Suspiciously low SWR for downspout antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N4VDI, Apr 28, 2021.

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  1. N4VDI

    N4VDI Ham Member QRZ Page

    @W9JEF, oops, I think you looked up my FCC address & found my PARENTS' house. I've been trying to update it, but the electronic bureaucracy has been a nightmare. The email address on file with the FCC hasn't existed since ~1998, so I can't receive the confirmation code to complete the update, but updating the email address on file requires delayed future action within a narrow window of time that begins "hours or days from now, after I've totally forgotten about it", then expires "before I stumble over it by accident or think about it again".
  2. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    I feel your pain.

    Since Google updated their street view, if you try to find our house, you're, instead, directed to US-412, a football field or so to the south of us. Not only that, but the aforementioned 4-lane divided highway is mislabeled as “Old Highway 68” (where our home actually is located).

    Jim . . . . . . . . . EARTH: LOVE IT OR LEAVE IT
  3. N4VDI

    N4VDI Ham Member QRZ Page

    Oh, also... a flagpole wouldn't really be viable.

    Under Florida law, the HOA can't legally prohibit flagpoles... but it can & does require that any permanent flagpole be used daily between 9am & 5pm AS a flagpole (and conversely, requires the flag to be removed between 9pm & 5am).

    The HOA's official solution to "I don't want to be enslaved by daily flag-flying requirements" is to install an approved permanent foundation (surrounded by dense landscape) & take the pole down when you aren't going to use it daily. For a literal flagpole, that might semi-work. For a fiberglass cylinder with antenna inside... not so much.

    Also, the flagpole's location is prescribed by the HOA. In my case, it would have to be inside a 20x20 foot U-shaped area with 2-story concrete walls to the north, east, and south.

    Effective radials in that area would be almost impossible for a whole host of reasons that include "lots of concrete sidewalk & wood deck, very little bare earth".

    Finally, getting a feedline to the antenna would be seriously problematic. The radio would be at the rear of the house on the second floor, and there's no good route for 75 feet of thick RG8 coax from there to the living room on the first floor at the front of the house (where it would exit through a wall & continue to the feedpoint).

    It would also be impossible to keep trespassing kids playing 'hide & seek' away from it, and it would be about 12-18" from the sidewalk leading to the front door.
  4. W9IQ

    W9IQ Ham Member QRZ Page

    I wouldn't complete discount your SWR measurements. The antenna is essentially a top load, vertical antenna with a counterpoise structure. Given the complexity of the real world issues, it would be difficult to model but it would be interesting to give it a try. You may have more issues with household RFI than with SWR.

    Good luck with the project.

    - Glenn W9IQ
  5. N4VDI

    N4VDI Ham Member QRZ Page

    @AA5MT, the ground path looked something like this:
    • NanoVNA
    • thin blue coax cable
    • SMA to UHF adapter
    • UHF chassis mount
    • thin red wire connected to ground side of UHF chassis mount
    • aluminum-framed screen enclosure cage (2" x 2" north-south frame elements, 2" x 4" east-west frame elements, see photo from above to get the general idea)
    • route of approximately 6-8 feet along the screen enclosure frame, to the point above the north wall where I illustrated the location of the ground rod.
    • 8 foot AWG12 copper wire between bottom of screen enclosure frame (resting on top of 6' high north wall) to ground rod
    • bronze ground-rod clamp
    • steel-core copper-clad ground rod
    The ground rods are likely to present a long-term practical problem I hadn't remembered until yesterday (when I tried to hammer one of them in) -- my yard only has about 12 inches of actual soil. Below that, it's all 3-6" chunks of crushed limestone (until, apparently, you get down to the level of the natural terrain, where it becomes literal solid limestone bedrock).

    I suppose I could pay someone to drill holes for the ground rods... but it seems like I'd get a better "ground" by just digging a horizontal trench down to the limestone layer & burying the rod horizontally. At least then, it would be making contact with 8 feet of soil, instead of passing through a foot of soil and sitting inside a loose hole bored through rock & secured with little more than limestone dust.

    For what it's worth, the measured resistance between almost any two points of the screen enclosure frame that are within arm's length is around 2.5 to 3.5 ohms.

    When the frame is floating and not bonded to the ground rods, the measured resistance between them is (unsurprisingly) "infinity", and the voltage difference appears to be 0.015v.

    When the frame is bonded to the ground rod with the 8-foot AWG12 wire, resistance falls to 0 ohms along the wire itself, and returns to ~2.5 to 3.5 ohms when the path includes the aluminum frame. The voltage difference measures "0.0v".

