[600MRG] Antenna Filters, E-Field Antennas

[Frank Lotito]

A question or two about the application of low-pass filters in E-Field antenna designs -

The E-Field antenna literature, my personal communications with a few E-Field antenna designers, and my limited first hand experience with E-Field antennas indicates a significant limitation of E-Field antennas.  They are quite susceptible to overload from strong nearby signals, such as standard AM Broadcast.  Additionally inserting a band-pass or low-pass filter between the probe antenna and gate of the first stage (a RF grounded common source FET) is not practical, so I thought.  My recent review of two articles about the NASA Inspire VLF-3 E-Field receiver surprised me!  The Inspire design does indeed have a low-pass PI-configuration filter between the probe antenna and gate of the FET first stage.  In addition, the Inspire VLF-3 uses a second low-pass PI-configuration filter between the first stage's drain output and base of a RF grounded common emitter NPN transistor second stage.

I can well appreciate the second low-pass filter between the FET drain and NPN transistor's base.  However I am having heart burn over the filter used between the probe antenna to very-very high impedance FET gate.  I tried to use my  circuit analysis software package to model this filter; I am not convinced that the input filter works to "limit bandwidth."  As a lossey attenuator, yes.  Or, maybe I did not properly model the filter and / or correctly use a transmission-loss to voltage conversion to determine the probe-to-FET filter's voltage gain (actually loss.)  My analysis also suggests a very strong resonance around 700 KHz.  In my geographic area, that's is a NO-NO !!!

Has anyone examined the NASA Inspire VLF-3 receiver's band pass characteristics below 500 KHz?  Did you use a bench measurement employing a signal generator and "dummy antenna" or a circuit analysis software package? Are there other E-Field designs employing a low-pass filter prior to the first stage's gate circuit?

ref: (1) http://theinspireproject.org/downloads/pdf/VLF_Instructions_Apr2011.pdf

(2) http://theinspireproject.org/downloads/pdf/inspire%20Theory_of_Operations.pdf

73 Frank Lotito  K3DZ / WH2XHA