Calculating the "ball-park" range of a transmitter including dBi
The standard link loss equation is: -

Link loss (dB) = 32.4 dB + 20log(MHz) + 20log(kilometres)

If your RF sensitivity is -121dBm (about right for 1 kbps) and you can transmit at +20 dBm then it could theoretically (in outer space with very little interference and without earth getting in the way) work with a link loss of 141 dB (20 dBm - (-121 dBm))

So, we now have 141 dB = 32.4 dB + 59.2 dB = 20log(km)

Therefore 20log(km) = 49.4 dB which means distance = 295 km = 183 miles.

All sounds great until you factor in real-world problems such as fade margin - this is a kind of rule of thumb that suggests that link loss (at any given distance) is usually degraded by at least 20 dB. I'm not going to justify this BTW.

This now means distance is 29.5 km = 18.3 miles.

By the way, 0 dBi antennas usually imply isotropic antennas (a theoretical device) but straight wires usually imply a quarter wave dipole and these have a small gain of about 1.7 dB.

If you were transmitting at 28 kbps, your receiver sensitivity will be decreased to: -

154 dBm - 10log(data rate) = 109 dBm - this reduces your range to about 7.5 km = 4.6 miles.

If you are in a built-up area with buildings and other forms of interference you might lose another 10 to 20 dB so be aware of this.

Here is a very approachable document that largely covers what I've said in my answer.