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

    After wringing my hands about it for a while, I decided to go ahead and electrically-isolate the 93-inch segment of downspout between the gutters and top of the aluminum-framed screen enclosure over my back yard, just so I wouldn't have to spend the next 10-15 years regretting not doing it while I had the opportunity.

    After getting it secured in place, I decided to pull out my NanoVNA and sweep its SWR... and was shocked by the apparent results. Despite being only 93 inches long, it somehow has SWR below 3:1 on 30, 20, and 17 meters.... and 4.70:1 on 40m.

    Inexplicably, though, its SWR on 15 and 10 meters was shockingly high... greater than 6:1 on 15m, and most surprisingly... greater than 8:1 on 10m, even though the downspout's length is (in theory, at least) almost precisely a quarter-wavelength of 10m long.

    Now... don't get me wrong. I'll be absolutely delighted if NanoVNA's opinion is actually legit... but this also has me questioning almost everything I thought I knew about vertical antennas, because it seems like these SWR readings are totally implausible.

    My hunch is that something about the test environment just completely broke all of NanoVNA's assumptions (like maybe 50-ohm impedance at the feedpoint?), and badly-skewed all of its interpretations of the data as a result. But... I have a tiny grain of hope that there's a perfectly good explanation, and maybe the SWR readings are real:
    • Perhaps the large surface area of the 3" x 4" downspout is making it act like a much longer one?
    • Perhaps the aluminum frame of the screen enclosure is so spectacularly good as an aerial counterpoise/ground plane/radial field, it's giving me some kind of bonus performance?
    • something else?
    Note that I DID explicitly calibrate NanoVNA for each range of frequencies, and got consistent results from subsequent sweeps... both over large ranges AND relatively narrow frequency ranges.

    Anyone have any explanations? (I'm going to reply to this post using my phone and post some photos I took of it). Here are some of the measured SWR and frequency values:
    • 40m, 4.95:1 to 4.67:1 between 7.000 and 7.300Mhz
    • 30m, 2.46:1 to 2.22:1 between 10.100 and 10.150MHz
    • 20m, 1.74:1 to 2.08:1 between 14.00 and 14.350MHz
    • 17m, 2.85:1 to 2.96:1 between 18.068 and 18.168MHz
     
  2. N4VDI

    N4VDI Ham Member QRZ Page

    Here are some photos 20210427_192910.jpg 20210427_195408.jpg
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    SWR and performance are not well related.

    Forget SWR measurements: How does it work, to make contacts?:)
     
    AK5B likes this.
  4. AJ4GQ

    AJ4GQ XML Subscriber QRZ Page

    What does your rig and antenna tuner say when you try to transmit? Can you get your hands on something like a RigExpert AA-30 or bettter?
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The Rig Expert is unlikely to be any more accurate than the miniVNA.

    The real issue is, lots of loads exhibit great SWR(50) but don't radiate worth a darn.

    The real proof is in the contacts made, and whether people hear you when you call them.:)

    My old saw, "I'd rather have a G5RV than a 3-element beam." Provided the G5RV is strung between the tops of two 120' towers, and the beam is at 20 feet on a mast.
     
    AK5B and PU2OZT like this.
  6. KH6AQ

    KH6AQ XML Subscriber QRZ Page

    I suspect coupling to other objects and high loss are contributing to the measured input impedance.

    The downspout is mounted up against what looks like a stucco wall. Is it? If so, it has metal mesh inside and who knows what electrical wiring behind that.

    It is grounded to some kind of metal thing and the cage. Where are those grounded? The RF return to current flows thru those objects to ground somewhere and radiates; it is the other half of the antenna. It too can couple to the building, finally terminating in possibly nothing more than the building ground rod.
     
    AK5B, K2CAJ, W6MK and 2 others like this.
  7. N4VDI

    N4VDI Ham Member QRZ Page

    There's no rig (yet). I just opportunistically insulated the downspout because I could, and because after tomorrow, I wouldn't have been able to, and I didn't want to spend the next 15 years regretting it.

