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#1
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Good DIY antennas for 33 cm use?
I recently lucked into a bin-full of Kenwood TK-981 mobile radios at a
ham swap meet, and bought the lot. One seems to need a new final and pigtail. The other four had "Works" notations on their yellow stickies, and all transmit with a respectable amount of power. Pending further testing I think I've got some useful kit here. These are, of course, FM radios, mostly intended for repeater operation. They also have simplex "talk-around" capability. Whomever tested them, had cloned/programmed in a codeplug configuration which seems to cover most of the West Coast ham repeaters and a few simplex frequencies as well. The likely use for these would mostly be as part of an emergency- response kit, for helping set up command-net and message-net structures during an exercise, drill, or an actual event. Due to the scarcity of 33 cm radios I would not expect that many (if any) of the individual ARES/RACES operators in our area would be taking part. So... now I need to buy or make antennas for them. I'm interested in ideas and suggestions. I've seen a number of DIY designs on the net: - Simple ground-plane antennas using an N connector and a few wires soldered on are easy to make, and quite predictable. Unfortunately the wires sticking out in several directions makes them not all that "go-kit-friendly". I could figure out a design using (e.g.) banana plugs and jacks to connect the elements, so the antenna could be easily "unplugged and knocked down" for storage, but that seems like a lot of fiddling. - Several people have published designs for 33 cm J-poles, often with a collinear structure for higher gain. Fairly predictable but need some tuning during construction. Go-kit-friendly if installed in a fiberglass or PVC radome tube. - Simple sleeve dipoles (fold back the coax braid, trim for SWR, heat-shrink) are easy, cheap, go-kit-friendly, and should be fairly robust. Yagis would also be useful but aren't as go-kit-friendly due to their size; Kent Britain's "cheap Yagi" design is easy and the ones I've made for other bands have worked well. So, any other suggestions for simple-yet-effective DIY antennas for this band? Are there commercial antennas good and cheap enough that I should just buy a bunch? Good mobile antennas? |
#3
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Good DIY antennas for 33 cm use?
On Wed, 15 Apr 2015 21:05:10 -0700, Jeff Liebermann
wrote: Oops. I "send" before I was done. Continuing. So... now I need to buy or make antennas for them. I'm interested in ideas and suggestions. Hint: 900 MHz radios don't like high VSWR antennas. I suggest you optimize the antenna for the 902 MHz TX area. I've seen a number of DIY designs on the net: Allow me to take some pot shots at these: - Simple ground-plane antennas using an N connector and a few wires soldered on are easy to make, and quite predictable. Unfortunately the wires sticking out in several directions makes them not all that "go-kit-friendly". I could figure out a design using (e.g.) banana plugs and jacks to connect the elements, so the antenna could be easily "unplugged and knocked down" for storage, but that seems like a lot of fiddling. Y'er right about that. Worse, the banana jacks and such tend to make the length of the elements rather variable, which is probably ok for a simple antenna, but not so great if you want to build something better, like a double skirted ground plane. I would save the ground plane for the antenna of last resort. - Several people have published designs for 33 cm J-poles, often with a collinear structure for higher gain. Fairly predictable but need some tuning during construction. Go-kit-friendly if installed in a fiberglass or PVC radome tube. A PVC radome will detune the antenna, often in an unpredictable manner. Also, before you build something inside a plastic or fiberglass pipe, put a piece of the stuff in the microwave oven and see if it gets hot. If it does, it's an RF absorber. I have a bad attitude about J-poles. I'll let someone else proclaim what a wonderful antenna they can be. I haven't had much luck with them. - Simple sleeve dipoles (fold back the coax braid, trim for SWR, heat-shrink) are easy, cheap, go-kit-friendly, and should be fairly robust. The simple coaxial antennas have a common problem. There needs to be a gap between the outer sleeve, and the coax cable braid. If you look at commercial coaxial antennas, the sleeve ground diameter is huge compared to the center mounting pipe. Just peeling back the braid and burying it under some shrink tubing is kinda marginal. Yagis would also be useful but aren't as go-kit-friendly due to their size; Kent Britain's "cheap Yagi" design is easy and the ones I've made for other bands have worked well. The problem with mounting vertically