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Building A Simple Dipole FM Antenna

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

  1. musichal

    musichal poet emeritus Subscriber

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    Not using twin lead. RG6. You're saying I need a 1:1 balun? The only ones I see are very pricey compared to the 300/75 (4:1?) examples. I infer from your closing sentence that despite the apparent impedance match, the unbal/bal issue makes it an impedance mismatch?
     

     

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  2. dewdude

    dewdude I fix stuff.

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    You probably wontw find a 1:1 bakun cheap...that's why folded dipoles are usully 300 ohm. You CAN make a standard dipole with a 300ohm feedpoint and run that in to the standard balun.

    300 to 75 is a 6:1.

    I wouldnt worry about imoedance with an RX signal. You can almost ignore the balanced and unbalanced aspect if you want....reception is more of a "see what works". You're not putting power through it and I don't think you're going to find a signal that needs it.

    You can always compensate with "gain" at the antenna.

    A dipole is a balanced antenna in any form...regardless of impedance a proper solution is to convert from the balanced to unbalanced. Most think a balun is just for impedance transform but it's actually a transformer.
     
  3. c.coyle

    c.coyle Crisis actor Subscriber

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    An indoor, un-folded dipole right? For receiving, and unless your feedline is very long, no balun necessary. EDIT: Hook the center connector to one leg, the braid to the other.

    RG-6 is mighty big and bulky. You can probably get by with RG-59.
     
  4. musichal

    musichal poet emeritus Subscriber

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    Already have the RG6. Yes, indoor, unbalanced, 25 ft run, so short. And yes, I know how to connect it.

    Didn't realize that it technically (perhaps) should have a 1:1 balun, which apparently is not critical for this use, IIUC.
     
  5. I LIKE MUSIC

    I LIKE MUSIC Super Member

    The ferrite RF common mode chokes take the place of the 1 to 1 balun, with less loss of the desired signal.
    Depending on the quality of the balun transformer (1 to1 or 4 to 1) they can have an insertion loss of as much as 2 or 3 dB.

    A balun transformer can be made in any turns ratio (impedance ratio).

    The common TV type balun transformer is, as has be mentioned, 75 Ohms to 300 Ohms, a four to one impedance transformation ratio (simply 300/75 = 4).

    BTW, a 4 to1 impedance mismatch, can cause as much or even more than a 4 dB attenuation (loss/reduction) in the received signal. If one is trying receive a weak signal, this amount of attenuation may make the difference of going from a somewhat noisy signal to a listenable signal.

    The length of the coaxial cable, even if short ( in a 30 inch long piece of coaxial cable the outside of the shield would be quarter wave length for the FM broadcast band and act as an antenna element that can skew the reception pattern of the antenna), can have an impact on the performance of the antenna if common mode RF currents are not dealt with.

    It can skew the reception pattern of the antenna, creating some relatively deep and unpredictable nulls (directions of poor reception). It can allow the outside of the shield of the coaxial cable to act as an antenna and pick up unwanted electrical noise and funnel it to the receiver/tuner.
     
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  6. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    Gee,
    You make my wire hanger foil antennas seem primitive ;)
     
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  7. I LIKE MUSIC

    I LIKE MUSIC Super Member

    There may be a few of the over 400 that have hit on this thread that have an interest in this, so here is some additional RF 101.

    The picture below shows the impact of common mode RF current on the performance of a dipole antenna.

    Because the common mode current is subtracted from the dipole element that the shield of the coaxial cable is connected to, the current in that dipole element is reduced as can be seen in the picture. Note how the signal on the right side dipole element is much less than the signal on the left dipole element.

    Also note the RF current on the outside to the coaxial cable shield, highlighted in yellow. This RF current will interact with the RF current from the dipole elements and it will add and subtract (causing unpredictable peaks and nulls in the signal reception pattern) depending on the phase relationship of the currents.

    upload_2018-7-11_14-13-16.png

    And because the shield of the coaxial antenna is acting as an antenna element, it can pick up unwanted electrical noise (EMI/RFI) and funnel this unwanted noise into the receiver/tuner

    To be clear, for stations of interest that are of moderate to strong RF field strength at the location of the antenna, the above becomes less important.

    But for those that have stations of interest with weaker RF field strength at the antenna, the above may and often does make the difference between a noisy signal and a listenable signal.
     
    Last edited: Jul 11, 2018
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  8. dewdude

    dewdude I fix stuff.

