How Well Does A Trap Dipole Work

Discussion in 'General Technical Questions and Answers' started by KJ4BIC, Nov 6, 2008.

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  1. KJ4BIC

    KJ4BIC Club Station

    I want to put up an HF wire for 80-40-20-15-10 meters but I want fairly good performance. So my question is, does a trap dipole perform better then an 80m wire & tuner?
  2. G4ALA

    G4ALA Ham Member QRZ Page

    No easy answer


    In general, a wire + tuner does not perform as well as a dipole, since, with the wire, half of the dipole is missing (yes, I know the current goes somewhere, but I am speaking in general terms.). The arrangement works, but also has the additional drawback of the polar diagram not resembling that of a dipole at higher frequencies. The field diagram can have peaks, much bigger than the maximum of a dipole, and pointing in particular directions.There are also more nulls. If the peaks and nulls point in the directions you want, the arrangement can be better that a dipole, and if they point in directions you do not want, the arrangement can be worse. Of course, a dipole has two big nulls, so a dipole also has a problem. With a dipole, the direction of the nulls is known.

    A trap dipole has shortened current carrying portions. Strength of field depends upon current carried and length over which that current is carried. While the current carrying portion always includes a maximum current portion, it is shorter. But, most trap dipoles use fatter wire or tubing and so are more efficient. Also, the trap dipole (more or less) preserves the dipole gain and polar diagram. Also, a trap dipole fits into a smaller space. Not so bad on all three of the last points.

    Most commercial antennas have fancy arrangements of this and that simply to provide an impedance match at the "resonant" frequency. Most fancy arrangements compromise far field strength. That is why, for example, non-fancy mono-band yagis out perform equivalent multi band yagis. A resonant dipole is simply a fancy arrangement of a generic dipole, chosen for impedance match. Insert an antenna tuner and the need for impedance match no longer exists.

    Balanced antennas generally have better polar diagrams and localization of radiation that unbalanced antennas.

    My recommendation to you is to use a doublet fed by balanced feeder with the antenna tuner at the base of the feeder and a common mode choke at the top of the balanced feeder at the doublet feed point. The doublet should be suitable for 80m.The polar diagram has the same properties as for a wire (see above) which may or may not be a problem.

    Sorry I can not give a definitive answer. I hope this helps.

    Last edited: Nov 6, 2008
  3. W5DXP

    W5DXP Ham Member QRZ Page

    Here's an efficient all-HF-band antenna that doesn't need a tuner, i.e. no tuner losses.

    Traps can be expensive, hard to adjust, somewhat lossy, and deteriorate with time. A trapped dipole has a typical dipole radiation pattern on all bands. An 80m wire has a multi-lobed "cloverleaf" pattern on frequencies above the EDZ frequency of 9 MHz.

    I use an 80m wire for 80m, 40m, and 30m. I use a 40 ft. high, ladder-line fed, 20m rotatable dipole for 20m, 17m, 15m, 12m, 10m, and 6m. Here are the max gain and take-off-angles according to EZNEC.

    20m: 7.8 dBi @ 24 deg
    17m: 7.6 dBi @ 19 deg
    15m: 7.7 dBi @ 16 deg
    12m: 8.8 dBi @ 14 deg
    10m: 9.5 dBi @ 12 deg
    6m: 9.4 dBi @6 deg (cloverleaf)

    That doesn't make sense to me. A doublet is a balanced antenna. A balanced feeder is balanced. What does a common mode choke for balanced line look like? Seems to me the choke should be installed at the output of the antenna tuner at the balanced feedline to coax junction.
    Last edited: Nov 6, 2008
  4. K1LLR

    K1LLR Premium Subscriber QRZ Page

    10db gain over a G5RV at 50 feet up***- NVIS-***

    10db gain over a G5RV at 50 feet up..I tried this and it worked very well..At 7 feet off the ground,,amazing!
  5. W4HAY

    W4HAY Ham Member QRZ Page

    I've been using the W3DZZ version (do a YAHOO! search) of the trap dipole for years. I did sling elements for 30 and 17 Meters beneath it to expand coverage. It's up 35 feet in a densely forested area and my IC-746 easily matches it on all bands. The ARRL Antenna Book has details.

    If you're not into DIY, Unadilla offers it in kit form as the W2AU/W2VS 5-band dipole. Amateur Electronic Supply offers it for $80.

    At the bottom of the sunspot cycle, I regularly work DX using 50 watts on CW. I also check into the SSB emergency nets barefoot, firing up the AL-811 only in cases of severe QRN from thunderstorms or hurricanes.

    I had a Yagi 25 years ago at a previous QTH, but sold it when I moved and never put up another. Not counting contests, I'll fill up an ARRL logbook in about a year.
  6. W8JI

    W8JI Ham Member QRZ Page

    The nice thing about a trap dipole is the pattern is a wide lobe with only two nulls on all bands. This is very important, because you won't have holes in coverage.

