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Circular versus linear polarization
The majority of antennas used today are (commercial) circularly
polarized Ham antennas remain in the linear domain (ala the Yagi and similar) There are many reasons espoused in CP advantages in "point to point" What is the main advantage hams hold over the more popular circular polarized antennas in its "skip" type useage versus "point to point" ? |
Circular versus linear polarization
On Sun, 9 Aug 2009 21:24:01 -0700 (PDT), Art Unwin
wrote: The majority of antennas used today are (commercial) circularly polarized Ham antennas remain in the linear domain (ala the Yagi and similar) There are many reasons espoused in CP advantages in "point to point" What is the main advantage hams hold over the more popular circular polarized antennas in its "skip" type useage versus "point to point" ? Huh? By "skip", I presume you mean for HF and DX. Numbers are always nice. There are CP antennas for HF: http://www.bruhns.us/CP_on_HF/CP_on_HF.html www.roke.co.uk/resources/datasheets/locate-sarsen.pdf www.ascsignal.com/images/content/gov_radar/pdfs/TA103.pdf http://www.antennaproducts.com/ht30detail.html The main advantage is that they deal with multipath better and don't have a deep cross polarization null. I've never tried one so I have no idea how well (or badly) they work. For what it's worth, we did some experimentation with CP on 146MHz repeaters in the 1970's. The results were an impressive reduction in "picket fence" type of fading for mobiles. However, the anenna gain was less than an equivalent size linear array, so there was some range reduction. Mo http://www.qsl.net/n/n9zia//cir_pol_rpt.html -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Circular versus linear polarization
Jeff/Art
Keep in mind too that cross polarized (circular) point to point links (ie with a CP ant at each end) suffer from odd reflection attenuation (ie the polarization sense gets reversed by reflection) In 70cm UHF experiments I did back in the 80's I found out that a horiz-horiz system (base to mobile) outperformed a circular-circular by at least 12dB when moving. This wasnt actually the base reason for the experiments so I didnt make accurate measurements. OT Art, but I hope interesting. How random is the propogated linear antenna HF wave polarization and does it vary much with single hop and/or high angle? That might be a starting point for determining how useful CP on HF might be. Tnxs for the link Jeff. Cheers Bob VK2YQA Jeff Liebermann wrote: On Sun, 9 Aug 2009 21:24:01 -0700 (PDT), Art Unwin wrote: The majority of antennas used today are (commercial) circularly polarized |
Circular versus linear polarization
On Aug 10, 6:29*am, Bob Bob wrote:
Jeff/Art Keep in mind too that cross polarized (circular) point to point links (ie with a CP ant at each end) suffer from odd reflection attenuation (ie the polarization sense gets reversed by reflection) In 70cm UHF experiments I did back in the 80's I found out that a horiz-horiz system (base to mobile) outperformed a circular-circular by at least 12dB when moving. This wasnt actually the base reason for the experiments so I didnt make accurate measurements. OT Art, but I hope interesting. How random is the propogated linear antenna HF wave polarization and does it vary much with single hop and/or high angle? That might be a starting point for determining how useful CP on HF might be. Tnxs for the link Jeff. Cheers Bob VK2YQA Jeff Liebermann wrote: On Sun, 9 Aug 2009 21:24:01 -0700 (PDT), Art Unwin wrote: The majority of antennas used today are (commercial) circularly polarized What stands out for me is the audio improvement. |
Circular versus linear polarization
"Art Unwin" wrote in message ... The majority of antennas used today are (commercial) circularly polarized That probably isn't true for VHF/UHF. Take a look at the website of a major supplier of professional antennas such as http://www.amphenol-jaybeam.com/base...nas-search.php. Entering CP in their search engine for base-station antennas yielded 2 results wheras entering VP yielded 365! One of the reasons for greater use of linear polarisations in professional applications is frequency re-use on the orthogonal polarisation some distance away - i.e. the value of the limited VHF/UHF spectrum. That's certainly true in Europe. Ham antennas remain in the linear domain (ala the Yagi and similar) There are many reasons espoused in CP advantages in "point to point" What is the main advantage hams hold over the more popular circular polarized antennas in its "skip" type useage versus "point to point" ? Extensive use is made of 'mixed polarisation' for transmitting VHF FM broadcast services in Europe. It may not be pure circular but it contains significant vertically- and horizontally-polarised components. The mobile and portable receiving antennas have whatever polarisation they end up with, more or less by accident, and fixed rooftop antennas are usually linearly polarised. DAB and terrestrial television are transmitted using V or H linear polarisation. Chris |
Circular versus linear polarization
On Aug 10, 9:04*am, "christofire" wrote:
"Art Unwin" wrote in message ... The majority of antennas used today are (commercial) circularly polarized That probably isn't true for VHF/UHF. *Take a look at the website of a major supplier of professional antennas such ashttp://www.amphenol-jaybeam.com/base-station-antennas-search.php. *Entering CP in their search engine for base-station antennas yielded 2 results wheras entering VP yielded 365! One of the reasons for greater use of linear polarisations in professional applications is frequency re-use on the orthogonal polarisation some distance away - i.e. the value of the limited VHF/UHF spectrum. *That's certainly true in Europe. Ham antennas remain in the linear domain (ala the Yagi and similar) There are many reasons espoused in CP advantages in "point to point" What is the main advantage hams hold over the more popular circular polarized antennas in its "skip" type useage versus "point to point" ? Extensive use is made of 'mixed polarisation' *for transmitting VHF FM broadcast services in Europe. *It may not be pure circular but it contains significant vertically- and horizontally-polarised components. *The mobile and portable receiving antennas have whatever polarisation they end up with, more or less by accident, and fixed rooftop antennas are usually linearly polarised. *DAB and terrestrial television are transmitted using V or H linear polarisation. Chris Hmm A bit too technical for me! When I model my antennas it shows gain for cp being 3 db above the gains of vertical and horizontal for the same antenna. Is that what you call "mixed polarization? In other words, it picks up all polarizations with a max deviation in signal strength of only 3 db. I find it hard to make any sort of comparison when using reflective waves at HF because it is not clear to me exactly what sort of rotations occur at the reflections on earth and of its layers together with possible rotation in transit in between in the absence of true comparison experiments. |
Circular versus linear polarization
christofire wrote:
"Art Unwin" wrote in message ... The majority of antennas used today are (commercial) circularly polarized That probably isn't true for VHF/UHF. Take a look at the website of a major supplier of professional antennas such as http://www.amphenol-jaybeam.com/base...nas-search.php. Entering CP in their search engine for base-station antennas yielded 2 results wheras entering VP yielded 365! One of the reasons for greater use of linear polarisations in professional applications is frequency re-use on the orthogonal polarisation some distance away - i.e. the value of the limited VHF/UHF spectrum. That's certainly true in Europe. Ham antennas remain in the linear domain (ala the Yagi and similar) There are many reasons espoused in CP advantages in "point to point" What is the main advantage hams hold over the more popular circular polarized antennas in its "skip" type useage versus "point to point" ? Extensive use is made of 'mixed polarisation' for transmitting VHF FM broadcast services in Europe. It may not be pure circular but it contains significant vertically- and horizontally-polarised components. The mobile and portable receiving antennas have whatever polarisation they end up with, more or less by accident, and fixed rooftop antennas are usually linearly polarised. DAB and terrestrial television are transmitted using V or H linear polarisation. Chris Off subject I know but as a matter of interest what happens to a TX CP antenna with a clockwise twist transmitting to a RX CP antenna with an anticlockwise twist, over short to medium vhf/uhf paths, I would have thought a reduction in signal ? Pedr GW6YMS |
Circular versus linear polarization
"P.R.Humphreys" wrote in message ... Off subject I know but as a matter of interest what happens to a TX CP antenna with a clockwise twist transmitting to a RX CP antenna with an anticlockwise twist, over short to medium vhf/uhf paths, I would have thought a reduction in signal ? Pedr GW6YMS About the same as going from a horizontal to a vertical antenna. You loose about 20 db of signal. One thing a CP signal will do that a horizontal or vertical signal won't is to reverse when it bounces off an object. That is if you are doing moon bounce and send up a signal using right hand CP , your receiving antenna will need to be left hand CP or you loose lots of db of signal. Also on the satellites it is helpful to be able to change the CP from left to right as the signal sometimes reverses depending on the angle the satellite antenna is pointing as it passes over. |
Circular versus linear polarization
On Aug 10, 8:23*am, Art Unwin wrote:
What stands out for me is the audio improvement. Only very high Q antennas should noticeably effect audio. Comparing a dipole to say a turnstile, you should notice no real difference at all. I've used turnstiles on 75 and 40 meters for years. They work very well, but audio improvement is not one of the usual features noted. I prefer a turnstile over a dipole on the low bands if I have my choice. They seem slightly more efficient overall, but I've never done any accurate testing. They also tend to fill in the nulls off the ends that a dipole can show. I've heard that running a circular polarized antenna like the turnstile on HF, only shows circular polarization at the higher angles. Which is what I'm usually using when working NVIS on the lower bands. |
Circular versus linear polarization
On Mon, 10 Aug 2009 06:29:03 -0500, Bob Bob
wrote: Keep in mind too that cross polarized (circular) point to point links (ie with a CP ant at each end) suffer from odd reflection attenuation (ie the polarization sense gets reversed by reflection) That's one of the big advantages of CP for point to point links. The polarization reversal on odd numbered reflections means that multipath is greatly reduced. In 70cm UHF experiments I did back in the 80's I found out that a horiz-horiz system (base to mobile) outperformed a circular-circular by at least 12dB when moving. This wasnt actually the base reason for the experiments so I didnt make accurate measurements. The book "Microwave Mobile Communications" by Willaim C. Jakes Jr (1974) has a few words on the subject. As I recall, Ma Bell concluded that neither linear or cirucular polarization is good enough and that some form of diversity is required. Incidentally, "The Practical Handbook of Amateur Radio FM and Repeaters" (1981) (Tab 1212) by Pasternak and Morris, has chapter 31 on CP tests on the WA6VQP repeater on Loop Mtn. They draw a polar plot of the measured repeater antenna pattern and note that they get the typical "flower" pattern, with attendent deep nulls with linear polarization. With CP, the nulls are far less pronounced. My tinkering in the 1970's was specifically to reduce severe multipath fades along a section of freeway in Smog Angeles. It worked, but with some loss in signal stength from mismatched linear and CP (theoretically -3dB). OT Art, but I hope interesting. How random is the propogated linear antenna HF wave polarization and does it vary much with single hop and/or high angle? That might be a starting point for determining how useful CP on HF might be. I did some tinkering with measuring the polarization of skywave signals using a rotating loop antenna. Including Faraday rotation, my guess(tm) is that it's quite random and changes rapidly. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Circular versus linear polarization
wrote in message ... On Aug 10, 8:23 am, Art Unwin wrote: What stands out for me is the audio improvement. Only very high Q antennas should noticeably effect audio. Comparing a dipole to say a turnstile, you should notice no real difference at all. I have though that also, but I have noticed that on 75 meters there sometimes seems to be a differance in the voice of the other station when I switch from my off center fed antenna at 50 feet and a dipole at 20 feet. This is when signals are around the same on either antanna. Around the s-9 point. I am using an Icom 746pro and an external switch to change the antennas. |
Circular versus linear polarization
I also did some experiments in the early '70s to see if CP would reduce
fading. I built a couple of types of omnidirectional CP antennas -- a "skew planar", and a copy of a commercial FM BC antenna, for mobile use with the local 450 MHz repeater. I soon discovered that as soon as I placed the antenna over the top of the car, the polarization became nearly linear. I've since learned that it's because of the nature of the reflections from the ground plane, and it's easily seen with EZNEC+. When I put the antenna far enough away from the car to minimize reflections, the lowered gain offset any possible advantage. Overall, they worked out worse than a conventional vertically polarized antenna. It might have been interesting to try CP at the repeater, but that was never done. The problem with ground reflection ruining the circularity makes it very difficult to achieve circular polarization for HF skip communication. A second problem is that the majority of CP antennas, such as the quadrature fed crossed dipole "turnstile", are circular only directly broadside, and increasingly elliptical as you move away from that direction. Roy Lewallen, W7EL |
Circular versus linear polarization
Art Unwin wrote:
The majority of antennas used today are (commercial) circularly polarized Ham antennas remain in the linear domain (ala the Yagi and similar) There are many reasons espoused in CP advantages in "point to point" What is the main advantage hams hold over the more popular circular polarized antennas in its "skip" type useage versus "point to point" ? Is there a CPOL advantage for atmospherically propagated HF? I can think of some disadvantages. |