polarized Yagi antennas on a pole is that the coax cable gets in the way of the pattern. If you use a metal pipe for mounting, that too gets in the way. So, any other suggestions for simple-yet-effective DIY antennas for this band? Of course. I wouldn't have taken pot shots at the other ideas without having a favorite solution available. It's called an AMOS or Franklin antenna. Here's some I've done for 1090 MHz: http://802.11junk.com/jeffl/antennas/AMOS-5-1090MHz/ http://802.11junk.com/jeffl/antennas/AMOS-7/ I build the 2.4Ghz variety inside ABS rain gutter downspout pipe. Mo http://www.qsl.net/yu1aw/Misc/vhf_ant.htm http://www.brest-wireless.net/wiki/materiel:amos While 900 MHz results in a longer and larger antenna, it's not that big. The same methods popular with sector antennas on 2.4/5GHz can be used. Note the vertical radiation pattern of the 1090 Mhz antennas above. It's totally horizontal because the antenna is vertically symmetrical. That means you're not sending as much RF into the ground or into the air as you would with an end fed antenna, which tends to have uptilt problems. Since the back of the AMOS antenna is a strip of sheet metal or wire mesh, the coax cable can be fed behind the strip without trashing the antenna pattern (as in a yagi or vertical dipole). Since it's fed with a balun, the coax cable doesn't radiate. You can also get very creative with the construction and still end up with a decent antenna. I've made them on 2.4Ghz out of a KD pine 2x4, some #12 hose wire, and assembled with a staple gun. For shielding, a strip of aluminum duct tape trimmed to the proper diameter. I'm also working on one that's made out of copper stained glass tape glued to the outside of an inflatable vinyl tube. Use your imagination. Be prepared to have some method of measuring VSWR versus frequency on 900 Mhz. Leave your Bird Wattmeter at home and look into a return loss bridge, RF sweep generator, DC amp, and a scope: https://www.google.com/search?q=return+loss+bridge Or, just build one: http://www.wb.commufa.jp/ja2djh/html/e_rlb.html Basically, what an RLB give you is a display of the VSWR versus frequency without any indication of whether the mismatch is inductive or capacitive. It's not a VNA (vector network analyzer) but for this project, you don't need one. If you want, I can grind out the numbers for most any configuration you want for 900 MHz. Are there commercial antennas good and cheap enough that I should just buy a bunch? Good mobile antennas? Of course. Most commercial mobile wire antennas cut for the 800-900 Mhz band can be trimmed to 902 Mhz. I have a mess of Motorola NMO mount 800/900 Mhz antennas that should work. Here's one: http://802.11junk.com/jeffl/antennas/Misc/slides/Motorola-850mhz-NMO.html I could probably send you a few if you want them. To use for portable operation, put some kind of ground plane under it. The downside is that you'll end up with some uptilt, which is great for talking to airplanes, but not very good for talking to the horizon. Low gain fiberglass antennas are usually marked 902-928 MHz. However, once the gain goes above about 6dBi, they end up cut for specific frequencies. Since they're sealed, they can't be retuned. Yagi's are ok, because they can usually be retuned. You can also find 900 Mhz patch and panel antennas. These do not handle high power very well, but if you have the 15 watt flavor of TK-981, it should be ok. The catch is that they're directional, which might be a good thing. I use one on my Motorola GTX 900 MHz radio in my palatial office. http://www.microcom.us/mt262006trhak.html Note the 9dBi gain. Also note that this one is circular polarized, which produces an automatic -3dB polarization mismatch loss. CP is common for RFID tag readers. 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 |
#4
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Good DIY antennas for 33 cm use?
Dave Platt wrote:
So... now I need to buy or make antennas for them. I'm interested in ideas and suggestions. Not sure if you want an omni antenna, but when you are interested in a small directional antenna I would recommend the double-quad, which on that kind of frequency is very easy to reproduce and works well. |
#5
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Good DIY antennas for 33 cm use?
In article ,
Rob wrote: Not sure if you want an omni antenna, but when you are interested in a small directional antenna I would recommend the double-quad, which on that kind of frequency is very easy to reproduce and works well. Thanks - that's a good suggestion! I build one of these years ago for WiFi experimentation, and doing something similar for 900 MHz ought not to be difficult. As Jeff suggested, I really need a good way of measuring reflected power at low levels, so I can trim-and-tune up properly. |
#6
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Good DIY antennas for 33 cm use?