    I rigged up the antenna connection to test it with NanoVNA because afterward, I was dying to get an idea how good (or bad) it might actually be.

    Hopefully, I won't have to depend on it as an actual antenna... but on the other hand, if it actually DOES work well on 20m, and decently on 30m & 17m, that would give me a HUGE amount of added flexibility & slightly reduce the urgency of spending $1k+ on a screwdriver antenna and Tarheel motorized tilt mount. :)
     
  8. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    How good is the electrical bond between sections of the downspout, and its bond to the gutter at the eve?

    Why not feed the downspout from the very bottom (more radiator length)? If the AC plug of your rig's power supply has a ground pin, then the house wiring becomes a part of the counterpoise “ground” system (actually becoming radials). Any meaningful SWR reading should be from the shack, and include the powerline neutral as ground reference

    You might try to contact “Wink” @WA8KOQ for advice. He uses a downspout antenna, and I've worked him a few times on 30 meters (last QSO, just 3 days ago).

    73,
    Jim . . . . . . . . . EARTH: LOVE IT OR LEAVE IT
     
  9. N4VDI

    N4VDI Ham Member QRZ Page

    @W9JEF There's no "bond" within the 93" upper downspout segment between the white vinyl insulators... it's a single continuous segment. The different color is because whomever originally put it up replaced the lower portion & didn't want to cut the new piece, so they just rammed 3 feet of it into the new one.

    I'm actually taking the whole thing down tomorrow to replace the dented lower section & (re)paint both segments. What you see in the pics was just today's science experiment prototype to analyze with NanoVNA.

    As for not using the whole length, the lower (now insulated) portion of the downspout lies below the plane of the grounded screen-enclosure aluminum beams serving as its radials/ground plane, so extending the element below it would be like wrapping half the antenna in a Faraday cage.

    Theoretically, now that the top is insulated from both the part below the screen cage & gutter above it, it should just be like a 93" ground plane antenna over a partial aerial radial field (to the south & east, but not north or west) that -- in theory -- is close to 10 meter's quarter wavelength.

    What surprised me isn't that it might perform poorly (I just took THAT for granted), but that it seemingly ended up with low SWR on bands where a 93" element ought to have terrible SWR, and high SWR on 10m (where it should be low, since 93" is close to 1/4 wavelength near ~30mhz).

    Also, I cheated slightly... I also added a connector near the top for a future whip wire, figuring it would allow me to tune it perfectly for 10m with maybe 5-10 inches of wire, or even 15m if I added 3 or 4 feet & curved it towards the roof to minimize its visibility. Not quite a pure "downspout" antenna, but more like a vertical disguised as a downspout.

    At one point, I contemplated whether someone who needed a "stealth" antenna could make a literal trapped multi-band vertical by combining aluminum & vinyl downspout segments, with coils wrapped directly onto the vinyl segments... then installing it as a FAKE downspout (butting the vinyl top against the gutter, without actually creating an opening for water to flow through). Basically, so it would LOOK like a downspout from a distance, but not actually BE a working downspout.

    It wouldn't work for ME (since I only have 8 feet of clear height above the ground plane of the aluminum screen frame), but I could totally see it working in a neighborhood with 2 story McMansions & high rooflines. In a place with a walk-out basement, you could probably stealth a literal full-size quarter-wave 40m antenna by disguising it as a fake downspout extending from basement-windows to attic ;-)
     
    Last edited: Apr 28, 2021
  10. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    A Faraday cage must totally surround the area it shields from RF. Feeding the downspout from its base would lengthen the radiator. To prevent cancellation, you could run a well-insulated ground lead UP THROUGH the downspout, then out through a hole at the level of the aluminum beams.
    You should try bonding the top of the downspout to the roof gutter. The top loading could result in a much better antenna for the lower bands.

    73,
    Jim . . . . . . . . . EARTH: LOVE IT OR LEAVE IT
     
    WB5YUZ likes this.

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