Phased W6NBC 40m Verticals

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K1LH, Aug 30, 2020.

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  1. K1LH

    K1LH Ham Member QRZ Page

    A while ago I ran across W6NBC's 40m vertical, in simple terms is a 1/2 wave element with loading to shorten it to around 40ft, but results in similar performance to a 1/4 wave vertical with radials. See http://www.w6nbc.com/articles/2014-QST40mvertical.pdf

    Recently 40m has taken my interest for enjoyable SSB phone contacts during the daytime and evenings. I currently have an OCFD at 55ft, but with the local noise floor (S3 to S5) there are a layer of DX signals that don't quite get over it.

    That got me thinking about employing something with F/B ratio at the lower elevation angles that would be simple and cost effective to try out. I have a 1/4 acre lot, having multiple wire antennas up isn't an option due to the selection of trees available, the one wire antenna I have up must be multi band.

    I decided to model a pair of John's verticals with Christman Phasing in mind, I'm pretty new at modeling phased arrays, but I'd like anyone that's had more experience to take a look and offer suggestions or point out any mistakes.

    I have done two, one is 1/4 wave spacing with 90 degree phase shift, and another with 1/8 wave spacing with 135 degree phase shift.

    To calculate the line lengths I used VA7ST's calculator http://www.va7st.ca/christman.htm for the more common 90 degree setup. I saved a local copy of the page and edited the JavaScript formulas to calculate the 135 degree phase shift based on information in one of ON4UN's Low Band DX books (not sure which edition, I have 5th ed and its not in that one) that featured this by way of using 57 and 139 degree lines. 1/8 wave spacing is attractive due it less space needed.

    Assuming this is viable, I'm not entirely sure about how to do the matching network for either.

    The two EZNEC models are attached.
     

    Attached Files:

    AK5B likes this.
  2. AK5B

    AK5B Ham Member QRZ Page

    Sounds good; Lonney; I'll be watching this thread with interest---lots of John's designs offer viable solutions in a interesting package, so to speak. Good luck with your build.

    73,

    Jeff
     
  3. K1LH

    K1LH Ham Member QRZ Page

    Well, I've tinkered with the models some more, when I add in the coax loss the F/B degrades further.

    I also tried W7EL's current forcing method using Arryafeed1 mentioned on https://ac6la.com/zpapndx2.html , that seemed to have slightly better results, also taking into account the feed impedance and reactance of each element as reported by the Src Data.

    When using 2 sources and entering the desired phase shift directly, the patterns are perfect, but after translating that into transmission line lengths via the calculators and inputting that into EZNEC trans lines the patterns lose a lot of F/B, I'm not sure why the big difference.

    The impedance where the two lines meet (source) also very low except for the 1/4 spacing with Christman phasing.

    Anyhow, I'm just trying to replicate what's in the books, and what others have done, to better understand how to get this right and what to expect. But I'll admit I'm dumb as a brick with it comes to the math, I get lost in it pretty quickly.

    Another possibility is to switch to a driven / passive reflector arrangement and making it reversible with open stubs.

    But I like the idea of current forcing etc as I believe the gain, F/B and pattern will be more "resilient" to other objects in the near field interacting with the antenna.

    Hopefully I've got the terminology right :)
     
  4. K8DO

    K8DO Ham Member QRZ Page

    Make one element parasitic (what you call passive) and get on with it.
    90% of the pattern improvement promised by phasing harnesses is never achieved in the ham world - too many compromises, too much loss.
    Whereas it is almost impossible to stop a parasitic element from coupling.
     
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  5. K1LH

    K1LH Ham Member QRZ Page

    I think you may be right here, it would also explain why some have poor results with these methods in practice.

    I gave some thought to a method called Opposite Voltage Feed by OH1TV which is in the 5th ed of Low Band DXing.

    OH1TV details several examples at http://www.kolumbus.fi/pekka.ketonen/ His system employs a 1/2 wavelength of coax between the two elements, added inductance at the rear element (resonates it lower in freq), added capacitance at the front (resonates it higher), and a low pass L network to match to 50 ohms resistive. I emailed OH1TV and got a quick reply with some suggestions on how to dial in the models of this system in EZNEC which is simply running through iterations of values until the pattern is as good as it's going to get. Then add on the low pass L network to match it. He also suggested running two half wave lines from each element to a phasing box which houses all the components + relays to do the reversal etc. I'm going to explore this to see what shakes out. I enjoy the puzzle solving aspect of it and learning along the way.

    Otherwise as you suggest, parasitic reflector. I've explored making 2 element center fed arrays electrically reversible by means of open coax stubs of the right length to add inductance reactance to the reflector while the feed-line is connected to the driven. A switch at a common point can reverse the system by swapping which element is fed, and which has the open stub.

    Thanks for your input. Much appreciated!
     
  6. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    40 meters is the band
    Verticals work great phase verticals work better and four squares are the best I have four squares on 160 and 75. Phase verticals even receive very well. 9 times out of 10 I don't need the beverage antennas.

    After the last sunspot cycle I built my rhombic farm to operate on 40 in it turned out to best time I've ever invested.

    Get something up and start making contacts I hope the work you sick
     
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  7. W5VQ

    W5VQ Premium Subscriber QRZ Page

    I bought a Comtek 40 meter four square controller years ago, made 4 full size elements out of scrap tubing. Put down 40 radials under each element 35' long. This antenna has worked great. It's really nice to switch in four directions instantly and the F/B is very good. I think DX Engineering offers a unit like the Comtek now. Worked 291 countries on 40 cw using this antenna.
     
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  8. K1LH

    K1LH Ham Member QRZ Page

    Sounds like a good setup. If I had the space and less neighbors I'd have something reasonable installed for 40m :) While we have a 1/4 acre lot, the house is right in the middle of it, so that leaves the backyard, which is partly sloped and half of which is obstructed with 100ft Fir and 50ft Maple trees. The OCFD runs front to back down one side of the property between a 55ft tree at the front and branch half way up 100ft Fir tree, the short leg has to angle off towards the opposite back corner to avoid the Maple tree.

    So that doesn't leave much room to play with, limits me to a pair of verticals on 40m, 15ft spacing is easy, getting them 30ft much apart more effort. Which was why I'm keen on 1/8 wavelength spacing, or something near that.

    Right now I have enough on hand to build one of John's verticals. 40ft fiberglass mast from Spiderbeamwhich I can mount on the rear 2nd level deck, wire, coax etc. This weekend I'll have a shot at building it time permitting.
     
    Last edited: Sep 1, 2020
    AK5B likes this.
  9. K1LH

    K1LH Ham Member QRZ Page

    Made some progress with OH1TV's OVF feed system in EZNEC with transmission lines modeled with loss and VF based on LMR-400 spec.

    In this one I've used two loads in each element vs the one in John's vertical.

    The patterns are pretty nice, the bandwidth is narrower than I'd like, remote switching some values in the L network might be an option.

    I've attached the EZNEC file, some plots below:

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    Attached Files:

    Last edited: Sep 12, 2020
  10. N7WR

    N7WR Premium Subscriber QRZ Page

    Years ago, wanting to finish DXCC on 40 from the west coast and knowing that I needed to work some EU to do so I built a phased 2 vertical array for 40. Built it out of some spare aluminum I had from other projects. The base of each vertical was at 8 feet and I used 4 elevated radials with each. Once the phasing harness I built was right I worked a lot of EU quite easily
     
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