Circular versus linear polarization
On Aug 10, 5:21*pm, dave wrote:
Art Unwin wrote: The majority of antennas used today are (commercial) circularly polarized Ham antennas remain in the linear domain (ala the Yagi and similar) There are many reasons espoused in CP advantages in "point to point" What is the main advantage hams hold over the more popular circular polarized antennas in its "skip" type useage versus "point to point" ? Is there a CPOL advantage for atmospherically propagated HF? *I can think of some disadvantages. I am all ears. Go for it |
Circular versus linear polarization
Art Unwin wrote:
The majority of antennas used today are (commercial) circularly polarized Ham antennas remain in the linear domain (ala the Yagi and similar) There are many reasons espoused in CP advantages in "point to point" What is the main advantage hams hold over the more popular circular polarized antennas in its "skip" type useage versus "point to point" ? Please provide examples of commercial antennas that are CP. Space communication antennas are not ok to include. Please give counts of CP versus linear in commercial use. tom K0TAR |
Circular versus linear polarization
On Aug 10, 11:14*pm, tom wrote:
Art Unwin wrote: The majority of antennas used today are (commercial) circularly polarized Ham antennas remain in the linear domain (ala the Yagi and similar) There are many reasons espoused in CP advantages in "point to point" What is the main advantage hams hold over the more popular circular polarized antennas in its "skip" type useage versus "point to point" ? Please provide examples of commercial antennas that are CP. *Space communication antennas are not ok to include. Please give counts of CP versus linear in commercial use. tom K0TAR Use your own thread to ask for assistance on your question. Be prepared to answer why you are requesting this personal service . I asked a question on this thread with respect to the main advantage for hams that linear polarization has over CP. I have no resistance to change if it can be justified. I see that it can pick up signals that linear antennas cannot hear because of a 30 db attenuation where as CP has only a 3 db attenuation! So what is it on the other side of the coin is what this thread is posing to those who are familiar with respect to radiators. |
Circular versus linear polarization
"Art Unwin" wrote in message
... On Aug 10, 11:14 pm, tom wrote: Art Unwin wrote: The majority of antennas used today are (commercial) circularly polarized Ham antennas remain in the linear domain (ala the Yagi and similar) There are many reasons espoused in CP advantages in "point to point" What is the main advantage hams hold over the more popular circular polarized antennas in its "skip" type useage versus "point to point" ? Please provide examples of commercial antennas that are CP. Space communication antennas are not ok to include. Please give counts of CP versus linear in commercial use. tom K0TAR Use your own thread to ask for assistance on your question. Be prepared to answer why you are requesting this personal service . I asked a question on this thread with respect to the main advantage for hams that linear polarization has over CP. I have no resistance to change if it can be justified. I see that it can pick up signals that linear antennas cannot hear because of a 30 db attenuation where as CP has only a 3 db attenuation! So what is it on the other side of the coin is what this thread is posing to those who are familiar with respect to radiators. Cross polarization can be used to provide isolation for duplex links or co-channel sharing. |
Circular versus linear polarization
Use your own thread to ask for assistance on your question. Be prepared to answer why you are requesting this personal service . I asked a question on this thread with respect to the main advantage for hams that linear polarization has over CP. I have no resistance to change if it can be justified. I see that it can pick up signals that linear antennas cannot hear because of a 30 db attenuation where as CP has only a 3 db attenuation! So what is it on the other side of the coin is what this thread is posing to those who are familiar with respect to radiators. Cross polarization can be used to provide isolation for duplex links or co-channel sharing. I think that you are looking at it from the wrong point of view. CP does not have 3dB attenuation, but a linearly polarized signal received on a CP antenna will be 3db down (whether that be H, V or slant). So for amateur use CP is useful in that with another CP station multipath due to reflections will be reduced (reflections taking on the opposite hand CP and thus attenuated). However, if a CP antenna is used to receive H or V you may well not see the full 3dB reduction due to twisting of the polarization on the path. In fact over some paths the signal on a CP antenna will be better than on a linearly polarized antenna due the a greater attenuation due to polarization twisting. Of course the best of both worlds is to have a crossed yagi with a phase switching box so that you can select CP (both hands) H, V or slant 45 both ways. Commercially (terrestrially) I don't think that CP is used much except perhaps on microwave point to point links. Jeff |
Circular versus linear polarization
On Aug 10, 12:24*am, Art Unwin wrote:
The majority of antennas used today are (commercial) circularly polarized Ham antennas remain in the linear domain (ala the Yagi and similar) There are many reasons espoused in CP advantages in "point to point" What is the main advantage hams hold over the more popular circular polarized antennas in its "skip" type useage versus "point to point" ? Where did you get the info that most commercial antenna use is circular polarity? Jimmie |
Circular versus linear polarization
snpOn Aug 11, 10:16*am, "Jeff" wrote:
Of course the best of both worlds is to have a crossed yagi with a phase switching box so that you can select CP (both hands) H, V or slant 45 both ways. Jeff, I want the" best of both" worlds but before one jumps one needs to know the advantages and disadvantages of both first. To be frank, I do not know what happens to all signals when they are reflected including the reflection off the ground to a receiver or a direct hit on the receiver from our upper layers so I have no way of undertaking a comparison. Now most hams are resistant to change or will only follow the majoratory thus the questions "who has the majoratory". For me I see CP as an ideal approach for smaller volume antennas which is the "holy grail" for hams, but one has to determine if smaller is important enough to overide possible faults.Now I have a whip antenna ( no ground plane) on my tower that is CP for top band but the bands are poor it is difficult to determine its worth especially since there are no similar radiators out there. So I fall back and ask for "what is known already" as there is no point in re inventing the wheel. Is that so bad? Now before the catwalling about the possibility of having a whip (a 5 foot pole) for top band my intention is to release all that during the next month. So lets not waste energy in side tracking comments and stay on the essence of this thread. Remember the last time I intended to release it was evident that most did not want to hear it........ so I went along ! Commercially (terrestrially) I don't think that CP is used much except perhaps on microwave point to point links. Jeff |
