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5/8 wave coil

Discussion in 'CB Antennas' started by Simon004, Aug 16, 2009.

  1. Simon004

    Simon004 Member

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    This is my first post here so hi to everyone,

    There are a some familiar names here I recognise from other forums I use and no doubt some of you may know me....

    anyway,



    I have a 1/2 wave silver rod that I removed the matching coil from years ago to use on 20M as a 1/4 wave with radials added, I play around with antenna's quite a lot and decided it would be more usefull to me now as an 11M antenna again, as I want to do something like in THIS THREAD

    What I also want to do is to turn this into a 5/8 wave by changing the pole at the base of the antenna for one about 3 feet longer than the origional. I could just add a new coil and play around changing where the tap is, but the problem I have is I can't put ANY antenna's up at home :thumbdown: , so any experementing has to be done away from the house, usually on top of a big hill somewhere. I'm going to be using 16 guage enammeled wire for the coil, so my question is what size coil would you start with, ie. how many turns on what diameter former, and how many turns in would you put the tap onto the coil?

    I've also heard about a "magic length" of antenna being 0.64 wavelengths long, a 5/8 wave is 0.625 wavelengths long, so I find it hard to believe that this small difference in size will make much diference at all, but as I am extending the antenna anyway, I could go for either length of element, if one really is superior to the other. I don't operate from home, only portable, so this antenna will be mounted around 18 foot above ground (the size of my portable mast).

    I realise that I could go and buy a 5/8 wave antenna pretty cheaply and just add ground planes, but I want to use what I already have, a) because I'm a cheapskate and b) because the more things like this I try the more I learn about antenna's.

    I look forward to your replies....

    73

    Simon 26OD004
    #1
  2. HiDef

    HiDef Active Member

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    You see a lot of ideas on the www about antennas but the brakes get slammed on when you ask exactly how to build. That's why I like this forum. A lot of hands on going on here.

    No way .64 verses .625 will amount to a hill of beans in this crazy world except in some people's minds. I'd worry more about what happens when you QSY higher and that high angle lobe starts taking low angle energy away because the stick is too long.

    A guess for a coil made of #16 would be to wind 10 or 12 turns 2-1/2" long on something the size of a D-104 stand diameter. As you probably already know, two things are happening here. The cold end of the coil gets the coax shield and hopefully resonant radials. Center of the coax feed goes up anywhere from maybe 3/4 of a turn to 2 or so turns and provides the impedance matching ratio. The rest of the coil is there to resonate (provide a conjugate match to) the stick. You have to experiment. The coax tap will be very tricky and 1/8 turn difference in tap point will change things. The two taps will also interact. You want a coil that will not change it's shape. Consider a solid permanent coil form. Do not let unused turns at the coil's top hang in the breeze or you will see fireworks at QRO. Progressively short from the coil top down towards the cold end to establish your tap.

    My guess is only a guess from something I saw over 30 years ago. It might be off. It also isn't the optimum coil form factor (length to diameter ratio) if you land up using the whole thing.

    I'm pretty sure someone else here has the exact numbers. Maybe he's on vacation.
    #2
  3. W5LZ

    W5LZ Crotchety Old Bastard

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    If you're looking for exact number, I'm afraid you're out of luck. :)
    Lot's of variables in that situation, so 'close' maybe, but exact, forget it.
    I think the example given is close enough that with some adjusting here-n-there, it ought'a work just dandy. There's always more than one way to do something, a little bit of difference typically makes no difference at all. (The only -sure- exception is with females and nuclear fission. I would respectfully suggest do that right the first time.)
    - 'Doc

    And just 'because', the shape of a typical matching coil is better if it's "square". Meaning as long as it's diameter, 2" across X 2" tall. that's not an absolute thingy either, but in general works out pretty good.

    And NO, I don't mean you should make the thing into a rectangle, it's still round.
    #3
  4. C2

    C2 Well-Known Member

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    I tried a 5/8 with a tapped coil and it did not work as well. It tuned up fine, but was not as sensitive as a dipole.

    I redid the match to a simple series inductor and it worked much better than the dipole.
    #4
  5. Simon004

    Simon004 Member

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    Thats a very good point and one I'd not really considered, if I go for a .64 wave, purely out of habbit I would have calculated the length of the pole for the centre frequency of the bandwidth I usually operate in. Its very rare that I will QSY more than 100KHz away from the design fequency anyway. Now I think I will calculate a .64 pole for the highest frequency I'm ever likely to use, but still tune the coil for the centre of the band, so I'll end up somewhere beween a 5/8 and a .64 in reality.

