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>>>>> DIRECTIONAL YAGI FOR 92.9 MHz (180 degrees)<<<<<<<<<
Hello,
I just finished building this one: http://www.drslick.org/Temp1/yagi929.jpg It's quite unusual, in that there are no directors and two reflectors. Here's the H-plane plot (the pattern looking down on the antenna from above, when the elements are vertical) in the yagi optimizer program that i used: http://www.drslick.org/Temp1/yagi929.jpg It's an interesting method that my friend used to get the pattern we wanted. We couldn't use the optimization algorithms, because these are designed to maximize the frontal lobe (can become quite narrow in span), whereas we wanted something with a 180 degree pattern. Notice it's about only about -2dB at 90 and 270 degrees. Also note that the outer edge of the pattern is about 4.51 dBi, so the dBd (decibals over a dipole) will be somewhere around +2.4dBd or so. This isn't that much gain, we realized, but the constraining factor was to make the yagi as compact as possible, so as to keep the landlord quiet. So we ended up with 3 elements minimum, in order to achieve the pattern and get close to 50 ohms. This antenna was designed for use when you are on the edge of your city, and you don't wanna waste ERP into the mountains or whatnot. As you can see in the Jpeg, it's built with HomeDepot stuff: 2" pvc and 5/16" treaded stock rod, which comes in lengths of 3 feet, with nuts used to attach lugs that are soldered to an SO-239, and for attaching to the PVC itself. Buy the connecting "link" nuts (they are longer than regular nuts), and you can put these on the ends to tune the lengths for best SWR. Here's the basic plans: Position (inches) 1/4 wavelength (inch)x two Driven element: 0 29.4" Reflector 1: 12" 38.01" Mast: 48" try for non-resonant length Reflector 2: 58" 32" So you will obviously have to make a two times the 1/4 wavelength that you see, to make a dipole. The DE has a space between the elements, but the reflector sections can be attached inside the PVC. Supposedly, the mast has no effect at a distance of 48" from the DE, but when i simulated this in the YO, the length of the mast DID matter, so it's advised that one make the mast at a non-resonant length, which for 92.9 megs would be avoiding a multiple of 5 feet. I didn't get a chance to measure the F/B ratio, nor do i really have the proper equipment to do so, but it's certainly directional, as a field test has proven. Questions and comments appreciated. Dr. Slick (not a real Doctor) |
Dr. Slick wrote:
Here's the H-plane plot Nope, it's not but an H-plane plot would be interesting. -- 73, Cecil, W5DXP |
Interesting photo but NO PATTERN
Dr. Slick wrote: Hello, I just finished building this one: http://www.drslick.org/Temp1/yagi929.jpg It's quite unusual, in that there are no directors and two reflectors. Here's the H-plane plot (the pattern looking down on the antenna from above, when the elements are vertical) in the yagi optimizer program that i used: http://www.drslick.org/Temp1/yagi929.jpg It's an interesting method that my friend used to get the pattern we wanted. We couldn't use the optimization algorithms, because these are designed to maximize the frontal lobe (can become quite narrow in span), whereas we wanted something with a 180 degree pattern. Notice it's about only about -2dB at 90 and 270 degrees. Also note that the outer edge of the pattern is about 4.51 dBi, so the dBd (decibals over a dipole) will be somewhere around +2.4dBd or so. This isn't that much gain, we realized, but the constraining factor was to make the yagi as compact as possible, so as to keep the landlord quiet. So we ended up with 3 elements minimum, in order to achieve the pattern and get close to 50 ohms. This antenna was designed for use when you are on the edge of your city, and you don't wanna waste ERP into the mountains or whatnot. As you can see in the Jpeg, it's built with HomeDepot stuff: 2" pvc and 5/16" treaded stock rod, which comes in lengths of 3 feet, with nuts used to attach lugs that are soldered to an SO-239, and for attaching to the PVC itself. Buy the connecting "link" nuts (they are longer than regular nuts), and you can put these on the ends to tune the lengths for best SWR. Here's the basic plans: Position (inches) 1/4 wavelength (inch)x two Driven element: 0 29.4" Reflector 1: 12" 38.01" Mast: 48" try for non-resonant length Reflector 2: 58" 32" So you will obviously have to make a two times the 1/4 wavelength that you see, to make a dipole. The DE has a space between the elements, but the reflector sections can be attached inside the PVC. Supposedly, the mast has no effect at a distance of 48" from the DE, but when i simulated this in the YO, the length of the mast DID matter, so it's advised that one make the mast at a non-resonant length, which for 92.9 megs would be avoiding a multiple of 5 feet. I didn't get a chance to measure the F/B ratio, nor do i really have the proper equipment to do so, but it's certainly directional, as a field test has proven. Questions and comments appreciated. Dr. Slick (not a real Doctor) |
Dave Shrader wrote in message news:CA44c.6318$C51.52257@attbi_s52...
