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  • Cheap FM Yagi

    I find aluminum tubing for low prices at local surplus metal dealers. But not everyone has such a place nearby, and they may not have the exact size or quantity you need.

    Internet prices for new 3/8" aluminum tubing seem very high. For example, the cheapest I could find a 72" piece of 0.049-wall 6061-alloy tubing was about $24. That will make just one thin-wall element. An antenna with more than a few elements becomes expensive fast.

    I found several places that sell and ship 3/16" solid aluminum rod for much less than 3/8" tubing. The cheapest price I found for a 72" piece was $3.16 here:

    http://www.speedymetals.com/pc-2434-...-extruded.aspx

    The 48" and 60" sizes are useful for shorter elements and cost even less. I didn't check shipping cost. They have a note that lengths 59" or greater incur additional UPS charges.

    I think 3/16" 6061-alloy solid aluminum rod would be strong enough for an FM antenna. I've decided to optimize a Yagi based on these elements. Using 3/16" instead of 3/8" elements costs roughly 0.3 dB in forward gain and 1 dB in F/R. I think the cost savings are well worth the performance hit.

    You can use 1" square aluminum for the boom, but this too is expensive. 1-1/2" PVC (1.9" OD) is much cheaper, can be bought at any hardware store, and eliminates the need for insulated element mounts. I don't think a 10-foot length will sag much, but it's easy to add boom guys if necessary.

    I'm currently looking into designs that use 10-11 elements on a ten-foot boom. F/R will be at least 30 dB across the FM band and forward gain will be whatever I can get (currently 6.5-8.5 dBd).

    If you're interested in building such a Yagi, speak up. If there's little or no interest, I'll drop the idea.

    Brian

  • #2
    I've seen the VHF / UHF Hams use a piece of wooden closet pole inside of the PVC boom to add rigidity. Any holes drilled though this type of boom should be carefully sealed (you don't want the wood to soak up water) before the antenna is put into service. PVC caps can be used to seal the ends of the boom.

    Brian... does the modeling program take into account the boom to mast assembly (clamp) and vertical (metal) support mast?

    73, Ed NN2E
    Owner / Operator - Murphy's Law Test Site & Thunderstorm Proving Grounds
    "You Might Be a Redneck If...
    Your TV is on 24/7.
    Your TV has been permanently on for over a decade.
    The only time your TV is off is during a power outage.
    Your TV gets 512 channels, but you go outside to use the bathroom.
    Your new TV is sitting on top of your old TV.
    Your TV costs more than all of your other furniture.
    Your deer-stand has a TV antenna on it.
    Your cable provider has no idea that you exist."
    Jeff Foxworthy

    Comment


    • #3
      The mast and bracket won't affect things unless they happen to be very close to an element.

      I forgot to mention that the design will have three or four reflectors. Use a PVC Tee at the end of the boom and a vertical PVC piece to hold the reflectors. With this on the end, you may need boom guys.

      Gain currently is 6.7-9.0 dBd across the band, which is not bad. 11 elements total, 4 reflectors, horizontal folded dipole. But two reflectors are 73" and the other two are 78". I think I will have to use a short center section of 3/8" tubing to get those lengths using 72" 3/16" rod.

      Brian

      Comment


      • #4
        Ah, OK, I had a standard Yagi design in my head... wasn't thinking of a multi-element reflector.

        As you know, my one big issue with modeled antennas is, eventually, you have to take them out of the safety of the computer and put them to use in the real world. It's at this point the antenna system becomes the antenna plus it's environment. Most real world antenna environments introduce,"gremlins" into the performance of the antenna. This is where the disclaimer, "You're results may vary" comes in handy.

        I look forward to reviewing your design.

        73, Ed NN2E
        Owner / Operator - Murphy's Law Test Site & Thunderstorm Proving Grounds
        "You Might Be a Redneck If...
        Your TV is on 24/7.
        Your TV has been permanently on for over a decade.
        The only time your TV is off is during a power outage.
        Your TV gets 512 channels, but you go outside to use the bathroom.
        Your new TV is sitting on top of your old TV.
        Your TV costs more than all of your other furniture.
        Your deer-stand has a TV antenna on it.
        Your cable provider has no idea that you exist."
        Jeff Foxworthy

        Comment


        • #5
          The only thing that makes much difference is ground proximity and other nearby antennas. I am currently optimizing this design in free space just to see what's possible. I may optimize the final design over ground since that's the way it will be used.

          I don't know what to do about the long reflectors. A 12" piece of 0.058"-wall 5/16" OD tubing would allow a tight fit when used as a center section, but the place selling cheap 3/16" rod doesn't have that wall thickness. Everything else I've thought of is a kludge.

          compg.png

          compf.png

          9.2 is 9 elements on an 81" boom. 15.12 is 15 elements on a 147" boom.