Circular versus linear polarization
On Mon, 10 Aug 2009 11:44:52 -0700, Roy Lewallen
wrote: I also did some experiments in the early '70s to see if CP would reduce fading. I built a couple of types of omnidirectional CP antennas -- a "skew planar", and a copy of a commercial FM BC antenna, for mobile use with the local 450 MHz repeater. A "halo" type of antenna? Some of the commercial broadcast FM antennas are eliptical polarized. Most of the signal is horizontally polarized, but there is a small vertical component in order to improve performance in vehicles. We tried several antennas at the repeater end. Something like this one seemed to work best: http://iris.nyit.edu/~sblank/VPFMfig5.gif We had 4 elements with a coax cable phasing mess. I soon discovered that as soon as I placed the antenna over the top of the car, the polarization became nearly linear. I've since learned that it's because of the nature of the reflections from the ground plane, and it's easily seen with EZNEC+. When I put the antenna far enough away from the car to minimize reflections, the lowered gain offset any possible advantage. Overall, they worked out worse than a conventional vertically polarized antenna. I tried to use CP on both ends and eventually gave up. Thanks for the explanation, but I have a different theory. The polarization changes sense (direction) every times it's reflected. We standardized on RH CP. When the RH CP signal hits the car, it is reflected as LH CP. If the LH CP signal arrives at the repeater antenna, which is RH polarized, they cancel. If it became linear, it would theoretically only present a -3dB polarization loss, which is not huge. It might have been interesting to try CP at the repeater, but that was never done. I can testify that it worked quite well for solving the specific problem. We were trying to eliminate picket fencing (frequency selective fading or Rayleigh fading). While there were some half hearted experiments with various CP mobile antennas, the major effort was at the repeater end. This was about 1971 so the technology used was rather crude. One student was doing his senior project (reqd for graduation) around this test. Several of us were enlisted to help. When was in the land mobile radio biz many years later, I repeated the tests with similar results. We hung a thermal chart recorder onto the first limiter testpoint (on a Motorola Sensicon T43 receiver) and plotted signal strength versus time as a mobile drove through the problem area with the xmitter keyed continuously. The test was repeated with various tower mounted antennas. The linear antenna had more signal (gain) than CP, but also had many more fades, what were far more pronounced. In all, I would call it an improvement in quality, but not in range. The problem with ground reflection ruining the circularity makes it very difficult to achieve circular polarization for HF skip communication. Well, I supplied several examples of commercial HF antennas that are circularly polarized. I'm tempted to try building one, just to see what works or breaks. A second problem is that the majority of CP antennas, such as the quadrature fed crossed dipole "turnstile", are circular only directly broadside, and increasingly elliptical as you move away from that direction. That's why high accuracy GPS antennas use choke rings at the antenna. It widens the pattern so that it picks up more of the sky, but also maintains some semblence of CP at the horizon. Any interest in me scanning and posting the chapter on circular polarization repeater antennas from the TAB book? 13 pages with some low quality pictures. Roy Lewallen, W7EL -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Circular versus linear polarization
On Mon, 10 Aug 2009 16:23:09 -0700 (PDT), Art Unwin
wrote: On Aug 10, 5:21*pm, dave wrote: Art Unwin wrote: The majority of antennas used today are (commercial) circularly polarized Ham antennas remain in the linear domain (ala the Yagi and similar) There are many reasons espoused in CP advantages in "point to point" What is the main advantage hams hold over the more popular circular polarized antennas in its "skip" type useage versus "point to point" ? Is there a CPOL advantage for atmospherically propagated HF? *I can think of some disadvantages. I am all ears. Go for it Applied CP made simple: 1. Circular polarization is like a drill. Instead of bouncing off the ionosphere, it drills right through it. Therefore, little or no skip with CP. 2. Circular polarization is sensitive to the direction of rotation. If you're sending CP with your right hand, or using a microphone in your right hand, then you can only hear right hand circular polarization. 3. CP changes sense every times it bounces off something. Therefore, you can only hear even numbered bounces with the same sense. Odd bounces disappear meaning you can hear short skip signals (NVIS), and long path, but nothing in between. 4. Right hand CP is the most common, because most tuning screws use a right hand thread. If you want to use left hand CP, you'll need to find some rather scarce left hand threaded coil forms and tuning slugs. I hope this helps. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Circular versus linear polarization
You need to read up on it. It does have it's uses and I know hams that use
it. Usually by Satellite guys that talk to mobiles too. It gets used by quite a few commercial broadcast FM stations. The theory is that a mobile antenna will hear either the main path or reflected path but to a lesser extent, both so multi-path distortion is minimized. Typically the reflection cause a reversal of polarization. There is no substitute for talking to the far station in whatever polarity is agreed on. |
Circular versus linear polarization
Jeff Liebermann wrote:
On Mon, 10 Aug 2009 11:44:52 -0700, Roy Lewallen wrote: I also did some experiments in the early '70s to see if CP would reduce fading. I built a couple of types of omnidirectional CP antennas -- a "skew planar", and a copy of a commercial FM BC antenna, for mobile use with the local 450 MHz repeater. A "halo" type of antenna? Some of the commercial broadcast FM antennas are eliptical polarized. Most of the signal is horizontally polarized, but there is a small vertical component in order to improve performance in vehicles. The "skew planar" looked like a cloverleaf antenna with each "petal" rotated 45 degrees. The other was a copy of a broadcast antenna advertised to be circularly polarized. I used a simple hand held dipole and field strength meter to judge polarization. I know now it was subject to a number of shortcomings, but I feel it did a reasonable job of indicating circularity. Both antennas were reasonably circular. . . . I tried to use CP on both ends and eventually gave up. Thanks for the explanation, but I have a different theory. The polarization changes sense (direction) every times it's reflected. We standardized on RH CP. When the RH CP signal hits the car, it is reflected as LH CP. If the LH CP signal arrives at the repeater antenna, which is RH polarized, they cancel. If it became linear, it would theoretically only present a -3dB polarization loss, which is not huge. No, that's a common misconception. A circularly polarized wave produces a circularly polarized wave of the opposite handedness only when reflected from a plane normal to its direction of propagation. That's seldom the case in a communication environment. When reflected from surfaces at other angles, the result is a change in circularity, from elliptical to nearly linear depending on the angle of reflection and the reflection coefficients of the surface. A short while with the modeling program of your choice will confirm this. . . . A second problem is that the majority of CP antennas, such as the quadrature fed crossed dipole "turnstile", are circular only directly broadside, and increasingly elliptical as you move away from that direction. That's why high accuracy GPS antennas use choke rings at the antenna. It widens the pattern so that it picks up more of the sky, but also maintains some semblence of CP at the horizon. I wasn't aware of any GPS receivers using crossed dipole "turnstile" type antennas. All the ones I've seen use either quadrifilar helix or patch antennas. Can you point to a reference or two regarding the choke rings -- I don't know what these are or what they do, and would like to learn. . . . Roy Lewallen, W7EL |
