Suspiciously low SWR for downspout antenna

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

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  1. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    Just wanted to see if posting a reply without a quote would bump the thread back to the New Posts page.

    EDIT: It didn't appear for me on New Posts (because it was me?). Do others now see this on the New Posts page?

    EDIT 2.0: Okay, I now see what's happening here. As soon as I read a post, it's no longer a “new” post. Its status then changes to a Recent Post. :cool:

    73,
    Jim , , , , , , , , , EARTH: LOVE IT OR LEAVE IT
     
    Last edited: Apr 28, 2021
  2. N7EKU

    N7EKU Ham Member QRZ Page

    Hmm,

    No need to be shocked. You know there's nothing special about the center conductor feeding the antenna? Nothing saying, "All the RF is going out here!" or "This is the most important part of the antenna!" An antenna consists of two parts. Yours has one part 93" long and another part (that big aluminum metal frame thing) how long (maybe pretty long/big)? What you have is some kind of off-center fed set up, with some or both legs kind of lossy due to stucco and whatever else is near or attached to the frame.

    I guess some downspouts could work fairly well as antennas. But likely this would be one on a wood-sided house, and then supported by a large radial field, or by tuned elevated radials.

    73
     
    AK5B and W9JEF like this.
  3. N4VDI

    N4VDI Ham Member QRZ Page

    @W9JEF, so you're saying that I should
    • eliminate the insulating vinyl connector I added to insulate the downspout from both contact with the screen enclosure's grounded frame and the portion of the downspout below the ground plane formed by it (but find some effective other way to insulate the metal downspout itself from contact with the aluminum frame where it passes through the opening)
    • drill a hole through the downspout at the same level where it passes through the opening framed into the screen enclosure's top, run an insulated wire in through it, down inside the downspout, and out the bottom opening, then connect the top end of the wire to the grounded frame, and the bottom end to the ground rod?
    That's interesting. I was under the impression that nothing good could possibly come from having a portion of the vertical radiating element below the aerial ground plane.

    Does the wire need to be separated from the downspout at the point where it exits through the hole near the middle by something like a cable gland, or is the insulation enough separation from the downspout?

    Here's what my original rationale was for insulating the downspout away from the gutter along the roofline:
    • The gutter always has at least a quarter-inch or more of standing water in it. My air conditioner has a condensate pump that routes it up to the roof, through a pipe, and ultimately dumps it into the same gutter used to collect rainwater from the roof. I figured that water inside the downspout is somewhat transient due to gravity, but that the perpetual presence of water in the gutter would detune it too badly.
    • As noted, I made provisions to connect a wire to the top of the downspout so it will emerge up through the hole in the gutter & can be bent and positioned as desired. I figured this would give me more control over its resonance than mere luck of the draw from whatever length (approx. 19 feet, actually) the gutter itself happened to be.
    • I figured that if I ever really wanted to, I could use that same wire to simply bond the downspout to the gutter. The difference being, it would be something I could control (and more importantly, alter in the future from up on the roof, without having to tear out and replace the screening to get at it from below).
    • The gutter runs north-south. I figured that portion of the antenna would behave somewhat like a dipole. The northeastern US megalopolis would mostly fall within the dead lobe to the north. That would be a Very Bad Thing.
    • By adding a wire to the top of the downspout instead, I can orient it east-west, improving my coverage of the eastern US to the north. A wire laying on top of a flat roof wouldn't be great, but it would probably be no worse than a gutter with an inch of standing water (surface area notwithstanding). In theory, I could also use something larger-diameter, like a one-inch diameter aluminum pipe laying horizontally on top of the roof in an east-west orientation (so the signal would be constricted to a wire for a foot or two, but could then have the larger-diameter pipe for the remainder of its journey along the surface towards the end).
     
  4. N4VDI

    N4VDI Ham Member QRZ Page

    backyard-gutterview.jpg @N7EKU, Here's a marked-up photo of the back yard taken from the roof looking down. "Main Antenna" is where I'm planning to put my primary antenna during normal use (I sewed a zipper into the screen panel there, so I can unzip it and poke the antenna up through it so it'll be entirely above the plane of the grounded screen enclosure frame).

    If it's not obvious, it's a townhouse. The back yard is surrounded by 6' concrete walls to the north, east, and south. To the west is the two story rear wall of the house itself. The screen's top is approximately 10 feet above the ground for the west and center three panels. The eastern 3 panels slop downward to transition it from being 10 feet high to 8 feet high.

