[Lowfer] active antenna input Z, etc.

[Steve Ratzlaff]

Hi Bill,
Thanks for your comments.
I've built lots of Natural Radio receivers for audio monitoring of 
whistlers, chorus and other phenomena that is below VLF, that use jfet 
inputs (e-field) and their design is quite different from the active whips 
we're talking about here for use from VLF-HF. That goes along with your 
comments of input impedance, needing to be much higher at audio frequencies 
for best results.
While it might be theoretically possible to build an e-field antenna that 
works down to audio frequencies and has response to HF, I've never seen one 
actually built and working.
73,
Steve

----- Original Message ----- 
From: "Bill Ashlock" <ashlockw at hotmail.com>
To: <lowfer at mailman.qth.net>
Sent: Friday, December 10, 2010 6:16 PM
Subject: Re: [Lowfer] antenna location (K2ORS remarks)


>
> Steve,
>
> In order to get a good feel of what the input Z characteristics are like 
> one has to understand that the input Z of the typical E-probe has a lot to 
> do with the input capacity of the FET amplifier, which is usually about 
> 10pf.  If for example you are operatining at 185KHz a 10pf capacity would 
> be a reactance of approx 100K ohms, so you are correct in stating that the 
> input Z is not 100s of megs but less than 100s of Kohms. Only at very low 
> frequencies would the FET probe exhibit 100s of Megs (assuming an input 
> resistor very high in resistance).
>
> Bill