Interesting photo but NO PATTERN Opps! Here is the H-plane plot: http://www.drslick.org/Temp1/yagiplot.jpg It was an interesting usage of the program, in that we used a "dummy" reflector that was very far away from the DE, and then used two directors as reflectors (notice on the plot that the lobe is actually pointed in the 180 degree direction). Taking a look at the photo again: http://www.drslick.org/Temp1/yagi929.jpg You will notice the 4 turns of 4" diameter coils in the RG-213 coax, just before the feedpoint (SO-239), using a plastic tie-wrap. You can use a balun instead, but i have never done this before, perhaps one of you out there has something i could try in the future. It would be a 1:1 in this case. You'll also notice that i didn't just leave the coax hanging off the feed point, as this might ruin the pattern and impedance. I tried to keep the coax at 90 degrees to the elements by tie-wrapping it to the PVC. It would be nice to borrow an anechoic chamber with the proper field-strength meter, and see how closely the actual unit matches the YO program in the H-plane. Great fun, and it should be able to handle up to 500 watts or so, but i don't recommend using anything more than 1 mW or so... hehehe! Questions and comments appreciated. Slick |
If you put the array on a rotator and put a receive antenna with some
sensitive output indicator beyond the near field, you can plot relative field, and thus accurate F/B by adjusting the power to the antenna for same received F/S at, say, 10º increments. Nothing else should change, so the relative field strength in the various directions should relate to each other. Ground reflections and absorption, and all the other artifacts will be constant unless you change something else. -- Crazy George Remove N O and S P A M imbedded in return address "Dr. Slick" wrote in message om... Dave Shrader wrote in message news:CA44c.6318$C51.52257@attbi_s52... Interesting photo but NO PATTERN Opps! Here is the H-plane plot: http://www.drslick.org/Temp1/yagiplot.jpg It was an interesting usage of the program, in that we used a "dummy" reflector that was very far away from the DE, and then used two directors as reflectors (notice on the plot that the lobe is actually pointed in the 180 degree direction). Taking a look at the photo again: http://www.drslick.org/Temp1/yagi929.jpg You will notice the 4 turns of 4" diameter coils in the RG-213 coax, just before the feedpoint (SO-239), using a plastic tie-wrap. You can use a balun instead, but i have never done this before, perhaps one of you out there has something i could try in the future. It would be a 1:1 in this case. You'll also notice that i didn't just leave the coax hanging off the feed point, as this might ruin the pattern and impedance. I tried to keep the coax at 90 degrees to the elements by tie-wrapping it to the PVC. It would be nice to borrow an anechoic chamber with the proper field-strength meter, and see how closely the actual unit matches the YO program in the H-plane. Great fun, and it should be able to handle up to 500 watts or so, but i don't recommend using anything more than 1 mW or so... hehehe! Questions and comments appreciated. Slick |
If you put the array on a rotator and put a receive antenna with some
sensitive output indicator beyond the near field, you can plot relative field, and thus accurate F/B by adjusting the power to the antenna for same received F/S at, say, 10º increments. Nothing else should change, so the relative field strength in the various directions should relate to each other. Ground reflections and absorption, and all the other artifacts will be constant unless you change something else. ok, that sounds like a good idea, with the rotator, keeping everything else the same. This site: http://radioproshop.com/antennaconcepts.htm Says that the near-field is defined by Rmin=(2*D**2)/wavelength, where D is the largest dimension of the antenna. So if we roughly say one wavelength is 10 feet, and the largest dimension is the dipole, then we get Rmin=5 feet, or about a 1/2 wavelength. This sounds a bit too close to me, perhaps the far-field is better defined as, like, ten wavelengths or so? Also, i don't really have a transmitter that I can vary the power that much...maybe that's something i should make or get. Something variable at least by 20dB or so. Slick |
If the second reflector is actually doing anything, which I doubt, you
should be using a non metallic mast, and not bringing the coax down between the reflectors. Tam/WB2TT "Dr. Slick" wrote in message om... Hello, I just finished building this one: http://www.drslick.org/Temp1/yagi929.jpg It's quite unusual, in that there are no directors and two reflectors. Here's the H-plane plot (the pattern looking down on the antenna from above, when the elements are vertical) in the yagi optimizer program that i used: http://www.drslick.org/Temp1/yagi929.jpg It's an interesting method that my friend used to get the pattern we wanted. We couldn't use the optimization algorithms, because these are designed to maximize the frontal lobe (can become quite narrow in span), whereas we wanted something with a 180 degree pattern. Notice it's about only about -2dB at 90 and 270 degrees. Also note that the outer edge of the pattern is about 4.51 dBi, so the dBd (decibals over a dipole) will be somewhere around +2.4dBd or so. This isn't that much gain, we realized, but the constraining factor was to make the yagi as compact as possible, so as to keep the landlord quiet. So we ended up with 3 elements minimum, in order to achieve the pattern and get close to 50 ohms. This antenna was designed for use when you are on the edge of your city, and you don't wanna waste ERP into the mountains or whatnot. As you can see in the Jpeg, it's built with HomeDepot stuff: 2" pvc and 5/16" treaded stock rod, which comes in lengths of 3 feet, with nuts used to attach lugs that are soldered to an SO-239, and for attaching to the PVC itself. Buy the connecting "link" nuts (they are longer than regular nuts), and you can put these on the ends to tune the lengths for best SWR. Here's the basic plans: Position (inches) 1/4 wavelength (inch)x two Driven element: 0 29.4" Reflector 1: 12" 38.01" Mast: 48" try for non-resonant length Reflector 2: 58" 32" So you will obviously have to make a two times the 1/4 wavelength that you see, to make a dipole. The DE has a space between the elements, but the reflector sections can be attached inside the PVC. Supposedly, the mast has no effect at a distance of 48" from the DE, but when i simulated this in the YO, the length of the mast DID matter, so it's advised that one make the mast at a non-resonant length, which for 92.9 megs would be avoiding a multiple of 5 feet. I didn't get a chance to measure the F/B ratio, nor do i really have the proper equipment to do so, but it's certainly directional, as a field test has proven. Questions and comments appreciated. Dr. Slick (not a real Doctor) |
OK, one alternative is to use a known accurate (1dB) step attenuator in the
receive line to get the relative numbers. Always go for the same strength indication on the receiver. Start at the azimuth with lowest indication, and add attenuation as it rotates. -- Crazy George Remove N O and S P A M imbedded in return address |
"Tam/WB2TT" wrote in message ...
If the second reflector is actually doing anything, which I doubt, you should be using a non metallic mast, and not bringing the coax down between the reflectors. Tam/WB2TT Actually, if you believe the yagi optimizer program, the second reflector is indeed not doing TOO much, but it does affect the back shape of the pattern, and improves the impedance bit, and also increases the dBd of the outer edge of the pattern by tiny bit too. And i did simulate having the mast at 48 inches from the DE, which i was advised was the distance where the mast would have the least affect. Changing the mast length in the YO program does affect the pattern shape though, so i tried to make the mast a non-resonant length. At any rate, we could probably survive without the second reflector, you are most likely correct, but maybe for the next design. It first, i considered having the "T" section of PVC pipe move outwards at a 90 degree angle, and then attach another section of 2" PVC that would be sticking out, so that the coax would be totally perpendicular to all the elements. But, this would be a bit unwieldy. Slick |
"Radio913" wrote in message ... If you put the array on a rotator and put a receive antenna with some sensitive output indicator beyond the near field, you can plot relative field, SNIP Also, i don't really have a transmitter that I can vary the power that much...maybe that's something i should make or get. Something variable at least by 20dB or so. Slick You don't need to pump power to measure antenna patterns. A simple signal generator with a milliwatt or so will work fine. If you want to be really serious about designing antennas, then you need to start acquiring serious tools. See eBay; start bidding on a sig gen, some pads (attenuators), a rotor, a field strength meter. Ed |
Slick,
The only thing about making the 48 inch mast non resonant is what's on the other end of it? A ceramic insulator is not the same as a Radio Shack steel house bracket. Since it is behind the first reflector, probably doesn't matter in the real world. Tam/WB2TT "Dr. Slick" wrote in message om... "Tam/WB2TT" wrote in message ... If the second reflector is actually doing anything, which I doubt, you should be using a non metallic mast, and not bringing the coax down between the reflectors. Tam/WB2TT Actually, if you believe the yagi optimizer program, the second reflector is indeed not doing TOO much, but it does affect the back shape of the pattern, and improves the impedance bit, and also increases the dBd of the outer edge of the pattern by tiny bit too. And i did simulate having the mast at 48 inches from the DE, which i was advised was the distance where the mast would have the least affect. Changing the mast length in the YO program does affect the pattern shape though, so i tried to make the mast a non-resonant length. At any rate, we could probably survive without the second reflector, you are most likely correct, but maybe for the next design. It first, i considered having the "T" section of PVC pipe move outwards at a 90 degree angle, and then attach another section of 2" PVC that would be sticking out, so that the coax would be totally perpendicular to all the elements. But, this would be a bit unwieldy. Slick |
"Crazy George" wrote in message ...
