[600MRG] ERP, EIRP, How & Why

[Pat Hamel]

There are historical reasons why the radio regulatory situation exists.

The radio industry worldwide has always had politically set limits on the
transmitted power. 
The experimental service (FCC Part-5) was intended originally to allow
manufacturers to legally transmit to verify performance required by their
customers or the FCC.
Commercial stations generally have a maximum and minimum power level, 
During a yearly required "proof of performance" LF & MF stations were
required to show they met the license requirements.
The way that the power was measured at startup of a new station was by
paying a professional engineering company to study maps and determine the
signal strength at certain locations and then measure to be sure that the
calculated conditions were met. From then on, for a non-directional AM
broadcaster, the originally determined local meter readings were the way to
insure the conditions continued to be met.
The regulations always required the results to be kept and filed with the
FCC.
Hams were exempt as were the experimental stations.

Years ago, the government paid to have experiments performed on transmitting
antennas. The results were the same for antennas of the same electrical
length (repeat Electrical length).
The results of these experiments are that the performance of any antenna
over perfect ground and/or in the clear (based on antenna type) can be
predicted mathematically.	
The antenna modeling programs use this mathematical foundation to do their
work.

A convenient way to explain the efficiency of an antenna was adopted - known
as the radiation resistance.
The radiation resistance of an antenna is the value of a resistor that would
dissipate as much power as the antenna radiated. 
In a perfect antenna, the radiation resistance and measured "R" (of R+Jx)
would be the same, but that can't happen.


We all know that physical length can be measured in meters or feet. 
Radiated power can be measured in effective radiated power (ERP) or
Isotropic effective radiated power (IERP).	

Most real antennas exist in real spaces, with trees, bad (lossy) soil,
sheds, houses, roads, railroad tracks, and people changing things.
This reality leads to the logical conclusion that a real antenna must be
less efficient than the ideal calculated antenna.

For example the 600 meter band 50 foot high antenna at WD2XSH/6 calculated
to have Zero.Point Seven Seven ohms radiation resistance.
The measured input "R" was in the vicinity of 28 ohms. 0.77 / 27.33 =
efficiency of 2.8 %. 
This quick ratio calculation shows that less than three percent of the power
from the transmitter would be transmitted - only if the antenna was perfect.

The antenna was supported by a tree which had sap alive almost all year and
the neighbors would not allow lawns to be torn up to lay long radials, so it
is logical that limiting transmit power to that which would be legal in a
perfect antenna must also be legal in the real case.
I know of no experimental stations who have been visited and examined by the
FCC, but I have my notebook with all the printouts from our coordinator, my
measurements and calculations, and my logs.
My four 811's loafing along at 30% efficiency class "B" linear caused no
interference and made 15 watts ERP from the antenna regularly. 

The question posed on the reflector was anticipating FCC granting a new ham
band at 630 meters. 
The FCC has traditionally chosen ERP measurements. 
Until they decide LF & MF ham allocations, we experimenters are limited to
the conditions specified in our licenses.

Use the method of determining your power that applies. 
Forward this to anyone who may be interested, but (disclaimer) my 74 year
old brain makes mis-teaks.
Pat W5THT & WD2XSH/6