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  • OT - AM Antenna - help!

    Ok, so the new shop is fantastic. 40x60 can take a while to fill up.

    I have the same old problem as others that enjoy some AM radio. Yes, I know I should just serve my internet connection out to the shop and forget this but something happened the other day and now I'm curious.

    The shop has florescent lighting. It has reflective aluminum insulation board for the ceiling. Kinda like a big Faraday cage.

    So I found a roll of coax and ran it up to the peak of the roof on the inside. With the one end hooked up to the antenna input on the stereo I had planned to attach the other end on the remote am antenna which came with the radio. It is the type that looks like a short cylinder with wire wrapped around it. Open in the center, no core. That didnt work at all. In fact it made it worse.

    I then tried a long piece of copper wire and hung it in different positions horizontally. Nothing. I look around the internet a bit and I find that I can make a 4000 foot long wire antenna, if I can find the room. There must be a better way.

    So for the interesting part.
    The coax is stripped and attached to the radio and the reception is terrible. I unhook it and the reception gets better. Now, I'm standing on an aluminum ladder with hard rubber feet. I am about 17' in the air. The coax hits the ladder and I notice the copper conductor laying on a rung. The reception is a little better!?!?!?

    So, I bolt the conductor to the ladder and nearly perfect reception. But to be honest, I dont want to leave the ladder in the middle of a bay as an antenna. But why a vertical direction and why the ladder? Something about the distance between the uprights? Or the rungs?

    I'm not much (if at all) of an antenna engineer but I know that there are a few ham guys around here as well as others that will have a solution. I bet I'm going to get at least one response on grounding the rig differently, which my be my next direction.

    Thoughts?
    Thanks rock~
    Civil engineers build targets, Mechanical engineers build weapons.

  • #2
    Start by grounding the shop, not the radio. If you have metal siding then tie it all together and ground it. Coax isn't what you should be using for such low frequencies. You need ladder twinlead with about an inch spacing to give around 370 ohm impedance. This is known as a balanced conductor and does not pick up local radiation well. You use a dipole antenna outside with wires in opposite directions as long and high as you can manage and run the twinlead from the center of the dipole keeping it as far from other conductors and metal as possible. Bring it to the radio and at that end you connect an air core coil wrapped around a frame about the size of a paperback book, again with as many turns as you can manage. Then fiddle with the location and orientation of the coil in respect of the radio for best reception.
    Last edited by Evan; 03-18-2009, 10:59 AM.
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    • #3
      An antenna has several parts and each has a purpose. The antenna itself is intended to be excited by radio waves. A well made antenna will be excited by radio waves of a particular range of frequencies and be less sensitive as the frequencies move away from the desired range. Antenna design should consider the polarization of the transmitted wave as well. Most if not all broadcast antenna are towers so the receiving antenna will work best if it is also vertical.

      The well crafted antenna will also accept radio waves from a particular compass direction and azimuth angle so that it will not receive waves from unwanted sources.

      Antenna length is a consideration, too. Broadcast band frequencies are quite low and would require a long antenna - quite impractical. Length can be electrically achieved by placing an inductor at the base of the antenna such that it's resonant frequency is centered approximately in broadcast band. Electronic antenna tuners are available that will handle this requirement. A company down the street from me, SGC, made excellent antenna tuners for years - very robust, weather resistant, and intended for boats and ships. They turn up on Ebay from time to time and will tune a 21' vertical antenna from nearly DC to blue light

      Antenna are balanced devices - they are either symmetrical as in a dipole (think large rabbit ears) or as ground-plane verticals where the base of the antenna is located at the hub of long radials stretched out across the ground. When radials on the ground are not possible, a mobile home works well as does metal roofing. Other configurations are available but all approximate this balanced design.

      Various antenna designs have an impedance at the feed point which brings us to the next component, the transmission line. Transmission lines are intended to conduct the signal from the antenna to the receiver with minimal signal loss but not act like an antenna while doing so. That after all would undo the desired characteristics of the antenna. Transmission lines are ladder lines as Evan has suggested, coax cable, or wave guide. They also are designed to match the impedance of the antenna so as to reduce loss.

      Improperly terminated transmission lines are useless - worse than useless, they are often better antenna than what they are attached to at attracting unwanted signals and that would include noise from local equipment such as lighting and machinery. In the case of coax the terminator should be a purpose-built connector that ensures the shield is grounded to the radio chassis and that the center conductor terminates within the radio chassis away from local noise radiation. Plastic radios are challenging in this regard, but not impossibly so. The receiver input impedance should match the transmission line. When this is not directly possible by design a matching device can be used.

