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
feed
with a lossless 50 ohm match"
"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."
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 halfwave 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 freespace 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.
73
Andy G8LUG
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