Radio: it's not just a hobby, it's a way of life

Current Operating Frequency and Mode

CQ 474.5 kHz CW

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

– Posted in: 630 Meter Instructional Topics, 630 Meters


Radiated power can be tough because there is so little calibrated equipment that is readily in the possession of the average amateur.  Making useful measurements rounds out the problem.  Here is how I make measurements of my system – note that there are a number of different approaches to accomplishing this task and this is just one:

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.

Capacitance is the value you measured in picoFarads for the isolated antenna. (you can also apply the typical rule of thumb of 8 pF/m for thicker conductors or 5 pF/m for thin conductors, like a wire)

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.

Also note that it has been reported that radiation resistance calculations in this spread sheet have been considered “optimistic” by some meaning that your reported EIRP for a given power level and resistance measurement could be high (meaning you have some additional headroom).  Some work is being done to resolve this in the long term but just know that depending on your antenna system, there could be some available headroom after these calculations.  If you want to get closer, use Neil’s, W0YSE, spreadsheet, found here, to determine a better, probably more realistic value of radiation resistance then edit the spreadsheet I specified earlier (the dropbox link higher up) and enter this value as a static value.  Neil does a very good job describing Laport’s approach to these calculations (here- scroll down) which seem to agree with models developed by Rudy, N6LF.

Note that 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.

What do I really think about EIRP, ERP and radiated power, in general, for amateur applications?  I think its pretty nebulous for a backyard vertical at these frequencies.  In the end, we will be better off with a rule of thumb to ensure compliance much like the early days of 60-meter activity rather than trying to measure and calculate all of these values which may or may not be accurate to begin with.  That is only my opinion and I say that feeling pretty good about my own personal station metrics.  It took me about five years to really be comfortable with my numbers, however, and I revised them constantly.

Note that a representative from the ARRL’s experimental group that was qualified to make the statement suggests that if we are within 3 dB of our maximum EIRP limit, that may be as close as we get in practice.  Given that this value may change from day to day and season to season for many of us, I tend to agree but as my comments earlier suggest, I am a bit more cynical and suggest it may be as much as 6 dB.  The main thing is to be able to justify what you are running on paper.  Make good notes and calculations and check your measurements again regularly.