Any type you like. It comes with a SMA Connector that you can hook up to Any antenna you have. There is a rubber duck antenna that comes in the box It is usless unless you are close to a strong repeater.
The TxWARN systems are well documented in the Home Patrol; both the analog, older system, and the newer TxWARN P25. I did see that the Fort Bend Simulcast site was updated today, If you need to make a change, addition, or other update between the weekly database updates, you can do that by editing your Favorites list in the Sentinel software.
For the antenna on my HP-1, I got an SMA to BNC adapter (see item 0292 on this page). For in-home use, I use a 90-degree BNC adapter, with one of the Radio Shack 800Mhz antennas. For longer distance (at home), I have a couple of rooftop antennas. Mobile, I use either a mag mount antenna, or one of these if I'm going to be out in a rural area where Vhf-high predominates, as it still does in the areas west and south of DFW.
This antenna is a telescoping one, with markings to indicate how far it should be extended, depending on the frequency you're interested in. It also has a swivel base, and comes in an SMA version, so that it could be attached directly to the side of the HP-1, yet have the antenna still extend vertically.
Which of the two systems are you trying to receive?
And which city are you in? (If not in a city or town, how about a rough idea of your location?)
I would definitely suggest a better antenna, such as the Radio Shack 800Mhz I mentioned above. That made all the difference in the world for my HP-1. I went from mostly getting the closer 800Mhz trunked systems here in the Dallas area to picking up some farther away, including one that neither my 396XT nor my PSR-500 will get with any consistency. That last one is the TxWARN P25 700Mhz site in Kaufman.