make a fair number of contacts with a dummy load. I know, I've done it by accident. I also made a fair number of contacts on one Field Day using a coax cable that was not connected to any antenna. In some cases, a dummy load or no antenna would be an improvement over some of the home brew antennas I've seen. One of the Manufactured loops is made of aluminim strips bolted together, and it seems to get good ratings on EHam. You can test that yourself with your MFJ antenna analyzer. Build a loop that way. Measure the VSWR curve and impedance at resonance. Then, rattle the loop a bit to move the bolts a little. Measure again. My guess is you'll see substantial changes every time you bang on the antenna. It might also be interesting to use an ESR (equivalent series resistance) meter to measure the DC resistance. That's also going to vary. On hand, I also have a yoyo antenna, a Miracle Whip antenna, and the MP1 which will be on the air as soon as I can measure the radials and see how it works indoors. Anything with amazing, magic, miracle, ultimate, or other superlatives is usually over-rated. Did I mention two Slinky's that I want to try -- first as a short vertical, and then as a helical magnetic loop! A what? Never mind. I don't have time to model a pretzel made from a Slinky. I can hardly wait to get going -- and I'm getting closer every day! Oh-oh. Slow down. It might be more fun doing cut-n-try, but it's much easier if you plan your antenna carefully, and build it once, not 20 times. Thanks for the tips Jeff -- your contributions to the group are always great. Y'er welcome. Good luck. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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