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    Ok. I goofed on the 6:1. I was thinking about it's application in ham radio for QRP work....where it's 6:1 to 50 ohms. That's habit anymore, I forget 75 ohms exists.

    One thing I will point out is a tuner cannot actually differentiate a 3dB difference in signal. Yes, 4dB is a lot...but what that will get you are stations already on the super extreme fringe...and it's questionable if those will decode even with that extra couple of dB. When you're talking amplitude modulation schemes on HF...that's when you can get a little nuts.

    But now that I'm not at work and constantly distracted..I read something on the first post that I felt I needed to address:

    "Better" is subjective...and I'd be willing to bet this person was ultimately trying to sell something. The diameter of your material matters less than it's length. All a bigger diameter does is give you "more bandwidth"...as in you will have a larger section where that antenna matches a specific impedance rating at it's feedpoint. But the reality of this is vastly overblown. Going from the twin-lead of a folded dipole to 3/8" copper tubing isn't really all that much of an improvement as you might think. I ran an antenna that used magnet wire as it's dipole elements; when I moved to an antenna that used 14 AWG wire I saw no noticeable difference in performance. When they talk about larger conductors being "better"...you're usually talking the difference between a simple piece of wire and a piece of metal a couple of inches in diameter. I thought moving from 14AWG cable to 1" tube on my antenna would make a drastic difference...I was then informed, by guys who know this, that I was "a few factors of 10 off". I think one guy told me I wouldn't see a noticeable difference in my antenna bandwidth until it got at least 6" or so. But again...when transmitting I need to be as close to 50 ohms as possible or the radio gets unhappy; with reception, it's not that important.

    Just remember...they call RG-6 a 75 ohm cable...and F-connectors are called 75 ohm...but the reality is that's all just a good approximation and since these are designed for CATV use...impedance mismatch isn't a concern due to the fact you have a very large link budget.

    Compared to your 3/8" tubing...there's no difference between it and whatever is in a folded dipole. I think the main reason people hate folded dipoles is they *never used them correctly* and equate bad performance to bad design as opposed to operator error. The fact is a properly installed folded dipole is going to perform EXTREMELY well on FM. I picked up WRDU 100.7 from Raleigh NC in Lynchburg VA on a cheap SDR using a folded dipole designed for the Japanese FM band going in to a 9:1 balun. It was strong enough I not only had stereo but the RDS identified the station before I even had to listen for a station ID. (https://dewdude.ath.cx/WRDU.mp3)

    The only time diameter is really important is when you're trying to put a LOT of power into that antenna...and you need something that won't get hot enough to burn up.

    The other thing to consider is that FM was originally horizontally polarized, not vertical. Incorrect polarization alone can cause 3dB of loss. Stations are putting out some vertical component to the signal...and that was the result of cars. Vertical may not be the best choice... the majority of the power is in the horizontal polarization.

    Anyway...common mode is a problem with improperly done feedlines...I just think of it as an issue more with transmitting than receiving. The reality is I have a couple of end-fed antennas here that *should* be choked but aren't...I'm not getting zapped when I transmit and the radio is happy. Chokes are...for all intents and purposes...the same as a 1:1. I do have a HF antenna tuner that basically uses a piece of choked coax for it's balun. That being said...you can create a common mode choke by just a few turns of coax. Google "ugly balun" for the details. They'll talk about HF designs...but the same basic principle will work for VHF, you just need less turns. If you look around at stuff for the 2m ham band that'll give you an idea.

    You could also start to look up fresnel zone. It's hard to think a cluster of trees 30 miles away could be affecting your reception...but it's true!


    Keeping the coax at a "right angle" as it comes out of a dipole is..pfffft; it's applying old thinking to a place it doesn't apply. A balanced/twin-lead system NEEDS to be perpendicular from it's elements for at least 1/8 wavelength, otherwise it can cause an unblance and throw the antenna system off. Coax is shielded...so the direction it comes off a dipole...provided it's choked...doesn't matter.
     
  9. musichal

    musichal poet emeritus Subscriber

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    I don't hate folded dipoles - since I want the antenna to be self-supporting and rigid for my application, a vertical simple dipole of copper tubing answers nicely. I already bought the ferrite chokes and will use them. Thank you for your input. :)
     
    Last edited: Jul 11, 2018
  10. I LIKE MUSIC

    I LIKE MUSIC Super Member

    It depends on the amount of the signal and quieting slope of the receiver/tuner. See the picture below.

    upload_2018-7-11_18-57-40.png

    Note how a small increase in RF signal can improve the quieting of the signal a fair amount.

