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Thread: High-Gain Hardware Store FM Antenna

  1. #1
    k6sti Guest

    Default High-Gain Hardware Store FM Antenna

    Since outdoor directional FM antennas seem to be no longer available and aluminum tubing has become expensive, I've designed an antenna you can make from cheap materials available at your local hardware store. It's a 5-element cubical quad that uses #14 copper wire supported by PVC or ABS pipe. The antenna takes advantage of the widespread use of circular polarization for FM broadcast signals by recovering some of the cross-polarized transmit power that a linearly polarized antenna ignores. Forward gain for a right-circular signal is 3-5 dB greater than that of an Antennacraft FM6 with the booms 20 feet above ground. I've added the design to my circularly polarized cubical quad writeup here:

    http://ham-radio.com/k6sti/cpquad.htm

    I'll be updating the writeup with additional notes.

    The antenna will receive right-circular, horizontal, and vertical signals well. It will not do well with left-circular signals, but at least in my area, there are hardly any of them. Unlike a linearly polarized antenna, the circularly polarized quad will attenuate multipath reflections since they change circularity sense.

    1/2" PVC pipe (0.84" outside diameter) should work fine for the loop spreaders. It costs $2.03 for a 10-foot length at Home Depot. A 10-foot piece of 1-1/2" pipe (1.9" OD) should make a good boom. It costs $6.49. You'll have to figure out how to mount the spreaders on the boom. The antenna requires about 30 feet of #14 bare copper wire. Home Depot wants $6.47 for a 25-foot roll of insulated stranded wire. Use solid wire if you can find it to ensure that the RF diameter is correct. It's not hard to strip the insulation off solid wire.

    Use an insulated mast section in the vicinity of the antenna. Use a 300:75-ohm halfwave coaxial balun to minimize loss and add current baluns at 30" intervals along the feedline in the vicinity of the antenna. These precautions maximize performance for any antenna that intercepts the vertical field. See this reference for other tips:

    http://ham-radio.com/k6sti/notes.htm

    Brian

  2. #2
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    That sounds great. I don't know why aluminum tubing is expensive when copper costs less. except for the weight issue, I do now know why a 7-8 element FM yagi could not be built with copper tubing. It should work as well. If you got thin enough copper tubing, the weight should not be too much of an issue. I wonder if copper tubing has been tried? I use copper tubing (pipe) for ground rods and they work great for that. A few 10 foot ones work good here in our soft soil.

  3. #3
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    Feb 2006
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    Copper is A LOT better conductor, than aluminum ! Weight and price, is usually the set back. I've built scanner antennas, out of Brass welding rod. It was cheap and the antenna, worked great. usually comes in 3' sticks.

    Gary H.

  4. #4
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    If I built an FM6 type antenna out of copper/brass piping and had them side by side, I wonder if there would be any difference in reception? On another note, does the size of the tubing matter as far as the reception goes? I have always used about the same size of what commercial antennas use. I wonder how an antenna would work with slightly smaller or larger tubing? I would think thin copper pipe would not be a whole lot heavier than good aluminum piping, but I never compared them that closely. I know the cheap thin aluminum piping is a lot lighter, like in the case of the FM6. Copper piping as Home Depot is less than than aluminum piping.

  5. #5
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    Increasing element diameter increases the electrical length which should not make much difference for a wideband design like a wide bandwidth, 88- 108 MHz antenna.
    I think copper must still be much more expensive than aluminium, isn't it? And the differences in conductivity make no significant difference in this application.
    I've added Brian's cubical quad to my list of summer projects since I didn't bring any FM antennas with me from Colorado. I've built several 144 and 432 loop yagis using #14 copper wire and 1/2 ir 1 inch PVC tubing. Very economical.

    Doug Allen K4LY EM85wb Inman, SC

  6. #6
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    I was looking at the quad loop and it looking interesting. Home Depot has solid bare copper wire down to #4, so making a loop out of heavier wire would probably hold the shape without the PVC pipe. The bigger the diameter the more the cost, but using #8 should work well. But again for me, keeping anything up in in our winds is a challenge The over all gain is nice, even if the F/B is not as good has with a yagi. It is worth a try I think. Making a bigger one might have better results. (I go to Home Depot a lot).

