ad: SDRKits-1

DX reciprocity?

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

Thread Status:
Not open for further replies.
ad: L-HROutlet
ad: l-rl
ad: Left-2
ad: L-MFJ
ad: FBNews-1
ad: Left-3
ad: Subscribe
  1. Pushraft

    Pushraft Banned

    These are all DX questions :

    1) Is a reduction or increase in power directly applicable to the inverse square power law? For example, if a contact is made with 5 S units at the receive side and the person transmitting increases power from say 100W to 1000W, would the receiver see an increase of 10dB from 5 S units to almost 7 S units assuming there is no significant fading during the power change?

    2) For someone like me that is currently only doing Rx "work" (more like fun to me), can I assume if I hear someone using any antenna (beam on 100 ft tower pointed at me, mobile, attic dipole...) that they would hear me back assuming they are close to my 100W limit? For example, if I am hearing someone well and they claim they are using 100 watts, if I were to Tx back to them with 100 watts, would it be safe to assume they could hear me about as well? If not, why not? Also note here I am saying any antenna insinuating that doesn't matter since I already hear them. That is, I am asserting this reciprocity property works for any type of antennas used. More simply stated, if an antenna tweak is performed on only 1 side of the link, both sides will benefit equally well for that particular link between them.

    3) Is it possible that the DX paths between points are different depending on who is transmitting and who is receiving such that one person has an advantage over another? Specifically, are there certain DX phenomena that are biased more towards one direction than another at certain times during the day making assumption # 2 above false?

    I know we can't much control the Ionosphere but it is interesting to analyze it's properties by making observations and asking questions.
     
  2. EI6GXB

    EI6GXB Ham Member QRZ Page

    Yes, this interests me too. I figure if someone is higher on the other side, the chances of their takeoff angle being lower are increased, and subsequently the received signals as well. It is a reasonable assumption that if I can clearly copy someone, they will copy me. Gain works both ways.

    I worked a few stations in eastern europe from (wait for it) a mobile antenna stuck on the radiator in college. They had beams etc, so yes, that kind of proves reciprocity.! eh?
     
  3. KJ4AUR

    KJ4AUR Ham Member QRZ Page

    Removed May 07, 2009
     
    Last edited: May 7, 2009
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    >Pushraft;1402193]These are all DX questions :

    1) Is a reduction or increase in power directly applicable to the inverse square power law? For example, if a contact is made with 5 S units at the receive side and the person transmitting increases power from say 100W to 1000W, would the receiver see an increase of 10dB from 5 S units to almost 7 S units assuming there is no significant fading during the power change?

    ::Yes, the change will always be 10 dB, however that's not any particular quantity of "S" units. Could be 1/2 S unit, could be 4 S units, could be almost anything -- however it will always be 10 dB. The reason you can't predict "S" units is because S meters vary in calibration all over the place.

    >2) For someone like me that is currently only doing Rx "work" (more like fun to me), can I assume if I hear someone using any antenna (beam on 100 ft tower pointed at me, mobile, attic dipole...) that they would hear me back assuming they are close to my 100W limit? For example, if I am hearing someone well and they claim they are using 100 watts, if I were to Tx back to them with 100 watts, would it be safe to assume they could hear me about as well?

    ::No.

    >If not, why not?<

    ::Good question. The reason the answer is "no" is because while the theory of reciprocity is nice, the only thing that really matters for receiving anybody is S/N or signal-to-noise ratio. Forget about S meters, they're completely irrelevant. If you hear somebody just over your noise level and he's running 100W, and you transmit back to him with 100W, he may hear you fine or he may not hear you at all, not even a peep.

    This would be caused by the other station having a higher noise level than you have. Which, by the way, happens a million times a day.

    Not only does noise intensity vary with geography, it varies with precise and specific location, antenna orientation, antenna radiation angle, time of day or evening, and a dozen other variables. The only one you might possibly have control over is local noise generated in your own home, and you can eliminate that if you disconnect your household from the AC mains and operate your rig using battery power.

    Other than that, you really have no control over all the other things. Or, at least it would seem so.

    In reality, you *can* control antenna directivity and beamwith, along with radiation angle -- by antenna design and installation (deployment). But these are complex matters for experimentation and to do justice to the experiments requires a large area in which to try different antennas.

    Often, if you have a "poor" antenna, your S/N ratio will be better than someone's who is using an excellent antenna! Problem is, this advantage only works for receiving, and doesn't help your transmitter a bit.

    >Also note here I am saying any antenna insinuating that doesn't matter since I already hear them. That is, I am asserting this reciprocity property works for any type of antennas used. More simply stated, if an antenna tweak is performed on only 1 side of the link, both sides will benefit equally well for that particular link between them. <

    ::Nice thought, it isn't true.

    >3) Is it possible that the DX paths between points are different depending on who is transmitting and who is receiving such that one person has an advantage over another? Specifically, are there certain DX phenomena that are biased more towards one direction than another at certain times during the day making assumption # 2 above false?<

    ::Not much, that is rarely if ever the issue. The real issue is S/N ratio being the only important factor in establishing a communications link, and that highly effective and efficient transmitting antennas often receive more noise than very poor antennas do, making "one way communications" a fairly common phenomenon -- due to this matter, and this matter alone.
     
  5. Pushraft

    Pushraft Banned

    If noise reduction is of paramount importance, then should stations be concentrating more on that rather than increasing power and getting an antenna higher up in the air? I hear many mobiles on my rig but they are buried enough in the noise I cannot hear their callsigns. If the noise was much less, I could probably hear a bunch of them.

    I heard someone on the radio today talking about noise cancelling "boxes" where they take the noise, "invert it", and "put it back". Do those actually exist? If so, how can they distinguish noise from non-noise?

