Mounting a HF vertical antenna on the building rooftop

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by OK8EI, Jun 3, 2021.

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  1. AK5B

    AK5B Ham Member QRZ Page

    Paul; Lucky you living in Prague---what a fantastic place---and atop a ten-story building there, no less!

    That SE-X80 looks just like a Comet CHA-250B clone, another very similar end-fed vertical with a lossy unun in the base. All antennas are a compromise in one way or another, but these types sort of lead the way.

    Without delving further into all the reasons why I will simply suggest that you try a more standardized, tried-and-true antenna that won't be difficult to install, tune and find rewarding in its performance;
    either a center-fed half-wave dipole as high as you can install it (the classic Hertz) or an elevated vertical monopole (the classic Marconi). Since I like to chase dx and also don't have any high supports necessary for best horizontal antenna performance, I like verticals (and I live close to the sea which helps their low-angle radiation, too).

    Narrowing my recommendation down to an easy-to-erect multi-band antenna, I would suggest one of the Hustler BTV series trapped verticals---and if the base is elevated just a few feet above the roof you'll only need one or two tuned radials per band of operation (eight, ten or dozen total would be great!).

    However, since the BTVs are all clasic Marconis---they will require a lot more radials if mounted on the ground (or directly on the roof which likely has sheet metal or similar underneath the surface). Mounted thusly you would need a bare minimum of 16 evenly spaced radials while 32 or 64 would be far better.

    Without going further into details I'll leave you with my recommendation for now and hope that it is of help to you in that magical place you now call home. I wonder if you have discovered the little underground pivnice many just call "the cave" close to the streetcar line/main street parallel to the Vltava yet? They served some of the best beer in the world there and it was only 5 Korunas for a big frosty mug when I was there years ago...

    Btw, a photo of one of my favorite places not too far from the "cave" is near the bottom of of my long QRZ page, too.

    73 and have fun!

    Last edited: Jun 3, 2021
    M0AGP likes this.
  2. AK5B

    AK5B Ham Member QRZ Page

    Jerry Sevick's (W2FMI, SK) shortened vertical experiments also bear this out; he found that the difference between a full-size Marconi and a short, fully optimized (via 120 radials, center loading and large capacity hat) was merely 1 db or so!
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    Then please explain my actual measured results (not calculated). I wasn't there when Sevick made his measurements. I know what I did when I made mine. I believe Sevick's claims to be unrealistic and not indicative of what a ham can expect from a shortened vertical antenna.

    I suspect that Sevick's measurements make the same mistake that Richard makes; namely assuming that local field strength measurements made within a few tens to hundreds of feet from the antenna predict how that antenna will work over a sky-wave path of several hundred to several thousand miles. That is why I discount Richard's overly optimistic claims for vertical antennas...

    Based on hundreds of rapidly-switched A/B comparisons (on 40m) over a sky-wave path from Arizona to Utah, California, Washington DC, Pennsylvania, and to other remote web-SDRs, the 8ft (shortened) screw-driver vertical is between -6 to -12dB worse than a full size 33ft vertical. A 33ft full-size 40m vertical (on the same steel roof near where the screwdriver was placed for testing) is 6dB worse than a 40m dipole at 55ft agl. The full-size vertical is 6 to 12dB worse than my ~48ft agl horizontal wire loop.

    The shortened screwdriver vertical has been moved to its permanent home in Woods Cross, UT, so I can no longer do the rapid A/B switching with it in the mix. I can, however compare my 40m vertical to my 40m dipole or to my loop used on its 4th harmonic. Those comparison results have held up week after week, year after year.

    I have one other basis to know that Sevick's (and Richard's) claims for loaded, shortened verticals are overblown. I used to run a mobile antenna efficiency contest at a yearly major Western States Hamfest in the 1970s. On 80m, the typical mobile antenna (best ones were center-loaded with large bug-catcher coils with large top hats) are 3 to 5% efficient! The fact that my "8ft screwdriver" worked out at about 15% on 40m is just about what this engineer expected. 15% efficiency is -8dB!

