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. OK8EI

    OK8EI Ham Member QRZ Page

    I am moving into a new apartment soon and it is on the 10th last floor of the building, in a higher altitude part of the city. The apartment has a terrace, and it's possible to step up on to the rooftop of the entire building from there.

    I would expect that this is an ideal place to mount an antenna. I plan to install a HF multiband vertical antenna there to operate on 80 m to 10m.

    However, I'm reading alot that this is not ideal and won't work well because of grounding issues and the requirement for radials.

    I am very interested in this antenna SE-X80
    : https://www.thunderpole.co.uk/amateur-radio-homebase-antennas/se-hf-x80-vertical-hf-antenna.html

    It states that no radials are required.

    Bottom line is, is this a good idea? Would it work well on the rooftop? Any suggestions appreciated. I'm still a sort of a noob when it comes to antenna theory, grounding etc. Thanks
     
  2. NE4ME

    NE4ME Ham Member QRZ Page

    If that's all you can do, it's all you can do. Never hurts to try unless lightning comes along. Hopefully lightning is not a big thing in your part of the world. If you are trying to work DX you may really like it, but if you can get a ground radial system you will likely like it better. The only time that I have ever really been impressed was with an old Hustler 10,15,20,40, 80 trapped vertical was when I was on 75M working DX. On the rest of those bands is beat my old 102ft G5RV, but I put up the double sized G5RV and it beat the vertical on all of those bands. If you want to talk close in on 40M and 80M you will have a hard time doing it with any vertical. With as many floors as you will be up, you may have a great signal on 10-15-20 with minimal work. As small as that vertical is, it's always going to be a compromise on 40M on up. I saw where that antenna boasts pretty good SWR on some bands. The saying I have been using for years is that SWR is a function of match and not efficiency. If low SWR rocks, then dummy loads would be great all band antennas ;) A dipole is really hard to beat on each HF band and you can run some extra power and not worry about smoking non-resonant antennas, voltage baluns, antenna tuners, etc. The antenna you mention may just be the "right now" antenna for you, but as you get more time in the hobby, you will see how much better some other stuff will work for you.

    On a slightly different topic. I don't know if you do much VHF/UHF in that area, but 10 stories up would be amazing if you could get up some small yagis up and work SSB on 6M, 2M,222, 432, etc. I have worked VHF/UHF from many mountains and then once from a 15 story building and they were all so much fun. Those top floor apartments cant be had all that often, so use it while you can.
     
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  3. OK8EI

    OK8EI Ham Member QRZ Page

    NE4ME .. thank you for the reply.

    I'm still learning here.. just a couple of things regarding your experience if you don't mind

    You state "if you can get a ground radial system you will likely like it better".. why exactly? What would be the significant differences here, with and without a grounding system?

    If the antenna states that no grounding is needed, why?

    I believe on the rooftop there is a common ground point from other tv antennas there going down the building to ground, could I use this as my ground point also? Thanks.
     
  4. W9XMT

    W9XMT Ham Member QRZ Page

    Short answer: A vertical monopole must be driven against conductors either buried in soil directly beneath it, or, if the base of the monopole is located atop another structure, then a set of horizontal "counterpoise" wires symmetrically arrayed around the base of the elevated monopole.

    Otherwise there is no path for r-f current to flow efficiently between the two output terminals of the source (the transmitter), and the radiation efficiency of the monopole antenna transmit system will be very poor.

    A "grounded" conductor/structure having a long vertical path to reach the Earth is a radiating element of the antenna system, and does not have the same r-f characteristics at its top as at its base.
     
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  5. OK8EI

    OK8EI Ham Member QRZ Page

    NE4ME .. thank you for the reply.

    I'm still learning here.. just a couple of things regarding your experience if you don't mind

    You state "if you can get a ground radial system you will likely like it better".. why exactly? What would be the significant differences here, with and without a grounding system?

    If the antenna states that no grounding is needed, why?

    I believe on the rooftop there is a common ground point from other tv antennas there going down the building to ground, could I use this as my ground point also? Thanks.
     
