A no Coax Test
With this test I want to show, that the transmitter "sees" the EHA as a 50 Ohms load when it is properly phased on its operating frequency. For the test I used my QRP transmitter and the third EH Antenna prototype. The QRP Transmitter is connected to a 12 V, 2 Ah battery and the short battery cable has a 2 turn winding on a ferrite toroid to isolate the battery and Amperemeter from the transmitter.
The QRP Transmitter is in its turn internally supplied via a 7809 to step down the voltage to a stable 9 Volts. There is an ON/OFF Switch to operate the QRP TX.
The plastic box containing the transmitter measures 120 (h) x 90 (w) x 35 (d). The connection to the PL-259 inside is 3 cm long, and the chassis is soldered to the PL-259 connector directly.
Three current measurements were first made without the EH Antenna:
On a 50 Ohms load (the Stabilock 4040), then with the PL-259 open (no load) and att last with the antenna connector shorted. The output in 50 Ohms was measured to 0.53 Watts and the current was measured for these three cases:
|Transmitter terminated with 50 Ohm
|Open circuit (no load)
Then the EH Antenna was placed on the QRP Transmitter as shown in the lower picture. Two receivers were used as monitors for the maximum ERP from the EH Antenna: A HP 8559A Spectrum Analyser with a pickup antenna and the Radiometer AFM 3 with the glassfiber whip. Both instruments have 1 dB resolution. The capacitors and coils on the EH Antenna were fine tuned for optimum radiated power and the current was observed on the milli-am-meter. Another two measurements were made, just for my interest, to observe the current consumption: C1 was tuned from its optimum position to its MAX and MIN positions respectively.
|Transmitter terminated with the EHA
|C1 set to MIN position (6 pF)
|C1 set to MAX position (35 pF)
With C1 at maximum, the radiation was more than 30 dB down and at minimum more than 15 dB down compared to the maximum ERP point.
It was noted that while tuning C1 from maximum capacitance, just before the ERP reached maximum, the current reached a minimum point at just less than 200 mA indicating a shortcircuit state. From this point the current rises sharply and the maximum ERP point is reached just after the minimum current point. The ERP then decreases and the current rises further.
The fact is, that I can clearly see a maximum ERP at a current which is equal (or very close to equal) to the current drawn when the transmitter is connected to a perfect 50 Ohm load.