"Telamon" wrote"
"Arthur Harris" wrote:

The "free space path loss" between Earth and Mars at 15 MHz would be 211
dB.
That's a HUGE loss. At UHF and microwave frequencies the path loss is
even
greater, BUT the use of very high gain dish antennas both on Earth and
Mars,
as well a low noise figure receivers, makes communication possible. At
HF,
antenna gain of more than about 10 dB is hard to obtain. And the
atmosphereic noise at HF is a killer for weak signal reception.

snip

How did you calculate this loss? I'm assuming you mean the 211 dB to be
an absorptive loss? Maybe you are considering the antenna on Mars to be
a point source off a 180 degree ground plane so the loss figure is power
distributed over a half sphere with the Earth - Mars distance?

The Free Space Path Loss Equation is:

Path Loss (dB) = 36.6 + 20 Log F + 20 Log D

Where F is freq in MHz and D is distance in miles. This assumes isotropic
antennas as both ends. The "path loss" represents the portion of the
transmitted signal that is NOT captured by the receiving antenna. It does
not include absorptive losses (which should be negligible in free space).

Some will argue that this is not a true dissipative loss, and that is
correct. But by knowing the "path loss" you can determine your needs as to
transmit power, receiver sensitivity, and antenna gain in order to assure
successful communication.

Art N2AH

Arthur Harris wrote:


The Free Space Path Loss Equation is:

Path Loss (dB) = 36.6 + 20 Log F + 20 Log D

Where F is freq in MHz and D is distance in miles. This assumes isotropic
antennas as both ends. The "path loss" represents the portion of the
transmitted signal that is NOT captured by the receiving antenna. It does
not include absorptive losses (which should be negligible in free space).

Some will argue that this is not a true dissipative loss, and that is
correct. But by knowing the "path loss" you can determine your needs as to
transmit power, receiver sensitivity, and antenna gain in order to assure
successful communication.

The key here is a theorem that shows that the "capture area" of a
perfectly efficient isotropic antenna is (wavelength)^2/(4 Pi).

-jpd