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160m Jumper Dipole - how many segments?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC8QVO, Jun 8, 2021.

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  1. KC8QVO

    KC8QVO Ham Member QRZ Page

    For what it is worth, the variance in the VF in that chart equates to a 6 foot leg length difference on 160m between the bottom of 80 and the top of 160. Thats a lot of room for error. I could attempt to split the difference, say toss a .978 velocity factor in (smack in the middle). That is pretty close to the .977 I got at 20m.
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Jumper?

    "Jumper," he said.

    So, jump her I did!

    That's why I don't work there anymore...:(
     
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  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    I took the length vs frequency plot in post #6 and divide the dipole length at each frequency by half of the wavelength at that frequency, which I think is what you are calling VF:

    upload_2021-6-8_17-6-54.png
    This is a far cry from 468/f.

    The "VF" is a function of several things, including "end effect", height of the dipole agl, shape of the dipole (flat vs vee), wire diameter, wire insulation (or bare), wire resistance, ground conductivity, ground dielectric, how the coax feeds the dipole (through a choke or no choke), etc...
     
  4. KC8QVO

    KC8QVO Ham Member QRZ Page

    The longer length in that chart is the 468/f calculation length. The shorter length is the measured (physical) length. I divided the physical length by the calculated length to get what I described as "velocity factor" in the yellow column.

    To add to the variables - there are spade connectors between each band segment, thus the "jumper" dipole, or sectional dipole as some refer to them as. The end result is a full length dipole on every band. On the feed point side of the wires you connect all the sections/jumpers that you need for the band segment you wish to operate, then disconnect the ones away from the feed point to terminate the active antenna elements at the 1/2 wave length of the band of operation. Shorter is higher in frequency, and longer is lower in frequency, of course.

    So your "Variable A" is, I believe, a twist to what I am calling "velocity factor". I am trying to find a "constant" to multiply in to the 468/f or 234/f calculations to match up to an exact frequency and not have to tune the wire - ideally. That does not look like it is possible to get that precise.

    So the next question is what is a good "guess"? Use the .978 that I figure is smack in the middle of the "velocity factors" calculated already?
     
  5. KC8QVO

    KC8QVO Ham Member QRZ Page

    This is what my numbers look like at the moment with .978. I think I will use that as my "guess" to start with.

    Yellow is the velocity factor. Green is the additional wire length for the segments/jumpers past the bottom of 80. The first one is close to 50ft. That, before when I calculated it over the weekend, was 55ft. I forget exactly what numbers I used for that - but the point goes to show just how much variance there is in what might appear to be slight number changes when you get down to 160m. Hence the whole point of this thread. Between my last calculations and this chart there is a 10ft overall difference in the size of the antenna (both legs combined). Note - these numbers are just calculations thus far, other than the bottom of 80m length as that is the actual antenna length as it sits today. After my "guess" with the calculations and setting up the question will be how close that "guess" is and what tuning will be required. I certainly hope I am not 10 feet off!!! I hate trimming wire to tune - it is a waste of good wire and the stubs I end up with aren't very useful when they are real short.

    Section additions for 160m to jumper dipole.jpg
     
  6. AA6P

    AA6P Ham Member QRZ Page

    I have a three band Jumper Dipole covering 40 meters, 30 meters, and 20 meters with the insulators made from ballpoint pens.

    The first version as shown below used aluminum magnet wire recovered from a motor.

    The antenna is now using 20 gauge stranded wire with Teflon insulation that I picked up years ago at the W6TRW swap meet. I really like this wire for antennas as it pulls very straight and is easy to install. The connections are now made with small ring terminals soldered to the ends of each segment. If not attached with a small machine screw, the excess wire is wrapped back on the associated leg of the antenna.

    The antenna is an Inverted V and there is not a lot of tension in either leg. It has held up so far better than expected.

    73, Kent
    AA6P

    Jumper Dipole.jpg
     
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  7. KC8QVO

    KC8QVO Ham Member QRZ Page

    Interesting idea with the bolt. I have done that for some power connections with several wires being tied together. It seems like it would be a bit of a nuisance for band changes, however.

    I have the elements made up for 160. I have not sealed up the joints or tuned them, though. If I don't get to set up the antenna before field day to tune I will just roll with it and use a tuner to span the difference for the time being. At least if it is up I can measure the frequency and compare that to the length to dial in the calculation to get how much to adjust the lengths by.
     
  8. G3YRO

    G3YRO Ham Member QRZ Page

    FWIW My Half Wave 160m Dipole, 50ft high, fed in the centre with LMR400 50 ohm coax, is resonant on 1860kHz.

    The 2:1 VSWR points are 1810kHz and 1900kHz.

    That suits me fine, as I rarely go above 1900kHz, as that's just used for local working here in Britain . . . all the DX and even European contacts are in the bottom half of the band.

    In terms of "velocity factor", to predict the actual length of wire to cut, in 50 years I have NEVER found you can cut it to the theoretical length ! There are clearly too many variables in practice . . . you have to make it long, and then trim to the desired frequency.

    Roger G3YRO
     
    Last edited: Jun 12, 2021
  9. AA6P

    AA6P Ham Member QRZ Page

    Here are the NanoVNA SWR Plots on my jumper dipole. I do not have a tuner and only use resonant antennas.

    It did take a little effort to trim the antenna length for each segment as they had to be done in sequence. I started with the 20 meter antenna and then moved on to the 30 meter and 40 meter sections.

    Each section would begin with more wire length than needed. I would try different lengths and fold back and wrap the excess wire on itself. The excess wire had more effect than expected so I had to be careful before deciding on the final cut. I also discovered that the antenna is quite sensitive to height, the angle of the two legs, and the height of the ends.

    The antenna is a relatively low Inverted V and it only takes a minute to lower the center insulator by releasing a cord. It does take additional time to install or remove jumpers. It is a small effort but I typically stay with one band for a period of weeks.

    73, Kent
    AA6P

    20m Dipole.png

    30m Dipole.png

    40m Dipole.png
     
  10. M0AGP

    M0AGP Ham Member QRZ Page

    Nice! What about a waterproof SPST?
     

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