Building my own antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE5UFH, Nov 10, 2009.

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  1. KE5UFH

    KE5UFH Ham Member QRZ Page

    I am a relatively new Ham, having got my ticket in 2008. I jumped head first into the hobby, but lost some steam after having to move to PA. I have now redeveloped my interest and am currently studying for the General license exam. I just purchased my first HF rig (FT-857D) along with the LG YT-100 auto tuner. I enjoy listening very much and currently have a 10 meter dipole up at about 20', and I receive quite a bit (at least I think so) on the 80 and 40 meter bands. I intend to build my own antennas, and have also purchased an antenna analyzer to ease the process. Dipoles are my focus, as it is what my current QTH is best suited for. I know I have enough room between my chimney and a large tree in my back yard, to stretch a 40m dipole out (appx. 69') at about 35' AGL. I have already set up a system of pulleys to raise and lower the antenna as I need to. I am reading a lot about different dipole antennas, but think I have read so much that I have confused myself. If someone can explain or direct me to good info on the easiest way to get on 80m with the room that I have. The normal dipole is what best suits my needs as I do not have the ability to hang an inverted V. Thanks in advance!!
     
  2. K7NNO

    K7NNO Moderator QRZ Page

    The first thing I would suggest is to get an antenna handbook, either from the ARRL or Orr, W6SAI. Reading bits and pieces off the internet will confuse you... you need a solid foundation of antenna theory in order to logically make decisions on antennas.

    Second, take any balun you have in your shack and throw it out the window.

    Good luck.
     
  3. AE5JU

    AE5JU Ham Member QRZ Page

    Last edited: Nov 10, 2009
  4. G3TXQ

    G3TXQ Ham Member QRZ Page

    If you follow WA7OET's good advice to get the ARRL Antenna Handbook, I suggest you read carefully the section on "Unbalanced coax feeding a balanced dipole" before you follow his second piece of advice to throw baluns out of the window!

    73,
    Steve G3TXQ
     
  5. AI3V

    AI3V Ham Member QRZ Page

    If all you have room for is a 40m dipole (66 ft of wire), then there are 3 basic ways to get on 80m

    1-Use the 40m dipole with a tuner.
    2-Add "loading"coils to the 40m dipole.
    3-Make a "folded dipole".Forgetabout it, this won't work.:)

    Pros and cons......

    1- if the tuner is at the transmitter end of the feedline, you will have a very high VSWR on the transmission line, and loose power.Antenna can be used on frequency's other than it's designed for with little change in efficency (I.E.will have some line loss due to VSWR).

    2-The loading coil will improve the VSWR, and minimize feedline losses.Antenna will be somewhat narrow bandwidth on 80m.Antenna can be used on other bands(somewhat dependent on location of loading coils).However, operating on other bands may have high VSWR/feedline loss.

    3-Good match, therefore low VSWR and feedline losses.However, the folded dipole will not work on frequency's other that it is designed for. A 80m folded dipole is pretty much only good on 80m.And will of course be too long to fit the space availible.:0

    Rege
     
    Last edited: Nov 10, 2009
  6. G3TXQ

    G3TXQ Ham Member QRZ Page

    Rege,

    I understand 1) and 2), but how do you get an 80m folded dipole into 66ft? Perhaps your meaning of "folded dipole" is different from textbook definition?

    Steve G3TXQ
     
  7. AI3V

    AI3V Ham Member QRZ Page

    Just fold it a couple times, of course!:eek:

    Shouldn't post in a hurry, you of course need more than 66 ft for a folded 80m dipole.

    For the record, a folded dipole is the same length as a regular 1/2 wave dipole, you just get a higher impedance.

    Rege
     
  8. KJ4IIF

    KJ4IIF Ham Member QRZ Page

    Dipoles are your main interest, and they work for what they are. Great for local rag chews and nets.

    An all band doublet fed with ladderline and a 4:1 balun will work for the lowest band your antenna length will allow and up through six meters. You did mention you had a tuner.

    If DX is also in your interest I suggest a vertical, the low angle of radiation is fantastic for working the dx.

    Many verticals available on the market and a 40 meter 1/4 wave( 75 meter for that matter) with a good ground system is definately home brewable and lots of fun.

    With an all band doublet and a vertical you can have the best of both, I use the vertical for transmitting for the lower angle of radiation and the wire for recieve as it is less noise.

    Which ever way you decide to go you will enjoy the HF bands.
     
    Last edited: Nov 11, 2009
  9. K7NNO

    K7NNO Moderator QRZ Page

    You know I was making a joke, right mate?
     
  10. W4PG

    W4PG Super Moderator Lifetime Member 279 Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    A dipole is one of the simplest antennas you can make. The length is 468/(freq in Mhz). So, for an 80 meter dipole for 3.8Mhz, the length would be 468/3.8 or about 123.16 feet. You want to actually cut it a bit longer (several inches) so you can tune it up to the frequency you want.

    Feed it with 50 ohm coax. You can actually feed the coax directly to the wire and it will work fine . . . center conductor to one side and braid to the other. However, it's better to use a balun made for dipoles at the feed point, like a W2AU balun. It helps keep RF from getting back into your shack among other things.

    The 2:1 SWR limits on 80 will be very narrow, however. While you can use a tuner, the losses in the coax with a high SWR makes it not worthwhile. If you want to use the antenna on other bands with a tuner, you'll need a different feedline, like open wire ladder line or twin-lead. This opens a whole 'nother can of worms.

    Same goes for 40, though you'll find you can work much more of the band between the 2:1 SWR limits.

    Have fun!

    ............Bob
     
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