Circular versus linear polarization
On Aug 11, 12:30*pm, "JB" wrote:
You need to read up on it. *It does have it's uses and I know hams that use it. *Usually by Satellite guys that talk to mobiles too. *It gets used by quite a few commercial broadcast FM stations. *The theory is that a mobile antenna will hear either the main path or reflected path but to a lesser extent, both so multi-path distortion is minimized. *Typically the reflection cause a reversal of polarization. *There is no substitute for talking to the far station in whatever polarity is agreed on. You bet that they have to read up on it. For those who consider themselves experts and others idiots. There is some sort of federation for antenna builders and the President of that group states we will have a disaster on our hands if we do not come up with a new technology for what is required in 5 years. Apparently cell phones will be divided into three bands and the big boys want access to all. This means that they need three separate antennas on the cell phone to cater for all (his words). They don't like that idea because it means three antennas in close proximation to each other. The response to that challenge is to group together the research facilities to find a "new" technology if ever there is one. A lot of money at stake and it is for any of you that are knowledgable in the field to apply for. Those who are really "knowledgable" already know there is no "new" technology only the one that they use and they all have researched it to death such that all the answers have been unfolded to them. It is also too late to ask God to be a bit more fair and provides some different options to what we already have. All I ask of them is to share with the rest of us exactly what happens to CP when it collides with anything in transit to a receiver. |
Circular versus linear polarization
On Aug 10, 11:11*am, Art Unwin wrote:
On Aug 10, 9:04*am, "christofire" wrote: "Art Unwin" wrote in message .... The majority of antennas used today are (commercial) circularly polarized That probably isn't true for VHF/UHF. *Take a look at the website of a major supplier of professional antennas such ashttp://www.amphenol-jaybeam.com/base-station-antennas-search.php. *Entering CP in their search engine for base-station antennas yielded 2 results wheras entering VP yielded 365! One of the reasons for greater use of linear polarisations in professional applications is frequency re-use on the orthogonal polarisation some distance away - i.e. the value of the limited VHF/UHF spectrum. *That's certainly true in Europe. Ham antennas remain in the linear domain (ala the Yagi and similar) There are many reasons espoused in CP advantages in "point to point" What is the main advantage hams hold over the more popular circular polarized antennas in its "skip" type useage versus "point to point" ? Extensive use is made of 'mixed polarisation' *for transmitting VHF FM broadcast services in Europe. *It may not be pure circular but it contains significant vertically- and horizontally-polarised components. *The mobile and portable receiving antennas have whatever polarisation they end up with, more or less by accident, and fixed rooftop antennas are usually linearly polarised. *DAB and terrestrial television are transmitted using V or H linear polarisation. Chris Hmm * * *A bit too technical for me! *When I model my antennas it shows *gain for cp being 3 db above the gains of vertical and horizontal for the same antenna. Is that what you call "mixed polarization? *In other words, it picks up all polarizations with a max deviation in signal strength of only 3 db. I find it hard to make any sort of comparison when using reflective waves at HF because it is not clear to me exactly what sort of rotations *occur at the reflections on earth and of its layers together with possible rotation in transit in between in the absence of true comparison experiments.- Hide quoted text - - Show quoted text - Describe the circular antenna you are modeling? With the gain you are claiming it doesnt seem to be an apples to apples comparison. I suspect you are comparing a fullwave loop to a halfwave dipole. Show the data you are using for your model or be prepared for most to suspect you of the usual handwaving. Jimmie. |
Circular versus linear polarization
On Aug 11, 3:28*pm, JIMMIE wrote:
On Aug 10, 11:11*am, Art Unwin wrote: On Aug 10, 9:04*am, "christofire" wrote: "Art Unwin" wrote in message .... The majority of antennas used today are (commercial) circularly polarized That probably isn't true for VHF/UHF. *Take a look at the website of a major supplier of professional antennas such ashttp://www.amphenol-jaybeam.com/base-station-antennas-search.php. *Entering CP in their search engine for base-station antennas yielded 2 results wheras entering VP yielded 365! One of the reasons for greater use of linear polarisations in professional applications is frequency re-use on the orthogonal polarisation some distance away - i.e. the value of the limited VHF/UHF spectrum. *That's certainly true in Europe. Ham antennas remain in the linear domain (ala the Yagi and similar) There are many reasons espoused in CP advantages in "point to point" What is the main advantage hams hold over the more popular circular polarized antennas in its "skip" type useage versus "point to point" ? Extensive use is made of 'mixed polarisation' *for transmitting VHF FM broadcast services in Europe. *It may not be pure circular but it contains significant vertically- and horizontally-polarised components. *The mobile and portable receiving antennas have whatever polarisation they end up with, more or less by accident, and fixed rooftop antennas are usually linearly polarised. *DAB and terrestrial television are transmitted using V or H linear polarisation. Chris Hmm * * *A bit too technical for me! *When I model my antennas it shows *gain for cp being 3 db above the gains of vertical and horizontal for the same antenna. Is that what you call "mixed polarization? *In other words, it picks up all polarizations with a max deviation in signal strength of only 3 db. I find it hard to make any sort of comparison when using reflective waves at HF because it is not clear to me exactly what sort of rotations *occur at the reflections on earth and of its layers together with possible rotation in transit in between in the absence of true comparison experiments.- Hide quoted text - - Show quoted text - Describe the circular antenna you are modeling? With the gain you are claiming it doesnt seem to be an apples to apples comparison. I suspect you are comparing a fullwave loop to a halfwave dipole. Show the data you are using for your model or be prepared for most to suspect you of the usual handwaving. Jimmie. NO |