    At the lower RF level the signal to noise ratio (S/N) is only about 42 dB. This would be a somewhat noisy signal. At the higher RF level the signal to noise ratio is 50 dB and that would likely be very close to noise free.

    Of course this is dependent on the quieting slope of the particular tuner/receiver being used.

    This is the calculation for cross polarization signal loss, for any angle of cross polarization.

    Loss (in DB) = 10*(log base 10)(cos(theta))^2

    I am not sure how one comes up with 3 dB loss. That was not the correct answer when I took the test for my first class radiotelephone operator’s license.

    In theory the loss is infinite, in the real world it can approach and exceed 20 dB.

    That said, if one looks at the FCC license data for FM broadcast stations, one will find that a lot of them broadcast the same amount of power in the horizontal and vertical planes. In fact circular polarization is not that uncommon.

    It is not uncommon for the FM signal to change polarization on its trip from the transmitter to the receive antenna, due to reflections and so on, so it is not a bad idea if one is having trouble receiving a stations of interest to change the orientation of the antenna.

    Below is a generic picture that shows what happens to the radiation pattern of a dipole antenna when a conductor (the shield of the feed line coaxial cable for example) is placed in the RF near field.

    Note the distortion of the pattern and the rather deep nulls (directions in which the antenna's reception is reduced).

    Because of the variables involved in each installation it is somewhat difficult to predict the exact RF pattern.

    upload_2018-7-11_21-9-54.png

    This is an example of what can happen if one just drops the coaxial cable next to and parallel with the element the dipole antenna. Doing this may also change the tuned frequency of the dipole antenna. This will happen even if one has taken steps to reduce the unwanted common mode current on the outside of the shield of the antenna.

    BTW, 3 ferrite common mode RF chokes are very likely to be better that the coiled coaxial cable choke across the full FM broadcast band. Just connect it to your RF VNA (vector network analyzer) to see the difference.

    There are now over 500 hits on this thread and to those that are following this thread, one can't just say that all of the signals that those following this thread may have will be powerful enough, that is have enough RF field strength that paying attention to some of the small details will not help.

    Again, this is just basic text book RF 101 stuff...The stuff that can make or break the reception of weaker stations. There is nothing wrong with just kludging together an antenna, but knowing about and paying attention to the small details can remove a lot of uncertainty from the antenna build and in doing so, possibly, if not likely, improve the performance of the antenna.

    And to musichal, just build your copper pipe dipole antenna, use 3 ferrite RF common mode chokes at the antenna feed point and run the coaxial cable at right angles from the dipole for as far as you can, at least a half wave length if you can. This will make your antenna perform about the best that it can.

    If one does this they can be fairly confident that their antenna is working about as well as it can for the amount of RF field strength (radio station signal strength) that is available to the antenna.






















    .
     
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  11. musichal

    musichal poet emeritus Subscriber

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    That is indeed the plan, just as we have worked out via your answers to my questions in this thread. It's K-I-S-S for me - meaning the plan, of course. As for the theory, it makes sense to me as I read, but putting it all together to achieve a level of useful critical thinking... well, I'm content to ask. But I will return to this thread again and again in the future, I am certain.
     

     

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  12. dewdude

    dewdude I fix stuff.

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    Ok..hams sometimes simplify stuff. We generally go by the 3db rule..especially when justifying a 1kW linear over a 1.5 kW linear.

    Same with cross polarization. That was probably on the amateur extra exam at one point. On August 6 2017 I could have probably not only told you... but calculated modulation index of FM. But then you jump on the DX portion of 40m for a few years and you forget this stuff.
     
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  13. I LIKE MUSIC

    I LIKE MUSIC Super Member

    To be clear I mean no disrespect to anyone.

    My posts tend to reflect my slight (well that might not be true) OCDness and to me it is not necessarily a bad idea that the information supplied to those that might want to build a dipole antenna is correct.

    The importance of the small details depends on the amount and quality of the RF signal strength at the antenna and the receiver/tuner being used (quieting slope and so on). For some it may be important and for others not so much.
     
  14. musichal

    musichal poet emeritus Subscriber

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    How much distance should be left between the proximal (center, as opposed to distal) ends of my simple dipole, the feed-point ends?
     
  15. I LIKE MUSIC

    I LIKE MUSIC Super Member

    An inch or two or three would be okay, but...

