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Can I use a 40 meter dipole on 20 and 15 meters?

Discussion in 'General Technical Questions and Answers' started by W6IRE, Apr 29, 2009.

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  1. W6IRE

    W6IRE Guest

    I'm looking at the Force 12 "Sigma 40" 40 meter vertical diplole...

    Presumably, this antenna is a half wavelength at 40 meters, so with an antenna tuner (or not) would I be able to use it on 20 meters? Also, since 15 meters is a third harmonic, could I use this antenna on 15 meters?

    Also, could I listen (not transmit) to 75 meters?

    Many thanks in advance!

    73...............Richard
     
  2. K9STH

    K9STH Ham Member

    IRE:

    The antenna should work fine on 15 meters as a 3/2 wave antenna. However, on 20 meters or 10 meters the antenna is going to present a very high impedance to a coaxial cable feedline. You might be able to use an antenna tuner to "match" your transmitter to whatever impedance the feedline presents at the transmitter end. However, the line losses in the coaxial cable will definitely be considerably higher than when operated on 40 or 15 meters.

    As for receiving on 80 meters, it should work fairly well. But, to transmit using the antenna you will definitely need to use an antenna tuner and the performance may be anything from OK to really dismal. You just will have to try to load the antenna with your tuner/transmitter arrangement and see what happens.

    Glen, K9STH
     
  3. WW3QB

    WW3QB Ham Member

    I've tried a horizontal wire 40m dipole. I could tune it with a tuner, but it was worse than dismal on 80m and 20m. Very good on 15m, and great on 40m (or it wouldn't be a 40m dipole). It was also pretty good with a tuner on 30m and fair on 17m. Not bad for a single wire antenna. The math should be the same with a vertical dipole.

    Edit: Since this vertical dipole is only 24 feet tall, I'm not so sure how it will perform on other bands (or even 40m). That is shortened.

    Edit #2: $749.00?!
     
    Last edited: Apr 29, 2009
  4. W6IRE

    W6IRE Guest

    Thanks guys! Yeah, that price is a bit high, reviews at eHam say guys are paying about $488

    Thanks again, guys!
     
  5. WA9CWX

    WA9CWX Ham Member

    people are paying nearly $500 for a 40 meter vertical dipole ?????
    I have a pair of phased 40 meter verticals, with 60 radials, and a 'perfect' :) matching system the whole shebang must have cost me about $300 including some nice aluminum and heavy duty mounting system. :confused:
     
  6. AI3V

    AI3V Ham Member

    Yikes, Thats a lotta $$$ ! :eek:

    You can homebrew a 40M vertical for WAY less $$$

    That sigma40 will work "OK" anywhere above 40M- The issue will be a high VSWR causing extra loss in the coax. The antenna is already "short" on 40M,but not too bad, 80 and 160 will be a exercise in frustration.


    Rege
     
  7. KJ3N

    KJ3N Ham Member

    Funny, sad, and amazing..... all at the same time.

    P. T. Barnum reigns supreme!

    For 1/10th of the cost, an ARRL Antenna Book (and some of your time) is a wise investment.
     
    Last edited: Apr 29, 2009
  8. KA4DPO

    KA4DPO Ham Member

    Unless you just like weird antennas, you can get a Butternut HF6V for a little less money and it's only 26 feet tall (2 feet higher than the Sigma). It will work 80 through 6 meters and my experience with that particular antenna has been very good. The only difference is you will need to put down a reasonable number of radials to make it work well. The best part is you won't need a tuner.:)
     
  9. AG3Y

    AG3Y Ham Member

    The best part is that it doesn't cost $500 ! ! ! Does it ?
     
  10. W5DXP

    W5DXP Ham Member

    Why is a 3/2WL vertical dipole on 15m a bad idea? Because the take-off-angle is high and most of the RF is lost in space. At the low of the sunspot cycle, 15m needs all the help it can get from low-angle radiation. For a vertical center-fed dipole, the length needs to be 1.25WL (or less).
     
  11. W0IS

    W0IS Ham Member

    For $488, I'll fly in and put up a dipole.
     
  12. G3TXQ

    G3TXQ Ham Member

    There may be some misunderstandings here.

    I believe that the Sigma-40 is a very short 40m dipole (24ft instead of 66ft) brought to resonance by a combination of end capacity loading elements and a central loading coil. You should not assume that it will resonate at 15m in the way that a full-size half-wave 40m dipole would - in fact the impedance could be very high.

    Steve
     
  13. K0RGR

    K0RGR Premium Subscriber

    We use these on 40 meters for Field Day, and they seem to be remarkable performers on 40. I would not expect them to work well anywhere else, except perhaps 15.
     
  14. G3TXQ

    G3TXQ Ham Member

    Why would we expect a heavily-loaded 40m antenna to work well on 15m? Try feeding a 40m base-loaded mobile whip on 15m and see what VSWR you get!

    Steve
     
  15. K0RGR

    K0RGR Premium Subscriber

    You're probably right - I said "perhaps", not that I expected it to work. Most likely, the loading coils would act like chokes at 21 Mhz.. The capacity hats add another dimension, too.
     
  16. K8JD

    K8JD Ham Member

    really loaded shortie

    And for a few bucks more I will sell you the Ambassador Bridge and then you can drive to Canada for free.
    I would argue that a 24 ft antenna is only a half wave on 17 meters !!!!!;)
    It certainly is not anywhere near a halfwave on 40, that would be 66 ft tall !
    It can't approach the efficiency in radiating a signal on 40 of, even , a quarterwave vertical !:eek: and that's only 33 ft tall.
     
    Last edited: Apr 29, 2009
  17. K8JD

    K8JD Ham Member

    I love my second hand Butternut HF6V ! I have a minimal radial system mostly for 40 and 30M (my son has pulled up my radials a few times with his lawn tractor:eek:) but worked many European countries on 30 M with 5 W.:D
     
  18. W6IRE

    W6IRE Guest

    Well I thank all youze guys for your responses, I have a very space limited back yard, about 40' by 10' so it's pretty tight. I would like a vertical antenna but just don't have room for radials, unless I can run them in a spiral pattern around and around the antenna...:confused:

    This vertical dipole sounded good because of the space needed, but I think I will take the advice of some of you and build my own antenna, probably an inverted V...

    Another question, If I put up a single band dipole for 40 meters, if I place a 20 meter dipole about 12" below it, will the 2 antennas in close proximity foul up their respective performance?

    Note to self: Order the ARRL antenna book.
     
  19. WA9CWX

    WA9CWX Ham Member

    Pay close attention to your note to self !!!
    Then......READ it.
    yep. on a 10x40' lot, low band antennas will be requiring creativity, and some experimenting too. My guess is that a conventional 40 meter 1/4 w vertical will work, just with a LOT of short radials.....maybe as many as 50 or so under 20' An inverted VEE will work, but does take a little room, keep the angle between the elements at less than 90 degrees, then,. to make it fit in the yard, taper (slope)down the ends if you have to.
     
  20. W6ORC

    W6ORC Ham Member

    Wheres the fun in buying an antenna?

    Do you have any idea how much coax and wire you could buy for $500? I'd never run out of projects. :D
     
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