I got a good answer in mail:
"J" == Jerry writes:

http://www.rdrop.com/users/billmc/phone_antenna is built around
a ".01uF capacitor", but what kind? Are big .01uF capacitors better
than small ones etc.?

J It isn't the size but the voltage rating that matters. The point of the
J capacitor is to protect the RF stage of your radio from the ringer signal of
J the phone, which will be anywhere from 40 to 70 volts. Considering that
J your radio may be sensitive to signals in the microvolt range, it isn't
J surprising that 70 volts could be a radio killer. I'd use a cap of 200 volt
J or more rating. There are a lot of different kinds of caps. A ceramic disk
J would be appropriate and dirt cheap.

Sure wish there would be that warning on that regular posting to
rec.radio.info . Yes I emailed McFadden.

Is .01uF critical or is there actually a range that is acceptable?

J The point is to have it big enough to pass all the RF signals your
J interested in, but not signals lower in frequency than you want. 0.01uf is
J a standard value, but double or half that would be OK.

Also the "RF connector" isn't clear. What are examples of this "radio
frequency connector"? Perhaps just coax to alligator clip?

J Just connect one end of the capacitor to the phone line and the other to
J your radio. If your radio has a telescoping antenna, just clip it to that.

Is the 50 ohm coax critical? How about a plain wire from the capacitor
to the radio's telescoping antenna? Should the telescoping antenna
still be extended, or retracted?

J You're making this too complicated. Just connect the capacitor to the
J antenna. Any wire will do. Considering that the phone wire may be miles
J long, whether or not you extend the telescoping antenna is probably
J irrelevant.

How do we know if our phone line antenna is working properly? Should
time signals on 5000, 10000 khz etc. jump out in perfect clarity?

J HF signals are rarely perfectly clear, no matter what the antenna. You
J ought to hear more than you do with the telescoping antenna alone.

Certainly there must be a slight advantage between one of red and
green vs. the other... or might we not even bother comparing, as they
are certain to be the same?

J If you have 4-wire phone, one of them will be ground, and that one certainly
J will NOT work. The others are probably equal as antennas. Just try them
J all.

By the way, I found a .01uF capacitor that the capacitor numbers web pages
don't seem to decode: 103K PE50. I can't figure out what the PE50 or
PE5Q means.

J Probably means 50 volts. Not enough. The first time the phone rings will
J damange this capacitor. A 1000-volt 0.01uF cap is only a quarter or so a
J Radio Shack.

Far from stores, I wonder if the necessary high voltage capacitor
could be found in my pile of broken radios. I suppose I would look
for meaty cylindrical types rather than pill shaped types. OK, will look.