    I have no idea whether I would, or should, see anything besides a 0.0v difference between a pair of ground rods ~18 feet apart. I'd expect that there would be none, and that the only reason I saw a difference between the floating frame and a ground rod was because it's presumably acting like a capacitor and storing a charge that expresses itself when I connected the enclosure and ground rod through the multimeter. My rationale for having two ground rods instead of one is to ensure that the enclosure frame closest to my adjacent neighbors is better-grounded than any point beyond it, and that the path of least-resistance for any stray current will always be "towards my own yard and the nearest ground rod, instead of any alternate path away from it".
  6. N4VDI

    N4VDI Ham Member QRZ Page

    Update: I did another round of experiments to measure the SWR (and generate Smith charts) for various scenarios, including:

    1. Original downspout segment (93" long, entirely above the screen enclosure top's plane, connected to the coax (mounted a foot below the enclosure's top plane) via ~15" of wire:


    2. New, longer downspout segment (~105" long, bottom is approximately 6" below the enclosure's top plane and 4" below the bottommost surface of the top's framing), connected to the coax via ~6" of wire:


    3. #14 wire approximately 112" long, running from the spot where the downspout would normally be attached to the gutter along the roof's edge down along the downspout's normal path, then connected to the same place as #1 and #2. For the record, it's not electrically connected to the gutter at the top.


    And got the following results:

    Downspout #1 (93"):

    SWR: 40m=2.2:1, 30m=1.43:1, 20m=1.37:1, 17m=3.01:1, 15m=5.05:1
    Smith: 30m=39.8ohm-12.5j, 20m=38.5ohm-8.20j, 17m=14.1ohm-13.8j, 15m=10.0ohm-7.07j


    Downspout #2 ( 105"):

    SWR: 40m=3.9:1, 30m=2.02:1, 20m=1.90:1, 17m=4.05:1, 15m=6.13:1
    Smith: 30m=27.3ohm-13.9j, 20m=26.2ohm-1.14j, 17m=12.8ohm-9.79j, 15m=8.24ohm-5.37j



    SWR: 30m=22.80:1, 20m=6.71:1, 17m=3.61:1, 15m=1.71:1

    Smith: 30m=1.60ohm-10.0j, 20m=4.1ohm-15.2j, 17m=17.4ohm-24.2j, 15m=30.6ohm-9.11j

    The main noteworthy points:

    • The plain wire behaves more or less as expected for a vertical wire of that length. There's obviously some stuff going on due to its proximity to the wall, but nothing particularly shocking. I could probably tweak its length to optimize it for 15m, but anything more is obviously going to require a loading coil.
    • The original, shorter downspout has a better curve for 17m and 15m than the longer one... but the longer one appears to be usable for 40m, and potentially good for 30m and 20m. Given that 40m, 30m, and 20m are my three favorite bands, this is a big deal.
    I just remembered that a couple of months ago, I found an article somewhere online that I can't find anymore that (assuming I understood it correctly) might explain the low SWR with the downspout segments vs wire.

    Apparently, when you have a short vertical element that's a hollow metal tube, it can exhibit the reduced SWR of a much longer element. If I understood it correctly:
    • The RF makes contact with the outside surface of the vertical element, and takes the path of least resistance: up towards the top, almost entirely along its outside surface.
    • Upon reaching the top, it has two paths to choose from:
      • Across the top lip, then down along the inside surface of the tube towards the bottom
      • Back down along the outside surface of the tube, flowing upstream as SWR
    • Being metaphorically "lazy", it mostly chooses to take the inside path.
    The net result is that it behaves like a hybrid of two parallel antennas in close proximity and an element that's twice as long, but has the top half "pulled inside" the bottom half (so the field generated by the "top half" gets uselessly focused almost "straight up" and "straight down" ... kind of like a metaphorical high-pressure jet of water erupting from a wall of mist (representing the "desirable" RF emission from the lower portion).

    If that's the case, would I really be better off replacing the aluminum downspout segment with vinyl, hiding a wire (or maybe 1/4" copper tubing) behind it (so the HOA wouldn't see it), and relying upon a bottom coil (like a W3FF Buddipole coil, or something along those lines) to make it behave like a normal "honest" coil-loaded short vertical element? Or should I just keep the downspout, enjoy the low SWR on 20/30/40m, and not worry about it?

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    Last edited: May 18, 2021

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