I am designing a switchable 75 ohm terminator circuit for a video
application using a Enhance Mode N-Channel MOSFET, ZVN4106F (ZETEX)
and a 75 ohm resistor. Please see the BMP attachment. The Source is
connected to ground. The Drain is connected to a 75ohm resistor and
the other side of the 75ohm resistor end to the video source-to-a
video analyzer coax line. The gate of the FET is connect to a TTL
source. The video source is sweeping video from 1 to 30 MHz. When the
TTL is high, the FET turns on and the analyzer sees the termination.
The frequency response is flat all the way to 30 MHz. When the TTL is
low, the FET turns off and the analyzer sees the unterminated
video,but the frequency response rolls down to 15.4MHz at -3dB.

-3dB down from what? Are you making an amplitude or impedance measurement?

Doing
my calculation using 15.4MHz, 75ohm load and fc=1/2pi*RC. The
capacitance of the FET I am using when it is off is about 137pF.

That's not right- the ZVN4106F should be something like 20pF.

Too
high for my application since I need to have a flat response within
3dB range all the way to 20MHz. I figure if I can get a low
capacitance Drain-to-source Enhance Mode N-Channel MOSFET of about
15pF or below, I'll be OK.

The ZVN4106F is one of the lowest available.

Any idea where I can find a FET like this?
By the way. In the past, someone sent me a 4 Bipolar/2 diode approach
to the switchable termination problem. This circuit worked great, but
with a cost. I need to use a -5v source that is not part of the
prototype design. The design will provide a +5v and a +12V voltage
source. Also, if i did provide a -5v souce, this source will need to
support 400mA load current since I would have to use 8 of these
switchable circuit(50mA each). Too high. The MOSFET approach on the
other hand draws very little current and can use an only +5v design
scheme. So I have to make the MOSFET approach to work, 1 to 20MHz
flatness. Any idea will be helpful.

The problem is most likely that the video is causing some amount of
conduction through the FET body diode on the negative excursions.