K7MEM Loaded Dipole Calculator

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AD8DT, Jan 10, 2019.

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  1. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I've worked 160m using an inverted-L and a loaded vertical. Both worked fine but the inverted-L seemed to work better overall. I've used different rigs on 160m including: Drake B line, Kenwood TS-830, Ten Tec Omni VI+ they all worked but the DSP noise reduction in the Omni VI+ was very helpful for pulling CW signals out of the noise on 160m.

    My comments above on the K7MEM calculator is about the calculator alone. It works, it produces a good starting point for different loaded dipole configurations. Whether a low 160m dipole is a good idea is a different conversation but the calculator does what it's supposed to do in terms of figuring out wire lengths and loading inductors.
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    So I began with a center-fed, uninsulated #14 horizontal wire, 120ft long, 60ft above sandy soil (2ms,10diel), no feedline, just to establish a baseline. The way the modeler works, it is not possible to place the coils exactly at 18ft from the ends; here is the best I can do:

    By breaking the wire into 61 segments, each segment is 120ft/61 = 1.967ft long. I can place the feeding source exactly in the center of segment 31, so the wire is symmetric around the feedpoint. EzNec allows you to place a "load" (coil) at the center of any segment, so to get as close to the 18ft in from the ends shown in Martin's figure, I tell EzNec to place the coil into segment #10, meaning that it is 9.5 segments in from the end, so that is 9.5*1.967 = 18.69ft in from the ends.

    With 143uH coils, at 1.93MHz, the feedpoint impedance is 21.8-j195, for an SWR50 of 38. So then I optimize both coil inductances to lower the SWR at the feedpoint and get that coil inductances of 153.8uH make the feedpoint impedance 24.6+j0, for an SWR of 2. This is pretty close to what Martin's design shows if we allow for the slightly different coil placement.

    Going back to the proposed antenna, I altered the model to account for the drooped ends, and the wire insulation. I change the model to have 30 segments in each half of the antenna (30 each side). Now I can place the coil into the center of seg #19, meaning that it is actually 9.5*60/30 = 19ft in from the end of each end.

    Rerunning the optimization, if the coil inductances that produce the lowest swr at the antenna feedpoint are 169uH each, which at 1.93MHz makes Z=24.7+j0, for an swr of 2.

    Here is a plot of R and jX centered around 1.93MHz:

    rjx.png

    Here is the swr50:
    swra.png

    If we now add 75ft of Vf=0.66 RG8, the SWR plot looks like this:

    cs.png
     
  3. AC6LA

    AC6LA Ham Member QRZ Page

    The trick is to put the loads on short 3-segment "sliding wires" on either side of the center.

    slider1.png

    Then, for example, to move the loads farther out the lengths of wires 3 and 5 are increased while the lengths of wires 1 and 7 are decreased such that the overall length of the dipole remains constant.

    With a few Sin and Cos functions thrown it is possible to do the same thing with a dipole that has a bend in the center. Here's a dipole with a "catenary sag", the same idea applies to an inverted Vee.

    slider2.png

    Now to change the lengths of wires 3/5 and 1/7, and hence move the position of the loads on the "sliding wires" 2 and 6, a bit of trigonometry is required to figure out the new coordinates in two dimensions. All of that is handled by using variables to set the XYZ wire coordinates of the model.

    The attached zip contains an AutoEZ format model with a variable used to set "exact position of loads on either side of center" regardless of whether or not the antenna has a bend in the center and regardless of the segmentation density.

    Dan, AC6LA
    https://ac6la.com/
     

    Attached Files:

    K7TRF likes this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    Thanks Dan,

    I was rushed this morning (house guests) so didn't have a lot of time to devote to this. However, here are the takeaways:

    It should be no surprise that if you shorten this horiz dipole to 120ft overall, that the real part of the feed impedance goes down from the theoretical 70 Ohms to ~25 Ohms, and it gets capacitive.

    In order to resonate it, the coils add inductive reactance to cancel the natural capacative reactance of the short antenna. The coils, however, can do only that, leaving the real part the same. This means that the feed-point resistance at resonance is such that the swr50 is ~2:1, so you will likely need a tuner full time.

    Also, notice how narrow the useable swr bandwidth is. Another way of saying this is that the Q of a loaded antenna is much higher than a full-sized one...

    Another takeway: the "tuning" of the antenna to place the lowest SWR at 1.93MHz is very dependent on the actual inductance of the coils, and where they are placed along the wire. Small changes in either one will move the SWR null. This means, have it on a pulley system so that you can make small changes and hoist it back up while initially pruning it.
     
    Last edited: Jan 11, 2019
  5. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    You can also change the sweet spot for SWR just by moving the antenna legs around.
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    Back to @AD8DT 's requirements:

    It looks to me that you have room for an antenna that is 60ft high in a space that is ~100ft wide. Your original proposal was an 120ft long invertedV centered in that space where the apex is at (0,60) and the ends are at (-52,30) and (52,30).

