DX reciprocity?

Discussion in 'General Technical Questions and Answers' started by Pushraft, Nov 20, 2008.

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  1. K7MH

    K7MH Ham Member QRZ Page

    It is all important and you never know which factor is going to be the important one at the moment you want to work someone.
    They all can be a huge benefit. Just depends what you are trying to do at the moment.
    I have experienced the one way propagation a number of times. More noticed it on 6 and 10 than anywhere else.
    Last edited: Nov 21, 2008
  2. K9STH

    K9STH Ham Member QRZ Page

    For all practical purposes "man made" noise is vertically polarized. Therefore, the approximately 20 dB "cross polarization" loss between a horizontally polarized and vertically polarized signals will result in the reduction of noise by approximately 20 dB where things like ignition noise, noise from appliances (including computers), and so forth are concerned. However, on atmospheric noises (including thunderstorms) horizontally polarized antennas generally do not have an advantage over vertically polarized antennas.

    There is a "trade off" in the fact that a vertically polarized (properly installed) has a lower angle of radiation which makes "DX" much easier to work especially where simple antennas are concerned. This is especially true on the 160, 80, 60, 40, and 30 meter bands where most horizontally polarized antennas consist of simple wire types (i.e. dipoles).

    Now for "close in" work, like up to between 700 and 1000 miles generally a horizontally polarized antenna will give a considerably greater signal strength. But, out over 1000 miles a vertically polarized antenna will generally give a greater signal strength. Of course this is where the simple types of antennas are concerned.

    For example, on the 80/75 meter band and on the 40 meter band I have both wire dipoles and a HyGain HyTower vertical. For QSOs out to around 700 miles the wire dipoles win "hands down". For QSOs more than 1000 miles away the HyTower wins "hands down". But, between 700 miles and 1000 miles I never know which antenna will work better because sometimes it is a dipole and sometimes it is the HyTower. Now for 40 meters where I can also use a pair of full sized vertical antennas phased 90 degrees then working stations more than 2000 to 3000 miles away is MUCH easier than when I run either vertically polarized antenna by itself.

    Because of the noise factor on 160, 80/75, and 40 meters some stations use a vertically polarized antenna for transmitting and a horizontally polarized antenna (often a shortened type, sometimes with a preamplifier) for receiving especially when working DX. That way they get the benefits of the low angle of radiation from the vertical and the benefits of lower noise from the horizontally polarized antenna.

    Glen, K9STH
  3. Pushraft

    Pushraft Banned

    I played around with my rig and got more noise reduction. It does have notch filtering and noise blanking but I dont see any DSP (it is a rig from the mid 80s). The rig is "grounded" to the center pin on te 3 way wall outlet. I figure it is better than nothing. On some bands, if I just lay my hand on top of the rig the noise drops by about 1 S unit.

    Also I remember reading somewhere that 2 way DX is not always the case. The route the signal takes from one direction is not necessarily the same exact route as from the other direction in a link.

    It would be very interesting if we could plot the actual paths for say 5000 mile 2 way DX transmissions in 2 different colors and see how close a path they actually take or if there are significant differences. What are the chances that they take the same exact path in both directions over the 5K miles? If the way it bounces off the reflective layers is slightly different in angle, that could make the difference in a contact or a miss.

    Also, someone with a high launch angle would seem to have the strongest path of their signal bouncing at a different skip angle than someone with a low launch angle. This could effect how strong a signal makes it to the destination. I would think that the signal that is "on path" more from the start would have a better chance of making it to the destination than one that has the wrong launch angle. I dont know for sure I am somewhat guessing but this is open for debate. One could argue so what if the launch angle is wrong because it will be wrong for both Rx and Tx so it should degrade both about equally.
  4. Pushraft

    Pushraft Banned

    How about this too:

    Someone with a good low angle launch from their antenna may have fewer skips than someone with a high launch angle antenna for a DX link between the 2. That may account for the non reciprocity of the link. The extra bounce from the ground could account for an additional 20dB loss for example making the signal only work one way with equal power on both ends. Possible?
  5. K7MH

    K7MH Ham Member QRZ Page

    Again, there are probably a lot of factors.
    You can notice a LOT of signal fading up and down sometimes over the time of a conversation. The reflective layer is not a smooth mirror like phenomena. More like the Bering Sea maybe at times! So a lot of things can probably happen to a signal once it gets there to vary its strength before you receive it.
  6. G3TXQ

    G3TXQ Ham Member QRZ Page

    Push, your logic is faulty. Let's say Station A has the antenna with the low take-off angle, and Station B has the antenna with the high take-off angle. If, when A transmits to B, the dominant path is a single hop even though the arrival angle doesn't "favour" B's antenna, it will also be the dominant path when B transmits to A.