OK, one alternative is to use a known accurate (1dB) step attenuator in the receive line to get the relative numbers. Always go for the same strength indication on the receiver. Start at the azimuth with lowest indication, and add attenuation as it rotates. Yeah, that's a good idea. I made an amplified RF sniffer from plans in the ARRL handbook that uses an op-amp, but i never calibrated the meter. i could do this if i had bought one of those step attenuators at that ham fest i was at. Even if i had bought the somewhat inaccurate one that rotated, i could have still measured how off it was on a power meter. Yeah, i can set the sensitivity of the sniffer with a pot that controls the feedback resistor, and i can set for full scale deflection at the rear lobe, which should be the lowest field strength, and then increase the attenuation to maintain the same needle position. I'd have to get it back from the guy and take it off the rooftop again! This experiment will have to wait! He's getting out just fine... :) Slick |
"Ed Price" wrote in message news:6wH4c.5720$uh.5339@fed1read02...
You don't need to pump power to measure antenna patterns. A simple signal generator with a milliwatt or so will work fine. If you want to be really serious about designing antennas, then you need to start acquiring serious tools. See eBay; start bidding on a sig gen, some pads (attenuators), a rotor, a field strength meter. Ed I've got an MFJ antenna analyzer, which is somewhere around 1 mW or so. If i calibrate my RF sniffer correctly, i think i can get a pretty accurate plot, accurate enough for my purposes. Slick |
"Tam/WB2TT" wrote in message ...
Slick, The only thing about making the 48 inch mast non resonant is what's on the other end of it? A ceramic insulator is not the same as a Radio Shack steel house bracket. Since it is behind the first reflector, probably doesn't matter in the real world. Tam, The 48 inches refers to it's position relative to the DE, not it's actually length, which in the ideal case would be at a non-resonant length. The yagi optimizer program i have shows that the length does indeed affect the propagation pattern. I'm not certain how the user has the mast mounted, but it's somewhere around 7.5 feet or so in length. At any rate, he is getting out very well! Err...i won't say at what wattage, someone might faint out there! hahaha! Slick |
where is the center of radiation on this antenna ?
where will the strongest field strength be ? |
i found this antenna page ,
can i use a coil for the use od decoupling the signal? http://www.packetradio.com/images/6mdipole.gif |
g subs inscribed:
i found this antenna page , can i use a coil for the use od decoupling the signal? http://www.packetradio.com/images/6mdipole.gif Yes. However you should investigate other matching methods - Google for the "Ross Dipole". Also look at Gamma matches and Delta matches - both are viable solutions for unbalanced feeder connection to a balanced antenna (like a dipole). -- BIAS COMMS Everything gets easier with practice, except getting up in the morning! |
(g subs) wrote in message . com...
where is the center of radiation on this antenna ? where will the strongest field strength be ? Looking at: http://www.drslick.org/Temp1/yagiplot.jpg You will notice the lobe is greatest at the rear (180), and this is because of an interesting use of the yagi optimizer program, where we actually used two directors to act as reflectors. So the strongest field strength will be where you point the driven element (dipole), and the weakest ERP will be where the two reflectors are facing. Slick |
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