    I have an alternative proposal: Fit this into the same space:
    dia.png

    Apex is at (0,60). Wire #1 (bottom wire) runs from (-35.33,20) to (35.33, 20), i.e. 70.66ft long, 20ft above the ground. Here are all the wire details:

    wr.png

    The feed is through a 1:1 current balun and is centered on wire#1 (where the red box is). In series with wire#1 (right next to the balun) is a wide-spaced variable capacitor (range: 10-50pF). There is any length of 50 Ohm coax between the balun and the station.

    With the wire dimensions above, and the capacitor adjusted to 35pF, at 1.93MHz, the swr50 is 1.009, Z=49.62+j0.25.

    With the wire dimensions above, and the capacitor adjusted to 44pF, at 1.82MHz, the swr50 is 1.5, Z=33.4+j0.34.

    With the wire dimensions above, and the capacitor adjusted to 31.5pF, at 1.98MHz, the swr50 is 1.21, Z=60.6-j0.53.

    So, now you have a 160m antenna that can be tuned anywhere in the 160m band just by adjusting one variable capacitor. Using stepping motor or DC gear motor, you could easily make a remote tuner. At the 100W level, the capacitor breakdown voltage needs to be about 4kV.

    The pattern, while not great, is better than the dipole:
    pa.png


    ps: It just occured to me that if I eliminate wire#2 and #5, make it a bit bigger, I could also make the feedpoint impedance 50Ohms at 1.9MHz. This would be a bit easier to build. Out of time today, I will get back to simulating that after taking my company to the Grand Canyon....
     
    Last edited: Jan 12, 2019
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    I
    I have tried two different approaches, but not concurrently, so a direct comparison is hard...

    First, inverted-L, driven against the steel roof of a 65x85 ft all-steel building (aircraft hangar). The tall part started at 20ft agl and went up to 55ft agl, then turned horizonatal for ~220ft. The distal end was about 50ft agl... Basically, a 1/4wl laying on its side...

    Now, a horizontal loop, 558ft in circumference, supported in three places at an average height of 50ft agl. Coax fed in a corner through a 1:4 unbal. The loop works well on 160m, 80m, 60m, 40m and 20m.

    The loop has a better snr on receive than the inverted L or any other vertical I have compared it against.
    On 160m, both antennas work well for SSB contacts in the Western USA from Canada to Mexico... On 160m WSPR, using the loop, I get spots all over the world.
     
    Last edited: Jan 12, 2019
  8. WB5WPA

    WB5WPA Ham Member QRZ Page

    re: "On 160m WSPR, using the loop, I get spots all over the world."

    I would like to "pull up that data" and take a look. Do you know what years you were active? I had some gaps in WSPR operating in past recent years owing to some now-resolved vision issues, otherwise I would have been collecting metrics then.

    The "big player" all along on 160 m seems to have been K9PAW from what I'm recollecting back to around the 2012 time frame. K5DNL did well using a 1/4 WL vertical from Oklahoma when he was active on 160m before going solely 630 meters. I've got nightly-run comparative data accrued when he was active during sometime around 2014 onward for awhile.

    The longest distance spots I've seen from those US hams that can make it (includes myself) have been to Australia.

    Going just about straight south (and a little east) the distance for me to the Antarctic station DP0GVN is 13398 km. They were spotting back in October 2018 and they seemed "easy to hit" by me. They are the 2nd most distant spot from the USA with AUS being the furthest I think.

    EU countries for me range at about 8,000 km plus distance thereabouts.

    The most spots I ever had was about 2 or 3 weeks back with 22 (twenty-two) individual WSPR reporting stations across the UK and EU mainland plus Iceland. 160 can be really hot at times, other times just blah, like recently, with only 6 to 8 spots overnight across the Atlantic.

    .
     
  9. WB5WPA

    WB5WPA Ham Member QRZ Page

    @WA7ARK

    Take for instance tonight - this spotter in way North and east Sweden is indicating hearing these stations the last 30 mins, including four from the USA on 160 meters (time is 8:50 PM CST, 0250UTC) in bold:

    SWLKQ40LS
    Hearing: 2E0ILY, DB9LG, DL9GCW, G3JKV, G3ZJG, G4BOO, IK5QLO, K9PAW, N1FXP, OE4LTB, OK1FCX, OK1KUN, OZ7IT, PA0O/P, SK3UB, W1CK, WB5WPA
    --------------------------------------------
    Overall, here's Hearing and Heardby callsigns (last 30 mins only)
    WB5WPA
    Hearing: AD5GH, K9PAW, KC9NBV, KD6RF, KJ4YBS, KN8DMK, N1FXP, W1CK, W4HBK, W5OF, W8OZZ, WA0JYF

