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Forward power and reflected power and transmitter and antenna matching

Discussion in 'Radio and Communications' started by sram, Aug 8, 2011.

  1. sram

    sram Member

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    If the impedance is matched between the antenna and the transmitter, from where does the reflected power come? And when could a transmitter damage an antenna? and when could an antenna damage a transmitter? And how can an antenna be damaged? isn't it just a piece of metal?


    Also, if reflected power is say 10 watts and forward power is 50, then the effective radiated power is 40 watts? right? And if I increase the power of the transmitter/amplifier, will the reflected power be increased proportionally?

    Thanks.
     
  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    If the transmtter, coax and antenna all match there is no reflected power.

    How can an antenna be damaged? Yes it is wire but wire can be dammaged. Too much current = heat=milt down. Too much voltage will cause sparks the reduces the power and in time eats up the metal.

    If you have a mis-match the transmitter will run hot! DC power supply = 60 watts, power out = 50 watts, transmitter heat=10 watts. It the transmitter has to eat up 10 watts that reflect back from the antenna it will heat to 20 watts or 2x.

    If 80% of the power makes it to the antenna and you increase the power at the transmitter, yes you will have more power at the antenna. proprtionally
     
  3. sram

    sram Member

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    I see. But, to be sure I understand your last comment, say my fwd power is now 330w and my ref power is 96 w(is that safe anyways?), and I go increase fwd power to 1000w, then ref power will be ~ 290w? and will it still be safe provided that the antenna is 1kw capable?

    Thanks.
     
  4. dave

    Dave New Member

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  5. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Save? If my car is running on 4 of 6 cylinders is that safe?
    In short-wave radio….OK. In broadcast radio….I have never seen 1/3 of the power reflect back. (maybe with one inch of ice over the antenna)
    I think you should de-rate the antenna a little with that mismatch.
    What antenna do you have?
     
  6. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I found a FM antenna that has a VSWR of 1.2
    With 330 watts you will have a reflection of 2.7 watts.
     
  7. sram

    sram Member

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    The antenna is this:
    http://www.labelitaly.com/website/aks1.html

    Actually 4 pieces of that in each direction.

    Bad choice?
     
  8. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    "each direction" ???? OK... tell me how you have them mounted, connected. 4+4????
    Is your idea to have a directional antenna? North/South not East/west.
    Do you have 4 mounted Vertical and 4 mounted horizontal?
    What is the distance between antennas?!!!!!!
    I see very little data on your antenna. Send anything you have.
    I have spent many hours 700 feet up working on FM antenna arrays.
    Where are you?
     
  9. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Four in each direction sounds a bad choice - how were planning on connecting them?, and why do you want multiple ones anyway? - it's already omni-directional.
     
  10. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Nigel,
    I don't know what he is doing! The antenna is omni-directional in one way and very directional in another. The picture shows it being used horizontally. That will be directional.

    Normally I space the antennas vertically on the tower at 1 wavelength apart. Each antenna is onmi-directional in the N. S. E. W. direction, and directional up/down. With each additional antenna the up/down directional gain increases.

    I think he has 4 pointing N. S. and 4 pointing E. W. (which could work if he really really knows antennas)
     
  11. sram

    sram Member

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    No No, I don't know antennas. Spent little time with them. But the way I have those 4 mounted seemed to be the only logical way. How else can they be mounted on an antenna mast?

    I'll post a pic soon. And yes the idea is to have a directional antenna for each direction(N,W,E,S). But I'm not sure about the separation between them. What should the distance be?

    And I'm now thinking if each antenna is 50 ohms, all four mounted the way I did it shouldn't add up to 50 ohms. Hmmm, do you just use regular resistor formulas (connected in parallel vs series)?
     
  12. sram

    sram Member

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    Okay, here is the antenna mounted on a 30 meters mast.

    [​IMG]

    Uploaded with ImageShack.us

    A 4-way splitter is used to connect them. Call it stupid or call me stupid, the most important thing to me is to get it to work properly eventually. It is working now, but it is not optimal of course as you can easily tell.

