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Re: [Antennaware] dBi conversion to Antenna Factor

To: "Guy Olinger, K2AV" <>,<>
Subject: Re: [Antennaware] dBi conversion to Antenna Factor
From: "Andy Ikin" <>
Reply-to: Andy Ikin <>
Date: Tue, 20 Nov 2007 23:12:22 -0000
List-post: <>
Guy Olinger, K2AV wrote on Nov. 20.

"Hi Andy,

dBi is relative to an "isotropic" antenna in free space.

If you place the isotropic antenna in free space relative to an unvarying
transmitted signal and get 50uv/50ohms of received signal at the antenna 
 with a lossless 50 ohm match"

"Given that dbi is an "ideal" relative figure independent of actual 
level, and dBm is the harsh reality of a received signal, there is no 
conversion".  The conversion is the relationship between concept and
accomplishment, and all that stands between the two, including how much 
the transmitter is running."

Guy, thank you very much for you reply and clarifying that the EZNEC 
modelling calculates the dBi assuming a 50 Ohm system.

I still have a problem with how you get 50uv/50ohms of received signal 
without knowing what the field strength is?

I have done a bit more digging into resolving the relationship of dBi to 
Antenna Factor. In sixth edition of the RSGB Radio Communication Handbook, 
Chp.11.3 Propagation. f ig.11.4 shows the relationship for field strength of 
a half-wave dipole to voltage at the Rx end of a correctly matched feeder 
connected to it. i.e. 1uV at Rx relative to 1uV/m. At 50MHz 1uV at Rx equals 
1uV/m (0dB), at approx. 1.8MHz Rx it is +30dBuV for 1uV/m. Therefore 
assuming that the above dipole is in free-space we can subtract approx. 
2.2dB to arrive at the dipole dBi? Hence, approx. +28dBi = an Antenna Factor 
of +30dB (1uV/m to +30dBuV). Assumming that my prognosis is correct,  the 
K9AY with -26dBi gain at 1.8MHz has an antenna Factor of +2dB.


Andy G8LUG

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