Building A Simple Dipole FM Antenna

Discussion in 'Tuners' started by musichal, Jun 16, 2018.

  1. musichal

    musichal poet emeritus Subscriber

    Messages:
    10,707
    Location:
    Norman OK
    I have 25' of 3/8" copper tubing and bad reception. Have had the tubing for several years, it's doing nothing, and I have no other plans for it, and after reading about the outdoor use of vertically mounted dipoles as pretty decent omnis, it occurred to me that I have three hall closets in a row on the other side of the wall behind my audio cabinet, so I should try out the tubing.

    I'd also read that thicker copper wire is better for reception than the super-thin wire of the common cheap folded dipoles. Well, tubing is pretty thick, and it is copper... can it be too thick? I don't know, but it is rigid, which is why I have decided to use it.

    You see, I live in a 70s ranch-style house on a slab. Hipped roof all way 'round, so even tighter as one nears exterior boundaries up there. I'm unable to climb even one rung of a ladder, much less crawl across ceiling joists, and the blown-in insulation is nasty stuff through which nobody likes to wade and wallow. Too much for Christine, who would also have a difficult time locating wall top plates, much less the right one.

    Hiding the antenna in the back of a closet, I figure I can drill a hole just big enough for the tubing through a top plate (2x4 lying flat, you know / more likely through the ceiling itself to make the job even simpler) thereby gaining some height, by having one of the T elements in the attic. Not ideal, but bound to be better than the thin wire dipole I'm using now, taped to the back of a cabinet that is less than 30" high. Nobody has to climb into the attic.

    I'm gambling that the top element wont push past any 110 wiring that might inject too much RFI by proximity (inserting from below, blind). Sure, I'd rather have more height, and I'd be willing to purchase a mast if I were able-bodled. Life is all about compromise quite often.

    So I'm planning to make each element 30.5" long, so almost 30" will be above ceiling height, in attic, meaning the tip of that one will be a smidgen over 10.5' above ground, near enough, whereas the lower end will just be about 5.5' above ground. Not good, but better than the whole antenna less than 30" above ground. I've made up my mind to try this. My experience with indoor antennas says you don't know what will work in a given location until you try, and this is an easy try.

    I have a couple questions.

    I've read that the first 20" or so of the 75-ohm RG6 should lead out from the middle of the T at a right angle. Would it be advisable to sweat 90-degree elbows to 20" lengths of the tubing and simply connect the balun to the ends of those, or does that 20" need to be shielded cable? Or just wouldn't make any difference anyway?

    Would it make any sense to make the upper element longer than the lower, for more height? Maybe make the overall length come out the same? I suspect that might mess up frequency matching, as dipoles are always the same length each element, as far as I've ever seen.

    The cable will run straight through the wall, and I'll use appropriate switch plates to hide the ugly holes, so there will be no other breaks/connections between the balun and the tuner back panel antenna input.

    The tuner is the little Sony HD. My home is brick, reception is lousy, and the Sony performs better than any other tuner I've had, or have, in this house (two Kenwood KT 615s, NAD 4130, Emotiva PT-100, Pro-Ject Box tuner, and a few others). I'm just trying to receive locals better. Only about five or six stations come in without noise now, but there are others that are trying. I'm not expecting dx miracles, just want the locals clean, and the Sony is a good bet for that, imo.

    Any comments, suggestions and answers appreciated.
     
    Last edited: Jun 17, 2018

     

    Please register to disable this ad.

  2. I LIKE MUSIC

    I LIKE MUSIC Super Member

    It appears that you are building a basic non-folded dipole, is this correct? If so then...

    The feed point impedance of a non-folded dipole is already 75 Ohms, so direct connection to your 75 Ohm feed line is correct. A 4 to 1 balun transformer is not needed and will increase signal loss (likely in the range of 3.5 dB or so) because of the impedance mismatch.

    A coaxial cable common mode RF choke as shown below will help decouple the feed line from the antenna in terms of common mode RF current. This will help prevent the shield of the coaxial cable from picking up unwanted electrical noise and help reduce the impact of the position of the coaxial cable on the pattern of the antenna.

    upload_2018-6-16_23-48-4.png

    The coil is wound out of the coaxial cable as close to the antenna feed point as possible.

    You could also use three or four ferrite chokes clipped on the coaxial cable as close to the feed point as possible.

    upload_2018-6-16_23-48-4.png

    The feed line should run perpendicular to the antenna for at least one half wave length.
     

    Attached Files:

    musichal likes this.
  3. musichal

    musichal poet emeritus Subscriber

    Messages:
    10,707
    Location:
    Norman OK
    Yes. Simple dipole, meaning not folded.

    So I don't need a balun at all? Even better.

    Where do I get ferrite chokes that clip on to the cable? Use them in addition to coiling the cable? I think I read somewhere just coil the cable into three to five loops of about six inches in diameter, is that correct?

    Am I correct in inferring extending 20" more tubing horizontally from the mid-point is not necessary? Possibly detrimental?

