March 2025 Presentation-Antenna Gain

 

MARCH 2025 PRESENTATION

 

Slides by Dave Harris

Questions:  Email me at jushamn@gmail.com. Put CSERG on the subject line.

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ANTENNA DEFINED

• Antennas divert, direct, or concentrate RF energy in one or more directions.

This directional property is called GAIN.

The amount of gain is referred to as a number of dBi.

• Example 1.8 dBi

Or as a number of dBd.

• Example 2.3 dBd

You will sometimes see an antenna's gain stated in dB.

• This does not tell you anything useful about the antenna's gain.

dBi DEFINED

• Decibel gain referenced to an isotropic radiator
• Also referred to as a “point source”

ISOTROPIC RADIATOR

• A spherical point from which energy (RF) radiates equally in all directions.
• An infinitely small point in free space.

dBd DEFINED

• Decibel gain referenced to a dipole.
• A perfect dipole in free space.

DOUGHNUT SHAPE

• A dipole’s energy is diverted away from the ends and bulges in the middle by 2.15 dBi.
• Dipole in free space.

dBi vs dBd

• Gain (dBd) = Gain (dBi) – 2.15
• Gain (dBi) = Gain (dBd) + 2.15

ANTENNA COMPARISON

• Antenna A claims forward gain of 4.0 dBd
• Antenna B claims forward gain of 4.5 dBi

WHICH HAS THE MOST GAIN?

• First convert antenna A’s gain to dBi.

dBd to dBi

• To convert to dBi add 2.15 to antenna A’s dBd gain

CONVERT A’s GAIN TO dBi

• 4.0 dBd + 2.15 = 6.15 dBi
• Antenna A's gain in dBi is 6.15

A vs B

• 6.15 dBi (antenna A) – 4.5 dBi (antenna B) = 1.65 dBi
• Antenna A has 1.65 dBi more gain than antenna B

- -  Most of use think in terms of Watts or radiated power. Radiated power comes in two varieties. - -

ERP OR EIRP

• ERP – Effective Radiated Power (Assumed Effective Dipole Radiated Power)
• EIRP – Effective Isotropic Radiated Power

RADIATED POWER IN WATTS

• 1 Watt applied to each antenna’s feed point
• Antenna A’s ERP is 2.51 Watts or EIRP 4.12 Watts
• Antenna B’s ERP is 1.72 Watts or EIRP 2.82 Watts

NOTE!!!

• dBi and dBd apply ONLY to antennas.
• Coax, amplifiers, filters, attenuators, sound, etc use other forms of dB notation.
• Wikipedia lists 60 different dB notations for various disciplines.
• dBW, dBm, dBz, dBsm, dBhl, dBpp, dBvu, etc.

Links

 Power Calculator — db /Watts/ EIRP
https://www.qsl.net/v73ns/dbwcalc.html
Worth a look, includes VE ham license app
https://www.qsl.net/v73ns/
Another worth the look
https://m0ukd.com/calculators/

FINAL THOUGHT

• Anything metal is a potential antenna. It will radiate or reflect (re-radiate) your signal.
• Yes, Grundalclitch, even the bed springs.