      The transmission line has to be properly terminated at the antenna as well as at the receiver. In the case of a dipole antenna the center conductor is attached to one side of the dipole and the shield is connected to the other (technically this is an unbalance design but it works). Better is to use a small BALUN (BALanced to UNbalanced) device to match the coax to the dipole. In the case of a ground-plane vertical, the antenna is also unbalanced electrically so the coax shield can be connected directly to the radials/trailer roof, and the center conductor to the vertical element.

      The final reason for having an antenna is to improve the signal to noise that reaches the receiver. The single most important factor for that is locating the antenna away from noise sources. A perfectly functional well crafted dipole antenna can be build in your loft but if it is sitting over your machinery room it won't work for beans as it will pick up the local noise easily and be hindered by the building from picking up the broadcast signal. So you would like it outside the building away from power lines and noise sources such as rooftop refrigeration units and Tesla coils. Not that many of us have rooftop high Tesla coils http://www.liveleak.com/view?i=3b6_1187215494

      And finally the antenna needs to be insulated from the attachment points. Dipoles often use "egg" insulators at the center and both ends. I built a nifty vertical antenna in high school by putting 4 10' sections of TV antenna pole on a large 7-Up bottle sitting on the ground. My ground was the water pipe that ran in to the house at that point. It was guyed in place with Nylon parachute cord which is self-insulating unless it rains (which it does a lot in Berkeley).

      Comment


      • #4
        Improperly terminated transmission lines are useless -
        Not quite. If the transmisson line has very low loss then the reflected signal bounces up and down the line with a portion of it passing the termination point each bounce. Open wire ladder line when properly installed is almost lossless compared to coax or an unbalanced line. A high standing wave ratio due to mismatched termination makes very little difference.

        As for polarization, unless you are in the near field of the transmitter the signal on the AM band is more than likely to be random polarization during the day and horizontal at night because of the influence of edge diffraction from such obstacle as ridges and mountains and reflection from the ionosphere at night. Also, a lot of smaller stations, especially local repeaters, use horizontal longwire antennas to transmit.
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        • #5
          Originally posted by Evan
          Not quite. If the transmisson line has very low loss then the reflected signal bounces up and down the line with a portion of it passing the termination point each bounce. Open wire ladder line when properly installed is almost lossless compared to coax or an unbalanced line. A high standing wave ratio due to mismatched termination makes very little difference.
          But an improperly terminated transmission line is just a piece of expensive wire. Doesn't matter if it is ladder line, twin-lead, or coax. In my professional career in radio (maritime services, ship board and shore stations) I've never installed a ladder line as they have too much windage to go to sea, and are vulnerable to icing no matter where they're found (ship vs shore). They can be problematic with structure penetrations as well, but they do have excellent electrical characteristics.

          As for polarization, unless you are in the near field of the transmitter the signal on the AM band is more than likely to be random polarization during the day and horizontal at night because of the influence of edge diffraction from such obstacle as ridges and mountains and reflection from the ionosphere at night. Also, a lot of smaller stations, especially local repeaters, use horizontal longwire antennas to transmit.
          Most listeners are by design inside the ground wave zone for large stations but for remote listening the polarization takes on strange characteristics and of course selective fading from multipath. Back in the day when I used to install Omega receivers (10 kHz) you could easily watch the diurnal shift at work as stations would go into the night and others would emerge into daylight.

          Regarding rebroadcasters - they are designed to illuminate multiple smaller target areas with low-power and multiple sites. Urban traffic tunnels, for example. This often involves directional antennas. In the modern broadcast receiver world polarization is rarely a consideration because of the proximity to and power of the station. It is only where noise reduction becomes an issue that polarization (for broadcast reception) becomes of interest.
          Last edited by dp; 03-18-2009, 12:41 PM.

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          • #6
            If I was that desperate to listen so you'll know when to dial 1800 282-2882 I'd build this loop antenna. Loops are famous for lowband reception and Google spits out hundreds of sites but this one looked like the less messing around to me.

            I'm in the sticks but get my dose of reality from one strong source and one back-up source 1 hour delayed

            http://www.mymorninglight.org/loopant.htm
            Last edited by Your Old Dog; 03-18-2009, 12:59 PM.
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            Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

            It is true, there is nothing free about freedom, don't be so quick to give it away.