    Oh, and, thank you.
     
    Last edited: Jun 17, 2018
  4. I LIKE MUSIC

    I LIKE MUSIC Super Member

    Ferrite chokes are available on line. Depending on what stores you have local to you, they might available locally. They would be used instead of the coaxial cable coil.

    There is a technique to making the loop type RF choke. The size and number of turns depends on the coaxial cable used for best performance. In the picture I posted, for the cable used, the length of the cable between the pieces of scotch tape (you can see it if you look closely) is 27 inches, then the coil is wound flat so that the pieces of scotch tape are across from each other.

    The length of the cable in the coil and the diameter of the coil determine the operating frequency of the coaxial cable RF choke. Using random sizes of coils may or may not work well.

    The coiled coaxial cable common mode RF choke is cheap and easy to make (although not necessarily accurately
    unless one has access to an RF vector network analyzer) thus is it quite often recommended.

    I prefer the ferrite chokes. Using more than one improves the common mode current decoupling, a good thing.

    Technically...

    The dipole antenna is a balanced antenna. Coaxial cable is unbalanced, so a method of making the connection correct is needed. Balun is short for balanced to unbalanced. The typical TV type balun not only does this it but it also provides a 4 to 1 impedance transformation which you do not need.

    But...

    A method to make the balanced antenna compatible with the unbalanced coaxial cable is still needed for best performance. Even though there is no need for an impedance transformation, there is still a need for a conversion from balanced to unbalanced. The coiled coaxial cable and the ferrite devices are forms of balanced to unbalanced (balun) devices.

    Depending of any number of variables, using common mode RF chokes may or may not make a large difference, but the ferrites are cheap and easy to use and will likely do no harm to the signal and more than likely will have a positive impact, even if it is small.
     
    EddyR and musichal like this.
  5. musichal

    musichal poet emeritus Subscriber

    Messages:
    10,707
    Location:
    Norman OK
    Chokes it is, and thank you for the clarification... Antenna 101 for dummies. Amazing how I can search online for basic info (on this topic for years) and get tons of technical jargon above my paygrade but can't find an answer to the simple questions. Appreciate your help.
     
  6. c.coyle

    c.coyle AK Subscriber Subscriber

    Messages:
    3,079
    Location:
    40.333434 -76.423714
    Thin wire will work as well as copper tubing in your situation. On the other hand, fooling around with antennas is fun, and anything worth doing is worth overdoing.
     
    mgosdin and musichal like this.

     

    Please register to disable this ad.

  7. rmp

    rmp New Member

    Messages:
    28
    musichal likes this.
  8. I LIKE MUSIC

    I LIKE MUSIC Super Member

    The above link contains some confusing statements. The antenna is made one half wave length not one wavelength in over all length.

    upload_2018-6-17_16-14-24.png

    In the example below, the author indicates that each arm should be shortened, but in the picture he shows an off center fed dipole with unequal arms. By the way this is called a Windom antenna. This type of antenna has been around for a long time. It has is pros and cons.

    upload_2018-6-17_16-23-28.png

    Again, some additional confusing information.

    He states "Figure 3 shows a collinear wire antenna. It consists of three half-wave sections, kept in phase with each other by half-wave phasing sections of transmission line.

    It is actually a quarter wave length section and a half wavelength phased together on each side of the feed line as shown in his picture. Considering the two center quarter wavelength sections (on one each side of the feed line) as half wave phased elements is not correct.

    upload_2018-6-17_16-56-4.png

    To be clear, I am not saying that experimenting with antennas is a bad idea. It just that it is not uncommon to run across confusing information on the internet.

    BTW, musichal is building a self supporting vertical dipole. Thin wire does not necessarily lend its self to being self supporting.

    Oh, and another BTW.

    The diameter of an antenna's elements has an impact on what is called the antennas fractional bandwidth.

    In round numbers the fractional bandwidth of an antenna made with thin wire may be around 5 percent and when made with a large diameter element such as copper tubing may be as high as 15 percent (the higher the better). This means that it will have a somewhat greater operating frequency range, although the difference in real world performance across the FM broadcast band may not be large. But it might, depending on the quieting slope of the receiver and the amount of signal involved, make a somewhat noisy station more listenable.
     
    musichal likes this.
  9. I LIKE MUSIC

    I LIKE MUSIC Super Member

    Another BTW for musichal.

    The basic dipole is such a consistent performer that it is used as a reference standard for other antennas.

    The second "d" in dBd when used to specify the gain of an antenna means the gain is referenced to a standard, basic dipole in a specified installation.

    For a simple antenna the basic dipole is hard to beat.
     
    musichal likes this.
  10. musichal

    musichal poet emeritus Subscriber

    Messages:
    10,707
    Location:
    Norman OK
    And, of course, the self-supporting aspect was important to me, since I want to extend it blindly into my attic. I appreciate that you get my purpose and read closely. Good reading comprehension. When I get it done will report back on the results.
     
