you need an adapter. direct tv sells them or you can buy them at most retailers such as walmart, best buy etc...
the signal has to be downgraded from digital to analog and only a convertor box can do this.
second possibility is if you have an old vcr with rca inputs (the receiver should also have an rca output), then you can ouput to the vcr and then use the vcr's coax output to the tv.
one of these 2 ways is about the easiest way with the convertor box being the cheapest if you have to buy something. i think directv sells the box for $20 but it has been a while since i have used one so it could have gone up by now.
I am not aware of DirectTV receivers' lineup, but that-other-company' receivers have analogue outputs for second TV which will be much better than RF output. So, check what your TV supports, and what your receiver can give, before buying anything.
i have the newest genie receivers and they only have rca and hdmi outputs. i don't think there has been a coax output on directv boxes in 15 years or more. i recall buying a nice vcr back in about 1998 when i first got directv since the box only had rca outputs on it. i did what i suggested above and used the vcr as a convertor box. even then coax was not available. the "other company" might use them but i have never used that company so can't say anything about them and their boxes.
of course since vcr's are pretty much not sold anymore, unless you have an old one lying around, the convertor box is the way to go. they are the same boxes they made when over the air was changed to digital from analog.
and since he is asking, i assume that he already looked and saw no coax output to use.