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#11
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A few Helical Antenna question
Chris W. wrote:
"I am unclear which polarization to use if I want 2 helical antennas to talk to each other." Unless you are bouncing the signals off a reflector, use the same polarization for both. The axial-mode helical antenna is circulaly polarized and fires along its axis away from its ground screen. It can be either left handed or right handed depending on which way the helix twists. It responds to vertical, horizontal or diagonally polarized waves. It does not respond to waves with the opposite twist to its helix. Circular polarization of the opposite twist is rejected. This is like machine bolt threads. A bolt can screw into a nut from either side if it has the same itch direction, but a left hand bolt won`t screw into a right hand nut. Circular polarization of the opposite rotation is rejected. Refer to any edition of "Antennas" by Kraus for the details. Kraus invented the helical antenna after other experts said it could not work. Best regards, Richard Harrison, KB5WZI |
#12
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A few Helical Antenna question
Dale Parfitt wrote:
"Chris W" wrote in message news:ku88f.1019$ZP1.519@dukeread11... Dale Parfitt wrote: PS I would like to build 2 of these one is for 2.4ghz wireless network and one is 432 mhz amateur band EME and or satellite work. So the 2.4ghz version will likely never see more than 100 mW but could see as much as 1 watt. The 432 mhz version could see as much as 1000 watts, but if I did that it would be to an array of 4 of them so I guess each one would only see 250 watts? -- Chris W EME on 432 is linearly polarized (although switching polarities to make up for Faraday rotation can be advantageous) - you'll be throwing away a huge 3dB, have higher sidelobes than a well designed Yagi (nullifying the low sky temp advantage of deep space) and less gain per boom length than a good Yagi- all in all, a dismal choice for EME. Dale W4OP The calculation in the ARRL Antenna book say that a 12 foot boom with a circumference of 1.33 wavelengths will give me a 19.9 dbi gain this should make up for the 3 db loss I think. I haven't seen a yagi that gives that much gain in with a 12 foot boom. The book also says that the side lobe problem can be significantly reduced by creating an array of 4 helical antennas. That should up the gain to 25.9 db. That's pretty good for EME isn't it? Take it to 16 and you get over 30 db gain. And the only need to be spaced at 1.5 wavelengths, that's only 3.4 feet. -- Chris W The closest optimized Yagi I found was a K1FO on a 14' boom- 17.9dBi which compares quite well with your 19.9dBi on a 12' boom once we adjust for the polarization loss making the helical 16.9dBi linear for the helix vs. 17.9dB for the K1FO My experience with EME is at 23cM where we use dishes- but I can tell you that 1dB is to kill for.. The circularity could be an advantage as it would be insensitive to Faraday. Another advantage is that they are non critical to build- at least from a gain standpoint. But I think the real issue will be weight and weight ditribution. Between the helical tubing, non conductive boom and the reflector screen a helix is going to be a clydesdale compared to a Yagi. Another problem might be the necessity to mount the array from behind the reflector- that's going to be quite a moment arm. Bottom line is, 432 MHz EME is well explored- I am not aware of the use of helices for EME there- although, again I am not active on 432 EME. If they are not being use, there's probably a reason. Dale W4OP I guess unless both stations were using a helical antenna, an array of yagies would be better. Since the helical antennas would have to have opposite polarization, that would be pretty inconvenient unless both stations had an array of RH for TX and LH for RX, in which case you would have to build twice as many antennas. I did have an idea for mounting them though. I was going to find something to use as a form to wrap the helix around and then cover it with fiberglass, making a big fiberglass tube, then remove the mold from the inside. I think that would make for a pretty strong and lite structure. I would then make some kind of fiberglass structure to mount the antenna at it's CG. Of course such an antenna would have a pretty high wind load, so you would need a way to easily take it down in case of high winds. -- Chris W Gift Giving Made Easy Get the gifts you want & give the gifts they want One stop wish list for any gift, from anywhere, for any occasion! http://thewishzone.com |
#13
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A few Helical Antenna question
"Chris W" wrote in message news:MQN8f.5728$ZP1.1737@dukeread11... Dale Parfitt wrote: "Chris W" wrote in message news:ku88f.1019$ZP1.519@dukeread11... Dale Parfitt wrote: PS I would like to build 2 of these one is for 2.4ghz wireless network and one is 432 mhz amateur band EME and or satellite work. So the 2.4ghz version will likely never see more than 100 mW but could see as much as 1 watt. The 432 mhz version could see as much as 1000 watts, but if I did that it would be to an array of 4 of them so I guess each one would only see 250 watts? -- Chris W EME on 432 is linearly polarized (although switching polarities to make up for Faraday rotation can be advantageous) - you'll be throwing away a huge 3dB, have higher sidelobes than a well designed Yagi (nullifying the low sky temp advantage of deep space) and less gain per boom length than a good Yagi- all in all, a dismal choice for EME. Dale W4OP The calculation in the ARRL Antenna book say that a 12 foot boom with a circumference of 1.33 wavelengths will give me a 19.9 dbi gain this should make up for the 3 db loss I think. I haven't seen a yagi that gives that much gain in with a 12 foot boom. The book also says that the side lobe problem can be significantly reduced by creating an array of 4 helical antennas. That should up the gain to 25.9 db. That's pretty good for EME isn't it? Take it to 16 and you get over 30 db gain. And the only need to be spaced at 1.5 wavelengths, that's only 3.4 feet. -- Chris W The closest optimized Yagi I found was a K1FO on a 14' boom- 17.9dBi which compares quite well with your 19.9dBi on a 12' boom once we adjust for the polarization loss making the helical 16.9dBi linear for the helix vs. 17.9dB for the K1FO My experience with EME is at 23cM where we use dishes- but I can tell you that 1dB is to kill for.. The circularity could be an advantage as it would be insensitive to Faraday. Another advantage is that they are non critical to build- at least from a gain standpoint. But I think the real issue will be weight and weight ditribution. Between the helical tubing, non conductive boom and the reflector screen a helix is going to be a clydesdale compared to a Yagi. Another problem might be the necessity to mount the array from behind the reflector- that's going to be quite a moment arm. Bottom line is, 432 MHz EME is well explored- I am not aware of the use of helices for EME there- although, again I am not active on 432 EME. If they are not being use, there's probably a reason. Dale W4OP I guess unless both stations were using a helical antenna, an array of yagies would be better. Since the helical antennas would have to have opposite polarization, that would be pretty inconvenient unless both stations had an array of RH for TX and LH for RX, in which case you would have to build twice as many antennas. I did have an idea for mounting them though. I was going to find something to use as a form to wrap the helix around and then cover it with fiberglass, making a big fiberglass tube, then remove the mold from the inside. I think that would make for a pretty strong and lite structure. I would then make some kind of fiberglass structure to mount the antenna at it's CG. Of course such an antenna would have a pretty high wind load, so you would need a way to easily take it down in case of high winds. -- Chris W Hi Chris, You're right on the polarity sense- it gets reversed upon lunar surface reflection. At 23cM we are using a scalar or septum horns that have 2 feedpoints- CW and CCW- much easier to to do with a horn as it only involves a 2nd orthogonal probe. A set of 4 capacity stubs arranged longitudinally along the guide generates the 2 senses. If you are seriously interested in EME- take alook at 23cM. Gain is easy to come by in the form of dishes; LNA NF are now in the 0.2dB and power from the GS series tubes and Mistubishi modules are stable and quite easy. Not unusual to hear SSB stations ragchewing on activity or contest weekends, no Faraday issues and lots of big gun activity. Dale W4OP |
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