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#1
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Help with 900Mhz turnstile....
I would like to build a 908Mhz turnstile.
How does one normally go about testing and adjusting an antenna at that high of frequency? For a lower frequency I can hook up a MFJ antenna analizer or an SWR meter,. I want to build a 900Mhz antenna with a hemispherical pattern. It is going on an RC plane, so a normal vertical style antenna will be problematic as it will have a null straight down. So I though a turnstile antenna would be perfect. Any advise on designing such a device? Paul Kl7JG |
#3
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On Tue, 15 Jul 2003 11:56:58 GMT, Dale Parfitt wrote:
Is it my browser or are the images on that page next to useless? I could not figure out the antenna's configuration, or read the graph ordinates? Text was less helpful. Is there another site that discusses this antenna Richard? Dale Hi Dale, Graphics straight from EZNEC which has served this group for years. Select any graphic and simply ask the browser to view that page. If you are unfamiliar with the drawing conventions I would suggest getting the free version and becoming adept. The SWR drawings show 40 data points quite clearly between 440 and 460 (every 0.5MHz) no more resolution is required, there are no surprise resonances hidden between data points. This is a very simple antenna, the description of the main radiator: "This is a low, long antenna roughly quarter wavelength from the ground point, up to the bend, out to the end. The drive point was found by trial and error." is quite explicit but I already admit may be confusing, thus the attending illustration. There is only one ground, there is only one bend, there is only one feed. The name describes it as well: "Inverted F" The letter F certainly is descriptive. The only embellishment is that it is constructed with a sheet rather than a wire, but both sheet and wire models' SWR are also shown and discussed: "Because of the sheet metal form, this qualifies as a thick radiator. The SWR curve on the left confirms this when compared to the SWR curve below." "This is the SWR curve for a one wire model of the design modeled here." The design couldn't be simpler for those already adept at building grounded verticals with a gamma feed - and without the cap of the gamma feed. The vertical is simply folded over at the feed. 73's Richard Clark, KB7QHC |
#4
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I'm not antenna adept, so in trying to read between the lines I have a few questions
Here is my ASCII art diagram.... XMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM M F M F M F M F M F M F M F M F GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG GGGGGGGGGGGGG M= metal strip. X= corner where metal strip is bent. F = feed G = ground plane. How is the feed setup? If I am driving it with coax the ground braid ends at the ground plane and the center conductor goes up to the FM junction point? The strip is 1/4 wavelength off of the ground plane, but how long is it from the bend to the end? How wide is it? What frequency did you build it for? How did it work? Paul KL7JG guy, so On Tue, 15 Jul 2003 05:06:54 GMT, Richard Clark wrote: On Mon, 14 Jul 2003 20:41:53 -0700, wrote: I would like to build a 908Mhz turnstile. How does one normally go about testing and adjusting an antenna at that high of frequency? For a lower frequency I can hook up a MFJ antenna analizer or an SWR meter,. I want to build a 900Mhz antenna with a hemispherical pattern. It is going on an RC plane, so a normal vertical style antenna will be problematic as it will have a null straight down. So I though a turnstile antenna would be perfect. Any advise on designing such a device? Paul Kl7JG Hi Paul, 1. build a 90.8MHz version and scale it down; 2. build an Inverted-F see: http://www.qsl.net/kb7qhc/antenna/In...%20F/index.htm 73's Richard Clark, KB7QHC |
#6
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Use the drawing (viewed as its own page)
Maybe I'm not getting it, but you seem to imply that there are things on your page to click on to open in a larger window???? When I load the webpage there is not "Larger" link and the pictures on the webpage are too small to read. Paul (KL7JG) On Tue, 15 Jul 2003 17:06:25 GMT, Richard Clark wrote: On Tue, 15 Jul 2003 09:25:07 -0700, wrote: I'm not antenna adept, so in trying to read between the lines I have a few questions Here is my ASCII art diagram.... XMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM M F M F M F M F M F M F M F M F GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG GGGGGGGGGGGGGGG yup M= metal strip. X= corner where metal strip is bent. F = feed G = ground plane. How is the feed setup? If I am driving it with coax the ground braid ends at the ground plane and the center conductor goes up to the FM junction point? perfect The strip is 1/4 wavelength off of the ground plane, but how long is it from the bend to the end? How wide is it? What frequency did you build it for? How did it work? Paul KL7JG Hi Paul, Total radiator length (M) is ¼ wavelength (approx). Use the drawing (viewed as its own page) to print a copy and scale all dimensions by wavelength. Do not presume any correction factors, simply use one Meter to be one Meter (not 0.95 Meter) at one Meter wavelength. I deliberately left the segments visible to aid in scaling. You should be able to perceive that the structure folds at roughly 0.04 wavelength, which is also the height above the ground plane. The foreshortened width of the sheet metal is also on the scale of 0.04 wavelength. For your application at 900MHz be aware that even thin wire (as in the feed) becomes significantly thick enough to change resonance and feed resistance through slight geometrical variations. This is true for any construction. That is why I provided the caution that it was chosen (or discovered) by trial and error. Given the 10MHz wide 2:1 BW, discovery covers a lot of turf. By purposely using thick components, this becomes less an issue. At these very short wavelengths, it is a rather simple matter to build deliberately wide resonant (low Q) structures. 73's Richard Clark, KB7QHC |
#7
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#8
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A while back, QST had an article on building a GPS turnstile antenna. The
article might be useful. Check with the ARRL for a reprint. Randy ka4nma |
#9
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#10
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On Thu, 17 Jul 2003 05:55:36 GMT, Richard Clark
wrote: Hmmmm, Not quite what I expected. I will do this the old fashion way, e-mail. 73's Richard Clark, KB7QHC |
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