DX-LB Dimensions

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KK5JY, Mar 15, 2017.

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  1. KK5JY

    KK5JY Ham Member QRZ Page

    I have an Alpha Delta DX-DD [dxe] that works quite well on 40m and 80m. Now that the sunspots are getting scarce, I would like to build a DX-LB [dxe] to use as a low dipole for the last two NVIS bands that still work with low sunspots -- 80m and 160m.

    I don't mind swiping the card for a DX-LB, but I would like to build it if possible. I didn't mind spending $130 for the DX-DD, because it allowed me to see how they did their construction, but I don't really want to do so twice. The construction techniques AD uses are simple, and very home-brew-friendly, so building the longer version should be easy.

    Do any of you have the measurements for the DX-LB? The main thing I need to know is the length of wire between each loading coil. If any of you know the number of turns on each of the two inductor types used, that would be great, too, ;) Once I have the wire spans properly measured, I should also be able to use EZ-NEC to experimentally find the value of the inductors.

    Thanks in advance for any info you can provide.
  2. KI8DJ

    KI8DJ Ham Member QRZ Page

  3. KI8DJ

    KI8DJ Ham Member QRZ Page

    I suggest you look at high power antennas. They have many shortened multiband and monoband antennas. They also sell the load coils seperately and give the dimensions required.These antennas are very well made and reasonably priced. Oh by the way the company is Hypower antennas.
    KK5JY likes this.
  4. KB6HRT

    KB6HRT Ham Member QRZ Page

    Have used both, there both Good antennas companies Hy Power antenna will take a little more Power on its coils than AD will both use NR 12 wire
    for the long runs of wire, a good thing for some antennas, if you have questions about the about the antenna Barry at HyPower knows his antennas
    abd will help you an a nice guy.....................73s...............kb6hrt
    KK5JY likes this.
  5. K8JD

    K8JD Ham Member QRZ Page

    This DX-LB ant, only 100 ft long according to ads, not the best for either 80 or 160M, may be pretty good for 40M !
    The loading coils Q make it a pretty narrow Low SWR bandwidth on 80 and 160M !
    To get on 160M you need some real estate for either a long wire dipole or radials for a big vertical !.
  6. KK5JY

    KK5JY Ham Member QRZ Page

    I think I have the vertical problem solved. This is really just for NVIS. I don't mind being a few dB down for a shortened dipole, since NVIS sigs on 160m are really strong, at least in this part of the world. I built a 160m-only antenna, and the in-state signals were extremely strong. I was just hoping to add 80m to it, in a single antenna, since 80m can still do occasional NVIS for the time being.
  7. K4SAV

    K4SAV Ham Member QRZ Page

    Since you know how to use EZNEC why don't you design it yourself. Shouldn't be too difficult. Start with an 80 meter dipole of about 140 ft. Add a trap on each end. Make the traps resonant on 80 meters. Then add two more wires on the ends of the antenna. Change the length of those wires until the antenna is resonant on the 160 frequency you want.

    You will discover that the length of the wires at the ends depend on the LC ratio used in the traps. There are an infinite number of combinations of LC that can be used. Use a value of C that you can get. If you run high power, this C will have to be a large doorknob cap (maybe a couple in parallel). Find a cap that is rated for delivering reactive power. Look for the KVA rating and compare that to the data that EZNEC says you have. Add some margin to your design. Small caps won't have a KVA rating.

    You can purchase traps if you want, but if the LC ratio is not specified, you will have to wait until you get the caps to finish the design.

    Traps can be designed without adding a separate cap. The L can self resonate on the desired frequency using its own distributed capacitance, however these will be very tricky to build and their value is likely to change with a little moisture or anything you cover them with. I would not recommend this approach.

    You can also implement the cap by winding the coil over a metal tube (with insulation) and let the wire-to-tube be the capacitance. This will require some experimentation to get the trap to the right frequency. Then you will have to measure the LC ratio to finish the design.

    The biggest problem you will have with a 160 / 80 meter trap dipole is bandwidth. Those are two very wide bands and even half wave dipoles have bandwidth problems. A trap dipole will have less bandwidth, but with EZNEC you can see what it is going to be before you build it.

    It will be a good learning experience and you can say you designed it yourself.

    Jerry, K4SAV
    KK5JY likes this.
  8. KK5JY

    KK5JY Ham Member QRZ Page

    That's the thing, the DX-DD and -LB don't use traps, and I would really like to avoid them. I may end up needing to go that route, but since I know it can be done without traps, I would like to, since there are that many fewer reactive components (no capacitors) to buy, burn up, or heat away my power. I really like AD's designs that use only inductors.

    The -LB has two sets of inductors... I could determine them experimentally in EZNEC, but it's very time-consuming to guess at initial values, since there are so many potential combinations. If I had some measurements from someone who has one, or has built one, that would get me very close to good initial values, and I can just fine-tune from there.

    This is very true, and for my purposes, this is fine. I want to center the resonant point in the digital sub-bands for some PSK or Olivia or similar.
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The A-D "Isoresonators" actually are traps. They use winding capacitance as the capacitors and perform the function of traps without parallel discrete capacitors.

    All the loss in traps (or the Isoresonators) is in the inductors, so this isn't "less lossy," it's just simpler but requires actually more inductance be used so it performs "more shortening" than traps having a lower L/C ratio.

    The DX-LB (I have one) is very heavily loaded on 160m as it's only 0.192 wavelengths long overall: Its 2:1 SWR bandwidth is about 20 kHz. However it does "work" and has proven to be a reasonably good performer for its size.
  10. KK5JY

    KK5JY Ham Member QRZ Page

    That's what I thought, too. And there is no doubt some significant interwinding capacitance, because of the way they wound their inductors.

    But when I measured the -DD version, and put the values into EZ-NEC, modeled only with inductance, it still worked out with resonances on both 80m and 40m. The current distribution made it look like the antenna was a "normal" loaded dipole on 80m... but on 40m, the current distribution was that of three collinear dipoles. The center dipole was the driven one, and was full-size on 40m. The other two were roughly centered on the inductors, and were thus heavily inductively loaded and physically short. But the phasing all worked out so that the model matched without using any capacitance.

    I assumed that the -LB was using some kind of similar mechanism, but I'm not sure if the AD design assumes or requires any self-capacitance in the inductors. Their documentation on the subject is ... vague. ;)

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