    I wasn't expecting someone to tell me an exact figure, was just after a ball park figure from where I can start experimenting. Even if a few people replied and wildly dissagreed with each other at least it would give me a couple of idea I could try out and see which one worked best in my aplication.

    I have no radio set up at home, only in the car, if I had a setup at home I could probably get away with putting up the antenna for a few minutes for a quick test, but as it is, I only really get out to play radio for a few hours at the weekend, so any experimenting cuts into my playing radio time.

    Thanks for the replies guys,

    After I made my first post I had a look through the forum and found a thread where someone had made a .64 wave and said they used around 8 turns on a 1.5" former for the matching coil, which wouldn't be a bad place for me to start, that'll teach me for posting first and searching second, lol
    #5
  6. Simon004

    Simon004 Member

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    There's some food for thought, as its not DC grounded though, would that not be asking for problems with static build up?
    #6
  7. sp5it

    sp5it Master of puppets

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  8. W5LZ

    W5LZ Crotchety Old Bastard

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    Simon004,
    "...as its not DC grounded though, would that not be asking for problems with static build up?"
    Not necessarily. Sure, there can be more 'static' on the antenna, but who sayz it's going to be all that objectionable, or a problem? There are gobs of ungrounded antennas out there that people don't seem to be having much problem with. A 'static problem' isn't a 'sure' thing, neither is a DC grounded antenna a 'sure' cure. Nice to have, but not something to waste a lot of good worry on for nothing.
    - 'Doc
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  9. Simon004

    Simon004 Member

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    thanks Doc,

    reason I asked that is because any vertical I have disected uses the "tapped coil" method, and also I've noticed the specs on antenna's seem to make a point of mentioning that they are DC grounded to prevent static buildup. I've even seen claims by people on other forums that a build up of static on your antenna can even attract lightning :eek: If this is true or not I don't know, but I cant really think of any reason why it would be.

    I've never had a problem with static (that I know of) so I'll go for the series inductor method, it will make things easyer too, rather than having to remove the enamel from the wire to relocate the tap I can just start with a long coil and shorten the length until i get a good match. I'll be using this antenna for /p anyway so it won't be up long enough for static to be an issue.

    Simon
    #9
  10. C2

    C2 Well-Known Member

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    After some discussion in another thread (in the ham antenna forum), I realized what was more likely happening with the simpler base load on my supposed 5/8 wave antenna, even though it seemed to work well...

    I figured out it was a base loaded 3/4 wave antenna!

    I'm not sure what freq your antenna is designed for, but if I calculate the length of a 3/4 wavelength wire for 27 MHz, I get 26 feet.

    If I take your 22.8 length and add the length of the coil you made (9 turns around 1.5 inch diameter = 42"), I get 26.33 feet...looks like a 3/4 wave antenna, but someone will point out that is not really true either.

    HiDef also mentioned the the concept of their being an optimum length-to-diameter ratio for the coil, which I believe is also important. W8JI also mentioned this in one of his articles.

    Inductors

    Lots of other good reading there too...

    Anyway, in my case I'm going to rebuild the classic feed as it was, maybe changing the materials a bit.
    #10
  11. W5LZ

    W5LZ Crotchety Old Bastard

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    C2,
    I think the gist of what you said is correct, but I also think that one or two of the 'steps' along the way are not correct.
    Yes, a loading coil does change an antenna's effective 'electrical' length. The amount of inductive reactance furnished by that loading coil and the length of the radiator above it (and/or below it) will determine what that effective 'electrical' length will be.
    No, the length of the conductor that makes up that loading coil has no direct relation to the resulting effective 'electrical' length. You can't just pull that coil out straight and add it's length to the length of the rest of the antenna and come up with the answer to what that 'electrical' length of antenna is. I'm sorry, but it just doesn't work that way.
    That's also why there can be "an optimum length-to-diameter ratio for the coil", which means producing the 'right' amount of inductive reactance in the most efficient way.
    In general, it means that 'short + fat' coils with adequate turn spacings are 'better' than 'tall + skinny' coils with adequate turn spacings. And like all generalizations, there will be particular circumstances where that is not true because of some extenuating 'other' things involved in the whole thing. (And just like you predicted, somebody pointed that out as not being 'right'. But hey, everybody get's 'lucky' now and then! But then, who say'z it's really a 3/4w length equivalent?? :) I wish that were true, it'd make things soooo much easier.)
    What makes all this so much fun is that while what you originally set out to do may have failed, you find out so many other useful thingys that it makes the whole thing worth while. So, 'mistakes' are to be expected. They should be counted as a learning experience and remembered (so you don't do the same thing again, or quite so often).
    - 'Doc

