Dave Platt wrote:

You'd need an LM317, a modest-sized heatsink, two resistors (to set
the voltage), and a couple of capacitors (to ensure stability). The
two resistors will form a voltage divider network between the
regulator's output terminal and ground, with the LM317's "ADJ" pin
connected to the junction of the two resistors. You'll want the two
resistors to be sized so that they draw at least 5 mA of current from
the output (10 mA might be safer) - the LM317 needs a minimum load to
regulate properly.

The LM317 needs between 2.0 and 2.5 volts of headroom, between the raw
supply and the output voltage, in order to regulate properly - it'll
"drop out" if the supply voltage drops too low. For a 12-volt-nominal
battery and a 9-volt load, this shouldn't be a problem.

You could use a series/parallel string of 1N4002 rectifier diodes (or
similar) as a voltage dropper, but the regulation would be distinctly
poorer and I don't know that it'd be any cheaper.

Well I built it and it seems work exactly like I want expect for one
thing. Please excuse the terminology error here, but I'm going to refer
to the part of the lm317 that you attach to a heat sink as the "top". I
built my transmitter inside an ammo can with a BNC-BNC bulkhead
connecter on the top of the can so I'm have an external antenna. I also
figured that the ammo can would be perfect to affix the lm317 to for
heat distribution. You may already see the problem. Basically when I
installed bulk head BNC connecter to the ammo can it grounded the entire
ammo can. The problem is, the "top" of the lm317 that you attach the
heat sink to has positive voltage on it. Its the same voltage as the
output pin on the lm317. Is this normal? So when I connected the lm317
to the ammo can(not knowing the problem) its created a short and the
lm317 really really hot. I just assumed that the "top" of the lm317
would work if it was grounded, but l was wrong. Am I missing something
or is this normal. As long as I either unhook the lm317 or the bulkhead
BNC connector from the can everything works great. Any comments.