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Built a 3 element yagi for CB; every station had talking until I keyed the mike

Discussion in 'CB and Export Equipment and Accessories' started by RCA Dimensia, Sep 29, 2010.

  1. RCA Dimensia

    RCA Dimensia Member

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    I decided to re-put a CB radio in my truck since I had a bunch of them laying around. They are all old. I chose the Uniden Trucker because it was the best of them with ANL, SWR Cal etc. I couldn't find my old 10 dollar little antenna the same as the ones most people have (about two and a half feet with a loading coil in the middle) so I bought another one. I realized there wasn't much on CB anymore when I could hardly get anything on channel 19 so I decided to start messing around with old TV antenna rods to try to make reception better. First I basically stuck two together to make a 9 foot vertical ground plane (truck roof) dipole using the magnet from the boughten antenna. It was better than the boughten antenna for receiving but did not read quite the TX power but it was still up there.

    Eventually I decided to completely destroy the aerial and made a three element, half wave yagi tuned to 27 mhz. I built it and just set it up temporarily on sawhorses not more than two feet above the ground and right next to my truck and aimed in the direction it happened to be in when I built it and when I connected it to the CB it was like every channel had someone on it. None of the signals were strong but most were clear. I couldn't believe how well it worked and it was so easy to do. I just used generic spacing for the elements and ~5% +/- lengths. Then I keyed the mike to talk but let off when I realized the TX power was way down at one watt or so and I didn't want to burn out the transmitter.

    As soon as I let off I noticed the static was much quieter and I immediately knew something wasn't right. I'm sure the impedance of the antenna is way off but I didn't think it would be that bad that it would basically immediately ruin the radio.


     
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  2. 443

    443 W9WDX Amateur Radio Club Member

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    For a Yagi you need a gamma match unit attached to the driven element. You can buy one or make one, I am sure if you google around for "homebrew gamma match" you can find a HAM site that will tell you how to make one yourself.

    Keying the radio into the homemade antenna with no matching network on the antenna probably destroyed the final output transistor in the radio.
     
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  3. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    You didn't ruin the radio, not thereceiver anyway and I highly doubt you ruined the transmitter either. A high SWR will not harm ANYTHING in the receiver. I suspect you have a bad/intermittent connection that accounts for the sudden drastic change in the receive.
     
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  4. 443

    443 W9WDX Amateur Radio Club Member

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    Years ago when I did similar projects without knowing what I was doing, I destroyed the Final output transistor. Put a new one in, all was well again, but yeah, it shouldn't always destroy the transistor, and he may have a bad connection, LOL. Sounds like he had those bases covered though, doing all the work he did, I bet his connection is good, his impedance, is not...
     
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  5. gamegetter

    gamegetter Well-Known Member

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    I applaud your efforts RCA

    I too learned how to make my own 3 element yagi back in the 70's. Researched all i could at the library and the rest well was learned at the school of hard knocks. I never burned my radio, in fact I still have it and works great. (guess i was lucky--but mainly patient) I made my own gamma match and later added a balun. Still have the old yagi, in fact hope to replace a element or two and set her back up before the winter. BEST OF LUCK AND KEEP ON TRUCKING or TALKING!
     
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  6. RCA Dimensia

    RCA Dimensia Member

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    Okay I knew it would need some kind of matching device, and I was going to ask this before, actually. What should the impedance of a half wave yagi be? 300 ohms just like a half wave TV yagi/log periodic/corner reflector/everything else that's jammed into a TV antenna. If so then a common 300-75 ohm TV matching transformer would work okay, the 50-75 ohm mismatch would not be bad. I've heard that even if the antenna impedance is ridiculous it will still work it will just give you a very high SWR.

    "If we connect a 50 Ohm resistor at one end of a piece of 50 Ohm co-axial cable, and connect a transmitter and SWR meter at the other end, the VSWR will be 1:1. The resistor is NOT, by any means, resonant.