  7. #7
    k6sti Guest

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    Quote Originally Posted by Doug Allen View Post
    Increasing element diameter increases the electrical length which should not make much difference for a wideband design like a wide bandwidth, 88- 108 MHz antenna.
    Quote Originally Posted by mwdxer View Post
    Home Depot has solid bare copper wire down to #4, so making a loop out of heavier wire would probably hold the shape without the PVC pipe. The bigger the diameter the more the cost, but using #8 should work well ... Making a bigger one might have better results.
    ALWAYS BUILD A DESIGN AS SPECIFIED. Do not change anything that affects electrical performance. This includes conductor lengths, diameters, and positions. As a test, I increased element diameter from 0.375" to 0.5" for several antenna models. Forward gain degraded up to 1.6 dB and F/R up to 7 dB at the top of the band. This test included designs whose dimensions I consider to be noncritical. You don't want this performance degradation to occur. Element diameter affects element tuning, which affects everything.

    Modern antenna designs squeeze all available performance out of an antenna configuration. Some optimizations may calculate hundreds of thousands of individual antenna models to converge to the final design. Given the chosen trade-off between forward gain and radiation pattern, normally there's nothing left to be had. Any electrical change moves the design from the point of optimality, which means the antenna performs worse. Optimized designs have no degrees of freedom left. They've all been used up to maximize performance.

    If you need to change a dimension for mechanical reasons, you must reoptimize the entire design. Otherwise, performance will suffer.

    Brian

  8. #8
    k6sti Guest

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    Click image for larger version. 

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    The quad has more gain than I realized. All booms are at 20 feet over average-quality ground. Analysis for right-circular field at 1 elevation angle. Quad gain will drop for signals with nonzero axial ratio, which I believe may be common in irregular terrain. See this:

    http://ham-radio.com/k6sti/axr.htm

    Brian

  9. #9
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    [QUOTE=k6sti;34719]ALWAYS BUILD A DESIGN AS SPECIFIED. Do not change anything that affects electrical performance. This includes conductor lengths, diameters, and positions. As a test, I increased element diameter from 0.375" to 0.5" for several antenna models. Forward gain degraded up to 1.6 dB and F/R up to 7 dB at the top of the band. This test included designs whose dimensions I consider to be noncritical. You don't want this performance degradation to occur. Element diameter affects element tuning, which affects everything.

    Thanks for the info. If I can find thicker aluminum tubing at 0.375", would that also detune the the antenna? I just have a feeling that the high winds will damage the FM6. One other thing, the rivet that the wing nut connections go on, are not very tight. There is some looseness after I tighten the wing nut down. Replacing that rivet with a steel bolt and nut, the same diameter, would make the connection much tighter. Would even that small change effect the performance?

    Modern antenna designs squeeze all available performance out of an antenna configuration. Some optimizations may calculate hundreds of thousands of individual antenna models to converge to the final design. Given the chosen trade-off between forward gain and radiation pattern, normally there's nothing left to be had. Any electrical change moves the design from the point of optimality, which means the antenna performs worse. Optimized designs have no degrees of freedom left. They've all been used up to maximize performance.



    If you need to change a dimension for mechanical reasons, you must reoptimize the entire design. Otherwise, performance will suffer.

    I guess that also includes your CP antenna. Larger wire would stand on its' own. I would not have to use PVC pipe or other support.
    Thanks again.

    Patrick

  10. #10
    k6sti Guest

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    Current flows only on the outside of tubing so the inner diameter doesn't matter. But why waste good aluminum beefing up an FM6, which is a mediocre design? It would be much better applied constructing a good design.

    If you replace one conductor with another of the same length and diameter, performance won't change. Just avoid lossy materials like stainless steel or chrome plating.

    What can make a difference at the feedpoint is the connection length. A surprisingly short excess length can degrade performance. Some 300:75-ohm baluns have very long leads. They form a transmission line whose impedance is greater than 300 ohms. For example, parallel #18 leads 4" long spaced 1.875" add about 79 ohms of reactance. That will increase SWR for a perfectly matched 300-ohm antenna to 1.3, yielding a mismatch loss of 0.07 dB. That isn't much, but why compromise the match at all? Things are worse at 75 ohms. If you leave 2" of stripped coax leads 0.5" apart, 75-ohm SWR will rise to 1.66 and mismatch loss to 0.28 dB. That sort of thing is easy to do. Unless the lead length is specified (and sometimes it is because it's an easy way to add a compensating feedpoint reactance), keep balun leads and stripped coax leads as short as possible.

    In the case of the FM6, I wouldn't worry about the long leads of the Radio Shack balun typically used. The lead length does affect performance, but I can't remember whether it hurts or helps. However, that balun is particularly lossy. You can improve the gain by an amount greater than the average improvement due to the coil by using a halfwave coaxial balun. See my FM6 writeup for details.

    Brian

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