          Brian

          Comment


          • #6
            Thanks, Brian. That all sounds interesting.
            Danny
            Shreveport, LA
            Mexico/Latin America TV DX ID Tips http://www.tvdxtips.com
            Submit and read DTV Stats http://www.tvdxexpo.com/dtvdxrecords.html
            TV and DTV DX Photographs http://www.tvdxexpo.com
            My Photographs of 100 Mexico TV DX Local IDs http://www.tvdxexpo.com/100mexicotvids.html
            More than 1,100 TV logs since 1994

            Comment


            • #7
              I had overlooked Speedy Metals custom cut service. Four 80" 3/16" rods are $16 total, or $4 each. Not bad and a simple solution to the over-72" reflector problem.

              Brian

              Comment


              • #8
                As I look around my antenna environment I see several things... a bunch of other antennas, several metal barns / buildings, house ductwork & wiring, metal vehicles, overhead power / phone wires, fence wires, guy wires...

                On the RF side of things I see houses full of RF crud-producing devices, a cell phone tower site, a 100KW FM transmitter, a 57KW DTV transmitter...

                ... And I live out in the middle of nowhere. Throw all of the aforementioned into a bowl, along with your antenna, mix it all together and now you have some idea of what your antenna / receiver has to deal with. It's chaos out there!

                My point is... If you build one of these antennas, and get weird results, don't blame Brian.

                The charts & graphs look good, Brian. Now, who's going to be the first to build one? I'm not really an FM guy so don't count on me. :-)

                73, Ed NN2E
                Owner / Operator - Murphy's Law Test Site & Thunderstorm Proving Grounds
                "You Might Be a Redneck If...
                Your TV is on 24/7.
                Your TV has been permanently on for over a decade.
                The only time your TV is off is during a power outage.
                Your TV gets 512 channels, but you go outside to use the bathroom.
                Your new TV is sitting on top of your old TV.
                Your TV costs more than all of your other furniture.
                Your deer-stand has a TV antenna on it.
                Your cable provider has no idea that you exist."
                Jeff Foxworthy

                Comment


                • #9
                  Originally posted by NN2E View Post
                  As I look around my antenna environment I see several things... a bunch of other antennas, several metal barns / buildings, house ductwork & wiring, metal vehicles, overhead power / phone wires, fence wires, guy wires...
                  Those things won't affect the antenna unless they are close to it. Large objects like metal barns can cause reflections, but that's another matter.

                  I'm trying to figure out how to make a robust folded dipole. The ideal way is to bend a single length of rod, but it would be too long to send through the mail. I think flattening the ends and bolting it together will have to do. But those connections are vulnerable. They need to be tight and waterproof. Accessing the center for feeding is also a problem since ideally it's inside the PVC boom. A model with one side in the boom and the fed side on the surface did not degrade performance, which surprises me. You have to support that fed rod somehow, but at least you have easy access for connections.

                  Brian

                  Comment


                  • #10
                    You're correct... the metal barns / buildings create lots of multi-path issues... A few years back the barn lost a section of metal roof during a storm. After this, local WPSD 32/6 would no longer decode using the indoor antenna system. After the missing section of roof was replaced, WPSD reception returned to normal.

                    If y'all want to try an interesting antenna / receiver vs. environment experiment...

                    Set up a portable FM radio in the middle of the room. Tune in to a weak FM station and then, walk around the room. Listen to what happens to the received signal.

                    In my 'shack' I have a 'Chairman Mao' brand FM receiver with a 30-ish inch (roughly 1/4 wavelength) piece of wire hooked to the antenna terminal. As I listen to local 95.5 W238AN Mayfield, KY I can walk around the room and 'tune' the system. If I stand in just the right spot W238AN disappears and WSM Nashville (100 mi) shows up. Move out of that spot and W238AN returns. Give it a try. "Your results may vary." :-)

                    Update 9-4-15... this morning I was able to receive KJEZ 95.5 Poplar Bluff, MO (110 mi) over local W238AN by standing in just the right spot, about 15 feet from the receiver.

                    Brian...I'll have to go out and stare at the antenna / aluminum pile to see what ideas come to mind, in regards to solving the mechanical issues, but it's too hot right now...

                    73, Ed NN2E
                    Owner / Operator - Murphy's Law Test Site & Thunderstorm Proving Grounds
                    Last edited by NN2E; 09-04-2015, 10:55 AM.
                    "You Might Be a Redneck If...
                    Your TV is on 24/7.
                    Your TV has been permanently on for over a decade.
                    The only time your TV is off is during a power outage.
                    Your TV gets 512 channels, but you go outside to use the bathroom.
                    Your new TV is sitting on top of your old TV.
                    Your TV costs more than all of your other furniture.
                    Your deer-stand has a TV antenna on it.
                    Your cable provider has no idea that you exist."
                    Jeff Foxworthy

                    Comment


                    • #11
                      Ed and Brian, you both know I'm not an antenna expert.