    Since I have an attic dipole, my noise level is quite high sometimes. As high as 7 on my meter with AGC on. Sometimes I find it slightly easier to use my 20dB attenuator to hear people. I've found the noise is less at night when my neighbors go to sleep however it's been cold here in FL and they are probably running heaters so that means motors and that equals noise.

    I wonder if it is possible to do some type of real time computer noise filtering of the audio stream. That is, instead of listening to the rigs direct audio, can it be "washed" thru an audio filter in real time so the S/N ratio is better? On a fast computer it might be close enough to real time that it would be usable. I am a computer guy I can look into it. I think that would partially solve many hams problems. Maybe there is already something on the market that does this.

    Imagine if the background was totally silent. You might only need 1 watt to talk across the country. That is optimistic thinking but I would think there should be ways to lessen the noise. We know there are certain characteristics of male voices so a sophisticated device should be able to separate that from almost "white" noise. I occasionally hear clicks and pops too so again, that should be recognized and filtered out.
     
  6. KC2UGV

    KC2UGV Ham Member QRZ Page

    Funny thing you ask about the filtering Pushraft. There is already such a thing. It's called DSP (Digital Signal Processing) and many rigs have it built in, and others use their computer running a spectrograph software to do it.
     
  7. KA1MDA

    KA1MDA Ham Member QRZ Page

    "For example, if I am hearing someone well and they claim they are using 100 watts, if I were to Tx back to them with 100 watts, would it be safe to assume they could hear me about as well?"

    No, because you don't have a license and can't transmit.
     
  8. AG3Y

    AG3Y Guest

    Push, on the HF bands, you are never going to eliminate noise entirely. You might be able to filter some of it out, with DSP, or even analog methods, but remember that noise is primarily an amplitude modulated phenomena, and so, incredibly, are SSB voice transmissions, as well as CW ( on and off is a change in amplitude of the received signal ! ) Even digital transmissions are somewhat of a form of FM, but they are being received by a AM ( SSB to be sure, but still amplitude ) sensitive detector! Various digital soundcard modes can over-ride a fair amount of noise, but if the noise is stronger than the tones, they will be covered up by that noise! If you eliminated all the noise with signal processing equipment, you would also be eliminating much of the desired signal!

    To answer your question about the noise reduction units that sample local noise, invert that signal, and re-apply it in the IF chain to eliminate the amplification of that noise by the IF, yes, there are noise processors that work that way.

    I once owned a Collins KWM-2 that had just such a unit in it, and it was very effective in eliminating the type of noise that is generated by power lines, and motor brushes, etc. Anything that was in the immediate area , but not such forms of noise that would be transmitted from a long distance away, such as static bursts from distant thunderstorms.

    These units work by sampling noise in a portion of the spectrum that is above the usual HF band frequencies, but not so high that the amplitude would be quite small. I am not sure exactly where they were the most sensitive, but I would suspect somewhere between 30 and 50 mhz. Since local ignition, power line, motor brush noise, etc. is pretty identical on both the HF bands and at the higher frequency, inverting the phase from the noise limiter unit, and applying it to the IF stage at a carefully balanced level would pretty much eliminate the noise from getting through to the detector!

    I believe MFJ makes such a unit today, but I am not sure how the signal would be injected into the receiver. You would have to look up the details about its operation for yourself.

    Hope this helps ! 73, Jim
     
  9. K7MH

    K7MH Ham Member QRZ Page

    Somewhat related to that and rather fascinating is using WSJT6 software and it's different modes that can detect digital ham signals that are below the noise level and decode them.
    it is hardly like using voice and the transmissions have to be well coordinated but it can transfer info at very low levels of power and signal levels you can't even hear through the speaker.
     
  10. W0IS

    W0IS Ham Member QRZ Page

    Another important practical factor is that, generally, receiving antennas are much more forgiving in terms of impedence mismatch, etc. In other words, your 40 meter dipole will receive quite well on 20 meters, without any attempt at antenna tuning. However, the transmitter will not like this arrangement, and you will wind up with much less power being radiated. So your 100 watts might be equivalent to 1 watt into a better matched antenna. An antenna tuner would largely eliminate this factor.

    This is an oft-debated topic. The general consensus seems to be that propagation is completely reciprocal--in other words, the ionosphere doesn't care which way the radio waves are going. It refracts them equally in either direction. Others (myself included) occasionally witness a phenomenon which can only be described as "one way skip". It might be all in our heads, and the result of local problems with noise, etc., on one end. But it doesn't happen too often. In general, if you can hear 'em, you can work 'em.

    By magic. Or possibly, they subtract out noise sources that appear on a very wide range of frequencies, on the assumption that such signals are noise.

    Most receivers have some sort of "noise blanker", which is quite effective for some types of noise. Basically, they are very good at filtering out repetitive signals.

    We do. Getting the antenna higher in the air is helpful in reducing noise, since most of the manmade sources of noise are located on or near the ground.

    Ya' think so? Maybe I'll have to give it a try sometime. Oh, wait a minute. That's fairly routine, and I do it all the time. Actually, I usually use 5 watts, because I'm too lazy to turn the power down. (Yes, I'm admitting to a violation of the rules, because if one watt would be adequate, then I'm violating the rules by using 5 watts.) But I'm sure you can tell my be memory how many dB difference there is between 1 and 5 watts. :D

    Here's a list of stations I recently worked in a 90 minute period, using 5 watts into a dipole. As you can see, I didn't do particularly well:

    http://qrpfoxhunt.org/summer_fox_hunt_2008/hunt10.htm
     
Thread Status:
Not open for further replies.

Share This Page