RE: NEC-LIST: induced voltages

At 13:42 30.06.1998 -0400, you wrote:
>Joachim,
>
> You said, "Even if you use a high ohmic input ampifier, lets say a
> FET, you will
> find a much lower voltage because of the amplifier imput capacitance
> of typically 2 ... 5 pF, which becomes much worse with a cable
> between antenna port and amplifier!"
>
>True, but they usually put the FET right at the antenna terminals, so
>that is not a factor.
>
>Matt Taylor
>

Hi Matt,

As you pointed out, it's really the best way to integrate the
transistor into the antenna to get some sort of "active field
strength sensor". On the other hand you'll you relly have no
chance to overcome the input impedance problem. Neglecting all losses
and the real part of the FET's S11 you will find:

a) the capacitance of the antenna feeder gap
b) the capacitance of the soldering points and the
   transistor terminals
c) the gate-source capacitance of the FET
d) if there is a voltage gain the gate-drain capacitance
   with a higher value than C(G-D), called
   "dynamic capacitance"

The typical values of the sum all these are in between
2 ... 5 pF.

So, if for example you use a small sensor with 0.1 m dipole length,
you'll find a source capacitance of about 0.25 pF. With a load of
only 2 pF the ratio of load voltage over source voltage becomes
0.25pF/(2pF+0.25pF) = 0.11 This is actually a big difference but
compared with the situation using a 50-Ohm load not too bad.

We use active sensors for field strength measurements up to 1 GHz. The
active part consists of a symmetric differential amplifier and an
emitter follower as impedance transformer to get the 50-Ohm source
impedance needed for the test receiver. As the amplier's input
impedance is capacitive and the source impedance of the passive sensor
part i.e. the real antenna is capacitive too, the antenna factor is
frequency independent over a quite big range. (with an amplifier the
antenna factor has to be calculated of course from the ratio of
fieldstrength over amplifier OUTPUT voltage).

Calibration of the sensors is necessary and takes place at our antenna
test range.

Regards and vy 73
 Joachim Brose
     DL6MES
Received on Thu Jul 09 1998 - 09:48:41 EDT