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. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    I didn't say anything about loss Steve. In either case the losses are laughable so it's moot. Having made many a trap dipole in my youth I can tell you the following facts.

    1. Traps are lossy, they suffer from I/R losses like any other inductors (capacitors are practically lossless).

    2. SWR bandwidth for trap antennas is very narrow because of the loading effect of the traps. A flatop/tuner arrangement doesn't have this problem.

    3. Trap antennas must be tuned (pruned) to resonate on each band and that can be very laborious and frustrating.

    4. Coax is heavier than ladder line and requires more support.

    5. Coax is subject to moisture intrusion and great care must be taken to seal the end where it connects to the antenna.

    I'm not anti coax Steve and have used it for 40+ years in lots of applications. I assume that the people asking these questions are new and don't have an engineering degree therefor, I try to keep it simple for them. A flat top fed by parallel feeders and a decent tuner is very easy to implement and will work well with little or no hassels.

    There are hundreds of antennas that might work better but are more difficult to implement. Many require measuring equipment and advanced knowlege of antenna theory.

    I like to give them something that is easy to build, will work, and is very forgiving of hight and terrain.
     
    Last edited: Nov 6, 2008
  2. G3TXQ

    G3TXQ Ham Member QRZ Page

    Apologies, I must have misunderstood what you meant when you said:

    Steve
     
  3. W9GB

    W9GB Ham Member QRZ Page

    Personally, I always thought the Unadilla W2AU/W2VS 5-band dipole kit was a better HF antenna than the G5RV.
    http://www.unadilla.com/antkit.htm

    You can purchase the kit (no wire) for $78 direct from Unadilla or from AES.
    http://www.unadilla.com/products.htm

    We have this dipole in our Field Day kit, as well as 2 or 3 club members who now use this antenna for their HF operations.
    The Unadilla/Reyco design has been around a LONG TIME, and this a popular single antenna choice for Novices (80,40,15) in 1960s and 1970s -- that only used one set of traps (40 meter).

    w9gb
     
    Last edited: Nov 7, 2008
  4. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    Depends almost entirely on the Q of the traps. I've had many many great trap dipoles in my "career".
     
  5. W8JI

    W8JI Ham Member QRZ Page


    There is no disadvantage to using coax with traps, especially on the higher bands.

    The real killer on a multiband antenna using balanced line is the messy pattern on higher bands. Too many nulls.Too narrow on the maximums. Bad pattern.
     
  6. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    I don't know if you remember Unadilla/Reyco Traps. They were very high Q, best on the market, I still have several of them. A four or five band trap dipole using these traps worked fine except the bandwidth was really narrow. You couldn't work the phone and CW portion of the band because you could only get about 100 Kc of useable bandwidth.

    A ladder line fed flat top with a tuner gives you the full use of the bands. As for pattern, that's very over blown. RF is not coherant like laser light and the lobe pattern that everyone quotes is close proximity. They spread out and get emulsified in the ionosphere a good bit. I have worked long path DX using antennas more than four wavelengths long where I should not have been able to.

    Those of you who have built many antennas know what I'm saying. Mostly I'm trying to help new hams get on the air in the most simple and effective way without a lot of expense and trouble. In short, a trap dipole works but it is a poor choice from a number of aspects.
     
  7. NA0AA

    NA0AA Ham Member QRZ Page

    I think if I had to forgo a full size 80 meter dipole, I'd go with something like the DX engineering antenna that only has one set of [whatever] and those shorten the 80 meter band only - 40 meters is full size inside the [whatever], and so you get 40 and 15 on those wires, plus 80, then it's got parallel dipoles for 20 and 10, so you get all 5 traditional bands in one coax feed line.

    If I had the room for a full sized doublet, I'd use that and feed it with balanced line and a tuner if I could only have one antenna.

    My principal objection with shortened antennas is their limited bandwidth.
     
  8. W8JI

    W8JI Ham Member QRZ Page

    I'm not sure what that means. The pattern is almost perfectly the same at 100 miles out as it is at a few wavelengths out. I think you are confusing atmospheric scattering of light with much lower frequency radio waves.

    The pattern in EZnec for example is calculated at hundereds of wavelengths out from the antenna, and it will be the same as the pattern at a few wavelengths.

