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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

    All,

    I have a Jumper Dipole I started making last year. It covers every band 6-80m (including WARC & 60m) and on 40-80 I put 2 segments per band in so it is resonant both for the CW and SSB portions of the bands.

    That brings me to my question. I am adding 160 meters this season for Field Day. I had intended to do that this weekend, but things didn't work out and I didn't get to start adding to the antenna to sweep it and see what the SWR curve was.

    My intent was to make a segment centered around 1850kc and see what the bandwidth was then make a determination of if I wanted 3 segments or if 2 would do it.

    What does the group here suggest?

    The question simply is in order to cover the full 160m band with adequate resonance should there be 2 segments - as I did on 40 and 80 - or should I add another segment in the middle to better cover the, say, 1800-1875kc, 1875-1925kc, and 1925-2000kc ranges? Or do you think 2 segments to cover the 1800-1900kc and 1900-2000kc portions would be plenty?

    Why I question it is how much the calculated length of a resonant dipole varies between the band edges - using the 234/f calculation (neglecting velocity factor) that would be 130ft at 1800kc and 117ft at 2000kc (these numbers are for 1/2 the dipole, each leg, not the total length of both legs combined). In any event, that is a difference of 23 feet on each leg, or 46 feet total lengths combined.

    If I had a measured SWR curve to see on the antenna I have that would help me tremendously. However, I do not have that and at this stage of the game I need to calculate my best guess on the velocity factor, build the additional segments, then only fine tune. That means I need to know how many segments to build ahead of time without the data to do it the "right way", as far as my original idea was on how to do it.

    For what it is worth, the jumper dipole works absolutely beautifully. I've used it mostly on 40 and 75. I couldn't ask for a better portable antenna. This one came out of a few other precursors. I decided to use 18 gauge Silky wire from The Wireman on this one - a bit thicker wire than the 26 gauge I used earlier (still awesome stuff). I figured I learned enough from the previous versions to do this one kind of a "one and done" deal - and so far it wins with flying colors. The 160m segments are going to be removable - so there will be double insulators between 80 and the 1st segment for 160 that can be separated. The antenna goes with me on all kinds of adventures and I don't want to carry all that extra wire for 160 all the time. The primary band the antenna was built for (of course it covers all bands) was 75 meters so that is already a good amount of wire. Add all the insulators for all the band segments and it all adds up. But - it has been well worth it.
     
  2. SM0GLD

    SM0GLD Ham Member QRZ Page

    According to EZNEC:
    On 160m with a #18 wire dipole placed at 30ft agl your VSWR 3:1 bandwith is 100kHz. The VSWR 2:1 bandwith is 60kHz.
     
    KC8QVO likes this.
  3. KC8QVO

    KC8QVO Ham Member QRZ Page

    Thanks!

    With the velocity factor being a bit goofy on what I have made so far I suspect all the spade connectors and junctions in the wire at the insulators is affecting things there quite a bit. My suspicion is that will also change the SWR curves.

    Can you manually input velocity factor of the wire used in the EZNEC calcs? I could come up with those on the higher bands - the numbers to find those are in my spreadsheet, but I stopped at 20m at that point in time. I can add 80 (bottom of it) from what I measured this weekend but don't have 30, the 2 segments on 40, or 60.
     
  4. KC8QVO

    KC8QVO Ham Member QRZ Page

    This is a chart a pieced together based on the 2:1 and 3:1 bandwidths given earlier. So this would be 3 segments to cover the whole band - with those numbers.

    SWR prediction 1 for 160m on jumper dipole.jpg
     
    WB5YUZ and SM0GLD like this.
  5. SM0GLD

    SM0GLD Ham Member QRZ Page

    No, not the VF but cable dielectric constant witch is needed to calculate the resulting VF.
    For transmission lines you can enter VF.
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    Here is a cutting chart for a 160m wire dipole:
    upload_2021-6-8_12-26-1.png
    Assumptions: Bare, Cu, #16awg wire, at 25ft agl, over average dirt (5mS,13).

    Here are three Swr50 plots for 259ft, 251ft, and 245ft:
    upload_2021-6-8_12-41-27.png
     
    KC8QVO likes this.
  7. W4HWD

    W4HWD Premium Subscriber QRZ Page

    160m in June? Good luck!
     
    K7JOE and AK5B like this.
  8. KC8QVO

    KC8QVO Ham Member QRZ Page

    @WA7ARK - thanks for the plots. That resembles the chart I came up with, though based on center frequency and not length, but you can interchange them somewhat - with the velocity factor they are directly related.

    Here are some numbers I ran on the velocity factors calculated off my spreadsheet. What I don't know is the exact center resonant frequencies measured. These are the lengths I got when I built the antenna to closely match the supposed center frequencies, however I would need to sweep the antenna again to find the 2:1 bandwidth then correlate a more exact center frequency based on that. I do not believe the exact center frequencies are what the chart shows.

    None the less, this is the result. Yellow column is the velocity factor the numbers point to:

    Velocity Factor Calculations Jumper Dipole.jpg

    I was hoping when I plotted the numbers like that I would be able to see a trend. However, they bounce all over the place so it is not possible to make any correlations from that. They vary from .955 to 1.001 in no particular order. Again, I don't have the numbers for the bands/band segments between the 20m and 80m CW portions shown.

    Perhaps if I set the antenna up and measure the exact center frequencies I can get the velocity factor to correlate better. However, setting up the antenna is the challenge - the idea is to build it as close as I can get it prior to setting it up at this point as I likely won't be able to set it up prior to Field Day and take the time to play with it. I don't have the room to set it up/assemble here. I'd rather spend my time during Field Day operating and enjoying eveyrones company than "playing with my antenna".
     
  9. KC8QVO

    KC8QVO Ham Member QRZ Page

    Stirring the pot I see. You can make 160m contacts all year 'round. Yeah, the band is in rough shape. That doesn't mean you can't make contacts. Plus once the antenna is set up for 160m I have that option all the time going forward if I am in a position where I can get on that band - not necessarily specifically for Field Day although that is the rush on doing it at the present time - to use it for Field Day this round. Overall the adding of 160m adds to the "one and done" comment earlier - once its done I don't need to mess with it again.
     
  10. W4HWD

    W4HWD Premium Subscriber QRZ Page

    AF02C8A7-5A1A-44D7-A12D-F2FEC552CE02.jpeg

    In case you don't know what this is, it's today's lightning map, courtesy of blitzortung.org. This is what the map will look like 2+ weeks from now when field day pops around, so, again...good luck with those S9+30 static crashes.

    I'll go down to 160 on field day too...maybe I'll hear you.
     
    Last edited: Jun 9, 2021

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