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Trying to figure out how to measure the EIRP of your short vertical antenna? Here’s how!

– Posted in: 630 Meter Instructional Topics, 630 Meters

Radiated power can be tough for a lot of guys because there is so little calibrated equipment that is readily available to the average amateur.  A fundamental lack of understanding  of just what and how to make measurements rounds out the problem.  Here is how I make measurements of my system:

Make sure your system has been resonated on the frequency of measurement.

Disconnect the matching network or ground connection between the loading coil and the radial / ground system at the antenna.

Adjust the system for resonance again.  It will probably change a bit when the matching network is disconnected due to loading effects.

Isolate the input of the loading coil from the feed line at the antenna.

Connect the center pin of your trusted analyzer to the input of the loading coil.  Connect the ground connection of the analyzer to the ground / radial system of the antenna with a short, secure, heavy connection.  I use an AA30 and find it to be quite accurate under a wide range of load conditions.  Other analyzer, particularly those that have been modified, may not provide the quality results or may have calibration problems.  For making the connections, I use a banana plug with a soldered alligator clip for the center pin connection and the outer shell of a PL-259 with a heavy gauge wire soldered to it completed the ground connections.  The ground connection is very important and needs to be solid.

 

Center pin and heavy ground connection

 

While the antenna is isolated from ground quickly measure and note the value of R.  This is your total system resistance.

Isolate the vertical itself and quickly drain static from the vertical.  Connect the center pin of the analyzer to the base of the antenna and make a solid connection to the ground system.

Note the value of capacitance.  This is the value for the entire antenna.

Replace all of the connections to the antenna.  This includes the feed line, ground connection, matching and loading coil.

Download and open the excel spread at this dropbox link:  https://www.dropbox.com/s/wtl7ym2xnqdt7tj/TeeAntCalcs.xls?dl=1

Don’t change the Monopole Radiation Factor, it should remain 3.  For a description of what this is, see section 1.3 of this link written by Rudy, N6LF / WD2XSH/20.

Enter your measurement frequency in MHz.

Enter the effective height of the antenna in meters.  This is not the entire length of the vertical section.  It is the distance from the base of the vertical to the lowest point of any capacitive top loading connected at the top of the antenna.

Capacitance is the value you measured in picoFarads for the isolated antenna.

Total resistance is the measured value in Ohms through the loading coil with no matching network attached.

Enter your total output power in watts.  This value can be adjusted while watching the values change below to find the optimal value for your granted power.  Note that this is EIRP and what we expect to be the units for power on 630-meters and 2200-meters under Part-97 rules.

 

Clear as mud?  If you still have problems relating these steps to your system, contact me.  Every system is different.  The key is to remember you want to measure the resistance of the resonant radiator without any matching to the feed line.  Keep in mind that if your antenna is located in an environment with lots of foliage or your ground moisture levels change seasonally, the values of Rtot can change.  Sometimes the antenna capacitance can vary.  Its a good idea to check it regularly and keep a log book of the variations.

I plan on filming a video for my October 2017 or January 2018 column in CQ when I plan on covering practical radiated power measurement methods and will include it in the column bonus material accessible from the front page of this site.