                      Believe it or not, I've done the experiment Ed mentions with TV, FM, MW, and SW, using rabbit ears, long wires, and even the
                      front section of a large RS VHF/UHF antenna in the house.

                      My most recent experiments have been with rabbit ears in an attempt to decode DTV from my local KTAL-DT-15.

                      Ed is right.

                      My interest in Brian's and MW's FM antenna projects stems from the thought that I may get back into FM DXing again in the
                      future.

                      The FM-6 works well for Pat Dyer and Christopher, and it will work well for Mike P and amfm, because they are near the Gulf. It
                      may work well for many DXers elsewhere, in fact. However, I'm just far enough away from the Gulf, and inland where I have
                      stations between me and the Gulf, that my feeling is that I need a larger, more directional antenna. Fortunately, I have my
                      old, rebuilt Finco FM-5, and it is in good condition. Yet, that might not be enough to do the job here.

                      Therefore, down the road I might want to buy or build something more directional.
                      Danny
                      Shreveport, LA
                      Mexico/Latin America TV DX ID Tips http://www.tvdxtips.com
                      Submit and read DTV Stats http://www.tvdxexpo.com/dtvdxrecords.html
                      TV and DTV DX Photographs http://www.tvdxexpo.com
                      My Photographs of 100 Mexico TV DX Local IDs http://www.tvdxexpo.com/100mexicotvids.html
                      More than 1,100 TV logs since 1994

                      Comment


                      • #12
                        compf.png
                        compg.png

                        Adding one more director. Since they only cost a few dollars, why not? But it hardly seems worth the trouble.

                        When F/R exceeds 30 dB, I have told the optimizer to treat the number as 30. Therefore, it doesn't try to improve F/R since it sees no benefit. Instead, it focuses on improving forward gain. This is one way of exploiting the fundamental gain-pattern trade-off that exists for all antennas. I could set the F/R target higher, but then forward gain would drop.

                        Brian

                        Comment


                        • #13
                          Thanks, Brian.
                          Danny
                          Shreveport, LA
                          Mexico/Latin America TV DX ID Tips http://www.tvdxtips.com
                          Submit and read DTV Stats http://www.tvdxexpo.com/dtvdxrecords.html
                          TV and DTV DX Photographs http://www.tvdxexpo.com
                          My Photographs of 100 Mexico TV DX Local IDs http://www.tvdxexpo.com/100mexicotvids.html
                          More than 1,100 TV logs since 1994

                          Comment


                          • #14
                            compf.png
                            compg.png

                            Here I added one more reflector and raised the F/R target to 32 dB, which it didn't meet everywhere. I'm not sure which design I prefer. Probably the 11 elements.

                            Maybe someone could suggest a good way to mount the folded dipole. I just realized that even the nonfed part has a problem. You must insert it through boom holes before flattening the ends. That could be awkward. Then you have to support the fed part somehow. Maybe it's time for some PVC joints and such.

                            http://www.homedepot.com/p/Charlotte...00HD/203823993

                            The folded dipole for the 11-element design consists of two 3/16" rods 60-1/2" long separated 6-5/8".

                            Brian

                            Comment


                            • #15
                              I suppose the folded dipole driven element could be made from two 'J' shaped pieces. Two small holes would need to be drilled in the boom, as close together as possible. The two dipole halves would be inserted through the boom holes with a certain amount of overlap at the middle. The two element halves could then be clamped together, on each side of the boom, to hold the proper dimension and hold the element in place.

                              As for supporting the lower half (feed-point section) of the folded dipole... perhaps some plastic spacers / straps, slid over the upper and lower sections of each element half. The plastic oil containers and anti-freeze jugs are made from some pretty tough stuff and seem to last a while in the sunlight. A couple of straps could be cut from those containers with holes spaced at the correct distance to properly hold the driven element dimensions / support the lower (open) half of the element.

                              73, Ed NN2E
                              Owner / Operator - Murphy's Law Test Site & Thunderstorm Proving Grounds
                              "You Might Be a Redneck If...
                              Your TV is on 24/7.
                              Your TV has been permanently on for over a decade.
                              The only time your TV is off is during a power outage.
                              Your TV gets 512 channels, but you go outside to use the bathroom.
                              Your new TV is sitting on top of your old TV.
                              Your TV costs more than all of your other furniture.
                              Your deer-stand has a TV antenna on it.
                              Your cable provider has no idea that you exist."
                              Jeff Foxworthy

                              Comment

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