Historical use of the term dBi as antenna gain

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KX4O, Feb 2, 2017.

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  1. AI3V

    AI3V Ham Member QRZ Page

    Yes!

    A great sanity check is to compare claimed gain with actuall beamwidth.

    As noted, a high gain antenna has a narrow beamwidth.

    This does not equal a narrow beamwidth antenna has high gain.

    Don't fall for the trap and confuse the 2

    Rege
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Gain is always expressed at a given elevation angle and assumes "typical" ground; sometimes another set of data is provided for "over seawater," and those numbers always look better in terms of actual gain.

    Most antennas I've seen specified assume installation at 1WL over standard Earth, so it's difficult to be "too high" on 80m but very easy to be "too high" on 10m for ionospheric work.

    Re ground reflection gain and designs based on it, shortwave broadcast antennas always are -- at least in the design documents I've seen, which are quite a lot of them. As an example I was involved in transmitter engineering for WMLK in PA back in about 1977-78 when the station was being assembled. The antenna is a log periodic made of heavy gauge wires supported by two towers and aimed into the ground (it aims way below horizon) by design, and the engineering team who put that together was a very well respected antenna engineering group from D.C. The intended audience is a portion of the Middle East, and of course this was a Christian broadcast station.

    Prediction was it would be receivable with a small shortwave receiver and a whip in the intended area >12 hours a day, running 2.5 megawatts e.r.p. from the station.

    Transmitter was activated about a year later and reception reports starting coming in to confirm exactly what was predicted. A lot of hams would probably look at that antenna and wonder how it could work, since it's obviously aiming into the ground a few hundred yards in front of the towers.

    When you have a very specific intended reception area, antenna design becomes more predictable.

    That station was built in a cornfield in the middle of nowhere and when the transmitter was first activated, the corn in the area of the antenna lobe burned up -- some exploded. Fun stuff.
     
  3. KX4O

    KX4O Ham Member QRZ Page

    WB2WIK said:
    An example is here: http://www.m2inc.com/amateur/20m6-125/


    Very very nice example. It's quite clear this company knows what they are doing by carefully stating gains in such a way as to not be ambiguous... and complying with FTC truth in advertising requirements. Their specifications are mysteriously placed into a jpg thus...

    [​IMG]
    It's clear from the specs...
    • Gain (Freespace) = 11.19 dBi ~ 9 dBd
    • Gain (70' AGL) = 16.6 dBi ~ 14.5 dBd
    Another more SI compliant way to put this is...
    • Gain = 11.19 dBi ~ 9 dBd (based on the standard definition of Directivity/Gain)
    • Gain (70' AGL) = 16.6 dB (re isotropic equiv. 70 feet AGL) ~ 14.5 dB (re dipole 70 feet AGL)
    M2 has carefully prepared their data to be non-ambiguous so it can pass the smell test of the antenna engineering world.

    What's missing, of course, is the characteristics of the ground and to be above board that really should be part of this. Buy hey, an A for effort.

    So... M2 verifies for us they see dBi and dBd as both meaning isotropic circumstances, as used by those in the professional antenna community. They make clear where they deviate from these measurement constants. You know why they do this right? If they did not, they would eventually be flagged for erroneous specifications by purchasing agents in the know. This could become a serious embarrassment especially if they sell to government customers... which seems likely given their stature in the antenna biz.

    M2... correctly avoiding the use of erroneous sammy the hammy antenna jargon.

    ps
    Anyone else having a tough time posting to QRZ? I keep getting 500 errors.
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I was getting such errors also for a while, seems to be fixed now.

    I think.:p

    Many reputable antenna manufacturers specify gain in such an unambiguous fashion. They all seem to use "typical" or "average" ground for the models, which is to say not poorest nor best, but about median. Many use the FCC charts to determine soil conductivity for their area (the charts were developed for the AM broadcast engineering crowd a long time ago, and they are on line) and to me the findings are a bit eyebrow-raising, e.g., almost noplace "near the ocean" actually is good (the ocean itself is terrific), and the highest soil conductivity in the country is in northeastern Texas. That would be really hard to guess.
     
  5. KA0GKT

    KA0GKT Ham Member QRZ Page

    The area around Wichita, KS isn't bad either.
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I didn't just look, but I believe you.

    Funny part is, this isn't very "intuitive" at all. "Looking around" won't tell you anything about ground conductivity.

    For about 3 years I owned a home at the Jersey shore, only one mile from the Atlantic Ocean, and if you dug a hole in my backyard (which I did, for a tower) it would immediately fill with brackish water. I had to pour special cement for the tower foundation, but luckily the local cement yards knew exactly what to do.

    So, I thought "if a hole only 5 feet deep fills in five minutes with brackish water, the earth conductivity around here must be good."

    Well, that was wrong. It sucked. Wichita is likely far better.

    You just can't guess at this.
     

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