  7. Pushraft

    Pushraft Banned

    I am not convinced of this. How do you know the dominant path will be the same for both directions? Let's take a hypothetical example. Suppose antenna A is high up and has its maximum radiation at 20 degrees up angle from the horizon and B has a low antenna with its maximum radiation at 47 degrees up angle from the horizon and not so good radiation at 20 degrees. Let's also assume for this example that both angles will be aligned for a good skip distance, the 20 degree requiring only 1 skip and the 47 degree requiring 2 skips. I didn't do the trig on this but let's assume the skip distances are correct for these angles for this example. Why would B's path be the same as A's in this case? When B transmits, at 20 degrees up angle, the signal will be rather weak compared to the 47 degree launch and since both are lined up as far as skip distance, which one will dominate?

    I guess this is considered a multipath situation where there are competing paths. I would think it is common that one side of the link has a good low angle launch and the other has high angle so this situation is worth analyzing. I remember reading, I think in an ARRL book, that the DX path is not necessarily the same in both directions. I dont even know if the skip angle stays the same or if it is changed slightly after bouncing.

    Someone told me they had someone else they were talking to adjust the tilt angle of their antenna until the signal was peaked. It was found that 27 degrees uptilt produced the best signal. This may have made the skip distance just right whereas a lower angle may have skipped over one persons head so to speak. So my point here is normally for a good terrestrial non DX link, a low launch angle is generally good. However for DX, it seems like the angle and/or distance has to be right and a lower angle may be worse. I would suspect that someone with a tilt adjustment on their beam antenna might be able to dial in someone else possibly even closer in than normal by peaking the tilt angle. However for large beams a tilt adjuster is probably expensive and many people dont use it.

    My point here is the angle of DX seems important. I dont know which carries more "weight", the correct launch angle of the transmitter, or the correct "listening" angle of the receiver. It would be interesting if someone could take a small beam for say 10m when that band is open, establish a DX link, then monkey around with the tilt angle and record it and see if it is consistantly best when talking to that same person in a fixed location with a fixed antenna. That is, I wonder if the optimum uptilt angle is a constant or if it changes with atmospheric conditions. I would suspect with DX, many things change.

    Suppose at 20 degrees launch of B, the signal is down 10dB from its 47 degree peak signal but at A, the receive sensitivity at 20 degrees is 10dB more than at 47 degrees. In that case, the path with 1 less skip should in theory be stronger even though the signal starts out from B as a -10dB signal from it's optimal launch angle. This is because it would have one less bounce off the ground and atmosphere but I would suspect there are anamolies where a multiple skip would actually beat a single skip when the other person is not in single skip distance. Also, there is a good chance that the skip path and angle that is actually making the link is not using either optimal elevation angle of either side because the skip distance is such that it requires other angles to work.

    Pretty cool stuff to analyze but hard to verify. Comments?
  8. KC2UGV

    KC2UGV Ham Member QRZ Page

    Push, now you're arguing theory vs. real world experience. Real world experience usually wins all things being equal.

    Antenna polarization plays a factor, ionosphere conditions, local and remote electrical sources, cars moving down a throughway, etc.

    However, electricity follows path of least resistance. Always. Add that to the random walk theory and you will see the dominant path will be the same for both directions. But, you have to keep in mind a constantly changing environment, among other things.

    So, due to the constant environment changes, you run into fades and drop outs. Even with the ionized clouds constantly changing location and shape in the E-layer.

    But, one way to really piss off a lot of experienced operators is to say,"Well, I don't believe you experiential facts."

    The only way, if you take that tack is to get licensed and try it yourself.