    If we connect a resonant antenna that has a feed impedance of 272 Ohms to the end of that piece of co-ax (ignoring any resonance effects of the co-ax), the VSWR will be 5.44:1."

    That's from that Antenna Theory site by K9ERG
     
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  7. 443

    443 W9WDX Amateur Radio Club Member

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    Not sure of that. DC Resistance has nothing to do with impedance, as far as I know.

    This site used to have a lot of good info, have you read it all?

    The Ultimate Guide to 11 Meter CB Antennas
     
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  8. Moleculo

    Moleculo Administrator Staff Member

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    Depending on how you built it, the 3 element yagi is going to have an impedance of more like 25 ohms or so. It could vary quite a bit, though. Your 300-75 ohm balun for TV isn't going to work very well.
     
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  9. Beetle

    Beetle Well-Known Member

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    The only real-world way to know just what the feedpoint impedance of an antenna is, is to measure it with some super expensive lab-quality equipment, OR to measure it with something like an MFJ-259B antenna analyzer, which isn't so expensive. After my multimeter, my 259B is the most-used piece of test equipment I have.

    It'll tell you everything you need to know about your antenna.
     
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  10. mrgumby

    mrgumby W9WDX Amateur Radio Club Member

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    CB Beam Antenna
     
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  11. RCA Dimensia

    RCA Dimensia Member

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    Why would the impedance be 25 ohms? Why would it be so low? What would it be if I end fed it?

    In that site it says to get a piece of 3/8 tube and in the diagram it's labeled "gamma tube", but what is "gamma tube".
     
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  12. RCA Dimensia

    RCA Dimensia Member

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    How could the impedance be 25 ohms? Why would it be that low? What if it was end fed?

    In the website it says to get a 3/8 tube for a gamma tube but what is a gamma tube? What kind of tube is that talking about?

    Whoops I entered that twice. I didn't see it because it was on page two.
     
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  13. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    You don't need a gamma match necessarily. He could use a beta match or simply use direct feed and adjust the element length and spacing to achieve a good match. My Cushcraft A3 tribander has direct feed and no matching device whatsoever. My 6m beam has a beta match which works great. Lots of antenna have no matching device but are designed so they do not need them. Gamma matches are the most difficult type of match to build if you are a newbie to antenna building.

    As for blowing the finals, I ask why would the receiver suddenly go quite if the finals were shot?

    As for using a TV type 4:1 matching transformer, forget it. They won't handle any kind of power and even a barefoot 4w radio may smoke it.

    BTW, the only common antenna for TV or whatever that is 300 ohms is the folded dipole. A single regular dipole is about 72 ohms.
     
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  14. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    I also didn't see page two. :oops:

    Anytime you have an antenna element in the presence of other conductors ESPECIALLY resonant conductors like other antenna elements the feedpoint impedance will be lowered because the other elements influence the current flow in the driven element.

    I suggest you Google "antenna gamma match" and look at some images of them as well as the thousands of info pages about how to make them.
     
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  15. 443

    443 W9WDX Amateur Radio Club Member

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    It's been a while, been radio'ing off and on for 20 years now, so memory not so good, but I know at least once or twice I had a radio final go bad and lost receive. I believe because the output of the transistor is tied directly to the output SO-239 connector, maybe via coil or cap or both, and when the transistor went bad instead of opening it shorted legs 1-2 or 2-3 and grounded out all my signal for RX coming into the radio from the antenna connector... Perhaps that is why it happened. Just know for sure at least once I had that problem.

    Yeah, a Beta Match would be easier for a newb... I used to know a few things about antenna's but man I have been "out of radio" for so long now, at least 5, maybe 6 years, I totally forgotten a lot of the stuff... I could have built a beta match back then, but not now, would have to read up...

    I think he needs to center feed his driven element, sounds like he is bottom feeding it.

    Got pics of this antenna? That may help greatly.
     
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