Electrically reversable 3 element Yagi Uda antenna?

Discussion in 'General Technical Questions and Answers' started by Pushraft, Nov 26, 2008.

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  1. Pushraft

    Pushraft Banned

    I was wondering if it is possible to design a short boom Yagi Uda beam antenna (such as a 3 element) so that the director and reflector can be electrically reversed from the shack. That is, someone could basically "flip" the antenna around 180 degrees just by flipping some switch in the shack which would somehow reverse the non-driven elements functions from director to reflector and vice versa.

    Of course this is a compromise solution as all 360 degrees wont be covered as well as with a rotor, but for people that dont want those but want better coverage, maybe this idea has some merit. You could basically pre-aim the beam in your favored general direction and you would get similar coverage in the opposite general direction at the flip of a switch. The other directions would be compromized but still usable.

    I've seen beams with equal element spacing so it seems the only "tricky" part would be making the director and reflector so their roles can easily be reversed. I would assume both of them could be cut to the proper director length with loading coils to make the reflector length and of course those loading coils would have some type of on/off switch.

    Maybe someone else has already come up with this design but I was just wondering about it. I didn't research this it just popped into my head.

    Maybe the 3 element could be designed with the widest E plane coverage such as more than 100 degrees, so that when electically "flipped", it would give you another 100+ degrees coverage all with gain. It seems like this setup would beat an omni antenna and not require a rotor. It may be good for people with close neighbors so that they could position the 3 element beam so it doesn't overhang their neighbors property but they still would get decent coverage off the side of their beam and more than decent coverage in the 2 favored general directions.

    Since the reflector and director are close to the driven element on many Yagi-Uda designs, maybe this setup can be made to work and will keep the boom short like under 1/3 WL long so for example, at 20m, the boom would only be about 20 ft long which is reasonable.
     
  2. K7MH

    K7MH Ham Member QRZ Page

    I think the SteppIR beams can basically do that. EX-PEN-SIVE!
    I'll stick with a rotor. Much cheaper.
    Some contest stations just have several big Yagi's that are left on different areas and switch between them. Maybe one on JA one on Europe, one on Africa, etc.
    On CB they used to have the Antenna Specialists Super Scanner that was a vertical beam that could get switched between three broad directions.
     
    Last edited: Nov 26, 2008
  3. Pushraft

    Pushraft Banned

    I remember that CB one and heard someone use it on the air one time. I was thinking that since HF beams are so large, it would be kinda cool to get a few dB gain in multiple directions without actually having to turn the beam. Someone could leave it aligned so it is over their property and even over their roof and just use the special reversing property so they wouldn't have to work people off of the back of their beam.

    Something like this might be good for say a 40m beam since a 3 element boom would be maybe 40-50 feet long. It might be easier not to rotate that physically but rather electrically.

    I must say that HF certainly presents a lot of challenges for antennas much unlike VHF and UHF.
     
  4. KE7VUX

    KE7VUX Ham Member QRZ Page

    I've been studying this subject a bit, too.

    There's at least one article floating around about a 2 element "wire yagi" in an attic.

    Both elements were cut to the same length, and the driven element is electrically switched from one side to the other.

    My crude understanding at the moment is it makes the antenna less efficient than a properly cut / designed 2 element yagi, but it does exhibit gain over a dipole AND front-to-back.

    ..and you can switch directions without a rotor, or even without the ability to rotate the antenna at all (like an attic mounted HF antenna - even if you wanted to buy a rotator, most houses are longer than they are wide, so you might not having the turning radius - plus all of those trusses in the way)

    This is different from the *phased array* concept where they'd all be driven and you'd want to make sure the two elements are "x" wavelengths apart (I think 1/4 or 1/2 WL are common, though it might be possible to do a narrower spacing at greater cost/complexity to get the phasing right)


    Now, back to your 3 element beam..

    There's another article out there about a 40M cubicle quad that's switchable in direction.

    When you're getting into arrays of 40M loops (or 80/160) just building the thing is trouble enough - turning it gets difficult.

    The article I'm thinking of is a 3 or 4 element square-loop "beam" much like a yagi, but in 3 dimensions, not 2.

    One driven element, two directors, one reflector.

    ddDr

    Which could be switched to be

    rDdd

    Basic explanation is that all 4 loops were made the same size - and made "under size" like a director.

    When a loop needed to become a reflector (typically longer) a coax loading stub was added to make it electrically longer. For a director the two ends were just connected together into a single "solid" loop.

