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

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Old 09-29-2010, 05:57 AM
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Default Built a 3 element yagi for CB; every station had talking until I keyed the mike


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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Old 09-29-2010, 07:10 AM
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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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Old 09-29-2010, 07:10 AM
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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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Old 09-29-2010, 07:14 AM
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Quote:
Originally Posted by Captain Kilowatt View Post
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.
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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Old 09-29-2010, 07:59 AM
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Default 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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Old 09-29-2010, 12:43 PM
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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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Old 09-29-2010, 03:13 PM
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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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Old 09-29-2010, 05:08 PM
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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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