Can I use a satellite dish as a ham radio antenna? I was thinking that because there are two unused, working satellite dishes that have all appropriate wires attached conveniently to the roof of my house. Would there be a way to use the dish as a ham antenna? Maybe it cn be used with a few modifications. All google searches came up blank.
Most TV satellite signals are between 915-MHz and 2.1-GHz. The 23-cm band (1240-MHz to 1300-MHz) does fall in this range, and a few hams have repurposed dishes for things like EME (Earth-Moon-Earth or "moon bounce") operation, but it's not a band many hams use.
The feed line for TV is usually 75-$\Omega$, not the 50-$\Omega$ typically used with ham radios. The LNB (Low Noise Block, TV downconverter) probably has filters that reject most ham frequencies to cut down on interference.
In short, for traditional 2-meter VHF and 440-MHz UHF it would not work and would require extensive modification. It would be far simpler to put a J pole or maybe a Yagi if you want a directional antenna.
You can use a satellite TV dish antenna for amateur radio use but how you use it depends on what frequency you want to operate on.
The satellite dish antenna is two basic parts, the parabolic reflector and the feed (at the focal point of the parabola). The gain of the dish depends on what frequency you are using and the diameter of the dish.
A typical satellite TV dish probably has a LNA (low noise amplifier) at the focal point, along with a block down converter to change the received TV signal down to around 1 GHz so that it can be sent over a long coax cable to the TV with less loss (losses in cheap coax at 10 Ghz are pretty high).
Unfortunately you can't easily reuse the satellite TV LNA and down converter (LNB) because it is designed for receive only and you will want be able to transmit. So you will have to build your own feed for the dish.
Those satellite TV dishes are about 20 inches in diameter, too small to use for 220 or 144 Mhz VHF (unless you want to make a slot antenna, more below). It could work for 440 Mhz UHF but the gain would be so low (~6 dB @440 Mhz) that it would be easier to just build a yagi. It's much better at 1200 Mhz, where the gain would be around 14 db, or at 2.4 Ghz about 20 db. A small dish works great at 10 Ghz where you get about 32 db of gain. There is a gain chart is this answer here.
There are articles explaining how to convert a satellite TV antenna to a stealth 2m (146 Mhz) slot antenna, such as here and here. You cut a slot in the parabolic dish which changes the dish from a passive reflector into the active element of the antenna. A horizontal slot gives you a vertically polarized antenna. Search for "ham radio slot antenna satellite dish" to find other examples and instructions.
Apart from using it as a ham radio satellite antenna (eg. for QO-100) or a microwave terrestrial antenna, you can turn it into a VHF slot antenna. Here is an example: https://w6nbc.com/articles/20xx-dishslot.pdf (keyword: "satellite dish slot antenna").
I want to point out an important aspect that is only implicit in the other answers. At the frequencies at which they can be efficiently used, dish antennas are (by design) highly directional. That is, you have to point them quite precisely in the direction you want to transmit and/or receive; the benefit is that less power is needed for communication because power isn't "wasted" sending radiation in other directions. This is what it means to have high gain (mentioned in progrmr's answer).
Such antennas are convenient for receiving satellite TV because the signal is from a geostationary satellite, so the dish can be accurately pointed once and then left in place. (The satellite transmits in many directions, but as seen by each individual receiver it is a distant, weak source in a fixed direction.)
While ham radio antennas are often somewhat directional (somewhat high-gain) in order to work better with less power, taking this to an extreme can make them very difficult to get pointed right, especially if you don't know in advance exactly where you're trying to talk to. In the presentation linked by Brian K1LI about dish communication, note the statement "Making a QSO [a contact] is a very special event."
In other, lower frequency bands, it is normal to make many contacts in various places with a low-gain (more omnidirectional) antenna that does not have to be re-aimed as much or at all. That is the type of ham radio experience that may be more appealing to beginners.
So the big picture is that use of a satellite dish is a niche area of ham radio, for those looking for more of an exotic challenge.
The dish itself is pretty much universal - it will reflect and focus any radio waves that are short enough not to go around (say, 1/4 of the dish radius) and long enough not to be scattered off the paint (say, centimeter and up, but the particular dish may be good down to 1mm).
This interval contains few amateur bands.
It is up to you to fit some transmitter and/or receiver at the focus.