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SHUNT FEED MY TOWER FOR HF BANDS?

Discussion in 'Amateur Radio Antennas' started by Happy_Hamer, Jun 22, 2009.

  1. Happy_Hamer

    Happy_Hamer Administrator
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    Is it really worth it?

    How do I do it?

    Here is what I have,

    My Rohn 25 tower is 60 foot tall, bracket mounted to the apex of the garage. No guy lines

    All I have on it now is a Diamond Dual Band VHF/UHF vertical on a Hazer.

    Can I get some use out of shunt feeding the tower?


     

  2. Beetle

    Beetle Sr. Member

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    Sure you can. Depends on what bands you want to use. On 20 through 10 a yagi or quad would be better for DX, but on 80 and 160 a shunt fed tower could be bliss.

    Lots of information on Google.
     
  3. W5LZ

    W5LZ Crotchety Old Bastard

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    HH,
    Can it be done? Yes. How hard is it? Depends on how many bands you want to cover, and if you happen to have a tuner. Is it worth doing? Good question, and since you are the only one who can answer that, beats me. I think it'd probably be the equivalent of a 1/4 wave vertical on 80 meters which should work pretty good. I know I've heard 1/4 wave'ers on 80 meters that really did sound very nice! It wasn't shunt-fed, but it was primarily built as an insulated 1/4 wave vertical, and didn't require shunt-feeding.
    I did a 'google' for shunt-fed towers and this site had some 'typical' information about how it was done;

    Antenna Towers : Antennas: Towers

    According to the authors, it was worth the effort. I do know that the 'tuner' box was a little intimidating to me, but I'm also sure that directly copying it wouldn't be necessary, sort of. The thought of putting in a good ground radial system is more intimidating! I am not exactly thrilled with that idea, but it's a very good idea anyway.
    If I have a tower close to a 1/4 wave length on 80 meters, I think I'd have to try it just to see what happened. (I find it's real easy to expend other people's labor! :))
    If you do try it, I definitely want to know about it!
    - 'Doc
     
  4. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    On 80 and 160m a vertical is a blessing.Even a 60' tower will put out a nice signal but you do need radials to be effective. One word of caution is to make sure the point where the tower is bracketed to the house is well insulated and i don't mean by simply bolting it to the wooden eave.Depending on the freq. and impedance realy high RF voltages can be developed at various points along the length of the tower and you do not want to start a house fire. Most of the time guys use a thick piece of nylon or other plastic material (an old office chair mat is good and makes lots of insulators) between the tower and house.There are pix on the internet of such installations. Shunt feeding can be a pain to find the proper length of the gamma arm and setting things to obtain a good match but once it is done you will have a great DX antenna on the low bands. "Locals" out to about 600-1000 miles however may notice a slight drop in signals due to the lower takeoff angle.
     
  5. Moleculo

    Moleculo Administrator
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    Do you have an autotuner that you can put at the tower base and trigger/control remotely?
     
  6. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    That works well for series fed towers but not shunt fed. You need to find a point that is close to 50 ohms and then bring it into line with a capacitor. Tuning a shunt fed tower is very similar to feeding a gamma fed yagi element except the length of the gamma arm is much longer in proportion to the element length.

    Another idea that is simpler is to connect a wire at the top and spaced 2-3 feet from the tower. Connect the feedline between ground and the wire at the bottom and you now have a folded monopole antenna. Often all that is required is a series capacitor to tune it or you could use an auto tuner in this case. The heavier the drop wire the better BTW.
     
  7. Moleculo

    Moleculo Administrator
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    I guess I don't know what a series fed tower is?
     
  8. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Sorry mole. Flashback to my broadcast days. A series fed tower is a tower that is insulated at the base from ground like 99.9% of all AM broadcast towers are. They have a large base insulator and the feedline connects between the tower and ground thereby making the feedline in series with the antenna system. A shunt fed tower has the feedline shield connected to ground but the center part of the feedline connects at a point somewhere along the vertical section of the grounded tower where the impedance is near 50 ohms. A capacitor is needed to cancel out the inductance presented by the gamma arm just like a regular gamma match on a yagi's driver element.
     
  9. Moleculo

    Moleculo Administrator
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    OK, a series fed is just like what most of the guys who put up flag pole antennas do - insulate it.
     
  10. Moleculo

    Moleculo Administrator
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    This is what I was thinking about with the autotuner idea. Wouldn't that be the easiest if you wanted to use the tower for multiple bands?
     
  11. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Correct.Series fed means an insulated base.Shunt fed means a grounded base like a typical tower that holds other antennas.



    Yes it is easy.All you need to do is connect a wire at the top of the tower and drop it to ground level keeping it spaced a couple feet from the tower and connect the feedline between the wire and ground. You can also drop additional wires if you want and connect them to the base of the tower leaving just one ungrounded to connect to the feedline. This has the advantage of raising the radiation resistance which will in turn allow tha antenna to perform better with a minimal ground radial system.One added wire quadruples the impedance and three wires raises it by a factor of nine.When dealing with impedances in the couple ohms range it is an easy way to raise the impedance to something nearer 50 ohms or at least something that the coax/tuner can handle better.
     
  12. W5LZ

    W5LZ Crotchety Old Bastard

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    Wonder what would happen to the radiation resistance if you connected the guy wires to the ground system?? Gnarly... Dude! ;)
    - 'Doc
     
  13. CDX-007

    CDX-007 Transducer Fanatic

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    A local Ham I was visiting years ago used a 1/4 wave long piece of insulated copper wire wrapped around the bottom of his 50' metal flagpole and was able to work Europe on 40m using a tuner. He buried 12, 1/4 wave radials in his soil, 4 @ 80m, 4 @ 40m & 4 @ 20m.
    I thought that was a very interesting approach to having an antenna in a CC&R tract.
    He said he was "Quite pleased with the performance".

    Inductive matching? Hmmm, ...I wonder if the concrete the tower was in acted as a matching capacitor to ground?
     

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