[linrad] Re: Polarization question
Hi Leif, I have been thinking more about the Linrad polarization questions that I raised here two weeks ago. I wrote: Suppose they [the feedline lengths in X and Y channels] are not well matched. In other words, suppose that the complex gains and signal delays in the two polarization channels are not equal. Will Linrad's polarization-matching capability be compromised? As far as I can see, it still works well even with poorly matched feedlines. I suppose this must mean that Linrad solves for a differential complex gain, and that over a fairly narrow bandwidth a different delay can be treated as a phase shift. You replied: From a practical point of view the cables are well matched. I think it is a safe assumption to guess that the length differences will be very small and of no concern. Let's work in terms of the Stokes Parameters, with complex signals X and Y. I = |X|^2 + |Y|^2 (total power) Q = |X|^2 - |Y|^2 (horizontal linear component) U = 2Re(X^* Y)(vertical linear component) V = 2Im(X^* Y)(circular component) L = sqrt(Q^2 + U^2) (linear polarized component) Theta = 0.5*atan(U/Q) (polarization angle) If I shift the phase of X relative to Y (say, by inserting an extra piece of cable in the X feedline), the values of U, V, L, and Theta will surely change. I don't know what you meant when writing From a practical point of view the cables are well matched. In my station, at present, it would be a complete accident if the downlines from the tower-mounted X and Y preamps were the same electrical length. They are just two pieces of coax that I had lying around. Moreover, my xpol yagis have the H elements located forward of the V elements by some 10 inches or so (I forget the exact amount) -- maybe 1/8 of a wavelength. This will have a very significant effect, as well, no? So, it seems to me that the only way to get things calibrated correctly is the one you outlined: Listen to a linearly polarised signal that arrives with similar strength in both polarisations. (This is the strongest reason why the X configuration is so much better than the + configuration. It is easy to find a pure H-pol signal. Finding a 45 degree terrestrial signal is virtually impossible due to ground reflections so a + configured system has to be calibrated on EME signals.) Change cable lengths until the signal appears close to linear on the pol meter. Fine tune by tweaking the second RF amplifiers. (will affect both amplitude and phase, but there are two second RF amplifiers so it should be possible to find both amplitude and phase matching. On the other hand, a strong practical reason to use the + configuration is that one wants to use the antenna for tropo as well as EME -- and therefore wants the ability to transmit a horizontal signal. My array, therefore, is in the + configuration. It would of course be easy to add parameters for amplitude and phase balance, but I have not done it since I found it easy to do in hardware:-) To me it seems much easier to do it in software, and perhaps I will try this within MAP65. Suppose the gains in the X and Y channels are already matched. Then, while receiving a 100% horizontally polarized signal, shouldn't it be sufficient to multiply the complex signal for X (or Y) by a complex constant e^(i*phi), with the phase shift phi chosen so as to minimize Stokes Parameter V ? -- Joe, K1JT # This message is sent to you because you are subscribed to the mailing list linrad@antennspecialisten.se. To unsubscribe, E-mail to: [EMAIL PROTECTED] To switch to the DIGEST mode, E-mail to [EMAIL PROTECTED] To switch to the INDEX mode, E-mail to [EMAIL PROTECTED] Send administrative queries to [EMAIL PROTECTED]
[linrad] Re: Polarization question
Hi Joe, I have been thinking more about the Linrad polarization questions that I raised here two weeks ago. I wrote: Suppose they [the feedline lengths in X and Y channels] are not well matched. In other words, suppose that the complex gains and signal delays in the two polarization channels are not equal. Will Linrad's polarization-matching capability be compromised? As far as I can see, it still works well even with poorly matched feedlines. I suppose this must mean that Linrad solves for a differential complex gain, and that over a fairly narrow bandwidth a different delay can be treated as a phase shift. You replied: From a practical point of view the cables are well matched. I think it is a safe assumption to guess that the length differences will be very small and of no concern. Let's work in terms of the Stokes Parameters, with complex signals X and Y. I = |X|^2 + |Y|^2 (total power) Q = |X|^2 - |Y|^2 (horizontal linear component) U = 2Re(X^* Y)(vertical linear component) V = 2Im(X^* Y)(circular component) L = sqrt(Q^2 + U^2) (linear polarized component) Theta = 0.5*atan(U/Q) (polarization angle) If I shift the phase of X relative to Y (say, by inserting an extra piece of cable in the X feedline), the values of U, V, L, and Theta will surely change.Yes, of course, but any difference in length of the cables will be insignificant as compared to the differences in phase shifts that you have in the amplifiers. I don't know what you meant when writing From a practical point of view the cables are well matched. In my station, at present, it would be a complete accident if the downlines from the tower-mounted X and Y preamps were the same electrical length. They are just two pieces of coax that I had lying around. But I am sure that none of them is VERY long in terms of wavelengths. It means that the phase shift vs frequency the difference in cable lenghts introduce is very small and negligible as compared to the large difference in phase vs frequency that non-matched RF amplifiers might introduce. Over a narrow bandwidth, such as 2 MHz, even that can be neglected if the RF amplifiers are reasonably similar. In the end, you just have an unknown, but frequency independent phase shift between the two RF channels. Moreover, my xpol yagis have the H elements located forward of the V elements by some 10 inches or so (I forget the exact amount) -- maybe 1/8 of a wavelength. This will have a very significant effect, as well, no? It just adds to the undeterminated phase difference and it is independent of frequency over 2 MHz. So, it seems to me that the only way to get things calibrated correctly is the one you outlined: Listen to a linearly polarised signal that arrives with similar strength in both polarisations. (This is the strongest reason why the X configuration is so much better than the + configuration. It is easy to find a pure H-pol signal. Finding a 45 degree terrestrial signal is virtually impossible due to ground