    I will try to summarize my issues:

    I have got a Mizar FM transmitter that feeds its output to this FM amplifier:
    http://www.electro-tech-online.com/threads/ac-power-requirements-for-this-fm-amplifier.120852/

    Originally the radio station used to give 20 Km range. The Mizar was connected via a LMR-400 series cable to an omni directional antenna mounted vertically. Now, the mizar is fed into the above mentioned amplifier, and then the amplifier is connected to the label-italy antenna array via a 50 meters cable(couldn't find a name or rating for it).

    1- I understand it is just better to use 1000 watt amplifier than using an amplifier to boost the signal of an existing transmitter. Right?

    2-The amplifier has a 7/16 Din female RF out connector. I couldn't really connect it firmly due to lack of tools. I think that's an issue.

    3- The feeding cable itself could be just bad. I think I should replace it with a LMR-600 series cable.

    4- The antenna might not be 50 ohms so a mismatch is there. That only or maybe the antenna system should just be modified altogether as implied by you.

    Sorry I had to type this quickly...

    Thanks folks.
     
  13. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I would imagine you use what the manufacturer recommends and provides. It's obviously crucial that it's all matched correctly.

    Ronsimpson obviously knows more about this than I do, but mounting four vertical dipoles horizontally like that seems a really poor idea?, and VERY wasteful of power output. Why do you want horizontal polarisation anyway?, is it a condition of your licence?.
     
  14. RadioRon

    RadioRon Well-Known Member

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    2) yes, connectors must be correctly tightened
    3) cable loss is dependent on load and source VSWR. As the load mismatch degrades, more energy is burned up as heat in the coax cable. Therefore, a bad mismatch can damage your cable depending on how much power you are putting in to it.
    4) the antenna impedance is a function of the impedance of the antenna by itself plus the mutual impedances due to coupling of other nearby antennas of the same frequency. Your antennas are placed more closely than they should be, so the mutual coupling is very strong and so your antenna impedance is certainly not 50 ohms.

    I recommend that you space your antennas further apart up and down the tower, perhaps with about 3 meters of vertical separation.
     
    Last edited: Aug 10, 2011
  15. sram

    sram Member

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    Well, the local distributor suggested this antenna solution. And no, horizontal polarization isn't coming from my license regulations. It is just like I said, the mounting metal pieces that came with it allow only this polarization. I'll wait for Ronsimpson.

    Thanks Nigel.
     
  16. sram

    sram Member

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    Are you saying that the distance should at least be 3 meters between any of the antennas? Making the vertical separation between the first and last antenna 9 meters ?

    Thanks.
     
  17. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Didn't the specifcation of the aerial state it was for vertical polarisation?.

    Using horizontal polarisation is crippling reception for in-car listeners, most UK sites tend to use slant polarisation for that reason.
     
  18. RadioRon

    RadioRon Well-Known Member

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    Yes, that's right.
     
  19. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Polarization; For US and Canada H is more important with V less. This is because, in the old days, house hold radios had H antennas. Then cars got radios most stations moves to H+V. There are different names for that. I think this depends on your market. Now days V only is worth thinking about because of all the cars.

    I have never seen your type of antenna before. In broadcast. I question the antenna spacing.

    See pictures: I have something like the left pictures. (H+V) I have never used the BroadCom antenna. The bottom Right picture is from Ramse and V only. In all these cases I use 1 through 12 antennas mounted vertically like in picture bottom left. Care is taken so the antennas are fed with the same length of wire or so the wire is 1 wavelength different. These antennas are built to send power N.S.E.W. with the same power. They have a very tight pattern vertically. (up/down) There is no need to send power into space and it is a bad idea to send power down into ground.

    I can see you have a pole not a tower so you cannot put a big antenna up. My smallest station had only one loop like the left center picture.
     

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  20. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    That's why the UK moved to mostly using slant polarisation years back, which gives a slight loss to both, rather than a huge loss to one type.

    I don't know if it's still done or not?.
     
  21. sram

    sram Member

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    I perfectly understand your comments about polarization. But the mounting brackets that came with the each of the 4 antenna suggested the polarization you see. The brackets are something like this:

    [​IMG]

    Uploaded with ImageShack.us

    I can only mount them vertically if I have a horizontal pole up there. But then again, it will be omni directional. So why did they sell them 4 antennas? And will it make sense to mount all 4 vertically ?

    I think I will have to ask the guy they bought this stuff from. I hope he is knowledgeable though, not only sells stuff for money.

    Thanks to all again.
     

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