    It's an appealing design to me, though it is complicated and there's a lot to work out on getting the spacing right (so the phasing is advantageous) and the right loading under the right circumstances to get the correct electrical lengths in place, and so on.

    And, as you've noted, when you're done, you have two directions, and no chance to really fine tune it, so the poor stations just off to the side of your beam will always be just off to the side.
     
  5. KA0GKT

    KA0GKT Ham Member QRZ Page

    Yes.


    It has been done for decades. There is an interesting project in the ARRL antenna book for just such an antenna.
     
  6. KC0UUT

    KC0UUT XML Subscriber QRZ Page

    Bi-directional yagi

    You could just build the yagi with 3 equal elements, all set to the length of the driven element. You will still get gain above a dipole, but not as much as a typical yagi.


    Here are the results of some NEC antenna modeling I did for a 20 meter 3 element yagi with 10 foot spacing between elements located 25 feet above ground:

    Case 1: 3 element yagi with director and reflector length set at .95 and 1.05 respectively times the length of the driven element. Gain: 9.7 DBi at 30 degree take-off angle relative to the horizon.

    Case 2: Same as above, but all elements set at length of driven element. Gain: 7.0 DBi at 25 degree angle

    Case 3: Dipole (Removed director and reflector, still 25 feet above ground) Gain: 5.4 DBi at 35 degrees

    Hope this helps & have a happy Thanksgiving.

    John
     
  7. Pushraft

    Pushraft Banned

    My idea was to design a 3 element Yagi-Uda antenna such that it has the widest possible azimuthal coverage (let's say gain over a dipole antenna for 120 degrees). The design would also allow the favored direction to be switched in the totally opposite direction. That would give you 240 degrees of gain coverage. For the remaining 120 degrees, there may be performance slightly worse than a dipole but you might have a 2/3rds chance of improved performance.

    For smaller antennas such as 6m and 10m it might be fun to try in an attic situation for the following reasons:

    1) a HF beam cannot be rotated up there.
    2) attic losses can be partially compensated for by extra gain.
    3) close spacing of ref, DE, & dir should fit for 10m and 6m bands.

    For 20m I cannot do this in my attic as there is barely enough room for my 20m dipole but for people that want some directivity without having to rotate a large antenna, this solution might work.

    I would be worried if I lived up north that ice would form on the rotor and cause problems. If you could electrically "steer" the antenna then you could bolt that bad boy in a fixed direction most acceptable by the neighbors and then work maybe 2/3rds of the directions with gain over a dipole and 1/3rd of the directions with slight loss.

    I think 120 degree beamwidth antennas do exist. I came across this link.

    http://www.patentstorm.us/patents/5629713/claims.html
     
  8. NA0AA

    NA0AA Ham Member QRZ Page

    See page 6-1 of the book MORE WIRE ANTENNA CLASSICS for a design for just the item Pushraft wants. It's for 80 Meters, but you know, that's 7.2727272727272727 wavelengths at 11 meters so it probably scales OK if you needed to [Just kidding, it's easily scaled if you want to to any band].

    It's complete with the equipment needed to switch directions.

    Enjoy.
     
  9. KA0GKT

    KA0GKT Ham Member QRZ Page

    A little more info...

    Here's the basics.

    1) Adding inductance (a coil) to an antenna generally makes the element electrically longer.

    2) Adding capacitance (a capacitor) to an antenna generally makes it electrically shorter.

    3) shorthand for Inductive reactance is X sub L, for Capacitance X sub C

    If you have a piece of transmission line shorter than a quarter weavelength and that transmission line is shorted at one end, the impedance across the other end will be capacative.

    If you take that same piece of transmission line and remove the short, the impedance will look inductive.

    By juggling the length of your director/reflectors you can make a three element Yagi which will switch front and back simply by removing the short from one piece of transmission line thereby making the element a reflector and shorting the other thereby making that element a director.

    Is the system as good as a three-element Yagi specifically designed to be mostly unidirectional? No, of course not. Does it provide gain in one direction or the other 180-degrees away? certainly.

    Most times these types of antennas are made of wire and are used on the lower HF bands. I knew one man who used to have a pair of such wire Yagis cut for 160 meters out on his farm. I believe his widow took down the antennas at the estate sale a couple of decades ago.
     
  10. K7MH

    K7MH Ham Member QRZ Page

    The idea is more useful in larger fixed wire beams for 40 or 80. A 3 element 6 meter beam should easily fit in even a small attic. My house is pretty small but I could turn a 4 element 6 meter beam in the attic, maybe larger. Glad I do not have to though!!:)
     
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