Circular versus linear polarization
Roy/Jeff
This is all interesting stuff that I have always wanted to experiment on some more. I note the comment (was it Roy?) about the positioning of a CP antenna over a vehicle roof and the effect that had on the total signal polarization. I slapped my forehead on that one! (ie I should have known and tested for it) I also used a skew planar loop. It was I guess maybe 400mm above the vehicle roof. It had 4 loops in phase. The halo/3 leaf HP (clover) was the same basic construction. At the time the results of H vs V really astounded me. In my case OTH UHF performance was maybe 12db "better" mainly from the reduction in flutter. No, that's a common misconception. A circularly polarized wave produces a circularly polarized wave of the opposite handedness only when reflected from a plane normal to its direction of propagation. That's seldom the case in a communication environment. When reflected from surfaces at other angles, the result is a change in circularity, from elliptical to nearly linear depending on the angle of reflection and the reflection coefficients of the surface. A short while with the modeling program of your choice will confirm this. Yes of course! (slaps forehead again!) This is something that has come out recently in some experiments I have been doing with mobile data comms on VHF SSB. I have a badly written and incomplete page; http://pages.suddenlink.net/vk2yqa Before getting into the data side of it I had noted that a lot of intelligence could be gleaned by looking at the Doppler effect from the moving vehicle. Only a tiny 30Hz or so was noted on 144MHz but it showed to my mind that a lot of signal comes from multiplathing and even some comes from double reflections. The level of shift over the Doppler "bandwidth" also showed that it was fairly evenly distributed. ie reflections off plane objects like oncoming vehicles were not really any stronger than from other directions. Such things as large oil tanks though are quite obvious when I correlate the trip timing with nearby objects. This also bought sense to the experiments I did back in the 80's where I tried a 3 el quad on the vehicle with dissappointing results. So where is this going? I wonder what the result would be Doppler "bandpass" wise if I used a CP antenna on the vehicle and base now? (The above mentioned results are all VP) Given the sense change reflections I wonder if I would get a "null" in the received bandpass due to movement. I am trying to visualize the result of this; http://pages.suddenlink.net/vk2yqa/img1.png during that test. Would I get a series of bands parallel to the envelope edges that would further indicate direction of travel relative to the base. I realize that the direct path already gives that info, just trying to get the likely scenbario in my head. Apologies to Art for taking his post so far OT. Cheers Bob VK2YQA |
Circular versus linear polarization
On Aug 10, 11:37*am, "Ralph Mowery"
wrote: "P.R.Humphreys" wrote in message ... Off subject I know but as a matter of interest what happens to a TX CP antenna with a clockwise *twist transmitting to a RX CP antenna with an anticlockwise twist, over short to medium vhf/uhf paths, I would have thought a reduction in signal ? Pedr GW6YMS About the same as going from a horizontal to a vertical antenna. * You loose about 20 db of signal. One thing a CP signal will do that a horizontal or vertical signal won't *is to reverse when it bounces off an object. *That is if you are doing moon bounce *and send up a signal using right hand CP , your receiving antenna will need to be left hand CP or you loose lots of db of signal. Also on the satellites it is helpful to be able to change the CP *from left to right as the signal sometimes reverses depending on the angle the satellite antenna is pointing as it passes over. Do you ever get to the point that your signal bounces off the upper layers instead of passing thru? I note that some users of the vhf have seen ducting effects (temp inversions?) |
Circular versus linear polarization
On Aug 11, 4:47*pm, Bob Bob wrote:
Roy/Jeff This is all interesting stuff that I have always wanted to experiment on some more. I note the comment (was it Roy?) about the positioning of a CP antenna over a vehicle roof and the effect that had on the total signal polarization. I slapped my forehead on that one! (ie I should have known and tested for it) I also used a skew planar loop. It was I guess maybe 400mm above the vehicle roof. It had 4 loops in phase. The halo/3 leaf HP (clover) was the same basic construction. At the time the results of H vs V really astounded me. In my case OTH UHF performance was maybe 12db "better" mainly from the reduction in flutter. No, that's a common misconception. A circularly polarized wave produces a circularly polarized wave of the opposite handedness only when reflected from a plane normal to its direction of propagation. That's seldom the case in a communication environment. When reflected from surfaces at other angles, the result is a change in circularity, from elliptical to nearly linear depending on the angle of reflection and the reflection coefficients of the surface. A short while with the modeling program of your choice will confirm this. Yes of course! (slaps forehead again!) This is something that has come out recently in some experiments I have been doing with mobile data comms on VHF SSB. I have a badly written and incomplete page; http://pages.suddenlink.net/vk2yqa Before getting into the data side of it I had noted that a lot of intelligence could be gleaned by looking at the Doppler effect from the moving vehicle. Only a tiny 30Hz or so was noted on 144MHz but it showed to my mind that a lot of signal comes from multiplathing and even some comes from double reflections. The level of shift over the Doppler "bandwidth" also showed that it was fairly evenly distributed. ie reflections off plane objects like oncoming vehicles were not really any stronger than from other directions. Such things as large oil tanks though are quite obvious when I correlate the trip timing with nearby objects. This also bought sense to the experiments I did back in the 80's where I tried a 3 el quad on the vehicle with dissappointing results.. So where is this going? I wonder what the result would be Doppler "bandpass" wise if I used a CP antenna on the vehicle and base now? (The above mentioned results are all VP) Given the sense change reflections I wonder if I would get a "null" in the received bandpass due to movement. *I am trying to visualize the result of this; http://pages.suddenlink.net/vk2yqa/img1.png during that test. Would I get a *series of bands parallel to the envelope edges that would further indicate direction of travel relative to the base. I realize that the direct path already gives that info, just trying to get the likely scenbario in my head. Apologies to Art for taking his post so far OT. Cheers Bob VK2YQA I don't see it as OT, if you get a response run with it cheers |
Circular versus linear polarization
