Fan Dipole or Doublet?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KF5LJW, Feb 13, 2018.

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  1. KF5LJW

    KF5LJW Ham Member QRZ Page

    OK I have some space restrictions to deal with on a friends house. No trees and a small lot, and HOA restrictions prevent outside antenna erection. The home does have an attic but is only single story, so height is another restriction he has to live with. He wants at least 40/20/10 which will be a challenge because his attic space is roughly 60 x 35

    I see two options either a Doublet cut to 66 feet and feed with 450 Ladder line, or a Fan Dipole with 3 elements cut for 40/20/10. My thinking is Doublet is the best option. Either way interior space with a length of 60 feet with 66 minimum requirement for either Doublet or Fan Dipole. At least with a Doublet he can get more than 3 bands but not sure how to handle the missing 6 feet. Make a 3-foot "Z at the ends?

    Whatcha say?
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I'd go with the doublet, but no need to make it a full half wavelength long. A good rule of thumb is to make a doublet at least 3/8 wavelength on the lowest band but it's actually better if the antenna isn't halfwave resonant on the lowest band. A true half wave resonant doublet on the lowest band makes it full wave resonant on the even harmonically related bands which is a hard antenna impedance to match. Cutting the antenna a bit long or short on the lowest supported band typically makes the harmonic bands easier to deal with.

    IOW, cutting the attic doublet to around 5o' to 55' long will make matching easier and it will fit a bit better into the available space even leaving some room for end insulators and support lines which is a very good idea as those ends are the high voltage points.

    The one story house and strict restrictions on outdoor antennas isn't great but assuming the house doesn't have a metal roof or metal backed attic insulation the antenna should get him on the air.
    VK3YE and NH7RO like this.
  3. KP4SX

    KP4SX XML Subscriber QRZ Page

    Sure, he could dangle 3 feet on either end of a 40 meter dipole and probably get it to work.
    Fanning the other two bands wouldn't be a problem and he could probably get by without a tuner. Getting ladder line from an attic to the shack could be problematic so I would choose a coax feed.
    BTW, doublet=dipole.
    NH7RO likes this.
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Technically true, but hams have long referred to balanced line fed not necessarily resonant multiband antennas as doublets and coax fed resonant antennas as dipoles.
    NH7RO and W7UUU like this.
  5. KF5LJW

    KF5LJW Ham Member QRZ Page

    Interesting because everything I read about Doublet Dipoles s they should be at least a half wavelength long as the lowest frequency. Example for 40 meters should be at least 66 feet, and say 70 feet is no problem. I will give it a try at say 58 feet.

  6. KF5LJW

    KF5LJW Ham Member QRZ Page

    Thank you for the input. I have build a Fan Dipole a few times and prefer not to make another one because they are tricky to trim and only good for a single band. 3 bands require 3 dipoles, one for each band. That is why I am thinking a Doublet cut to 40 meters. That should allow operating on all the higher in frequency of 7 Mhz.

    My real concern is mounting height of roughly 20 feet and the effect. FWIW I do not think getting the Ladder Line form the shack to attic will be a problem.
  7. W7UUU

    W7UUU QRZ Lifetime Member #133 Volunteer Moderator Life Member Platinum Subscriber QRZ Page

    I've recently modeled* my (intentionally non-resonant) 150-foot double at about 100 feet.... at least as far as the models are concerned, it looks good on all bands except 160, but with the 450-ohm window-line feeder (137 feet long in the model) shorted and with a radial field, makes for a decent "top band" antenna.

    Will be interesting to see how it works in reality soon, vs. the model


    [*Thanks to club member Randy, @WB4SPB , for developing this model - will be very interesting to compare model vs. reality]
    NH7RO likes this.
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Nahhh, some folks have advocated much shorter versions like Cebik's 88 foot 80m and up doublet or the 44' version for 40m and up. Nice antenna patterns on the higher bands IF you can match into them. Personally I try to keep them around 3/8 wavelength or longer but avoid actual half wave resonance.

    Here's W8JI's take on minimum doublet lengths:

    His rule of thumb works out to a tad longer than 3/8 wavelength but pretty close:
    As someone else posted, running the balanced feed line up into the attic may be a challenge since you want to keep it away from household surfaces, especially conductive surfaces. Your friend might try the antenna with a manual tuner up in the attic, if he likes the performance he could run a remote auto tuner up in the attic with coax running from the tuner to the shack. That's currently the way I've got my 40-10 meter doublet configured, the antenna is set up inverted-V style with the center held up by a 25' mast mounted in a roof top tower. Homebrew open wire feed line runs from the antenna feed point to a balun and an LDG RT-600 remote tuner. Coax runs from there down to the entry point at the shack. It works great, tunes nearly instantly with great on the air results.
    Last edited: Feb 13, 2018
  9. W5DXP

    W5DXP Ham Member QRZ Page

    Consider that without looking outside at the antenna, it is hard to tell the difference between a half-wave 50 ohm dipole and a full wave 4050 ohm Double Zepp because they have the same SWR450 on the ladder-line and therefore the same SWR=9:1 circle on a Smith Chart (neglecting losses).

    If one feeds the half wave 50 ohm dipole with an odd multiple of 1/4WL of ladder-line, one will see 4050 ohms in the shack.

    If one feeds the full wave 4050 ohm Double Zepp with an even multiple of 1/2WL of ladder-line, one will see 50 ohms in the shack.

    Moral: There are good and bad combinations of dipole length and ladder-line length. But given any length of dipole that is at least 0.4WL* on the lowest frequency of operation, there is a good combination of dipole length and ladder-line length that will usually result in an SWR50 less than 2:1.

    * Note that a minimum dipole length of 3/8WL (0.375WL) is usually recommended. For an SWR50 less than 2:1, we need to increase that recommendation to about 0.4 wavelength to decrease the SWR450 to less than 18:1.
    73, Cecil,
  10. KF5LJW

    KF5LJW Ham Member QRZ Page

    I apologize if I gave you the impression I doubted you. Was not my intention. What you are describing sounds a lot like a G5RV Jr? I had one of those things and really disliked it. Could not get it to tune worth a darn. I could get more contacts with a Dummy Load. Looking back on it I think the problem at the time was improper routing of the tuned stub.

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