Sommer Antennas  Antenna Gain 
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More than ten years ago a humorous article on antenna gain was published in the ham radio magazine "QRV." The article stated that there are three kinds of gain:
1. the dB/d and dB/i gain based on CCIR regulations (the true and honest real
gain!)
2. the "dB/ham radio gain," which is 6dB higher than the real one.
3. the "dB/CB gain," which is 10 dB higher than the real one.
Confused? Here is a little help so you can make your own calculations.
The gain of an antenna A compared to an antenna B is equal to the capture area
of antenna A compared to the capture area of antenna B.
The capture area of an antenna is determined by multiplying the antenna's radiator length by 1/4 lambda. See Figures 1 and 2.
Example:
Antenna A Fullsize 20m antenna (made from CUwire 2mm/1/16" O.D.
+/)
14 MHz dipole 1/2 lambda long: = 10m x 5m (1/4 lambda) = 50 sq meters
The capture area of a 20m dipole is 50 sq. meters or 540 sq. ft. The gain of this antenna is zero dB/d or 2.14 dB/i.
Antenna B Shortened 20m antenna (CUwire 2mm/1/16" O.D. w/big loading
coils)
radiator length 5m/16.5ft., tuned by two loading coils to 14.2 MHz = 5m x 5m =
25 sq. meter.
The capture area of antenna B is 25sq. meters or 270 sq. ft. Note: Antenna B has only half the capture area of antenna A and is therefore able to "catch" only 50 percent of the electromagnetic field; e.g., 50mV, compared to 100 mV/50 Ohms. This means 6dB less gain for antenna B in comparison to antenna A.
These calculations require extremely low loss coils. Since there is no such thing as "noloss coils," an additional loss of typically 3 to 8 dB has to be taken into account.
Imagine how much power is wasted while using such antennas for receiving/transmitting.
The fact is, that only 250 Watts of the 1,000 Watts HF in your feedline are radiated  and this with "extremelylowloss loading coils."
These kinds of calculations can be found in all good antenna books, usually presented in the form of rather intimidating mathematical formulas. We hope the explanation above prove helpful to you.
A typical vertical antenna with 0 (zero) dB/D gain in space is the Sleeven 1/2wave dipole. Other 0 dB/D gain systems are 1/2wave antennas such as the ZeppWindomFuchs and vertical radiators such as Ground PlaneDiscone (all in space).
Antennas shorter than 1/2wavelength (tuned by a loading coil or traps) have much less gain. Typical: 1/2wavelength = 1/4 $#34;power gain" ±loss of traps/coils. Following are some examples:
40 m GROUND PLANE 14 ft / 4.3 m high*, center loaded. Its
efficiency is 7%. Its gain is 12 dB/D (compared to a fullsize system without
losses). This means that only 7 watts of a 100watt signal will be radiated; 93
watts are wasted in this antenna. See Ham Radio Magazine, Sept., 1982, P. 18 ff, by W1GV. * A fullsize 40 m 1/4 wavelength ground plane is 34.6 ft/10.5 m high; it has 0 dB/D gain.

160 m GROUND PLANE 33 ft / 10 m high*, 12Ohm ground
Configuration Gain Efficiency 1. Top loaded (big capacitor) 10 dB 10.0% 2. Center loaded coil 19 dB 1.2% 3. Base loaded coil 20 dB 1.0% See Ham Radio Magazine, May, 1983, p. 36 ff, by W7DHD. 
20 ft / 6 m long MULTIBAND ANTENNA 1012151720 m Halfwave Zepp
system. Open wire feedline or matchbox on "F"
Configuration Gain Efficiency A=10 m 0.5 dB/D ~90% B=12 m 2 dB/D ~63% C=15 m 4 dB/D ~25% D=17 m 6 dB/D ~15% E=20 m 8 dB/D ~15% Note: The traps were air coils, made from #9 copper wire, with air capacitors.

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