Antenna Factor

Antenna factor is a measure of the transfer from an incident electric field, normally expressed in units of volts/meter to the voltage presented to an SDR or other measuring device of known impedance. If Antenna Factor is known, an SAS can provide absolute electric field measurements as well as comparison with other receiving systems worldwide equally well qualified. If a suitable antenna measurement range which can produce known fields were available this factor could be directly measured but for an extremely broadband system such as this one, one that can operate at very long wavelengths, a range of this sort is not possible. For this reason the antenna factor for the SAS is estimated using both measurement and theoretical modeling. These models are still in development!

To characterize the entire SAS a combination of methods, modeling and measurement is used.

The hardware, including the SAPreamp, CAT5 cable and ShackBoard can be assembled and the entire assembly measured using a VNA having suitable frequency range. A TinyVNA is able to operate over most of the SAS system's potentially useful 1 kHz-200 MHz range. But modern VNAs are normally 50 ohm systems. They give the most precise measurements when the DUT is also approximately a 50 ohm device. For the SAS this condition isn't met because the input impedance is extremely high compared to 50 ohms. Because of this the input shaping present in the SAPreamp has almost no impact on the VNA measurement. The essentially open-circuit load the SAPreamp presents to Port 1 of the VNA which is followed by measurement in the 50 ohm environment at Port2 results in an apparent gain due to the impedance difference. This does provide accurate voltage gain information but does not display the effects due to the high impedance frequency shaping present in the system when the monopoles are used as probes . A portion of that frequency shaping is created by the reactance of the dipole itself interacting with the RCR shaping elements. It is the open-circuit voltage of Ra,  modified by Xa and the shaping elements, that results in a frequency-shaped output from the SAS.

The antenna itself can only be approximated as a Short Dipole. Additional models and methods must be used to characterize it at higher frequency where it becomes a half wavelength or longer. As previously described, traditional models or measurement for matched antennas do not apply. However, once modeled and combined with the SAPreamplifier's input RCR shaping model, signal transfer from either the incident field or from a modeled radiation resistance to the SAPreamp input.

These subsystem models can then be combined with a VNA measurement (at 50 ohms) of the rest of the SAS system which follows; connecting CAT5 cable and ShackBoard, to produce an overall antenna factor that provides a reasonably accurate measurement of the absolute field strength anywhere within the entire frequency range of the system.

It should be recognized that the series capacitance of the dipole varies with dipole length so modifies the above response. This must be included in any antenna factor estimate. The circuits following the antenna and input filtering, including the SAPreamp, CAT5 connecting cable and ShackBoard taken together are seen to have approximately unity gain. Precise filtering shape changes with component values but generally looks as the red trace below:

Measurement from the monopole terminals to the 50 ohm SDR input using 100 feet of CAT5 cable interconnecting the SAPreamp to the ShackBoard is shown below. This excludes the effects of frequency shaping that are added by the dipole and shaping components. This measurement was performed using a VNA calibrated to a 50 ohm reference impedance.As shown the gain from Preamp input to ShackBoard output is approximately 0 dB from near DC to over 200 MHz.

With all of these factors put together, the dipole model, shaping model and active stage measurement might provide an antenna factor and thereby a reasonably good approximation of the incident electric field up through 5 MHz from measured power at the ShackBoard output.

The power within the radiation resistance can be obtained and used for comparison and study. Knowledge of a geographical region and comparison of the system's measured noise floor with worldwide ITU estimates for expected noise floors in a region of that same type provides a metric to gauge a system's performance. It provides a method to know when an expected regional limiting performance has been reached and identifies areas where that is not the case. This knowledge is essential for improving achieving and maintaining optimum receive system performance.

According to the short dipole model used with a matched antenna, the radiation resistance, Ra, becomes extremely small at short electrical lengths while the equivalent capacitance of the reactive portion of the impedance remains constant. For a 6m dipole this value is about 10.8 pF. Because the total impedance is so high, no significant current flows and this characterization can also be used to approximate the two monopoles of an SAS operating as a probe.

So although intercepted power, pattern and gain remain constant, the available voltage from electrically small antennas becomes very small. This creates a +20 dB/decade slope. Even so, from ITU  regional measurements, the spectral noise power density has a -28.6 dB/decade negative slope. To a degree these effects cancel to make low noise system design most difficult to achieve at the high frequency end of the range.

For reference, the theoretical model for the antenna factor of an electrically short 6m dipole in a traditional matched application at 5 MHz indicates a 7.55 dB gain from the incident field to a high impedance voltmeter at the dipole terminals.