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Antenna Placement ontop of Cement Structure

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC2SAH, Dec 9, 2018.

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  1. KC2SAH

    KC2SAH Ham Member QRZ Page

    Greetings all,

    I have been trying to do some research on antenna placement, but have come up short on a definitive answer and was hoping that I could get some feedback from here. I am trying to determine the best place to put my 40m and 20m dipole antennas. I live in a typhoon-prone area, so my home is an entirely cement two-story structure. Normally, wood-framed homes don't have a huge impact on the radiation patterns of dipole antennas (ignoring the effects of AC units, etc.), so an antenna placed on the roof of a two story home would act like it is 30' in the air. But what about placing those same dipole antennas on top of a two-story cement roof? Does the radiation pattern act as if it were only 5' off the ground?

    Thank you all and I look forward to your thoughts.

  2. AI3V

    AI3V Ham Member QRZ Page

    Acts like 5' off the ground.

  3. WG7X

    WG7X Platinum Subscriber Platinum Subscriber QRZ Page

    Let's think about that. Now, would a vertical antenna on top of a mountain work better or worse than one at sea level if both of them were "ground mounted"?

    Assuming the same amount of ground conductivity?

    I am guessing that the antenna that is much higher will work better than the one at sea level, all other things being equal. So my answer to your question should be obvious. Any antenna elevated high enough is probably out perform one at ground level.

    The antenna pattern will depend more on what kind of stuff is in the house under it and how high the antenna is above that stuff. Metallic items in the concrete like rebar will most likely affect the pattern, but it will be totally unknown how it will affect it since we have no idea where they are.

    Just put it up and see...
    KC2SAH likes this.
  4. K4SAV

    K4SAV Ham Member QRZ Page

    Your cement roof is likely to be fairly conductive and it will affect the antenna pattern. Portland cement is RF transparent but the stuff they put into it to make concrete plus the rebar or wire mesh should make it into a pretty good conductor..

    An antenna at 5 ft over the roof will have some very high angles lobes due to the roof but the antenna will also see the ground in the distance and that will help the low angle signals. The antenna pattern will not look like that of a normal dipole. Placement of the antenna on the roof and the roof size should also affect the pattern a lot. Placement on the edge of the roof should result in more low angle signals in the direction away from the roof. There could even be a null in low angle signals in the direction of the long area of the roof. Antenna placement for maximum amount of high angle signals and minimum low angle signals should occur in the center of the roof.

    Jerry, K4SAV
    KC2SAH likes this.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Get the dipole higher above the roof, or somewhere else in the yard where it's as far from the house (and above the house) as possible -- you'll see a positive difference.

    With a "cement" house, if restricted to an antenna that's only directly above the house and can't be anywhere else, I'd use a vertical and not a dipole. If it has a flat roof (?), all the better for a vertical as you have lots of places to easily lay down radial wires, and wire is cheap -- if it all blows away in the next hurricane, replace them.:)
    WA8FOZ, K2CAJ, KC2SAH and 1 other person like this.
  6. KC2SAH

    KC2SAH Ham Member QRZ Page

    The house does have a flat roof, with plenty of room to walk around. It's a quad, so approximately 40' by 120' in total. Like WB2WIK said, plenty of room for radials. I am out in Okinawa, Japan, so my signals have to travel a good distance to reach Hawaii or the U.S. west coast, which is why I was going to use a dipole vs. vertical. However, I think I am going to just experiment with using a dipole and a vertical with radials, and I'll see what happens. K4SAV, that's a good point about using the edge of the roof, so I will try putting the antenna in the middle of the roof as well as the edges and see what I can get for a difference. WG7X that was my thought as well... even though it's close to the wire mesh/rebar from the structure, the radiation pattern should be impacted by the fact that it's still 30' above the ground.

    Thank you all for the input. I'm still learning about antennas, so I appreciate the feedback and information! I will be sure to experiment a little and then post the results here at a later date!

  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    You might also reach out to Derek @KE0EYJ he has a similar situation and has built a respectable antenna farm up on top of his apartment building in Korea. He might be able to give you some very relevant first hand advice. He's pretty active on these boards and has built many antennas.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Actually, for "DX" a vertical over a roof can work a lot better than a dipole.

    "China here, Japan here, Okinawa here." - Pat Morita, aka Miyagi-san.
  9. WA4SIX

    WA4SIX Ham Member QRZ Page

    Ditto... Laying some radials will capacitively couple to the rebar in the structure. It should work well.

    WA8FOZ and WZ7U like this.
  10. WZ7U

    WZ7U Ham Member QRZ Page

    I'm in the vertical camp on this one, but a dipole will help with the A/B comparisons. There will probably be a good chance of interaction between the two, so best of luck with this endeavor and looking forward to the results.

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