On Fri, 17 Apr 2015 17:04:21 -0700, (Dave
Platt) wrote: Incidentally, what's with the 33 cm wavelength notation? I'll start using wavelength instead of frequency when HP, TEK, or some other reputable vendor supplies a frequency counter or generator calibrated in wavelength instead of the usual frequency or period. Awww, I'll bet you even say "2 meters" sometimes ;-) Sure, but when I feel like being a PITA, I often say "the 6 foot band". The EDZ is an omni (excepting pattern disruption from the tower or mast it's mounted on). I'm not sure whether it, or an AMOS, would be more useful in the sort of deployment I'm thinking about. Stations at the outside edge of the event might benefit by the directionality of the AMOS; the station in the center might want an omni. Oh swell. Take two problematic J-Poles and stick them together and hope that the result will be a better antenna. Two wrongs don't make a right. Ed Fong gave a presentation to the San Lorenzo Valley ARC a few months ago on J-Poles. Here's the slide show from the presentation. https://www.slvarc.org/wp-content/uploads/2015/03/dbj-2-slides_hawaii.pdf http://edsantennas.weebly.com http://www.work-sat.com/Antennas_files/FONG-DBJ1.pdf The AMOS/Franklin antenna is NOT an omni thanks the reflector. It's commonly used in sector antennas found in 800 MHz and up cellular panel antennas. The stubs can be replaced by coils but it's still an AMOS/Franklin antenna. Neat! I like being able to build good, useful antennas from indigenous materials (that's why I like the "cheap Yagi" design). Rule-of-thumb. It's easy to build a low gain antenna ( 8dBi ) out of junk parts, using sloppy construction, and loose tolerances. Things become more critical above 8dBi. You could probably build a biquad, AMOS-3, patch, or similar antenna using the aforementioned methods. An AMOS-5 or AMOS-7 requires more precision. What I have to work with at the moment is an 8640B (unfortunately without the doubler), I have 2 or 3 of those. Let me know when you need the gears fixed. http://802.11junk.com/jeffl/pics/HP8640B/ plenty of scopes, Yep. I have the same problem: http://802.11junk.com/jeffl/crud/scopes-to-be-fixed.jpg That's the "to be fixed one of these days" pile, which now has 4 more scopes in the pile. and a Systron-Donner spectrum analyzer which goes up well above 2 gig. You'll need a sweep generator. I have about 7, none of which work well. http://802.11junk.com/jeffl/pics/home/slides/BL-shop5.html The logic was to buy 3 broken generators on eBay and make one that works out of the parts. However, I goofed and bought one each revisions A, B, and C, which are mutually incompatible. Argh. I've been thinking that by adding a decent directional coupler, Yeah, that would work. However, I suggest you just get an RF sweeper and a reflection coefficient bridge and be done with it. The real problem with indoor antenna testing and the test equipment is that the position of everything, including your body, has an effect on the VSWR curve. You can move around my shop and produce almost any disgusting pattern desired. It's much like RADAR, where the reflected signal become the VSWR. If you want, I can grind out the numbers for most any configuration you want for 900 MHz. I'd definitely appreciate it if you could crank out an AMOS or two of varying lengths, for 902, and see what they look like. Done using 4NEC2. See: http://802.11junk.com/jeffl/antennas/AMOS-5-915MHz/ 12.7dBi gain. 150 degree horizontal beamwidth. 10 degree vertical beamwidth. VSWR = 1.15 to 1.50:1 in the 902-928 MHz range. The wire is #12 AWG copper without any insulation. Some details: 1. I changed my mind and made it for 915 Mhz instead of 902 MHz. 2. I goofed and made the reflector too short. It will work as is but I need to recalculate and re-optimize for a slightly longer reflector. 3. The antenna is designed for 200 ohms and will require a 4:1 coax balun. Something like this: http://www.brest-wireless.net/albums/AntenneAmos/balun.jpg but using 1/4 wave electrical coax at 915 MHz. 4. Most of the dimensions are a radius, not a diameter and that measurements are to the center of the wires, not the edge. 5. The numbers are all in wavelengths. Scraping the NEC file: SY W=.003 'Wire radius #12 AWG = 2mm dia. SY Rx=0.50/2, Rnx=4 'Reflector width / 2 , number of wires SY Ry=3.70/2, Rny=20 'Reflector height / 2 , number of wires SY Dl=0.335472 'Half of center dipole length SY Dh=0.216148 'Feed distance from reflector to dipole SY D=0.147626 'Stub Length SY E=0.183988 'Stub width SY B=0.605056 'Length of 2nd element SY C=0.512391 'Length of 3rd element To convert these to cut lengths, take the wavelength at 915 MHz, convert 915 MHz to mm wavelength = 328 mm, and multiple by the calculated wavelengths. For example, the reflector is: 0.50 wavelegths wide or 0.52 * 328 mm = 171 mm wide 3.70 wavlengths long or 3.70 * 328 mm = 1214 mm long (about 4ft) and so on. I also claculated a larger AMOS-7 (14.3dBi gain) which is about 6-7 ft long. That might be a too big for portable use. Might be worthwhile if you really need 1.6dB more gain. Bug me if you want it posted. I've got a few sheets of FR-4 and (certainly better) some Rogers low-loss PCB stock, and could probably etch or cut-and-trim to make a patch antenna of this size... once I figure out what the matching section would need to be. I don't think you'll be able to do much without a pile of test equipment. That works ok for 440 MHz, but is twice as tricky for 915 MHz. At 2400 MHz (wi-fi), cut-n-try is quite difficult. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#7
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Good DIY antennas for 33 cm use?
On Sat, 18 Apr 2015 18:35:45 -0700, Jeff Liebermann
wrote: 2. I goofed and made the reflector too short. It will work as is but I need to recalculate and re-optimize for a slightly longer reflector. Done. Antenna is now 4.2 wavelengths long or: 4.2 * 328 mm/wavelength = 1380 mm high (4.5ft) -- 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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