[Antennaware] dBi conversion to Antenna Factor

Guy Olinger, K2AV olinger at bellsouth.net
Mon Nov 19 20:44:48 EST 2007

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 feed 
with a lossless 50 ohm match, and then place the K9AY loop here on earth at 
the same distance to a transmitting source whose effective radiated power in 
the direction of the K9AY is the same as the free space transmitting antenna 
in the direction of the isotropic, with a lossless 50 ohm matching device on 
the K9AY, AND with the K9AY pattern maximum intercepting the incoming signal, 
EZNEC is specifying that the signal will be 26dB below that of the 50uv/50ohms 
or 2.5uv/50ohms

Noting that you probably do not have the services of the space shuttle to 
complete part one of the comparison, the dbi is usually used to compare one 
"real" antenna with another apart from the "literal" comparison with the 
unreal isotropic.

That is to say if a dipole has a gain of 7dbi at the incoming signal angle and 
azimuth, and the K9AY is -26dbi at the same incoming angle and azimuth at the 
same point, whatever was measured on the dipole matched to 50 ohms would be 
33db above the K9AY matched to 50 ohms, plus/minus any correction for feedline 
loss, etc,

Note that most special HF "receiving" antennas are particularly lossy compared 
to "decent" transmitting antennas, because they are either small compared to 
wavelength, or their design deliberately makes use of a lossy device for 
rejection of unwanted signals, as in beverages which make use of a close 
ground and a termination resistor to construct their performance.

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

73, Guy.

----- Original Message ----- 
From: "Andy Ikin" <andy.ikin at btopenworld.com>
To: <antennaware at contesting.com>
Sent: Monday, November 19, 2007 3:58 PM
Subject: [Antennaware] dBi conversion to Antenna Factor

Hello Folks,

I am a newcomer to the list.

I am still trying to become "aufait" with EZNEC, but I have some difficulty 
with Rx antennas' in that the gain quoted is in dBi. Whilst this is easy to 
understand for Tx antennas, it is only of comparative use for Rx antennas. How 
do I simply convert dBi to Antenna Factor ( dBV/m to dBm ). Otherwise it is 
going to be very tedious if I have to build some of the modelled antennas and 
compare them to a calibrated active loop antenna?!

Another question I have; for example with the EZNEC model of the K9AY the gain 
quoted is -26dBi; is this gain dependent on the matching transformer as this 
transformer is not in the model? Or am I missing something?


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