DX reciprocity?

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

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  1. W8JI

    W8JI Ham Member QRZ Page



    I'm reading this thinking....

    "Push is going to take a long long time to ever really understand how things work because Push thinks he knows a whole lot more than he really does, so everything anyone says to try to explain things turns into a big old long argument."

    You are pretty much wrong about the path. It is possible near the MUF or under some very special conditions to have a difference with direction, but is the exception by far rather than the norm. Several others gave you excellent advice about that that you simply chose to ignore. It is possible to build a one-way mirror, it just isn't the norm.

    You are absolutely wrong about antenna tilt. The G3 gave excellent advice that you choose to dismiss. If you built a big enough antenna far enough from earth so the earth did not influence the angle, then what you say would be true. But at HF at any height below a several hundred feet above ground and with a practical antenna for most people to build, G3TXQ is absolutely correct.

    To build an antenna with wave tilt here on 40 meters, I have to have antennas at 90 feet and 180 feet and phase them. The result is about 1% of the time something elese is better than both antennas in phase.

    But of course you know better. You know all a person has to do is tilt the antenna. You only ask the question to waste everyone else's time, because you learned all about antennas on CB.....

    Or do I have that wrong?
     
  2. Pushraft

    Pushraft Banned

    To be clear, I am not saying anyone is right or wrong about the same DX path theory. I am just saying I am not convinced of it yet. The mirror example is not really a good example because mirrors dont duct like the atmosphere. I dont know enough about atmosphereic ducting to make a good arguement but I would guess that the entry point is important and ducting may occur in one direction but not necessarily in the other direction or at least not in an identical path.

    In other words, from what I have read, there is not just simple refracting going on here where the signal bounces at nice angles like a cue ball on a pool table. If I understand correctly, the signal sometimes goes for a "free ride" up in the atmosphere before it comes down. That would be roughly equivalent to a cue ball hitting the corner bumper of the pool table, hugging the "bumper" for a bit (travelling parallel to it), then coming off at an angle again. Let's see you do that with mirrors!

    I dont even know if the angle it comes out at is the same as the angle it his the atmosphere at. It may be different! I dont know. My guess is the dominant path is constantly changing based on conditions. If that is correct, it would be changing from both link directions and thus would be very unlikely to be the same both ways. But even if conditions were not changing, because of the ducting described previously, identical paths may not happen.

    If someone tells me they are identical, my first question to you is how do you know that and how can you prove it or at least make a good arguement in favor of it? I dont think there is any way to prove it. I think this is just a mental exercise into the mysteries of DX propagation.
     
  3. Pushraft

    Pushraft Banned

    How about a 10m vertical antenna on a typical rooftop? Are you saying if I tilt that, the elevation pattern of the antenna does not change at all? I dont believe it. Just recently, someone helped me model a 3/4 WL GP vertical antenna on a vehicle. Tipping the antenna had a HUGE effect on the elevation pattern. It wasnt 10m but why would 10m be any different?
     
  4. Pushraft

    Pushraft Banned

    Another thought I had... Why should I even bother getting my ticket if you are telling me there is reciprocity? That is, if I can hear someone, there is a good chance they will hear me back assuming they have similar power as me and similar noise. Since I have less power than many but probably more noise on my end due to an attic antenna, things should balance out. That is, with their nice "quiet" beam antenna 100 ft up, they should be able to hear my 100W attic dipole just as well as I can hear them using 1KW. So my point is why even bother getting my license? I thought the main idea was to make contacts and see how far you can "get out". Well if they are "getting in" to me, then that is an indicator of how far I should "get out".

    Has anyone even seen and/or heard a situation where one person on the link was much stronger than the other and both stations were using similar power? If so, what would cause this?

    Another factor is SWR. One antenna on one side may be better at Rx than Tx if it has high SWR.

    I am not saying I am or I am not getting my ticket I am just wondering what would be the point? I dont need a license to receive and it is interesting to listen in as they talk about antennas, band conditions, propagation... Since I dont know "squat" anyway, what motivation do I have to talk? For example: "This is K3PSH on an attic dipole. Weather here is sunny and warm. That's all I got for you today. 73s, K3PSH shutting down". I'm not jumping out of my chair with excitement.
     
  5. G3TXQ

    G3TXQ Ham Member QRZ Page

    OK, I said I'd given up, but I'll have one last try.

    I notice you've now changed the rules again - originally you talked about tilting an HF beam, now it's a vertical monopole. Nevertheless, here are some facts:

    I took a 20m quarter-wave vertical and modelled it over average ground. I looked at it's vertical response. Then I tilted it over by 40 degrees from vertical. Here's what I found:

    1) The elevation angle for maximum radiation went up slightly - by 4 degrees from 27 degrees to 31 degrees.
    2) At any elevation angle between 5 degrees and 20 degrees there was less than 1dB difference between the two.

