Background:
  
  I am a limousine driver in New York metropolitan area. I have a HP
pocket pc (Jornada 720) in my car, which is wirelessly connected with
my base through antena, radio and modem which are installed in my car.
We communicate every thing  with my base through this computer. we
don't use two way radio to talk.
  
  The problem is:
  
  we must stay in the car all the time even when we are waiting for
the job( sometimes it may takes one or two hours to get a job at slow
hours). It quite often happens that when I left the for just a minute
for restroom, the job was sent to me and I missed it. In this case I
have to queqe in the list  from the begining, which means another one
to two hours lost...
  
  The question is:
  
  I have two HP pocket pcs (exactly the same including all the
softwares, one just for spare).  The one in the car is connected with
my base. When I leave the car, I can bring the spare pocket pc with
me. How can I connected these two pcs?  For more clearly, I want to
use the pocket pc to receive and send messages when I am walking on
the street or at home ( one or two blocks away from my car)

  I need the detail information including how to and where to buy the
necessary equipments.

Do you know how the pocket pc in your car communicates with your base? GPRS perhaps?

I don't really know what system they use to connct with my pc. Anyway
in my car I  have antena, radio and modem. It covers the whole Now
York city metropolitan area. The area covered  probably ranges from
60-80 miles in diameter.
  By the way. the answer is supposed to be described directly, not any
web site addres. I prefer buy the necessary equipments from stores in
new york area, like Radio shack,compusa.., to from internet.

Without knowing exactly what type of system the business uses I'm
afraid it would be impossible to know what equipment you'd need to do
this.

If you want to receive the signal directly from the system,  I can can
just buy another  antena, radio and modem, the ones exactly the same
as I installed in my car, but it would be too heavy to carry on the
street.
  
  There is a cable connected from the modem to my computer. I think
the signals have been transferred to digital already . Is there any
equipment that I can retransmit this signal from my car to the
computer  I carry on the street which is a couple of hundred feet
away?  The function is something like wireless earphone. Wireless
earphone is a one-way trasmision. But what I need is a two-way
transmision
  
  The equipment in the car can be a little more heavier, but the
equipment I carry on the street should be lighter for easy carry on
the street. Both of the equipment must be able to transmit and receive
signals.

The only option available to you is to use a wifi card for both PCs.
Unfortunately as your first PC is already connected to a modem/antenna
combo it'll probably be impossible to attach another modem/antenna to
it in addition to that one as most pocket PCs only have the one
expansion slot.

Another big problem you have is what format and protocols to use to
send the information from one device to the other, which would again
require a great deal of technical know-how to setup, if it's even
possible.

What I was going to suggest was to switch to receiving all the data
directly to your mobile PC, bypassing the need to retransmit whatever
is received by the car. However, without knowing exactly what type of
system the business uses to transmit it would be impossible to know
what type of antenna/modem combo you'd need for your second PC.

As I told you before. Receiving the signal directly would be too heavy to carry.

  After I examined my pc carefully, I found the cable from the modem
is  connected through a adapter and then connected to the dock which
is connected to the bottom of the pc( through 10 pins). So the pc card
slot is still available. The purpose of the adapter is only for easy
separation of the pc and the modem. I take home the pc every night.
The adapter is uncommon( 14 pins, radio shock don't have this
adapter). When I opened the dock, I found there are only 5 lines come
out  from the cable. I guess two of them are power lines, so only
three are for signals. ( the power of the pc comes from this cable).
  If connection via wifi is complicated, is it possible to intercept
the signal from the adapter before it is connected to pc in the car,
and retransmit wirelessly to the cable on the the mobile pc  before it
enter it.  So  we can use any format and protocols.  Am I right?

'As I told you before. Receiving the signal directly would be too heavy to carry.'

What's to say there isn't a smaller, more integrated, and essentially
more expensive alternative to the receiving equipment in your car that
would slot into the back of your pocket PC directly?

And no, you're wrong in saying you can just grab and repeat the signal
to your second device without any processing.

Verizon wireeless can be used inside the tunnel in New york area, is
that "just grab  the signal from  outside the tunnel and repeat it to
my cell phone inside the tunnel without any processing"?

Yes it is, but using waveguides. I'm pretty sure you *wouldn't* want a
waveguide trailing from your car to wherever the hell you might in
order to solve your problem - you might as well use a cable!

Yep, noggywoggy has it right there.