Miha Valencic of 2:380/106.10 wrote this to me in netmail.  I replied to
him there, and moved copied it here for all to see.
 
Hi, Miha;
 
 MV> * Reply to a message in HAM_TECH.
 
 MV> Ivy Iverson wrote in a message to Roy Witt:
 
 II> 4-section version which should get close to 12 dB... around
 II> 40W ERP from a HT!  That should put me up with the folks
 II> with a 50-Watt mobile and a ground plane for a home rig.  Of
 
 MV> What do you mean when you say/write ERP? AFAIK if you have 5W maximum
 MV> output that's it, isn't it? Or less, according to the SWR. Maybe I'm
 MV> wrong.
 
There are other losses to consider besides SWR, primarily the various
losses which occur in the transmission line.  There are also minor losses
in every connection between the transmitter's final stage and the antenna.
By the time a 2 Meter signal, (around 146 MHz), goes through a few yards
of coax, (I am guessing here), you will have about 4 Watts available at
the antenna, (assuming a perfect 1:1 SWR).
 
ERP means Effective Radiated Power.  It indicates how much power your
antenna WOULD be if you were using some standard antenna, such as a
dipole.  The gain comes from the design of the antenna.  For every 3 dB,
(DeciBels), of gain you EFFECTIVELY double the power.  So if you feed 5
Watts into an antenna with 3 dB of gain, your EFFECTIVE power is 10 Watts.
It's a logrithmic sale, so if the gain of the antenna is 10 dB, it
effectively multiplies the power 10 times, and the ERP is 50 Watts from
that 5 Watts into the antenna.  20 dB multiplies the effective power 100
times, etc.
 
Most common antennas, especially at frequencies below 30 MHz, typically
have gains around 5 to 10 dB.  Higher frequency antennas used for satelite
and moonbounce might have gains of 20 dB for a yagi to over 100 for a
large dish.  I forget the actual number, but the huge radiotelescope dish
at Aricebo I believe has a gain of several HUNDRED dB!  (That's ... DARN!
I can't even figure that high in my head, I think it's way over 1,000
times the transmitter power!  I once knew a fellow that had bounced a
signal off of the Moon using that antenna, and he was transmitting only
ONE TENTH OF A WATT - THE SAME POWER TRANSMITTED BY THE WIRELESS TELEPHONE
IN YOUR HOME!
 
Note that whatever antenna gain does to the transmitted signal, it also
does the same to the received signal, effectively multiplying the strength
of the other guy's transmitter by the same factor.
 
The way an antenna gets gain is by taking signal from unwanted directions,
such as up in the sky and down to the ground, and adding it to the signal
in the desired direction(s).
 
Most commercial antennas are rated compared to their gain over an
isotropic antenna.  An isotropic antenna radiates equally in all direction
and all planes, (up, down, North, South, East and West).  It's gain is 0
dBi or zero gain compared to an isotropic antenna.  (Duh!)  Since it
radiates (and receives) equally in all directions, it has the lowest gain
of any antenna.  This antenna DOES NOT EXIST!  It is purely theoritical.
IMO, a better standard to compare other antennas to is a dipole, which has
a gain of 0 dBd, or 0 gain compared to a dipole.  The reason that a dipole
has a higher gain than an isotropic antenna is because it has practically
no radiation off of the ends, and it adds that signal to the signal that
it radiates at right angles to it's elements.  There are many things which
can be done with the design of an antenna to further take signal away from
undesired directions and add it to desired direction(s), thus increasing
the gain of the antenna and thus the ERP.
 
73 DE KB9QPM
   Ivy
 
 
 
... Antenna... Isn't that Uncle Harry's wife?
 
        --
 
          ,-----> Ivy's WALL BBS Ä Home of Lakeshore Net <-----,
          |---------------> Ivy Iverson, KB9QPM <--------------|
          `-> Netmail me a request for info on Lakeshore Net <-'
 
___ Blue Wave/QWK v2.20 [NR]
--- TriToss (tm) 1.03 - (Unregistered)
---------------