    This whole mini-project came about because I'm re-screening, and for the first time since I bought the house 12 years ago, I have access to the area above the screen. My eventual plan is to put a screwdriver antenna with motorized tilt base up on the roof (rendering all the antenna plans above the screened area long-term moot), but for now, the screened-over back yard has a major advantage over the roof: accessibility, so I can take the antenna down when I'm not using it (to avoid provoking the HOA), and swap antennas whenever I want to. In contrast, getting up onto the roof is a major, time-consuming and laborious chore... so anything that goes up on the roof has to be capable of being left untouched by human hands for months at a time. The HOA is fairly mellow... but I wouldn't dare to put a ham antenna up there in full public view without a motorized tilt mount so I can lay it down and hide it when it's not in use. I've already learned that when something can't be 100% hidden, the next-best thing is to play what I call the "Batcave Strategy"... high "coolness" factor, and make it obvious that you've bent over backwards to appease them.
     
  5. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    Owing to the much greater conductivity of the gutter surrounding it on 3 sides, even a large amount of water, should have very little effect on the tuning.
    If you make the wire the same length as the gutter, and run it in an opposite direction, it would cancel any directivity introduced by the gutter. But the directivity would occur strictly in the horizontal plane, and diminish as the vertical angle increases. Due to rotation of charged particles, the ionosphere has a habit of changing the wave polarity (one cause of QSB).
    You have lots of possibilities, and a knack for experimentation. The heuristic approach to engineering--what the ARRL calls the “cut and try” method.

    You have the true ham spirit--learn by experiment, and have fun doing it. :cool:

    73,
    Jim . . . . . . . . . EARTH: LOVE IT OR LEAVE IT
     
  6. K8DO

    K8DO Ham Member QRZ Page

    Have some local ham with a portable rig and tuner drop by and put power to your antenna farm,
     
  7. PU2OZT

    PU2OZT Ham Member QRZ Page

    «HAM» spirit vs Radio, you nailed it!
    Methink philosophy here is 55% fun, nowaday's motto, why applying known working solutions, first, if you could become the newest Tesla on Planet Earth having fun, and getting others attention. Then there is 40% opportunism and remaining 5% is luck.
    Slider moves, can be 20% luck, if you like.

    Hey!!! adjust condensated water flow to propagation instead of air humidity! or, fill your Guttenna with heavy water and you could even turn yourself into Henri Becquerel of the millenium. Wouldn't be great, to win a trip to Stockholm next year?

    And ultimately and hopefully, your thingamajig feels so lossy would not even tickle a gutter cat.

    Oliver
     
    W9JEF likes this.
  8. AA5MT

    AA5MT Ham Member QRZ Page

    Did you check it with a ground wire attached to the ground side of the coax - like the antenna will have? If you did not, you may not see the same results when you connect a radio. The antenna floats - the radio does not, if it uses a grounded power supply.

    An old story. A radio operator decides to move his mobile radio to his house, rather than buying a new one. He checks the antenna on the car with the swr meter, and just to make sure, he checked it with his antenna analyzer. Both had a 1:1 swr shown. When he moved it to the house, the analyzer showed a good swr, but the radio would not load up, due to high swr. After much consideration, he decided that the ground wire added must have something to do with it. When he removed the ground wire, suddenly the swr was 1:1 on the radio. On this type of antenna, you cannot ground it without a proper ground plane under it, not just a ground. The ground plane takes the place of the vehicle body. Your mileage may vary.

    Tom
     
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  9. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    What about the capacitance of these vinyl (dielectric constant of ~3) insulators? The end is capacitively coupled to the gutter. Looks like you have a metal roof--assuming the gutter is insulated from that, there's that pesky capacitance to consider.
    My theory is that running the ground through the inside of the lower portion of the downspout radiator (bass-ackward coax with ground as inside conductor) might negate the cancellation due to its current opposing the current of the radiator. I'm no EE, so I could be wrong.

    Have you considered a flagpole (like the one I saw that's across the street from you, only taller)?

    73,
    Jim . . . . . . . . . EARTH: LOVE IT OR LEAVE IT
     
    PU2OZT likes this.
  10. AK5B

    AK5B Ham Member QRZ Page

    Great idea and away from the stucco!
     
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