    One other data point. The 8ft screwdriver antenna is now installed on a metal building similar to my aircraft hangar in No. Utah. On our weekly 40m schedule, we have one ham at that location, and another ham lives about 1mi away, and uses a 80m horizontal wire loop on its second harmonic. That loop is only about 25ft agl. Both hams run 100W transceivers. I copy signals from both here in Az (airline distance = 408mi). The guy with the loop is 2 to 4 S-units better than the 8ft vertical... You cant get something for nothing!
    Last edited: Jun 3, 2021
    SWL37632 likes this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    That might be useful if the OP had asked about "an unloaded, 1/4WL Marconi vertical antenna base-driven against a good set of buried radials"!

    ...And just so nobody gets the wrong impression: to get the quoted 80% efficiency, you have to include ground wave radiation in the efficiency calculation. You know, the radiation which never contributes to actually talking to someone that is more than a few miles away!
    2E0CIT, SWL37632 and AK5B like this.
  5. AK5B

    AK5B Ham Member QRZ Page

    OK, fair enough on most of what you said just now---but I have to wonder about that comparison of the low horizontal loop to the shortened vertical---since a 40 meter horizontal antenna at only 25 feet is much more likely to be a good NVIS radiator for that short (408-mile) path than any vertical, shortened or full size, right?
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    So your expectation is that if you had the shortened vertical, you would be receiving all kinds of DX on the shortened vertical that you could not simultaneously receive with a dipole. I had the screwdriver up on my hangar roof for several weeks after building it before flying it up to Utah. During that time, I was able to make lots of rapid A/B comparisons against my other antennas, even though they are higher than 25ft agl. It was always the worst antenna here, by far, for both transmitting and receiving, but hey, it worked really well for only being 8ft tall!

    I had a 40ft aluminum irrigation pipe vertical up at the same time (still up, recently shortened to 34ft). If verticals were such wonderful antennas for DX, don't you think that I would be using it to work DX. In fact, the vertical is always worse than the other two. The resting noise level on the vertical is 1 to 2 S-units higher than either of the horizontal wires, so even if the vertical got out a better signal, I would have to switch to the wire during receive for weak signals.

    I truly believe that vertical HF antennas are way over-hyped!

    I am blessed (cursed?) in that I have room for multiple antennas. If I only had one antenna, I would swear that what-ever the latest antenna is, it is the best antenna in the world... Since I have two, three or more antennas on some bands, I can quickly determine which antenna works "best". In retirement, I have been building, trying, comparing, and discarding antennas at a prodigious rate... Verticals are not high on my list...
    SWL37632 likes this.
  7. 2E0VSS

    2E0VSS Ham Member QRZ Page

    Except for top band of course...and for obvious reasons..
  8. W9XMT

    W9XMT Ham Member QRZ Page

    Never have I claimed that a Sevick-type, coil-wound, top-loaded, electrically short vertical monopole is an efficient antenna system. Recently I posted an analysis of such a system operating on 7.2 MHz (see below for a re-post).

    To WA7ARK: this time, please notice and remember what I wrote in the OBSERVATION comment in that graphic.

  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    The latest standard for comparison: picture taken during the full moon last week:
    The 40m vertical is visible, as is the remote tuner (at 55ft agl) for the loop at the tower top.

    The new antenna has two (folded) elements on 40/30m. Beats my former best (the loop) by about 1 S-unit in the directions where the loop has peaks, and about 2 S-units where the loop has nulls.
    Last edited: Jun 3, 2021
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  10. 2E0VSS

    2E0VSS Ham Member QRZ Page

    We envy you guys with all that real estate..
    If i were to put that tower up here, id be carted off to the nearest Asylum and the Key thrown away.
    In fact, that span would encroach on 5 back yards.
    In a Ass about way it gives us massive problems to overcome, yet oddly enough, it almosty could be said to test our grey matter that little bit more to come up with viable ways to get a competing signal out of a Shoe box, Hence our Captain Mr Louis Varney G5RV saved our bacon in the 50s.
    Your Captain, James T Kirk lived in a world that was unreachable, untouchable..a little like the Jaw dropping acres of Land that we can only dream of.
    Beam me up Scotty, im gettin outta here..
    God bless America.
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