    It also will be instantly ready to transmit, and is not mechanically complex.

    The loss in traps is greatly exaggerated. They are most lossy ONLY on the band they are trapping, and that loss is generally pretty small. So if you had a 80-10 meter trap dipole with traps for 40,20,15 and 10 the loss on 80 would be virtually nil. The loss on 40 would essentially only be with the 40 traps, and even very poor traps would have less than 1 dB loss.

    It is actually a very good system except for the cost and complexity of the traps.

    As a matter of fact, right now I have a ladder line fed dipole up 130 feet with a perfectly vertical feedline down to a tuner as a "spare" or multi-use antenna. I'm considering converting that antenna to a trap antenna because using the tuner is a PITA, and I don't want to bring ladder line several hundred feet into the barn or house. I also don't like the pattern of that antenna on 40 and higher because it has narrow lobes on 40 and multiple narrow lobes on higher bands....exactly what makes it less valuable for me.

    I would say overall with reasonable traps the performance would be MUCH better than it is now with ladder line and a tuner because I would get rid of the 10-15 dB deep holes in multiple directions on the higher bands.

    73 Tom
  7. G4ALA

    G4ALA Ham Member QRZ Page

    Common Mode Choke

    Dear W5DXP and everyone,

    I had presumed that, like all of the antenna tuners I possess, none of them expensive or flashy, there is a balance output through which the balanced feeder would be fed. My apologies for not making that clear. Putting a common mode choke on the balanced feeder at the ATU is certainly good practice. From your antenna article it seems that you use a ferrite ring 1:1 balun to force a transformation from unbalanced current flow in the co-ax feed to balanced flow in the balanced feeder. It illustrates the powerful properties of ferrite rings.

    The "common mode choke" I refer to is useful in overcoming various vices inherent in all antennas I have ever built.

    The "common mode choke" in its most basic form is just a ferrite ring through which the balanced feeder is threaded. I suppose it is a type of 1:1 balun. A 1:1 transformer will allow you to feed a balanced antenna using co-ax. In the case of co-ax,where balanced line conductor proximity effects are not a problem, you can wind the coax a few times through the ring. You can also coil the co-ax to form an inductor to make a common mode choke which suppresses common mode current. Such co-ax inductor common mode chokes are popular in balancing yagi beam antennas. It is the little coil of co-ax that can be seen up near the driven element.

    The common mode choke can be formed, as in your antenna article, by several ferrite rings together. It has a remarkable effect in forcing only differential mode current flow in any wire or cables threaded through it. A 1:1 balun at the antenna feed point forces balance in the antenna. Even an alleged balanced antenna has influences upon it that promote imbalance such as wire length differences, proximity of objects, etc.

    It is a fiction of horizontal antenna lore that the vertically rising and horizontally ground trailing feeder is invisible to RF. It seldom is. All sorts of currents are induced in the feeder and join in merrily with the polar diagram of the antenna to produce a result which is not the result in the book. Diferential currents are substantially invisible To suppress such unwanted common mode currents, provision of common mode chokes at intervals along the length of the feeder certainly helps. This can be done by coiling for a few turns any co-ax feeder or by clipping a split ferrite ring onto the balanced or unbalance feeder.A little experimentation can produce significant improvements.

    Your use of different feeder lengths certainly seems a good idea to allow "all band" working. Did a similar effect happen with variable co-ax lengths? That might overcome any conductor proximity effects to be found with twin line. The cost and size of so many co-ax switches and stowage and weight of so many inconvenient lengths of co-ax might be a problem though, HI!

    But I am straying from the original question, which is a major vice in thread replies. I apologize.

    73 to all,

  8. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    Last edited: Nov 6, 2008
  9. KC9MLL

    KC9MLL Ham Member QRZ Page

    Not a Wire guy..

    Hi, I have NEVER owned a Trapped Dipole, or a Wire antenna.
    But, I have definately worked several of them over the last year.
    When I sit back and compare the signal reports I gave out, and group them by distance from my QTH, The trapped Dipole seems to have gotten better signal reports from me.
    I am NOT saying that trapped Di-Poles are Better, I am just judgeing from my data at hand. I have worked over 35 Countries and all 50 states, but when I set up my station, I was limited as to what I could put up. No trees, and no tower at the time. So I run a Hustler 5BTV vertical Antenna. and it does great.

    Well, Enjoy whatever antenna you decide to run with.

  10. G3TXQ

    G3TXQ Ham Member QRZ Page

    Do the figures really support this?

    Let's say I have an 80/40m trap dipole which presents a reasonable match to 50 Ohms on both bands. Say I feed it with 100ft of RG213. The feeder loss will be 0.35dB on 80m and 0.50dB on 40m

    Now, instead, let's make it a simple 80m half wave fed with 100ft of 450 Ohm Wireman #551. The feeder loss will be 0.32dB on 80m and 0.84dB 0n 40m.

    Where's the disadvantage in using the coax?

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