    Per my OCDness, the length of the center conductor and the shield of the coaxial cable, once separated, become part of the length of the elements of the dipole antenna. :whip::D

    If you made the spacing 6 inches, then you might want to take into account the length of the center conductor and shield (each would be 3 inches or so long) when measuring your copper tubing for the elements because they would no longer be part of the feed line, but part of their respective antenna elements.

    upload_2018-7-13_0-11-39.png
     
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  16. musichal

    musichal poet emeritus Subscriber

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    Hmmm, playing with a dipole calculator I noticed that about a quarter-inch in length changes the (center?) tuning by 1Mhz, and while that probably isn't super-critical for my scenario/implementation, which is far from ideal, I can understand the OCDness. I'm shooting for tuning at 98Mhz, the center of the band. I have seen some confusing (to me) comments about actual lengths calculated from the formula needing slight adjustment. Now my OCDness is showing, too. The results indicate very near 28 and 3/4 inches per element, and that is how long I plan to cut them.

    Looks like I may get to work on this tonight... maybe. Surely tomorrow, if not. I hope. Christine and I are about to set me up an ersatz work table in the garage.
     

     

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  17. amptramp

    amptramp Active Member

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    For antenna tuning purposes, I have used the square root of the product of the end-of-band frequencies.

    SQRT (88 * 108) = 97.488 MHz

    This is the frequency you should be tuning for. This gives you a ratio of 1.1078 from 88 to this frequency and from this frequency to 108.
     
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  18. musichal

    musichal poet emeritus Subscriber

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    So my 98 was pretty darn close. And thanks for the formula.
     
    Last edited: Jul 13, 2018
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  19. I LIKE MUSIC

    I LIKE MUSIC Super Member

    Some additional OCDness general information.

    Below is a software modeling of the dipole antennas in free space. Note this is free space, nothing in the RF near field (especially at the ends of the dipole antenna). Real world antennas will be somewhat different, although not necessarily better.

    upload_2018-7-13_2-40-36.png

    Note how the gain falls off as the frequency of operation moves away from the resonant (tuned) frequency, based on the element lengths, for the dipole antennas. The omni antenna is somewhat different than the basic dipole antenna under discussion here.

    More information about the omni antenna can be found here, for those that might be interested.

    Because of the physics involved, the ends of the dipole antenna have a very high impedance, in theory it would be infinite, but in the real world it is several thousands Ohms. The means that the antenna will be sensitive to detuning as the ends of the antenna are brought within proximity of non-perfect RF dielectric materials, for example other conductors or metal objects and other materials that might not look like conductors at first glance, but do not necessarily have good RF dielectric properties.

    upload_2018-7-13_16-4-6.png


    Is very accurate tuning of the dipole antenna important? Maybe, maybe not, depending on the previously mentioned variables.
     
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  20. musichal

    musichal poet emeritus Subscriber

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    Finally got the "work table" set up in the garage at the proper height for the wheelchair I use out there (I use a walker inside). Something you guys could fashion in half an hour takes me a lot longer, with rest periods about every twenty minutes that last up to twice that and sometimes include a short power nap. I'm not whining, just explaining why it will take me so long to assemble such a simple thing. In fact, I'm enjoying it tremendously. I have a couple other projects to follow this one, too.

    As a night owl, this fits my non-schedule just fine as I get to work while it is cool - I wouldn't last five minutes doing anything in the heat of day. I see the gray through the glass of my front door, 0530 already. This is the peak of cool now, so I'd best get out there and complete one more step.

    Later Edit:
    Whew! I'm tired. Spent about an hour-and-a-half out there this time, but the good news is that I got a whole lot accomplished! If you include a whole lot of absolutely nothing. Spent much of that time looking for my WD-40, which I had just two days ago, and placed it on a shelf beside the D5 with Christine in agreement that this would be their new place.

    However, she'd been out there organizing stuff for me until about 0200 this morning and she organized my spray stuff right out of my ability to locate. Hey, she meant well, and did a lot of things right so I'll just have to ask her about noon when she awakes to go to work - 7.5 more months until she retires, and she's counting it down.

    Why do I need WD-40 to put together a simple antenna? Well, the answer to that is too stupid for me to want to reveal, but it will be smooth sailing once I get it. :D

    Oh, the rest of the time I spent figuring out I need WD-40.
     
    Last edited: Jul 14, 2018
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