reflections so a + configured system has to be calibrated on EME signals.) Change cable lengths until the signal appears close to linear on the pol meter. Fine tune by tweaking the second RF amplifiers. (will affect both amplitude and phase, but there are two second RF amplifiers so it should be possible to find both amplitude and phase matching. On the other hand, a strong practical reason to use the + configuration is that one wants to use the antenna for tropo as well as EME -- and therefore wants the ability to transmit a horizontal signal. My array, therefore, is in the + configuration. But that is a non-argument. I was using the X configuration for 10 years with only the option to put 50% of the power into each polarisation. In phase for vertical and out of phase for horizontal. (I also had +/- 90 degrees for circular, but although very good for aurora I found it useless for EME) I actually lost some interesting contacts because I could not put all the power in +45 or -45 degrees. I did hear GW0KZG/MM from the red sea and got QRZ from him. I knew 45 degrees would have given 3 dB more signal, but I could not do it because my X configuration was set up in the simple way with only H or V (or circular) as the emitted polarisation. It would of course be easy to add parameters for amplitude and phase balance, but I have not done it since I found it easy to do in hardware:-) To me it seems much easier to do it in software, and perhaps I will try this within MAP65. OK, but remember there is a performance penalty Suppose the gains in the X and Y channels are already matched. Then, while receiving a 100% horizontally polarized signal, shouldn't it be sufficient to multiply the complex signal for X (or Y) by a complex constant e^(i*phi), with the phase shift phi chosen so as to minimize Stokes
[linrad] Re: Polarization question
Sirs! Leif Joe, In addition to all the interesting observations and suggestions regarding the polarization (indication) challenges it is maybe useful to add that once we ignite our perfectionist behavior and begin to trim all of the feed lines and matching as near to ideal as possible, it is good to keep in mind that the phase-shift provided by a physical length of transmission line is only in good relation to the velocity factor when the line is terminated in its characteristic impedance (this being an additional source of error if input impedances of e.g .intermediate amplifiers or mixer stages are way off the nominal cable impedance). This can be an issue if one chooses e.g. two completely different builds of coax, like one with foam or teflon and the other with solid PE insulation (velocity factors possibly of 0.82 and 0.66 respectively), both at 50 ohms impedance though. Though Linrad would not have problems with the resultant phase shift, the indicated value for the polarization for the same station would be different, when jumping the communications frequency from say 144.020 to e.g. 144.380. Now if 40 meter long coaxes were used with an undefined termination somewhere near the operator's desk, then in addition to the phase change with frequency there would be the amplitude difference variation between channels, that Leif was referring to. Thus precision and symmetry in constructing (to eliminate error sources) is always a bonus, and a win-win starting point over commercial hardware, that generally shows considerable variation, when not sold as matched pairs (like some transistor components). Actually one can go quite far with phase differences. I have recently constructed a couple of boxes with mechanical switches (11-position decks) that can combine ANY two antennas to approximately line up their signal phases. Linrad can do without it for reception as the software by itself can line up those phases. But for transmit it is nice to be able to share full power into two antennas with the proper phase for that particular propagation condition. Changing the phase of two separate final amplifiers will not be sufficient or optimal for many circumstances. 73, Zaba OH1ZAA/2 At 00:44 26.6.2007, SM5BSZ wrote: Hi Joe, I have been thinking more about the Linrad polarization questions that I raised here two weeks ago. I wrote: Suppose they [the feedline lengths in X and Y channels] are not well matched. In other words, suppose that the complex gains and signal delays in the two polarization channels are not equal. Will Linrad's polarization-matching capability be compromised? As far as I can see, it still works well even with poorly matched feedlines. I suppose this must mean that Linrad solves for a differential complex gain, and that over a fairly narrow bandwidth a different delay can be treated as a phase shift. You replied: From a practical point of view the cables are well matched. I think it is a safe assumption to guess that the length differences will be very small and of no concern. Let's work in terms of the Stokes Parameters, with complex signals X and Y. I = |X|^2 + |Y|^2 (total power) Q = |X|^2 - |Y|^2 (horizontal linear component) U = 2Re(X^* Y)(vertical linear component) V = 2Im(X^* Y)(circular component) L = sqrt(Q^2 + U^2) (linear polarized component) Theta = 0.5*atan(U/Q) (polarization angle) If I shift the phase of X relative to Y (say, by inserting an extra piece of cable in the X feedline), the values of U, V, L, and Theta will surely change.Yes, of course, but any difference in length of the cables will be insignificant as compared to the differences in phase shifts that you have in the amplifiers. I don't know what you meant when writing From a practical point of view the cables are well matched. In my station, at present, it would be a complete accident if the downlines from the tower-mounted X and Y preamps were the same electrical length. They are just two pieces of coax that I had lying around. But I am sure that none of them is VERY long in terms of wavelengths. It means that the phase shift vs frequency the difference in cable lenghts introduce is very small and negligible as compared to the large difference in phase vs frequency that non-matched RF amplifiers might introduce. Over a narrow bandwidth, such as 2 MHz, even that can be neglected if the RF amplifiers are reasonably similar. In the end, you just have an unknown, but frequency independent phase shift between the two RF channels. Moreover, my xpol yagis have the H elements located forward of the V elements by some 10 inches or so (I forget the exact amount) -- maybe 1/8 of a wavelength. This will have a very significant effect, as well, no? It just adds to the undeterminated phase difference and it is independent of frequency over 2 MHz. So, it seems to me that the only way to get things
