Trap fan dipole

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KA8MNP, Nov 20, 2013.

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  1. KA8MNP

    KA8MNP Ham Member QRZ Page

    Currently using a home brew fan dipole on 80-40-20. Works OK. My lot is very small and I have to bend the 80 meter part of the dipole to fit. If I build a new antenna and put traps in the 80 meter portion will it still work? I'm thinking the 80 meter part would be a little less broad band with the traps but shouldn't bother the non-trapped portion.
     
  2. W5DXP

    W5DXP Ham Member QRZ Page

    I'm assuming you mean loading coils, not traps. Traps do not shorten the physical length of an antenna - loading coils do.

    The functioning of a fan dipole depends upon the dipole that is 1/2WL on 80m being a multiple of full-wavelengths on 40m, 20m, 15m, and 10m and therefore having a very high feedpoint impedance. Unfortunately, when one adds loading coils, one changes the feedpoint impedance on those higher bands to a lower (usually unknown) value. At the loading coil self-resonant frequency (and higher) the results will be unpredictable and Murphy's Law says they will be disappointing.
     
  3. K7MEM

    K7MEM Ham Member QRZ Page

    It has been my experience that the 80 Meter bandwidth will not be affected much by the existence of a trap.

    I have a trap dipole for 15, 40, and 80 Meters. The traps are made from RG8x on PVC forms. I was just doing some adjusting yesterday and did some testing. I found that the antenna covers the entire 15 and 40 Meter bands with a SWR of 2:1 or less. And it covers about 110 KHz of 80 Meters with a SWR of 2:1, or less. That's about the same bandwidth I get with single band dipoles.

    Overall the 15 Meter traps shorten the 40 Meter sections by 2 feet, each side. And the 40 Meter traps shorten the 80 Meter section by about 5 feet each side. Overall the savings should be around 14' of total length, or more. The savings will be less if all your doing is making a 40/80 Meter trap section. But it is still a substantial savings.

    There is a bit of loss, due to the trap, but it hasn't bothered me much. It is best if you have some kind of Grid Dip Meter handy. The trap should be checked for resonance before it is installed on the antenna. Once the trap is attached, first adjust the length of the 40 Meter section, then adjust the adjust length of the 80 Meter section.
     
  4. KA8MNP

    KA8MNP Ham Member QRZ Page

    Thanks guys. Just wanted to shorten the 80 meter part of then antenna and continue to use separate elements on 40 and 20. I'd forgotten that if I added what i call traps to it that I'd in effect be making a dual band antenna. Not my intention just wanted to physically shorten the 80 meter element. 80 meters just doesn't quite fit in my back yard and I have to add an insulator to put a bend in it about 20 feet from the end of one leg. Seems to work OK but I'd just like to have it completely straight. Maybe I should just leave it alone. The apex is about 30 feet above ground and the ends are about 15-20 so its more of an inverted vee I guess.
     
  5. W5DXP

    W5DXP Ham Member QRZ Page

    A trap is (usually) a parallel tuned LC circuit designed to block signals at a certain frequency. A trap does not appreciably shorten an antenna. A 130 ft 80m dipole with 40m traps installed will still be about 120 ft long.

    --------------------40m trap---------------------FP--------------------40m trap----------------------

    A loading coil is close to a pure inductive reactance used to neutralize the capacitive reactance of a short antenna. A 65' dipole with loading coils installed to make it work on 80m is only half as long as a full-size 80m dipole.

    ----------loading coil----------FP----------loading coil----------
     
  6. KA8MNP

    KA8MNP Ham Member QRZ Page

    Thanks Cecil the little diagram helps. Watched a video on you tube a while back where a guy made such an antenna so that it would work both 80 and 40. Ended up being shorter than the original, I'd have to watch it again if I were going to build one but it was pretty close to what you show in your diagram. Always wanting to experiment with antennas but I don't have much real estate to work with. I do have a river right behind my back yard and have been real tempted to put some kind of wire across it. The other side is mostly wilderness so nobody would complain about a small nearly invisible wire.
     
  7. N2EY

    N2EY Ham Member QRZ Page

    All depends what you consider "appreciably shorten"

    I have used 80/40 trap dipoles for many years and found that adding traps shortened the antenna from 130+ feet to about 110 feet. That's a shortening of 20 feet (my antennas are adjusted for the CW end of the bands).

    I use RG-58 coax wound on PVC pipe, a la N4UU's article in QST for December 1981 (IIRC).

     
  8. KA8MNP

    KA8MNP Ham Member QRZ Page

    10 - 20 feet would make a big difference in my situation. I plan on trying to go higher in the tree and get some space that way. That will however change the angle so not sure what will happen then. Of course, I'll make sure I can return to my original configuration if it doesn't work out so I can just go back to where i was. As I said it works, just want to see what happens if I straighten out the bend and maybe get a few more feet in the air. Really cramped conditions and I feel fortunate just to have a working antenna.
     
  9. K7MEM

    K7MEM Ham Member QRZ Page

    If you know the total length that will fit without bending, you can then use this web page to calculate the loading coil required.

    http://www.k7mem.com/Electronic_Notebook/antennas/shortant.html

    The closer the loading coil is to the feed point, the smaller the inductor will be, but the efficiency may not be good. As you move the loading coil further away from the feed point the required inductor gets larger but the efficiency goes up. The web page listed shows the inductor requirements for a variety of configurations.
     
  10. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    How much are you bending the antenna? Unless you have bends in excess of ninety degrees, it is entirely possible loading the antenna to shorten it will degrade the performance more than it would be degraded by a couple of slight bends to allow the antenna to fit in the available space.
     
  11. KA8MNP

    KA8MNP Ham Member QRZ Page

    Thanks guys.
     
  12. W5DXP

    W5DXP Ham Member QRZ Page

    Maybe a poor choice of numbers and words, Jim, to illustrate a concept. The purpose of using traps on an HF antenna is (probably) not to shorten it. OTOH, the purpose of using loading coils on an HF antenna is (probably) to shorten it. For instance, I've never seen traps used to get a 75m mobile antenna down to a legal length. But I realize that a loading coil is also a trap at a certain frequency.:)
     
    Last edited: Nov 22, 2013
  13. N2EY

    N2EY Ham Member QRZ Page

    OK Cecil, no worries.

    Point is, a trap dipole will sometimes fit in a location that a full-sized dipole won't, AND give multiband coax-fed operation in the process.

    Just another tool in the toolbox.
     
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