    73,
    Steve
     
  6. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    JI:

    If you carefully read my statement on noise you will see that I reference man made noise (i.e. ignition noise, etc.) is generally vertically polarized. Experiments undertaken by companies like Motorola and General Electric (their two-way radio divisions) over the years have proved that. Collins Radio found the same thing in the 1950s through the 1980s. However, atmospheric noise definitely has no preferred polarization.

    Being in a rural situation you definitely do not have the noise sources close to you that those of us who live in suburban or urban locations have. Therefore, a vertically polarized antenna is going to receive basically the same noise factor as a horizontally polarized antenna.

    Due to man made devices the noise floor in the urban and suburban areas has risen dramatically in the past couple of decades. In years past as you moved higher in frequency the ambient noise level dropped. Unfortunately, this is no longer true. In this area the noise level on the 70 cm band is generally worse than what I hear on the 40 meter band and sometimes even worse than on the 80 meter and 160 meter bands. Even Al Ward, W5LUA, who is a well known UHF and microwave operator is plagued with noise. Like me, Al tries to maintain an "S" meter calibration in line with the old 50 microvolts for S-9. Last night I worked Al on 432.100 MHz SSB and my noise level was running between S-3 and S-4. Al was experiencing a noise level between S-5 and S-6 and his location is somewhat more "open" than mine. That was with horizontally polarized antennas. If I switch to the antenna that I use for FM the noise level skyrockets.

    With all of the 47 CFR Part 15 devices that are now allowed within the 70 cm band the noise level is increasing every day. It is the same with the 6 meter band with all of the devices operating in the 49 MHz band.

    The cross polarization differences between horizontal and vertical are generally accepted as being about 20 dB. Now there are situations where that can increase to 30 dB or more. However, the average is pretty close to 20 dB. Where man made noise is concerned, since it is generally vertically polarized, using a horizontally polarized antenna does help considerably with the reduction in the received noise level when compared with a vertically polarized antenna. However, for other noise, since the polarization can be anywhere from vertically polarized to horizontally polarized and anywhere in between, the polarization of the antenna definitely will have little or no effect at all on the noise level at any particular time.

    Glen, K9STH
     
  7. W0IS

    W0IS Ham Member QRZ Page

    The smart electrons take the path of least resistance. The stupid electrons take the path of highest resistance. The bell curve of electron IQ is skewed toward the high end.
     
  8. Pushraft

    Pushraft Banned

    When I was talking about tilt, I was using an HF beam as an example but I wasn't saying it was only for HF and only for beams.

    When I had someone help me model my 3/4WL GP vertical antenna, I saw a big diffrence when the antenna was tipped. It probably depends on the fractional WL of the antenna how much difference. For example, a 1/4WL vertical might not be as "sensitive" to tipping as a 3/4WL vertical. Since they both tune up with adequate GP, why not change your model to a 20m 3/4WL vertical over average ground? That would be one big antenna but just for fun, "tip" it the same in your model (5 to 20 degrees and then 40 degrees) and see what happens.

    My guess is there will be more of a difference in the 3/4 WL than in the 1/4WL vertical. I believe a 3/4WL vertical with GP is already a high launch angle antenna like 47 degrees. Might be good for local DX work where a high launch angle is needed. Since 3/4WL vertical doesn't need a tuner (it is resonant just like a 1/4WL), it may be possible for local DX on the smaller bands such as 10m and maybe even 6m if the conditions were right. I dont know what the critical angles for those bands are. On my van, the 3/4WL vertical was much worse than just a 1/4WL vertical for ground contacts over flat ground as expected because the 1/4WL has 27 degree launch and the 3/4WL had 47 degree launch. I dont know the pattern at 0 degrees elevation but based on actual results, the 1/4WL "won".
     
  9. W0IS

    W0IS Ham Member QRZ Page

    By long experience. There seem to be some very rare cases where this is not true, although even their existence is disputed. But they are the rare exception.

    Incidentally, if different antennas are used for transmit and receive, then all bets are off. For example, if a vertical is used for transmit and a Beverage is used for receive, then it's more possible that the two signals are taking a different path.

    But with the same antenna used for transmit and receive, then the path will be the same, 100% or very close to 100% of the time.
     
  10. W0IS

    W0IS Ham Member QRZ Page

    That's a good analogy. To carry it a step further, there are no "RF diodes" in the sky, at least none that have been discovered yet.
     
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