DX reciprocity?

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

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  1. N4CR

    N4CR Ham Member QRZ Page

    Probably if you can get them spread out enough. But if you put a bunch of antennas close to each other, they all interact and make RF mud. I recently had up a 20 meter tribander 20 feet below my doublet. I noticed as soon as I hung it there that the doublet wasn't performing well. And as soon as I took it down, the doublet started working again. So, the beam was for 20,15,10 meters, yet it clearly disrupted the doublet on 40 and 80 meters. And hugely on 20 meters.

    Lesson learned. You can have many antennas, but you need to spread them out or they have an effect on each other and at least half the time it will be in bad ways.

    A fan dipole is no exception to this except for one thing. After you build it, you tune each segment to resonate even though it is in proximity with other wires. That is essentially negating the detuning effect of the other elements.

    This is truly a challenge on a multiple station field day setup.

    I would say that a good horizontal (I prefer a doublet) up as high as you can get it and a good vertical with a great radial field will be an excellent starting point. If takes quite a bit of work to get a better setup than that.
     
  2. M3KXZ

    M3KXZ Ham Member QRZ Page

    Remember, an antenna doesn't just exhibit one take off angle. It has an RF field that covers a range of angles, often with not much real difference in gain between the lower and the higher angles. If you mount a dipole or inverted V for 20m with the feedpoint at about 8m above ground, then you'll have a good RF field from 25 degrees to 90 degrees, with not much more than 2dB variation across the whole lot. 2dB is ONE THIRD of an S unit as a rough guide. Double the height of the antenna, then you'll get a couple more dB at the lower angle, but you'll lose a lot at the higher angles. Also, it's not easy always to get the antenna so high. So to get the lower angle (25 degrees and below) use a decent vertical.

    Like AI4OF said, a doublet you can use on multiple bands, and a decent vertical, will do it.
     
  3. Pushraft

    Pushraft Banned

    Yes I am aware the pattern is not a ray of radiation coming off an antenna. My question to M3KXZ would be if I put up a sloped inverted V 80m antenna from my window with the apex at 7m above ground and let it slope down to 2m above ground on the other end, will I get any good radiation at angles other than straight up? I am considering this antenna as a night only antenna and would need some efficient way to roll up this wire antenna when I was finished. Also, what directivity would it have?

    I am actually getting some 80m activity using my 20m attic dipole antenna so I am just using that currently but was wondering if this outdoor 80m antenna described above would be significantly better. It seems to me it will be way too low to the ground to get any good radiation at low elevation angles. Perhaps some loading coils on the 20m attic antenna for the lower bands or an outboard antenna tuner are my only real options for Tx on 80m when that time comes.
     
  4. Pushraft

    Pushraft Banned

    low radiation angles

    Another thing to consider when trying to predict optimal takeoff angle is the curvature of the Earth. For example, suppose you were trying to strengthen a 1000 mile single hop DX contact. Maybe a ham buddy several states away you chat with regularly. Assuming the reflective layer you will be using is 200 miles up, the signal will need to "rise" 200 miles as it "runs" 500 miles. Using trig, we are looking for an angle with a Tangent of 0.4 which is 200/500. This is about 22 degrees. If the Earth was flat over the 1000 miles then it seems 22 degrees would be the optimal launch angle.

    However as we all know, the Earth is not flat. It curves about 1 degree for every 70 miles. So in our example, the 22 degree launch angle is a little low from being optimal. It seems we would need a slightly steeper launch angle to compensate for the Earth's curvature. Perhaps 27-30 degrees launch would be better in this scenario.

    Since so many things change rapidly in the ionosphere and antennas generally have a broad range of radiation, this is not really a concern to the ham trying to make DX contacts at "random" distances. However, if someone is not getting any contacts under 1000 miles but would like to, raising the launch angle may be beneficial. Perhaps 2 antennas at different heights with a switch, one to work hams within 1000 miles and another to work up to 2500 miles on a single skip. Perhaps even a higher launch angle mobile antenna can be used for closer contacts.

    When I get licensed, I would like to make a few "local" contacts around 500 miles or so not just 1000+ miles like I am hearing now. With an attic dipole there is no easy way to do that. It depends on the other persons antenna in my case.

    A 20 meter 1/4WL vertical whip on my van would be 5m tall or about 16.5 ft which is possible when parked since I have a triple magnet mount that should hold it. Any idea how well that might work for 20m? I could try it on Rx only sometime.
     
  5. KJ4AUR

    KJ4AUR Ham Member QRZ Page

    Removed May 07, 2009
     
    Last edited: May 7, 2009
  6. Pushraft

    Pushraft Banned

    I am just putting out some ideas hoping to get a response. Maybe someone will read it and tell me their experience and whether the 20m mobile full length whip will work or not... things like that.
     
  7. EI8DRB

    EI8DRB Ham Member QRZ Page

    Not great, but it will work.
    Why not just use a hamstick or pro-am loaded whip? They are a good bit shorter and hence less of a danger to wildlife and low flying aircraft.
     
  8. AG3Y

    AG3Y Guest

    Push, I admire your sense of curiousity, and desire to learn. But my point is that you can learn so much more by actually getting on the air, and SEEING, FIRST HAND how such things as "reciprocity" work in the real world, rather than just theorizing about the many issues you bring up here on the ZED !

    Several of us have advocated that, many times over, and you seem to suggest that you have your "own way" of doing things ( do I hear Frank Sinatra, here ? ) and what has worked for the vast majority of us, is just NOT the way you intend to do things, yourself.

    The thing that so many others and I are finding so frustrating about this situation is that you seem to want to know these things, but you refuse to take our advice and actually get on the HF bands and EXPERIENCE THEM FOR YOURSELF!

    " A picture is worth a thousand words ! ! ! "

    73, Jim
     
  9. K9XR

    K9XR Ham Member QRZ Page

    Why should he really change anything? He has the "tech junkies" here on the Zed in the palm of his hand. They complain, but they keep coming back. Hey, it works for him.
     
  10. N5KRC

    N5KRC Ham Member QRZ Page

    Personally, I don't even see the reason for him to get licensed at this point. He can just live vicariously through the rest of us.

    I think he should change his name to Obamacraft. They both seem to think alike; Take what some have busted their ass for, and "spread the wealth" among those who are not willing to make a commitment or sacrifice.
     
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