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2 Element Quad

Discussion in 'CB Antennas' started by HomerBB, May 13, 2009.

  1. HomerBB

    HomerBB Sr. Member

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    I am working on a 2 element quad antenna. I noticed that raising it a mere two feet higher brought the SWR down a little. It is only twelve feet from the ground to the boom while working on it. Can I expect a definite improvement on the SWR when I get it up to operating height, 30 - 35 feet?

    Current SWR at present height is:

    ch 40 2:1:1

    ch 20 1:6:1



    ch 1 1:4:1

    See it here:

    2 Element Quad
     
    #1 HomerBB, May 13, 2009
    Last edited: May 13, 2009

  2. JDemoret

    JDemoret Member

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    Very nice Homer (y). Do you have details on the matching device? I really like those quad antennas.

    John
     
  3. RickC.

    RickC. Hopeless antenna junkie

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    Homer,

    How are you matching it? I had a 2 element quad for 11m for a year or so matched with a 1/4 wave of RG-59. VSWR was under 1.5:1 for the whole band. I wouldn't expect to see a significant difference in the VSWR, but it won't cost anything to raise it and see.

    I once built a 2 element Delta Loop for 10m and fed it direct with RG-11, figuring the feedpoint impedance should give a decent match to 75 ohms. I was wrong. There was no analyzer or noise bridge to be had so I don't know what the impedance actually was, but I ended up throwing together a wire gamma match that matched it perfectly.

    I don't recall the exact dimensions of the 11m quad, but the DE was just the standard 1005/f(MHz) and the reflector was 5% longer. Spacing was right at 5 feet. That was a great little antenna, fed in the corner of the diamond for vertical polarization.

    Is that another clothesline?

    Man, how do you have time for all this neat stuff? I'm doing good to keep my grass cut!
     
  4. JDemoret

    JDemoret Member

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    What Rick said.:headbang My grass is growing like somebody is yanking it out of the ground.
     
  5. W5LZ

    W5LZ Crotchety Old Bastard

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    SWR will change with height. Basically it's because you are decreasing the interaction between the antenna and all the stuff around it (inductive/capacitive reactances). Does it go up or down? Yep, one or the other, I've had it go both ways, so I'd just be guessing.
    - 'Doc
     
  6. HomerBB

    HomerBB Sr. Member

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    Thanks, Doc, we'll just have to see which way it goes...
     
  7. HomerBB

    HomerBB Sr. Member

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    Matching system:
    I started with a 1/4 wavelength 75 ohm section. It was not enough -

    SWR
    ch 40 --- 3:0:1
    ch 1 --- 2:5:1

    So I did some more reading and decided to try 1/12 wavelength coax transformer sections. The result was this:

    ANT-----75ohm----->-----50ohm----->-----75ohm----->-----50ohm-----TX
    ------(any length) ------ (1/12 wl) -------- (1/12 wl) ------- (feedline)

    I can not explain to you technically why it works, but the resultsspeak for themselves.

    This afternoon:

    SWR
    ch 40 --- 1:7:1
    ch 1 --- 1:4:1
     
  8. HomerBB

    HomerBB Sr. Member

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    No, not a clothesline.

    I mow very fast.
     
  9. RickC.

    RickC. Hopeless antenna junkie

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    Interesting.

    Do you have the usual DE/Reflector, or are you using a director (I've used both, they work the same)? What are the dimensions of the loops and what's the spacing. The feedpoint impedance of a quad (and a 2 element yagi for that matter) can vary quite a bit (40 to 125 ohms in the case of the quad) depending on the spacing.

    What's your source for the 1/12 section? I don't see how that would provide much of an impedance transformation but maybe I'm missing something. If your VSWR was that high with a 1/4 wave 75 ohm Q section it looks like there may have been some other issues. These antennas are not usually hard to match to 50 ohms. On your 1/4 wave section, was the coax poly or foam dielectric, and how long was it?

    Sorry for all the questions, I'm just trying to figure out why that didn't work in this case. I've used that matching method a bunch of times from 7 to 29 MHz and have never had a problem with it.

    Thanks,

    Rick
     
    #9 RickC., May 14, 2009
    Last edited: May 14, 2009
  10. HomerBB

    HomerBB Sr. Member

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  11. RickC.

    RickC. Hopeless antenna junkie

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    Thanks.

    The reflector looks a bit on the short side, resonant around 26.53, whereas 5% lower would be around 25.895 (assuming a 27.185 fc). Other than that, it all looks right. According to the numbers I crunched the feedpoint impedence should be around 100 ohms at that spacing with a reflector cut for 25.895. And your 1/4 wave section was about 7 1/2 feet? Hmmmm..

    Thanks for the reference on the 1/12, I'll dig more into that when I'm not supposed to be working (!).

    Your reflector is only 2 1/2% longer than the DE, do you think that's enough?
     
  12. HomerBB

    HomerBB Sr. Member

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    I'm beginning to think not.

    Raised it to 28' at the boom, and the SWR did not improve.

    ch 01 ---- 1:4:1
    ch 19 ---- 1:7:1
    ch 40 ---- 2:0:1

    Now back to the matching at new height.

    (new photos at same location)
     
  13. RickC.

    RickC. Hopeless antenna junkie

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    Yeah...

    Well, three things you could do:

    1. Make the reflector longer and totally re-do the whole spreader thing (a pain).
    2. Lengthen the reflector using a stub.
    3. Shorten it enough to make it a director (also a pain).

    I'd vote for 2. And I bet you the 1/4 wave transformer will do the trick for you then.

    I guess a 4th thing would be to load the reflector with a coil (Maco does that with their V Quad), which would be an interesting experiement, but the stub would be easier, and just tie it back toward the mast or along one of the spreaders. An analyzer or a noise bridge would help, but you can always just give it your best guess.

    You're doing some neat stuff-- and all at once!


    Rick
     
  14. HomerBB

    HomerBB Sr. Member

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    I'll have to make up my mind which way to go.
    Thanks for the help and compliments.
     
  15. RickC.

    RickC. Hopeless antenna junkie

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    Decisions, decisions...

    Are those spreaders movable (out from the hub)? If they are then lengthening the reflector might not be so bad.

    How about some details on the boom, hubs, and spreaders? You took an original approach to that, and I'd like to see how you did it (and maybe steal an idea or two...). I just used angle aluminum for the hub and 3/4" square wooden spreaders I ripped from straight-grained 1x6s on my 11m quad. Yours looks like it may be just a bit heavier and maybe a little more of a windload, but more rigid than mine was.

    Now if I could just manage to get a 20m quad up in the same place without it looking like a giant spider was eating the house...

    I don't know if this is the first quad you've had, but I predict you will really like it once you work out the kinks.


    Rick
     

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