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

    Stop punishing yourself. I have quit. You can too.
     
  2. G3TXQ

    G3TXQ Ham Member QRZ Page

    Tom, perhaps you are right. Anyway, it looks like Push has tired of this thread!

    Steve
     
  3. Pushraft

    Pushraft Banned

    It would really help people like me if the ARRL would publish in their antenna books not only stuff that works but stuff that doesnt work and index it too. That way if I was wondering about HF downtilt I could just go to the index, quickly find the page(s), and read about it. I think they should add a chapter and call it "Sh*t that doesn't work". That would be the first chapter I would read.

    As WB2WIK already mentioned, they dont bother to mention it because it doesn't work on HF even though it works fine on VHF and even better on UHF. It would be much more informative if they did this so someone like me that reads about for 6m or even higher band doesnt get mislead into thinking it will work on HF as well.

    I apologize if I wasted your time and/or made you frustrated but I was fairly convinced if it works on higher bands that it would work on the lower bands too.

    That person who wrote the ad about using a long boom to lower takeoff angle for 6m beams confused me too because why would anyone care about that on 6m? Any 6m antenna under 3/4WL is only 4.5m up max and that is only about 15 feet. How many people would rather erect a long boom at 15 ft then just raise short boom up to say 25 or 30 ft?

    Since most of my experience is in CB, VHF and UHF, this is an understandable error by me.

    Again sorry for doubting you I guess I just needed enough people to convince myself HF downtilt wont work. You guys are the antenna gods and I am just a peasant not even worth of a rig.

    So I guess it is safe to assume downtilt will work well on Wifi 2.45 Ghz since that has a wavelength of about 4.5 inches. Would it work better for horizontal or vertical polarization or wouldn't it matter much?
     
  4. W0IS

    W0IS Ham Member QRZ Page

    It would be a thick chapter, since there are an infinite number of things that don't work.
     
  5. M3KXZ

    M3KXZ Ham Member QRZ Page

    Don't confuse downtilt of a beam - to fill a null or whatever - with tilting a vertical element at ground level. Tilting a vertical close to ground level WILL have an effect regardless of frequency. This can easily be seen when a comparison is made between, for example, a vertical dipole, a sloping dipole and a horizontal dipole. We all know that a vertical 1/2 wave has a low angle of radiation and exhibits virtually no RF overhead, a horizontal has a higher angle of radiation with some strong overhead response (more so as it gets closer and closer to ground). A sloper falls in between with the RF field distorted preferentially in one direction at the expense of the other.
     
  6. G3TXQ

    G3TXQ Ham Member QRZ Page

    Yes, but we mustn't assume that a higher take-off angle means that the low-angle performance is poorer.

    Here are the elevation responses of a 20m half-wave:

    Blue - vertical close to the ground
    Green - vertical rotated through 45 degrees
    Red curve - horizontal at a height of 30ft

    http://www.karinya.net/g3txq/dipoles.png

    Notice that at angles below 5 degrees there's hardly any difference. Above 5 degrees the horizontal dipole is nearly always better.

    It's very important to compare these elevation responses on the same scale - simply looking at the optimum elevation angle can be very misleading.

    Steve
     
  7. K7MH

    K7MH XML Subscriber QRZ Page

    Not surprised about that!
     
  8. Pushraft

    Pushraft Banned

    I guess the question I should have asked is this:

    Assuming identical rigs on both sides of a 20m DX link with identical noise levels and identical power (let's say 100 watts each), with the only difference being one guy has a dipole low to the ground and the other guy has a 4 element beam 66 ft up or any "killer" 20m antenna, should they sound about the same on both ends?

    What I am getting at is if the only difference is one person has a great antenna and the other guy has a crappy antenna, will they sound the same or nearly the same? For example, if the "big daddy" is transmitting on his big antenna and sending out a 4/7 signal, can the "little guy" expect his signal to be 4/7 also back to that same person? Of course by 4/7 I mean R4/S7 (Readability 4 which is little difficulty reading) and 7 S unit of signal strength.

    This is an important question because people like me who can only have crappy attic antennas and are debating whether to get licensed need to know ahead of time (before we can actually transmit) if this will work. I could additionally ask a local ham to work my rig at my location and see what happens. I hope any contacts give an accurate signal report.

    I guess the best way to know is to have a local ham assist me and just try it.
     
  9. N6EAG

    N6EAG Ham Member QRZ Page

    Antenna gain is a two way street. It provides the same gain for receiving and transmitting.

    A higher gain antenna will hear better and transmit better also............ by the same dB numbers.

    A 10 dB gain yagi will make a 10 watt signal sound like a 100 watt at the receiving end if the receiving person has a unity gain antenna, like a half wave dipole. if the person on the receiving end switches to a 10db yagi, then your 10 watt radiated signal will sound like a 1000 watt signal to him.

    A db is a db...both ways.
     
  10. AB8MA

    AB8MA Premium Subscriber QRZ Page


    You have already spent more effort in this thread alone than any effort you might exert studying for and passing all tests required to get your extra class license.
     
    Last edited: Nov 22, 2008
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