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            • #7
              AM reception

              You have plenty of signal inside your shop; the "ladder antenna" worked, so will other designs.

              First, as suggested, try building a loop antenna. It has a balanced output that your stereo likes, and it can be made to be quite sensitive, and it's compact. Unfortunately, it is also quite directional -- you have to turn it broadside to the direction of the transmitter for peak signal. (Some WWII aircraft direction finders used hand-rotatable loops (outside the aircraft's skin) calibrated with the angle to/from the station. They worked well.)

              If you want a simple approach, try grounding one side of the AM antenna connection to any available grounded point. Hook a wire to the other terminal, turn the stereo on, tune it to your favorite station, and move the wire around until you get a good signal. Leaving the wire where the signal is best, try tuning around. Chances are you'll also get acceptable signals from other stations too.

              The previous contributors' comments generally true; transmission polarization is vertical, ladder line has low loss, poor terminations generate reflections, and reflections have little impact with low-loss line. Wrong items include the idea that coax won't work at low frequencies (it will, but just isn't necessary for you). Perfect impedence matching is also unnecessary unless you're trying to recover "all" of the signal (you aren't) and/or have a high-loss feed line. Ditto for balanced/unbalanced connections; they are lossy, but often that doesn't matter.

              Let us know what your final solution is.

              Comment


              • #8
                Originally posted by patty boy
                Perfect impedence matching is also unnecessary unless you're trying to recover "all" of the signal (you aren't) and/or have a high-loss feed line. Ditto for balanced/unbalanced connections; they are lossy, but often that doesn't matter.
                Imperfect matching causes the transmission line to act as a receptor thus injecting unwanted signal, typically from local noise sources. It also attenuates the desired signal from the antenna but that is not the greatest concern in reception (not the case for transmitting where high vswr can cause sparks in the transmitter). And since the OP's looking to eliminate noise, transmission lines are more interesting to explore and refine.

                Comment


                • #9
                  Regarding rebroadcasters - they are designed to illuminate multiple smaller target areas with low-power and multiple sites. Urban traffic tunnels, for example.
                  Heh. Not so much in Canada. Repeaters here are to service entire small towns, like Williams Lake. CBC has hundreds or even thousands of these across Canada. Usually a 5 watt or so box set up at the local phone company with a wire strung out the window to a nearby pole.


                  It doesn't matter if the termination of a balanced line isn't perfect as long as both sides see the same charactersistic impedance. In that case it will stay balanced and still reject local fields since both sides of the line see the same signal but in opposite phase. I have used ladder line for years for listening on my Yaesu FRG 7700 and only recently took it (the remains) down because of damage by a tree.

                  Imperfect matching most certainly causes problems with an unbalanced line such as coax which does not phase cancel local interference.
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                  • #10
                    Originally posted by Evan
                    Heh. Not so much in Canada. Repeaters here are to service entire small towns, like Williams Lake. CBC has hundreds or even thousands of these across Canada. Usually a 5 watt or so box set up at the local phone company with a wire strung out the window to a nearby pole.
                    I have a wifi-FM broadcast transverter here. It uses a Mac Mini with a wifi connection (or wired net connection) to drive a Ramsey FM-25B stereo transmitter that feeds a 5/8 wave copper tube J-Pole antenna. Works pretty well. There's about 8 hours of music stored on the Mini for those times a network isn't available.

                    http://www.apple.com/macmini/
                    http://www.ramseyelectronics.com/cgi...tion&key=FM25B
                    http://www.mycal.net/old/projects/mpr/jpole.htm

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                    • #11
                      No mention I have seen of whether the am radio works ok when outside the shop. If reception is lousy outside and antenna inside is not likely to help much.
                      Steve

                      Comment


                      • #12
                        Fancy antennas are all very well when you need them. Transmission line is all very well, when you need IT.

                        But receiving AM is usually not so complicated if your radio will do a decent job to begin with.

                        Your "ladder" antenna may have been a sort of non-designed "capacitively loaded" antenna.

                        1) the usual AM antenna is a loop, because a tuned straight wire antenna is totally impractical at AM frequencies. Generally on older stereo receivers it is a stick of ferrite core with a coil of wire around it, on the back panel. You have to turn it to get the best reception, so it normally is attached on a sort of universal joint.

                        2) it is reasonably rare to find a non-metal building which has sufficient shielding to block AM reception, particularly when a loop antenna is used. Usually there is not enough grounded metal, and it isn't all-enclosed. You can receive radio signals very nicely using a loop which is completely covered in copper pipe, with just ONE small slit where the pipe is cut, because the loop is magnetically coupled. (I HAVE had trouble in a metal-roof building, when on the "mezzanine, right under the roof).