  11. I LIKE MUSIC

    I LIKE MUSIC Super Member

    I wish you good luck with your project. I hope it works well for you.
     

     

    Please register to disable this ad.

  12. Dingman

    Dingman Do you know where your towel is? Subscriber

    Messages:
    2,380
    Location:
    Des Moines, Ia
    All I want to say is plumbing grade copper is not the same as electrical grade.

    Now, if you were using electrical ground rods... LOL.
     
    musichal likes this.
  13. musichal

    musichal poet emeritus Subscriber

    Messages:
    10,707
    Location:
    Norman OK
    At least I have the right metal to test my mettle and I'll meddle with it 'til I win a medal.
     
    triode17, Dingman and I LIKE MUSIC like this.
  14. c.coyle

    c.coyle AK Subscriber Subscriber

    Messages:
    3,079
    Location:
    40.333434 -76.423714
    It does, if you staple it to the wall :) Must have missed that, should have read more carefully.

    All true, but not likely to make any discernible difference in this application.
     
    musichal likes this.
  15. musichal

    musichal poet emeritus Subscriber

    Messages:
    10,707
    Location:
    Norman OK
    I'm going with (A), anything worth doing is worth overdoing. I like that. :)
     
    Last edited: Jun 20, 2018
  16. musichal

    musichal poet emeritus Subscriber

    Messages:
    10,707
    Location:
    Norman OK
    My build, though extremely simple, is temporarily delayed until Christine has the time to help me gather tools and set up a work table in our garage, which we have been emptying of junk, but still need to organize. If I could do it without her, it would have already been done.

    However, I have another question based on a possibly-dumb idea.

    If I stripped the outer dielectric and the grounding sheath off a length of coax, and ran the remaining inner white dielectric with center wire through the two arms of the simple dipole antenna tubing, then attached the wire to the distal ends of those arms, would it then be a folded dipole? Or would the copper tubing completely surrounding the wire shield it from acting as one? Even if I did it, would it be likely to yield an improvement? And would I then need a balun? Just how dumb is this idea?

    Yes, I'm here to ask the stupid questions so others don't have to. What do you think I LIKE SOUND, and others who've demonstrated knowledge about RF?
     

     

    Please register to disable this ad.

  17. I LIKE MUSIC

    I LIKE MUSIC Super Member

    I am not sure that I follow our description.

    You might be describing, at least in part a type of antenna that is called,believe it or not (and no I am not Mr. Ripply) a double bazooka antenna.

    In the picture below you can see that the coaxial cable feed line is connected to the shield of the antenna element and the center conductor of the cable used for the antenna element is continuous end to end.

    upload_2018-6-29_18-15-59.png

    upload_2018-6-29_18-26-27.png

    If I understand your description, your copper tubing would replace the coaxial cable shield. It is still a 75 Ohm (more or less) antenna and it still should have chokes for the common mode RF current.

    This type of antenna has somewhat of a mythical reputation among use amateur radio operators (not necessarily me).

    The Double Bazooka Dipole is a half wave dipole with an attempt at compensation of the reactance change that occurs around resonance for a half wave dipole.

    But when push comes to shove, the information below comes close to giving a good description of its performance.

    upload_2018-6-29_18-22-45.png

    I would stick with your basic design.
     
    musichal likes this.
  18. musichal

    musichal poet emeritus Subscriber

    Messages:
    10,707
    Location:
    Norman OK
    I will.

    I just wondered how to make a vertical folded dipole, and the two parallel elements seemed a bad idea to me for something trying to be an Omni, so the thought of running the coax through the tubing occurred to me. Figured I would just be replacing one shield with another, but thought I'd ask. I also figured someone else would have already tried it, as it is such a simple idea.
     
  19. I LIKE MUSIC

    I LIKE MUSIC Super Member

    In general the parallel elements will not have much impact on the directionality of a vertical dipole.

    Below is an example of a copper pipe folded dipole.

    upload_2018-6-29_19-1-56.png

    If you look at the RF current distribution on a folded dipole compared to a non-folded dipole, it will be similar. The magic of the folded dipole is the 4 to 1 impedance transformation (75 Ohms to 300 Ohms) and possibly a little wider bandwidth.

    upload_2018-6-29_19-22-46.png


    Note that the current flows the same direction in each conductor (top and bottom) for each half of the RF cycle. This picture shows 1/2 cycle. For the next half cycle the current flow would be reversed, the same as a non-folded dipole.

    The result is a pattern that is similar to a non-folded dipole.
     
    musichal likes this.
  20. dewdude

    dewdude I fix stuff.

    Messages:
    3,035
    Location:
    Manassas, VA
    A dipole in free space is about 73 ohms. But it is a balanced antenna. You would need a balun to properly transform to unbalanced. You can get a 1:1 balun to do this.

    Most twin lead antennas are designed to be a 300ohm at the feedpoint.

    Or just not worry. Impedance mistatch for reception is not a big problem and the signals are powerful enough to not matter much.
     

Share This Page