"Art Unwin" wrote in message ... Do you ever get to the point that your signal bounces off the upper layers instead of passing thru? I note that some users of the vhf have seen ducting effects (temp inversions?) I hae not noticed it, but will not say that it can not hapen. I have worked Texas from NC on two meters and lots of times way to the north on 2 and 432. I was not working the sats while that was going on. |
Circular versus linear polarization
On Tue, 11 Aug 2009 13:13:38 -0700, Roy Lewallen
wrote: A second problem is that the majority of CP antennas, such as the quadrature fed crossed dipole "turnstile", are circular only directly broadside, and increasingly elliptical as you move away from that direction. Sorry, my original reply to this comment was screwed up thanks to me talking on the phone while writing. Yeah, the problem with turnstiles CP is at the horizon. At the horizon, one element of the turnstile would be roughly perpendicular to me, thus acting as a simple linear dipole. The other crossed element would be seen from the end, resulting in no radiation in my direction. So, at the horizon, a turnstile is mostly linear polarization. That's why high accuracy GPS antennas use choke rings at the antenna. It widens the pattern so that it picks up more of the sky, but also maintains some semblence of CP at the horizon. I wasn't aware of any GPS receivers using crossed dipole "turnstile" type antennas. All the ones I've seen use either quadrifilar helix or patch antennas. Can you point to a reference or two regarding the choke rings -- I don't know what these are or what they do, and would like to learn. If you really want a turnstile GPS antenna: www.arrl.org/tis/info/pdf/0210036.pdf As for the choke ring, you've probably seen them in the center of C-band DBS/TVRO dish antennas. Links: http://www.javad.com/jns/index.html?/jns/technology/Choke%20Ring%20Theory.html http://www.trimble.com/infrastructure/gnss-choke-ring-antenna.aspx?dtID=overview http://www.gpsworld.com/survey/news/trimble-choke-ring-antenna-uses-dorne-and-margolin-dipole-3620 More detail: www.novatel.com/Documents/Papers/3D_choke_ring.pdf If you remove the radome in the center, it's a "pinwheel" antenna, with which I'm totally unfamiliar. Note the comments on "low elevation tracking", which is what I was mumbling about for improving the performance at the horizon. The above article don't show it, but the choke ring does maintain some semblance of CP near the horizon. Patent on the dual frequency (L1 and L2 for GPS) choke ring: http://www.google.com/patents/about?id=EiwIAAAAEBAJ&dq=6278407 |
Circular versus linear polarization
On Aug 11, 9:00*pm, "Ralph Mowery" wrote:
"Art Unwin" wrote in message ... Do you ever get to the point that your signal bounces off the upper layers instead of passing thru? I note that some users of the vhf have seen ducting effects (temp inversions?) I hae not noticed it, but will not say that it can not hapen. I have worked Texas from NC on two meters and lots of times way to the north on 2 and 432. *I was not working the sats while that was going on. I used to have a longwire that really liked to work into Louisanna from NC. One of the lobes must have been going right into Sldell. Jimmie |
Circular versus linear polarization
On Aug 11, 5:10*pm, Art Unwin wrote:
On Aug 11, 3:28*pm, JIMMIE wrote: On Aug 10, 11:11*am, Art Unwin wrote: On Aug 10, 9:04*am, "christofire" wrote: "Art Unwin" wrote in message ... The majority of antennas used today are (commercial) circularly polarized That probably isn't true for VHF/UHF. *Take a look at the website of a major supplier of professional antennas such ashttp://www.amphenol-jaybeam.com/base-station-antennas-search.php. *Entering CP in their search engine for base-station antennas yielded 2 results wheras entering VP yielded 365! One of the reasons for greater use of linear polarisations in professional applications is frequency re-use on the orthogonal polarisation some distance away - i.e. the value of the limited VHF/UHF spectrum. *That's certainly true in Europe. Ham antennas remain in the linear domain (ala the Yagi and similar) There are many reasons espoused in CP advantages in "point to point" What is the main advantage hams hold over the more popular circular polarized antennas in its "skip" type useage versus "point to point" ? Extensive use is made of 'mixed polarisation' *for transmitting VHF FM broadcast services in Europe. *It may not be pure circular but it contains significant vertically- and horizontally-polarised components. *The mobile and portable receiving antennas have whatever polarisation they end up with, more or less by accident, and fixed rooftop antennas are usually linearly polarised. *DAB and terrestrial television are transmitted using V or H linear polarisation. Chris Hmm * * *A bit too technical for me! *When I model my antennas it shows *gain for cp being 3 db above the gains of vertical and horizontal for the same antenna. Is that what you call "mixed polarization? *In other words, it picks up all polarizations with a max deviation in signal strength of only 3 db. I find it hard to make any sort of comparison when using reflective waves at HF because it is not clear to me exactly what sort of rotations *occur at the reflections on earth and of its layers together with possible rotation in transit in between in the absence of true comparison experiments.- Hide quoted text - - Show quoted text - Describe the circular antenna you are modeling? With the gain you are claiming it doesnt seem to be an apples to apples comparison. I suspect you are comparing a fullwave loop to a halfwave dipole. Show the data you are using for your model or be prepared for most to suspect you of the usual handwaving. Jimmie. NO- Hide quoted text - - Show quoted text - Sounds like the same old Art to me, wants opinions and help on an antenna and he is not willing to disclose what he is really talking about. I would Ploink him if I could. Jimmie |
Circular versus linear polarization
JIMMIE wrote:
On Aug 11, 5:10 pm, Art Unwin wrote: On Aug 11, 3:28 pm, JIMMIE wrote: NO- Hide quoted text - - Show quoted text - Sounds like the same old Art to me, wants opinions and help on an antenna and he is not willing to disclose what he is really talking about. I would Ploink him if I could. Jimmie No, don't even think of it. He is the best entertainment on the net when he is off his meds. tom K0TAR |
Circular versus linear polarization
Jeff Liebermann wrote:
Applied CP made simple: 1. Circular polarization is like a drill. Instead of bouncing off the ionosphere, it drills right through it. Therefore, little or no skip with CP. So the ionosphere knows the difference between CP and linear? Interesting, since horizontal and vertical both reflect, and CP is a combination of both. Could you please explain what's happening with CP? Thanks. tom K0TAR |
Circular versus linear polarization
On Tue, 11 Aug 2009 23:01:49 -0500, tom wrote:
Jeff Liebermann wrote: Applied CP made simple: 1. Circular polarization is like a drill. Instead of bouncing off the ionosphere, it drills right through it. Therefore, little or no skip with CP. So the ionosphere knows the difference between CP and linear? I suspect the decision making abilities of the ionosphere approaches zero. Attributing intelligence to inanimate objects is generally a bad idea. Interesting, since horizontal and vertical both reflect, and CP is a combination of both. Could you please explain what's happening with CP? Ummm.... no, I can't. Please read my other points and see if they pass a sanity check. Incidentally #2 should read: "If you're sending CW (not CP) with your right hand..." Sorry(tm). I just hate it when someone takes me seriously. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Circular versus linear polarization