    Last edited: Jun 3, 2021
  6. OK8EI

    OK8EI Ham Member QRZ Page

    Thanks. So a monopole vertical antenna designed with counterpoise radials at its base, should definitely perform better than an antenna without radials? I guess it's not as straight forward as that however..
     
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    Paul,

    there are two types of commercially produced vertical antennas that you might be looking at:

    1) the kind that is fundamentally a Marconi-style, resonant, shortened (loaded), 1/4wl vertical radiating element (monopole) which expects to be installed on full-length 1/4wl horizontal radials (minimum 2ea diametrically opposed on each band). The antenna is effectively 1/2wl in diameter (radial tip to radial tip) and what would otherwise be 1/4wl high monopole is reduced by loading to a fraction of that.

    2) the kind that is fundamentally a Hertz, resonant, shortened (loaded), 1/2wl dipole radiating element which is turned 90 degrees to vertical. In this case, both the part of the antenna above and below the feedpoint are shortened by loading. This makes the antenna occupy only the vertical dimension (no radials), but it will be taller than 1), above.

    Now think about this. 1/4wl at the 40m band is 10metres or 33feet. 1/2wl is 20m or 66ft. Can you expect to erect a 20m long vertical antenna on your roof? If not, how much can you shorten it and expect it to retain its efficiency. The cost of shortening a radiating element is loss of efficiency... In 1), only half the antenna is effectively being shorted, while the other half has to remain full size (the radials). In 2), both halves of the antenna are being shortened simultaneously....

    The dirty little secret of both 1) and 2) is how little of the power fed to them by your transmitter actually gets radiated. You can learn about that by reading some old posts of mine, here:

    Post #27,29, & 30 here is a type 1) motorized screwdriver shortened vertical antenna I built for on a metal roof at a height-restricted fixed location (airport). I already had the "ground plane". It is equivalent to tens of thousands of radial wires... I only had to shorten the vertical radiator part. Even though, the efficiency of this system is about 15%. (85% of the RF power never gets radiated).
     
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  8. OK8EI

    OK8EI Ham Member QRZ Page

    Thanks for the info.

    Now as we increase in frequency, let's say using the 20 meter band :

    If we have a 10 meter ( 33 foot) long antenna for example, no doubt efficiency will increase as ths will be a 1/2 wave for 20 meters. The radiating is element is now big enough to fit half of the wave. Would we still require a ground plane here in this case? My guess is no as the antenna is able to radiate all or most of the RF (correct me if i'm wrong :) )

    However, if we have a 5 meter long antenna, this will be a 1/4 length antenna for the 20 meter band. I guess a ground is required in this case as not all of the RF gets radiated as we wish and it needs to go somewhere?

    There are still many 1/4 wave antennas (for the 11 meter band for example) , that don't required radials or grounding. This is what confuses me .
     
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    There are two types of CB antennas that I am familiar with, those that have a 1/4wl (8.5ft) monopole and 1/4wl radials, or the type which have very short radials but the monopole is almost 1/2wl long (18-19ft).

    The basis for this is demonstrated by my previous post:
    https://forums.qrz.com/index.php?th...-vs-ground-radials.651001/page-6#post-5007151

    The reality is that unless the antenna is "loaded" (to shorten it), to a first approximation, the sum of a radial length plus the monopole length have to add to 1/2wl in order for the antenna to be resonant.

    If the antenna is vertical Hertz dipole, then its total length for resonance is still 1/2 wl. However, the feedpoint can be moved lower than the center. That changes only the feedpoint impedance, as described here:

    https://forums.qrz.com/index.php?th...-or-raised-radials.681510/page-3#post-5269934
     
    Last edited: Jun 3, 2021
  10. W9XMT

    W9XMT Ham Member QRZ Page

    A further reality is that an unloaded, 1/4WL Marconi vertical antenna base-driven against a good set of buried radials actually radiates (launches) more than 80% of the Z-matched transmitter power applied across its feedpoint terminals. See the graphic below.

    This radiation efficiency (or better) also applies to that vertical radiator when elevated, and driven against a counterpoise consisting of 2 or 3 pairs of elevated, horizontal wires around the base of the radiator.

    This radiation efficiency is much better performance than the ~15% value stated in earlier posts to this thread.

    [​IMG]
     
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