Gain is most likely the first aspect we look at when we evaluate a antenna.
Going through all the data provided by manufacturers, it could be sometimes difficult for the average user to interpertate what is right and what is wrong or how the manufacturer came up with `his` numbers.
If gain is stated in freespace and dBi it is realtive simpel:
One can compared without influences of earth to others.
Thankfully there are more and more manufacturers who do provide the so called “fee-space” gain numbers if we are lucky they have added nice antenna plots along with it.
But not all, and sometimes we see values indicating much higher numbers then what is possible!
One “trick” is to provide the gain over “real ground”, if this is done…the manufacturer should indicate the height of the antenna at which it was calculated.
Thankfully, sometimes we do see those with an antenna height expressed along with it.
The gain might look higher….and the manufacturer isnt lying, but it isnt really fair either as the average CB user might not have heard of things like “additional ground gain”.
Advertisment 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.
The amount one should subtract depends on the antenna height.
That additional gain we see here, is due to that antenna heigth and is known as “ground-gain”. It is esthablished due to the refelections 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 there for add up).
The amount of gain which should be subtracted can be found by using the below diagram (these are values for average ground conditions).
The antenna height is expressed and the “additional” ground-gain which should be subtracted in order to get the “feespace” antenna gain.
We can see the “extra” gain remains more or less equal when the antenna is higher then 10 meters above ground (5..5,6 dB)
Sadly, there are also still a lot of manufacturers who just “make-up figures…
Without providing any facts, without cheating hints just random numbers.
Advertised 4elements yagis on a 6 meter boom with 11dBI gain or a 5elements with 18dB gain etc etc.
Or advertising with just “dB”, so you do not have a reference…
Gain compared to what ? a neelde ?
All of this is just plain silly.
There is no way to “deal” with those in order to get a good indication.
That alone should get the alarm bells ringing.
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 influence of anything else.
But there is simple way to get a good indication for all Yagi antennas :
Gain versus boom length.
For over 40..50 years, the gain for a Yagi antenna actually hasnt changed that much.
Differences between good designs are less then 1 dB for a given boomlength.
Antenna gain primairily comes from boom length…and not the number of elements nor the shape of it.
The number of elements is almost negligible in aspect to gain.
That amount of elements does highly effect things like FB/FR/SWR bandwidth and antenna Q factor etc.
That is of course where we can find our difference between most Yagi’s !
Besides, peaked high gain Yagi’s (we are only speaking of say 0,5dB higher then expected)..should be handeld with caution.
Especially if they are narrow banded.
They come with problems…narrow banded means a high Q, opposite to a coil a high Q is not a good sign for our beam !
That antenna will be very sensitive to rain and obstacles, the impedance will become tricky to match and
eventually producing more issues then what could be gained from that so called high gain.
If you have a Yagi for CB band, and you would like to know what the gain might be from the antenna, the provided diagram will tell you.
It is to provide an insigth of which amount of gain one should expect for a specific boom length.
Its was made for 27,555 Mhz and about 30 NEC confirmed antennas were used as input.
Anyone who claims higher numbers then listed in the diagram should be taken with a grain of salt.