    Heard by: AD5GH, AE5LY, G0LUJ, G3ZJG, G4KPX, GM4OAS, K4ZAD, K6MCS, K9AN, K9PAW, K9YWO, KA7OEI-1, KB0VER, KC9NBV, KD2OM, KD6RF, KD7YUW, KD8WBZ, KG5ABO, KJ4RWD, KJ4YBS, KK1D, KK4MBI, KN4GDX, KU4A, KY5M, N1FXP, N2HQI, N2NOM, N3BBF, N3TCH, N6GN/K, N6GN/K2, N6GN/K3, N6RY, N8IT, N8UR, ND7M, PA8AD, SWLKQ40LS, TF3GZ, VE3GTC, VE6JYKIWI, W1CK, W1NJC, W3PM, W5OF, W8SGT, WA0JYF, WA2ZKD, WA3TTS, WA5DJJ, WA9WTK, WD4AH, WD8QLY, WQ2H
     
    Last edited: Jan 13, 2019 at 2:58 AM
  10. WB5WPA

    WB5WPA Ham Member QRZ Page

    @WA7ARK - Good showing on 160 meter WSPR last night! We both made it across the Atlantic.

    Stations showing as Hearing/Heardby just before 7:30 AM CDT (1330UTC)

    WB5WPA
    Hearing: AD5GH, K9PAW, K9YWO, KC9NBV, KD6RF, KJ4YBS, KM4RK, KN8DMK, N0ZHE, N1FXP, N3TCH, W1CK, W4HBK, W5OF, W8AN, W8OZZ, WA0JYF, WA5JOT, WA7ARK, WA7X, WA9WTK, WC0Y, WD4AH, WQ2H

    Heard by: AC9QR, AD5GH, AE5LY, EA1FAQ, G0LUJ, G3ZJG, G4KPX, GM4OAS, K4ZAD, K6MCS, K9AN, K9PAW, K9YWO, KA7OEI-1, KB0VER, KB9AMG, KC9NBV, KD2OM, KD6RF, KD6RF, KD7YUW, KD8WBZ, KD9KKA, KG5ABO, KJ4RWD, KJ4YBS, KK1D, KK4MBI, KK6PR, KM4RK, KN4GDX, KPH, KU4A, KY5M, N0ZHE, N1FXP, N2HQI, N2IXX, N2NOM, N3BBF, N3TCH, N4LQ, N6GN/K, N6GN/K2, N6GN/K3, N6RY, N8IT, N8UR, ND7M, NH6XO, OZ2JBR, PA0RDT, PA8AD, SWLKQ40LS, TF3GZ, VE3GTC, VE6JYKIWI, W0AIR, W0AY, W0GSQ, W1CK, W1NJC, W3PM, W4RYW, W5GHZ, W5OF, W5PSX, W8AN, W8QLF, W8SGT, WA0JYF, WA2ZKD, WA3TTS, WA5DJJ, WA5JOT, WA7ARK, WA9WTK, WB5DYG, WC0Y, WD4AH, WD8QLY, WF7T, WO7I, WQ2H, WS5W, WSLFN03
    -----------------------------------------------------

    WA7ARK
    Hearing: AD5GH, K9PAW, KC5QIH, KC9NBV, KD6RF, KJ4YBS, KM4RK, KN8DMK, N0ZHE, N1FXP, N3TCH, VE7BPB, W1CK, W4HBK, W5OF, W8AN, W8OZZ, W8QLF, WA0JYF, WA7KTM, WA7X, WA9WTK, WB5WPA, WQ2H

    Heard by: AD5GH, AE5LY, AG7NT, AI6VN/75, AI6VN/KH6, EA1FAQ, G8DYK, GM4OAS, K6MCS, K7MHI, K9AN, K9PAW, K9YWO, KA7OEI-1, KB0VER, KB9AMG, KC9NBV, KD2OM, KD6RF, KD7YUW, KD8WBZ, KF6JEP, KJ4YBS, KK1D, KK4MBI, KK6PR, KM4RK, KN4GDX, KPH, KU4A, KY5M, N0ZHE, N1FXP, N2HQI, N2NOM, N3BBF, N3TCH, N6GN/K, N6GN/K2, N6GN/K3, N6RY, N7TUG, N8IT, N8UR, ND7M, NH6XO, PA8AD, PY1EME, TF3GZ, VE3GTC, VE6JYKIWI, W0AIR, W0AY, W1CK, W1NJC, W3PM, W5GHZ, W8AN, W8QLF, W8SGT, WA0JYF, WA2ZKD, WA3TTS, WA5DJJ, WA9WTK, WB5DYG, WB5WPA, WD4AH, WD8QLY, WF7T, WQ2H


    Here's what that looks like for each call sign respectively for the 24 fr period (map filtered for each callsign prior to 'mapping'):


    Clipboard-9_WA7ARK_all_crop.jpg

    Clipboard-11_WB5WPA_all_crop.jpg
     
    Last edited: Jan 13, 2019 at 1:52 PM

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