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Antennas
It's antenna-decision-time for my Europa kitplane; though I purchased
a couple of Bob Archer antennae w/ my kit, my sparse knowledge on the subject (make that next to zero), has led me to reconsider the issue as a blank slate. I'm thinking I'll be needing antennae for 2 coms, 1 nav, transponder, and ELT. Not interested in wasting dollars, but usually willing to pop for something which assures trouble-free operations, particularly when I'll be having wing tip and tail mounted strobes (3) which I gather can sometimes complicate things. My research led me to RST's website...I've always enjoyed Jim Weir's articles, and years ago built one of his intercoms which served me well. His $29 antenna package w/ textbook for plastic airplanes sounded great. As I investigated further, I received this email from a pal who knows his avionics: Asking his opinion on RST's offerings, I wrote, "For $29, Jim can solve all the antenna issues for a plastic airplane...now, I know, I know...something too good to be true probably isn't...but...every now and then. Your thoughts?" And my pal responded: "You can make dipoles fairly easily, but they will have to be tuned for the specific frequencies that they will be used for. preferably once they are installed in the airplane. You will also need a "balun" to balance the impedance of the elements to the impedance of the feed line. It isn't as complicated as it sounds, but most folks don't have the gear to tune home made antennas and wind up relying on someone with a ham license who has the gear, or taking it to a radio shop. This is especially true of the transponder antenna, as you will be hard pressed to find a ham operator who has the capability to tune an antenna at 1090 Mhz. The COM frequency band (118-136 Mhz) is a bit more common to hams, though. Most 2 meter test gear will handle those frequencies. "FWIW, I have both the sweep test gear (I can sweep anything from 30 Mhz to 32 Ghz with my sweep gear) and the knowledge of how to make my antennas, but I chose to use the AAE antennas because I liked the material they were made from and it was one less thing I had to do to get my bird into the air. It would have been difficult for me to reproduce what AAE does for their price in terms of quality and performance. There's nothing wrong with a copper tape antenna, though, provided you tune the antenna to minimize the SWR (reflected power) across the band." Now my only "sweeping" equipment and experience is on the end of a broom...and I live on an island without a convenient neighborhood radio shop, and the only ham I'm familiar with comes in a sandwich. So my questions for this discussion (and for Jim, if he's monitoring this) are, taaa daaa, 1. What's involved in "tuning" an antenna? (Can proper tuning be assured by careful measurement and installation...or must one have both access to the antenna for trimming, and diagnostic equipment of some kind?) 2.What's involved in "sweeping" an antenna? What equipment is needed, and what is the nature of any alterations to the antenna which the process of sweeping reveals? 3. Can these issues of tuning and sweeping be ignored by buying off- the-shelf products? Humbly, Fred |
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Antennas
wrote in message
... It's antenna-decision-time for my Europa kitplane; though I purchased a couple of Bob Archer antennae w/ my kit, Bob makes a pretty good antenna. Brits say antennae; most of us rebels use antennas. my sparse knowledge on the subject (make that next to zero), has led me to reconsider the issue as a blank slate. I'm thinking I'll be needing antennae for 2 coms, 1 nav, transponder, and ELT. Not interested in wasting dollars, but usually willing to pop for something which assures trouble-free operations, particularly when I'll be having wing tip and tail mounted strobes (3) which I gather can sometimes complicate things. Dipoles are pretty good antennas when you can't be sure whether it will be somebody below you or somebody above you that you want to talk to. However, for transponder, a cheap "steel b-b" antenna that comes with every transponder ever made mounted on an aluminum ground plane (cut out of a pie plate with scissors if you wish) is the way to go, as the station (hopefully) will always be below you. My research led me to RST's website...I've always enjoyed Jim Weir's articles, and years ago built one of his intercoms which served me well. His $29 antenna package w/ textbook for plastic airplanes sounded great. As I investigated further, I received this email from a pal who knows his avionics: Asking his opinion on RST's offerings, I wrote, "For $29, Jim can solve all the antenna issues for a plastic airplane...now, I know, I know...something too good to be true probably isn't...but...every now and then. Your thoughts?" $36 by the time you tack on shipping. And my pal responded: "You can make dipoles fairly easily, but they will have to be tuned for the specific frequencies that they will be used for. That's true, but misleading. Using half inch wide tape makes the tuning rather trivial providing you can cut thin copper tape to within quarter of an inch of the length specified in the textbook. preferably once they are installed in the airplane. You will also need a "balun" to balance