    (That also explains the knots you see on antenna people's heads. That muttering you hear isn't formulas being raced through to check their accuracy, it's the guy saying, "I know better than that. Why'd I do that? Aw 'spit'!...did it again!", and so on. While they are banging their head against a wall.)
    Who, me? I ain't never!
    #11
  12. Simon004

    Simon004 Member

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    thanks for that bud, it gives me a good starting point. At the moment I have the main element I'm going to use, which is a shade under 22 feet in length, rather than change the bottom part of the half wave silver rod for a longer piece, I changed the top piece for half of the reflector from a 10M yagi that I've had sitting in my shed for a few years now, which gives me the length I need. I've used a piece of pine dowel to isolate the main element of the antenna from ground, and a 1.5 inch plastic tube (from one of my kids toys that they don't use) to wind the coil onto. The coil is at the moment about 15 turns, I found a piece of 16 guage antenna wire and wound it all on, with the intention of trimming it down untill I get a good match.

    For the ground plane I'm using some more of the 10M yagi aluminium tubing, which is 4 foot in length, but I am also adding another 5 foot of copper wire and attaching it to the next radial like you did on your half wave vertical. I'm using 4 radials so the angle for the wire won't be as accute as it would be only using 3 ground plane legs.

    I must admit I am a little confused now from the last post, allthough I understand that the matching coil on a 5/8th wave antenna does not replace the missing element length between a 5/8th and a 3/4 wave, the added inductance does simulate the antenna to give the same impedance match as a 3/4 wave would do.

    So if you have a 5/8th wave element with a base loaded coil (no tap), it then becomes a 3/4 wave base loaded antenna, but if you have a 5/8th wave element with a tapped coil it is a 5/8th wave antenna? It might just be me, but that is how I read C2's post.

    #12
  13. C2

    C2 Well-Known Member

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    Well, I had pretty good odds.

    And mistakes are often the cause of great discoveries...
    #13
  14. C2

    C2 Well-Known Member

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    The point I was really trying to make is that it is a coincidence...

    From what I've read (and interpreted) is that the matching device just transforms the impedance of the radiating element, whatever length that is...

    As we unravel this onion, I continue to seek the real truths...namely, why are we even interested in the 5/8 wave design? Because it has some magical property?
    #14
  15. W5LZ

    W5LZ Crotchety Old Bastard

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    What's the 'magic' about a 5/8w antenna?
    It relates to how antennas radiate, the shape of that radiation pattern. If you start with a very short antenna in terms of wave length, the radiation pattern tends to wards being a 'ball'. The longer that antenna gets to be the 'larger' that ball begins to be, and it starts to sort of 'flatten' out a bit. Think of a balloon and pushing the top down towards the bottom sitting on a flat surface. The more you push down, the larger around the balloon gets. That 'hole' you've been pushing your finger down into gets 'wider' at the top and narrows at the tip of your finger. (And that's about as far as that analogy will go without some heavy duty imagination, which I'm not capable of describing all that well, so I won't even try.) Figure on that 'ball' turning into a sourt of 'saucer' shape at some point. That point is very close to about a 5/8 wave length. From there, when the antenna gets longer the 'flatness' of it's radiation pattern's shape starts getting less 'flat', gradually changing to some odd shape that's about the equivalent of that 'ball' again, as far as lower radiation angles go. that's a really crapy way of trying to describe it, sorry 'bout that. The point being that that 'change over point' is somewhere around a 5/8 wave length. that change over point is NOT exact by any means. There are just too many other factors that can affect a radiation pattern (it's environment, for instance).
    If you want to consider 5/8w (.625w) that 'point', or 0.640w that point, suit yourself. There just won't be that much difference.
    - 'Doc
    #15


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