Gain versus boom length

 

When evaluating Yagi antennas, gain is often the first metric people look at.

Searching through all the data provided by manufacturers, it could be sometimes difficult for the average user to interpretate what is right and what is wrong or how the manufacturer came up with `his` numbers. Interpreting the gain provided by the manufacturer correctly is key. This page will help you:

  • Understand how much gain you can realistically expect from a given boom length.

  • Recognize how some manufacturers manipulate gain figures to mislead buyers.

 

     

    In a free-space environment (no ground reflections), comparing antennas should be pritty straightforward.

    And luckily some manufacturers do honestly publish dBi gain figures in that free-space including radiation plots.

    But not all are honest…

     

    So, lets have a look at what is possible:

    Which gain can be expected for a given boom length…

     

    If you’re using a Yagi on the CB (11m) band (27.555 MHz), this chart—based on over 40 NEC and AN-SOF modeled antennas—shows expected free-space gain vs. boom length.

    It is to provide an insigth in the amount of gain one should expect for a specific boom length.

    Important facts !

    • In all honesty, for over 40..50 years, the gain for a Yagi antenna actually has not changed that much.
    • Differences between good designs are less then 1 dB (gain) for a given boom length.
    • Antenna gain primarily comes from boom length…and not the number of elements nor the shape of it.
    • (It is true that by extending the element length you could enhance gain, but then we are speaking of say 15 meters long elements.)
    • Overall one could say: The number of elements is almost negligible in respect to gain.
    • The amount of elements does highly affect other things like the so called: Front to back /Front to rear and the SWR bandwidth and thus the antenna Q factor etc.

     

    And that is of course, where we can find the hugh difference between past days and modern design Yagi’s !

    Caution: “High gain” antennas (claims of +0.5 dB more) that are narrowbanded may become overly sensitive to moisture or nearby obstacles and harder to match properly.

    That will lead to an antenna which will be very sensitive to rain and obstacles. The impedance will become tricky to match and

    eventually producing more issues than what could be gained from that so called high gain.

     

    • Manufacturer Tricks to Watch Out For

    Some common misleading tactics are:

    • Starting with the worst situation: Sadly, there are still a lot of manufacturers who just “make-up” figures ! Doing so without providing any facts, without cheating hints just giving random numbers. Advertised 4 elements Yagis on a 6 meter boom with 11 dBi gain or a 5 elements with 18dB gain etc. Or advertising with just “dB”, so you do not have a reference… Gain compared to what ? a neelde ? There is no way to deal with that except to have a look at the provided boomlength and gain provided on this page. This is a red flag ! I would not recommend these kind of manufacturers.

    Another trick is:

    • To provide the gain over so called “real ground”. If this is done, the manufacturer should indicate the height of the antenna at which it was calculated and the groundtype he used. Thankfully, sometimes we do see those with an antenna height expressed along with it. The gain might look higher and the manufacturer isn’t lying, but on the other hand, it isn’t really fair either. As the average CB user might not have heard of things like “additional ground gain” and does not know how to interpret. Advertisement look like these:

     

    (example of 6el long boom Yagi from a random manufacturer)

     

    (And another one from a Cubical Quad manufacturer:)

     

     

    In these cases you should subtract roughly 5,5 dB in order to find the “feespace” dBi gain figure.

    That additional gain we see here, is due to that antenna heigth and is known as “ground-gain”. It is established due to the reflections of earth.

    (In short: The direct wave from the antenna, and the waves which reflect against the ground around your antenna are in phase and therefore add up).

    The amount of gain that should be subtracted can be found by using the below diagram (these are values for average ground conditions). the result will be the expected freespace gain.

     

     

     

    BE WISE ! 

    It is always wise to ask for antenna plots from the antenna manufacturer (if not listed) preferbly in “freespace” so you can compare them with others without the influence of anything else.

    Anyone who claims higher numbers then listed in the diagram should be taken with a grain of salt.

     



     

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