the impedance of the elements to the impedance of the feed line. I hate to contradict your friend, but "balun" is made up of the two words "balanced" and "unbalanced" (get it, bal-un?). A balun is used to go from a balanced dipole (two equal arms) to unbalanced coaxial cable (center conductor and outer shield). We used sintered powdered ferrite toroids (little ferrite donuts) slipped over the coax to keep the nasties from coming back down the line, and this is called (strangely enough) a ferrite balun. A perfect dipole (of which there ain't some) will have a center point feed impedance of 72 ohms. Coaxial cable for aircraft is 50 ohms. THe slight mismatch between 72 and 50 is actually of some use to us as it does what is called mismatch broadbanding. (and for those of you who think broadband is an all girl orchestra, go to the next topic). In other words, to cover the entire 118 to 137 MHz com band isn't particularly easy UNLESS you go to relatively wide elements (half inch tape) and mismatch broadbanding. The actual response curve across these frequencies looks like a parabola (flashlight lens) and the idea is to flatten the parabola to the extent possible. You do this by bringing up the middle a little which lowers the ends a little. As with all things aircraft, an antenna is a series of compromises flying in loose formation. It isn't as complicated as it sounds, but most folks don't have the gear to tune home made antennas and wind up relying on someone with a ham license who has the gear, or taking it to a radio shop. This is especially true of the transponder antenna, as you will be hard pressed to find a ham operator who has the capability to tune an antenna at 1090 Mhz. The COM frequency band (118-136 Mhz) is a bit more common to hams, though. Most 2 meter test gear will handle those frequencies. That's why you buy the cheap little b-b antenna if one doesn't come with your transponder. Cut the pie plate ground plane like the textbook shows and you won't have any problems. A general comment here ... antennas are the easiest thing in electronics to make and there is nothing magical about them. They perform damn near just like the theory predicts. However, there has grown up amongst the cognoscenti a nearly spiritual belief that antennas are best constructed during the light of a full moon with the help of powdered bat wings and frog tongues. IT just ain't so. I mean, I've had DOCTORS put these things together correctly. "FWIW, I have both the sweep test gear (I can sweep anything from 30 Mhz to 32 Ghz with my sweep gear) and the knowledge of how to make my antennas, but I chose to use the AAE antennas because I liked the material they were made from and it was one less thing I had to do to get my bird into the air. It would have been difficult for me to reproduce what AAE does for their price in terms of quality and performance. There's nothing wrong with a copper tape antenna, though, provided you tune the antenna to minimize the SWR (reflected power) across the band." Hey, as I said, if you can cut half inch copper tape to within a quarter of an inch or so of the 22 inches that we tell you to cut it, no problems. At least with the OTHER ten thousand that we've sold in thirty years. But, if you've already got the Archer antennas, by all means don't toss them away. Bob does a pretty fair job of designing antennas. Now my only "sweeping" equipment and experience is on the end of a broom...and I live on an island without a convenient neighborhood radio shop, and the only ham I'm familiar with comes in a sandwich. So my questions for this discussion (and for Jim, if he's monitoring this) are, taaa daaa, 1. What's involved in "tuning" an antenna? (Can proper tuning be assured by careful measurement and installation...or must one have both access to the antenna for trimming, and diagnostic equipment of some kind?) An exacto knife and a steel rule. 2.What's involved in "sweeping" an antenna? What equipment is needed, and what is the nature of any alterations to the antenna which the process of sweeping reveals? Well, it DOES keep the dust off of them. Use a very soft bristle broom. Just kidding. When we first design an antenna we do in fact sweep test them. This involves generating a whole lot of frequencies near the band of interest quite fast and looking at the VSWR response curve on an oscilloscope. For example, I might sweep the COM antenna from 100 to 150 MHz. and then concentrate on the 118-137 MHz. portion, making sure that there aren't any unintended squigglies (that's a technical term; you'll get used to it) in the band that we can't explain or deal with. After doing this ten or fifteen times with identical results, we sort of figure that doing it another fifteen times is just a waste of time and copper tape. Actually, we make the antenna about an inch longer than we think it will be and then peel off tape a tenth of an inch at a time (exacto knife tuning) from both ends until it is spot on frequency. 3. Can these issues of tuning and sweeping be ignored by buying off- the-shelf products? Well, I've got a couple of dozen rolls of tape and toroids on my shelf and I suspect Bob does too. Your call as to which one you want to use. I'm sure others will chime in. Humbly, Fred Jim Weir Humble Antenna Tinker |
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