                        Without knowing what your radio has for a built-in antenna, in fact without knowing what your radio is at all, or if it receives well outside the shop, it is hard to know what will work best.

                        Some thoughts.....

                        Old-time AM radios had a loop antenna built in on their back panel, typically. If you needed better reception, often you could use a couple dozen feet of wire outside, with a turn or two wound near that loop antenna and the end grounded.

                        It was even more effective to connect the wire to a tuned circuit if you were in a "fringe" area.
                        You used to be able to get from Radio Shack, or other such place (Lafayette, etc) a "standard" device known as an "AM loopstick", really just a ferrite core and a coil around it. With the standard 365 picofarad tuning capacitor (adjustable) it would tune the whole AM band, boosting the strength of whatever signal you wanted to tune it to.

                        AM is nearly forgotten by the high tech folks, so those parts are not common at all any more. But receiving AM still shouldn't be a big problem.

                        Start by checking reception OUTSIDE. If that is OK, but is bad inside shop with antenna in same orientation , try a length of wire and coil near your radio's AM antenna, per the above. If that isn't much help, then you might need to tune it.

                        Picture... the rod is the AM antenna, and is a ferrite and coil type. it conects with the white and black wires visible below it. The "twin-lead" is the FM antenna.

                        Last edited by J Tiers; 03-19-2009, 12:49 AM.
                        1601

                        Keep eye on ball.
                        Hashim Khan

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                        • #13
                          If reception is lousy outside and antenna inside is not likely to help much.
                          Sure it will. A loopstick antenna is far from being an optimal antenna. It is the only reasonable antenna for AM frequencies that will fit in a radio but it has a very small "capture area". The optimum antenna on any frequency below microwaves is a dipole, center loaded with a balanced line. Normally each leg would be 1/4 wavelength long but on AM this is unrealistic so a submultiple is used and the antenna may be tuned to appear to be a quarter wave by inserting capacitance/inductance in the legs. This isn't necessary to achieve good results but both legs of the dipole should be the same length at least.

                          There are various ways to match the transmission line to the dipole. In this case since it is receiving only and the frequency range that needs to be covered is more than a full octave a slightly lossy but broadband method should be used. Using open ladder line for the transmisson line the last few feet of the ladder line are spread out from the normal spacing to a width of perhaps 18 inches and connected to each end of the dipole. The ladder line forms a Y shape at the connection to the antenna legs.

                          Feeding ladder into a building is simple. Make a hole large enough to accomodate the line and cover it with a chunk of plastic like a breadboard. Use a pair of bolts at the same spacing and fasten the ladder line to the bolts on both sides. All metal other than the bolts should be kept at least three times the width of the ladder line away from the line.

                          Ladder line looks like this:


                          At the radio end simply make a loop as I already described and connect the ladder line to the loop. If by chance the AM antenna is accessible then winding as many turns as possible using fine wire would couple the antenna well. That isn't required though. There needn't be a physical connection to the radio. A loop of as many turns as possible on a frame placed next to the radio will work nearly as well as a wired connection since it acts as an air core transformer.
                          Last edited by Evan; 03-19-2009, 09:43 AM.
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                          • #14
                            Thanks gents, I'm reading all of this and letting it run around in my head.

                            The radio has fantastic reception out side of the shop and in the attic. So my logic was, get the antenna up where the radio sounds good and bring the box down to a working height. This was a reasonable plan due to the fact that the box had an antenna hookup on it.

                            My antenna is external. If I unhook it from the box I get no reception for any station. The antenna is a wire loop that came with the radio. No rod.

                            A loop of as many turns as possible on a frame placed next to the radio will work nearly as well as a wired connection since it acts as an air core transformer.
                            I have heard this. CCCrane has a setup that is similar. I always wondered how it worked.

                            Going out to try again for about 1/2 hour, then I have to get some real work done.
                            Thanks again.
                            rock~
                            Civil engineers build targets, Mechanical engineers build weapons.

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                            • #15
                              Try this first. Take two pieces of wire long enough to go outside and still have about 30 feet left on each piece. Very loosely twist them together with a drill for the part that runs inside. Hook them up to the radio and run the twisted part outside and then take each end and hang them in opposite directions from a handy tree or whatever. If needed add some twine to the end to make the reach to a support or just make the wires longer.
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