Help with baluns for non-dipole/non-beam antennas

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K3RW, Sep 26, 2015.

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  1. K3RW

    K3RW Ham Member QRZ Page

    I'm new to homebrewing antennas, and I can't find a definitive answer on this one. I did 'Google' it first and buzz through the forums but still don't know. Building a dipole and an end-fed was easy.

    I want to build a loop antenna--the design says to use a 1:1 balun. Ok. Now, which one, and how?

    All the baluns I've seen are either for dipoles or beams. Instead of using the wrong terms, I'll just say the dipole balun I'm looking at has a plug for the 'big coax' going into the base of it, and two posts coming out the sides of the top. In a traditional dipole, that's where both sides connect. Ok.

    But for a loop, or any non-dipole is that the right one to use? And how would I use it? Make the whole loop and connect it to only ONE of the posts? Make the whole loop connecting both posts like a big circle? Get a different balun that only has one post? I just can't figure out how to hook the smaller wire to the balun for a loop antenna.

    Some people have stripped the coax, built a torroid thing, wrapped some stuff, and made it work. I'd rather buy something then trust my skills on that.

    I'm also interested in building a single spiral antenna. Same issue here. Feeding in the 'big coax' to a balun to match to a much smaller feed line, but not sure how to hook up what would be a single wire. I'm guessing the answer is the same.

    Unfortunately all the antenna plans I saw just say use a 1:1 balun and don't specify a model or show the connection close enough to tell how or which one they did.
     
  2. K8JD

    K8JD Ham Member QRZ Page

    BalUn for a Loop antenna?
    The loops I made in the past were one wavelength on 40M and I used a direct soldered connection from the loop to a quarterwave of 75 Ohm coax and spliced to my 50 Ohm line to the shack. No BalUn.
    The last rebuild of my 40M loop used a MFJ 1:1 BalUn. So I had PL259 connector on the end of the feedline and the ends of my Q-section of RG59. Spliced with a "Barrel connector".
    The main reason for using the MFJ BalUn was the nice coax connector on the bottom and the hanging eyebolt on the top, that I used to rope hoist the feed line and loop wires up onto a high tree branch, for support also.
    I don't really know if the balun had any effect on how things worked electrically, since I never had problems before without a BalUn
     
  3. K8JD

    K8JD Ham Member QRZ Page

    What is the "spiral" antenna ? Do you mean a helically wound loaded vertical ?
    A BalUn is not for splicing large to small coax, if that is what you are refering to,
    it's for connecting an unbalanced feed line to a balanced antenna.

    Building and experimanting with antennas is a great part of this hobby, but you need to do a bit of research in the many fine books that are available about antennas and how they work.
    The internet is a place to look but there is too much misinfomation and opinion with no proof floating around.

     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    A loop antenna can be a full-wave or half-wave or something else; you didn't describe your loop.

    Usually on HF, it's a full-wavelength for the lowest frequency band you intend to use and the formula is 1005/f where f is frequency in MHz and the result is in feet. So, an 80m loop is about 270 feet in circumference or perimeter. You 'feed' both sides of the loop and connect the two wires where they come together to the two terminals of the balun, one wire to each terminal. It's a 'complete circle' with a very small break at one point, just a few inches, where the feedline attaches.

    A FW loop 1/2 wavelength above ground has a feedpoint impedance around 100 Ohms, so it's not a bad match for 70 Ohm coax or even 50 Ohm coax.

    I've fed many full wave loops with no balun at all and didn't have any problems I could find, but a balun couldn't hurt.

    If the loop is some random non-resonant length, it is often best to use balanced line like twin lead or window line instead of coax, and just use a good balanced tuner at the station end to tune it. Those can work fine if they're up high and big enough for the frequency you're using. Nothing magical about making it a resonant length, other than for matching purposes.
     
  5. K3RW

    K3RW Ham Member QRZ Page

    The loop I would make would be a 40m one. Design for the 40m one is 16 total feet of wire, so I'm not exactly sure of its wavelength, since it claims it would be usable on 20 and 10 as well.

    http://www.qsl.net/g4fon/MagLoop.htm

    The other antenna I saw is this one--the top of the page is for the 80m one. Further down the page is the 40m one--that's the one I'd build.

    http://www.nonstopsystems.com/radio/frank_radio_antenna_SM0VPO.htm

    Now, I've been 'told' that a 1:1 balun is what I should use, given the ohmic difference between the 50 ohm coax and the unknown value of the 16AWG. Sound right?

    And I was also told, no, I don't need to put on these these capacitors/trimmers/etc. since I am using an all-band antenna tuner.
     
  6. N7EKU

    N7EKU Ham Member QRZ Page

    Hi Gregory,

    The links you listed are for "small transmitting loop" antennas.

    Being a small loop (in realtion to the wavelength of use) means it must be tuned (that's what the trimmer capacitors are for) and has a very narrow bandwidth (meaning if you change frequency just a little bit, you will need to re-tune the antenna again).

    The first link design is only for very low power use because it does not use high voltage capacitors. See this link: http://www.66pacific.com/calculators/small_tx_loop_calc.aspx for checking the design. On 40M it only comes out as being 1% efficient (ugh).

    The second linked antenna first used small air variable capacitors for antenna use up to 10W. The designer had to buy vacuum capacitors for higher power use. He got a great price, nowadays those cost about $200 or more. But the design is most likely inefficient also. Mostly because the wire diameter is so small. Normally these are made with 1/2" copper tubing or bigger.

    For receiving only, any type of capacitor will work fine, and lots of people enjoy these types of antennas.

    These do not necessarily need a balun. Neither, in that case, do tuned dipole antennas necessarily need them. A great, and free, resource is the ARRL antenna book. If you google for the title you should find an online archived copy of a version. It is old, but the basics have not changed since then.

    73,


    Mark.
     
  7. K3RW

    K3RW Ham Member QRZ Page

    Wow--thanks for the excellent analysis. Jeez, if those antennas are that inefficient, it is truly a waste of time to even build one.

    Random question--but MFJ makes a portable loop (about $40) and a Super High-Q loop (about $470). The super high-q one is either for 120 or 220VAC. Functionally, what is the difference between the el cheapo portable MFJ one, and this super expensive one? And why would the VAC matter? Is it 'energized' like an electromagnet?

    I'm def picking up that book. I needed the reality check on that. Thanks for sparing me a lot of frustration!
     
  8. KB1CKT

    KB1CKT Ham Member QRZ Page

    Small transmitting loops can be very lossy. Or not so bad. Depends upon the construction. AA5TB has an Excel sheet where one can put in a few inputs to calculate loss.

    However... all antennas work, to some degree. Last night for fun i tuned 80m on my 6BTV. At best it is 25% efficient on 80m. I figure mine is no more than 17%, based on input impedance measurements (ground plane too little). Yet I got a 569 on a path from NH to IN.
     
  9. K3RW

    K3RW Ham Member QRZ Page

    I wanted to build a log periodic--homebrew style, and small. I saw plans for one that fit on a picnic table. Not realistic though.

    I just need something a bit more directional than the dipole I've got stuck facing SW-ish. I work mainly Asia from here and I could move the antenna I suppose. Ideally I'd have a yagi, but I have a wife too. LOL.
     
  10. KB1CKT

    KB1CKT Ham Member QRZ Page

    I came across this small 20m 2 element beam the other day. No idea on losses, but it seems small enough to erect and try out some time.
     

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