On Tue, 11 Aug 2009 13:13:38 -0700, Roy Lewallen
wrote: Jeff Liebermann wrote: On Mon, 10 Aug 2009 11:44:52 -0700, Roy Lewallen wrote: I also did some experiments in the early '70s to see if CP would reduce fading. I built a couple of types of omnidirectional CP antennas -- a "skew planar", and a copy of a commercial FM BC antenna, for mobile use with the local 450 MHz repeater. A "halo" type of antenna? Some of the commercial broadcast FM antennas are eliptical polarized. Most of the signal is horizontally polarized, but there is a small vertical component in order to improve performance in vehicles. The "skew planar" looked like a cloverleaf antenna with each "petal" rotated 45 degrees. The other was a copy of a broadcast antenna advertised to be circularly polarized. I used a simple hand held dipole and field strength meter to judge polarization. I know now it was subject to a number of shortcomings, but I feel it did a reasonable job of indicating circularity. Both antennas were reasonably circular. Sounds reasonable. Incidentally, the FM broadcast "cloverleaf" antenna was invented by Philip Smith, the inventor of the Smith Chart: http://www.ieeeghn.org/wiki/index.php/Philip_H._Smith_Oral_History#FM_Broadcasting_and_t he_Cloverleaf_Antenna I blundered cross this page on a 6/2 meter CP antenna design. Looks workable but very narrow band: http://www.wa7x.com/cycloid_info.html I tried to use CP on both ends and eventually gave up. Thanks for the explanation, but I have a different theory. The polarization changes sense (direction) every times it's reflected. We standardized on RH CP. When the RH CP signal hits the car, it is reflected as LH CP. If the LH CP signal arrives at the repeater antenna, which is RH polarized, they cancel. If it became linear, it would theoretically only present a -3dB polarization loss, which is not huge. No, that's a common misconception. A circularly polarized wave produces a circularly polarized wave of the opposite handedness only when reflected from a plane normal to its direction of propagation. That's seldom the case in a communication environment. When reflected from surfaces at other angles, the result is a change in circularity, from elliptical to nearly linear depending on the angle of reflection and the reflection coefficients of the surface. A short while with the modeling program of your choice will confirm this. With my limited abilities, it will probably take more than a "short while" with an antenna modeling program. I've never done any polarization studies. I'll take your word for this, but it would seem that there would be a gradual transition from total reversal in sense from a perpendicular reflector, to conversion to linear polarization with shallow reflection angles. I'll see if I can find some research on this. (I'm curious). I once did some crude experimentation on the degree of isolation provided by a reflective sense reversal. I just happen to have two big 2.4Ghz helical antennas, both RH CP. I separated them by about 15 meters and measured the received signal level. I then placed an obstruction (corner of building) along the line of sight, and supplied a flat plate reflector. I didn't think to try varying angles of incidence and reflection and just ran it at 45 degrees from the plane of the flat plate reflector. The signal dropped about -15dB which I guess is about all I could expect in an uncontrolled environment. The loss would indicate that the signal was still substantially circularly polarized at 45 degrees. I still have the helix antennas and can repeat the test if necessary (and if I can find the time). Thanks for the clarification. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Circular versus linear polarization
Jeff Liebermann wrote:
Ummm.... no, I can't. Please read my other points and see if they pass a sanity check. Incidentally #2 should read: "If you're sending CW (not CP) with your right hand..." Sorry(tm). I just hate it when someone takes me seriously. Ahh. I see. Should have read on rather than just reading the first and responding. tom K0TAR |
GPS/Choke Ring Circular versus linear polarization
Roy Lewallen wrote:
I wasn't aware of any GPS receivers using crossed dipole "turnstile" type antennas. All the ones I've seen use either quadrifilar helix or patch antennas. Can you point to a reference or two regarding the choke rings -- I don't know what these are or what they do, and would like to learn. . . . Roy Lewallen, W7EL For high performance GPS receivers and measurements, it's important that the apparent position of the antenna be be independent of the look angle to the various satellites. Partly this is by making antennas with a phase center that is look angle independent, partly this is by making sure you're not receiving a combination of direct and reflected waves. Remember that for precision GPS, what you're looking at is essentially the carrier phase within a single chip time (about 100ns). The carrier phase outside the correlator's time window doesn't contribute to the observable measurement (because it's got a random 180 degree phase shift superimposed on it). So what you're really worried about is interference that causes an apparent change in phase of the carrier (at 1.5GHz.. call it 20cm wavelength). In precision GPS, you're talking millimeter scale measurements, or, say, better than 1 degree of apparent phase shift. A reflected signal that is 35 dB down is enough to get this sort of error. The multipath from "distant" reflectors is fairly easily dealt with by putting the antenna on a pole. Distant, here, means a few meters away.. farther than that, and the multipath signal's code phase is far enough away that it doesn't contribute to the measurement. The chip length is about 100 ns, or 30 meters. So, putting the antenna on a pole a few meters up, with a plate underneath it that cuts off the "view" of the ground closer than a few meters away guarantees that you won't see any reflections from something closer than 20 meters path length different. You also choose an antenna that has very little gain below several degrees above the horizon. But, just any old flat plate won't work, because you can have a creeping wave propagate across the surface AND you don't want the plate itself to reflect a signal. Solution: make a plate that is a RF "black hole" at the frequency of interest.. it's a series of grooves that are carefully designed to attenuate the reflected and evanescent wave propagating across the surface (just like in a corrugated horn). The most common design is by Dorne and Margolin, and I guarantee you've seen these if you've seen surveyors doing GPS measurements. They're about 2' in diameter with several wide grooves around a small conical or hemispherical radome covering the actual antenna. Sometimes, the whole thing is covered by a hemispherical radome. http://facility.unavco.org/project_s.../antennas.html Is a photo of a typical geodetic installation (sub-mm accuracies) http://www.trimble.com/infrastructur...?dtID=overview is the actual antenna. |
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