  9. W8JI

    W8JI Ham Member QRZ Page

    Let me inject a few things.

    The noise polarization issue. It is NOT 20 dB or any other set amount. That is a myth. The reason a claim is made that noise is vertically polarized is because the earth acts like a filter. It does not support horizontally polarized propagation at lower frequencies where ground wave has an effect. So on low frequencies, if you are near a groundwave noise source, local distant noise propagates better to the receiver when it is vertically polarized.

    At my house, in a very rural location, verticals are just as quiet as horizontals. As amatter of fact the "low-noise" K9AY, Flag, Pennant, magnetic loop and other antennas are all typically vertically polarized.

    On VHF and higher, a horizontal antenna is just as likely to be quiet as a vertical in a noisy location when they have the same directivity. So the polarization thing is a myth, with the exception of longer distance ground wave noise at lower frequencies.

    Second, DSP. DSP systems in receivers we buy can only emulate analog systems. They can restrict noise by bandwidth. Noise power is directly proportional to bandwidth, while signal power is constant so long as the bandwidth of the signal is less than the receiver bandwidth.

    So if I am listening to a 100Hz wide CW signal with a 500 Hz wide filter, the noise power is five times higher than it needs to be.

    As for processing out noise, nearly all of the DSP systems are not very good. They often can be worse for very weak signals than an analog system. What they are good at is INCREASING the apparent S/N when the signal is already above noise. The only way to have a DSP system process a signal out of noise is to have a signal that is keyed to some time base or has error correction, or have the DSP system have a noise cancelling system to filter coherent noise like repeating pulses. Either the signal or the noise has to have a steady pattern, or it just can't work.

    I have one of the larger receiving systems in the world, and the only use I have for DSP in a radio is narrowing selectivity or summing two stereo channels from two phase locked receivers with variable phase to combine wide spaced antennas into a giant array.
    Otherwise DSP systems help enhance a signal that is already out of the noise, but they won't help a signal that is near or in the noise. DSP systems can act like a noise pulse blanker, but a good IF noise blanker is always better.

    In short, DSP systems are not what they are cracked up to be. In most cases analog systems are as good or better. (My K3 receiver uses a narrow analog filter to do the hard work, and then smooths things up a bit with a DSP system. Primary signal processing with DSP receivers are useless to me. They have terrible dynamic range.)

    Finally, the reason people can hear things unevenly is what WB2WIK outlined. The noise floor is different at each end. The ERP is different also. A very very poor gain antenna can make a wonderful receiving antenna, but a poor gain antenna will never make a good transmitting antenna.

    For receiving, it is all about directivity and antenna pattern.

    For transmitting, it is all about gain at the desired angle and direction.

    I'm sure there are some cases where the path is not perfectly identical in each direction, but for the most path it pretty much always is.

    What it usually boils down to is effective power transmitted, and receiver antenna directivity and site noise.


    73 Tom
  10. G3TXQ

    G3TXQ Ham Member QRZ Page

    Push, you're still not thinking it through logically.

    Let's take your example of optimum angles of 20 deg for Station A and 47 deg for Station B. Let' say that Station A's antenna is 10dB down when working at 47 deg, and let's say Station B's antenna is 15dB down when working at 20 deg.

    Let's call the 20 deg single-hop path, Path 1, and the 47 deg double-hop path, Path 2, and let's say that Path 2 has 20dB more attenuation than Path 1.

    When A transmits to B we have relative losses of 0+0+15 for Path1
    When A transmits to B we have relative losses of 10+20+0 for Path2

    When B transmits to A we have relative losses of 15+0+0 for Path1
    When B transmits to A we have relative losses of 0+20+10 for Path2

    Notice that Path1 would be the dominant path in both directions (15dB vs 30dB) - can you see why - the antenna gains are all reciprocal, so the attenuation in one direction is just the same in the other. So whichever is the prefered path from A>B will be the preferred path from B>A.

    And don't think I've chosen a "special case" - try changing the numbers around and convince yourself.

    Before the "purists" wade in saying this is over-simplistic, I'm just trying to get Push to understand some basic antenna reciprocity stuff.

    Last edited: Nov 21, 2008
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