    While it is true that the path can shift around and you might even have multipath or some scattering, it certainly is not true that a little scattering, path slewing, or multipath makes the nulls go away. I can find and hold a null on stations thousands of miles away, and the null is just as deep as it is locally when measured at the same angle and elevation as the skywave signal path.

    I'm not sure what that means either. It sounds like you are confusing being able to work stations in a null with the absolute signal level loss in that null.

    For example I can measure the pattern of my Yagi antennas at a few wavelegths distant, and that pattern repeats closely over an ionospheric path for DX stations. If the antenna is 20 dB down off the sides at 15 degrees elevation, it behaves exactly the same for skywave signals.

    Now if I couldn't turn the antenna to focus nulls and observe the changes as I rotated the antenna, I might become delusional and think just because I could work people off the sides the nulls weren't really there. But they are.

    We know this because we have tried to use a 130 foot long doublet to augment other antennas during contests, and the multiple nulls on higher bands just kill our signal for some azimuths and elevation angles. Even a little ground mounted vertical will be a few S units better than the high dipole when a signal falls into a null area.

    The claim is actually silly if we think about it, because if indeed the nulls filled in then all antennas of the same efficiency would be equal for receiving and transmitting. A 10 dB gain yagi stack would no longer have 10 dB gain, because it only obtains the gain through nulling of certain directions.

    I don't think VOA or anyone else would buy the claim that directivity magically vanishes on skywave paths, and I sure don't think OTHR systems or other HF DF'ing system would work if things were as soupy as you seen to think.

    Perhaps what you are not considering is the current taper in the element that blurrs the nulls in really long antennas. The attenuation along a very long conductor is actually quite significant, for example a longwire (like an element in a Rhombic) gain limits at about 3.3 to 3.9 wavelengths because of attenuation in the element. The signal level and gain actually starts to go down beyond that length since the efficiency decreases faster than the sin x/x beamwidth decreases. That attenuation ceratinly blurrs the pattern from a lossless radiator, but it does so at the expense of overall gain so you are no better off filling an undesired null just by adding loss to the system.


    73 Tom
     
    Last edited: Nov 7, 2008
  9. WA9CWX

    WA9CWX Ham Member QRZ Page

    There are good and bad, advantages and disadvantages.

    For ONE thing, I would completely dismiss loss comments that talk about anything LESS than 1 DB on HF....That is a plain silly number to even be thinking about on HF.

    IF you don't mind using a tuner, AND you have a REASONABLY clear path TO the tuner from the antenna, using ladder line is a nice way to get all frequency coverage.
    YES, on the HIGHER frequencies, you will have some kind of pattern, BUT assuming you have not mounted your antenna 100' or more in the sky, it is likely you will never realize WHAT those patterns ARE, let alone will they have any real effect on your operating until you start looking for rare contacts in tiny provinces where people eat people and wire is outlawed.

    In other words, it is going to be quite a while before whatever directivity exists on the higher HF bands even MEANS anything to you.

    On the OTHER hand, a coax fed antenna, although a bit more complex to set up and tune with traps, and a bit more of a problem to support, IS a more convenient antenna to USE, as long as you don't plan on a lot of band edge QSYing type operations. They CAN be rather narrow, and when SWR DOES go up, coax becomes more of a concern. NOT a big concern, but it CAN cause some trouble.

    Fan Dipoles work better, but involve MORE supports (to actually FAN the elements), and although they are more broadbanded, they too suffer from SWR trouble at their extremes of band edges, like ANY low frequency dipole.

    The only REAL advantage to coax, is that it is convenient, it can be coiled, run through a wall, next to gutter, underground, whatever.

    Ladder line cannot.

    Frank
     
  10. W5DXP

    W5DXP Ham Member QRZ Page

    Flip side of the coin is: What is lost in a null is gained in a radiation lobe. If one can "hold a null" in one direction, one can hold a QSO in another direction.

    OTOH, Louis Varney, G5RV, chose the 1.5WL length for his G5RV based on the cloverleaf pattern on 20m. I have found those "messy patterns" to be pretty good for world-wide DX. In AZ, I very carefully aimed my upper band G5RV lobes at Europe, Asia, South America, and New Zealand. Worked very well. Here's a graph of the East/West 20m G5RV pattern from my Texas QTH. The gray areas are the 3dB point coverages.
     
    Last edited: Nov 7, 2008
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