If you want to put up a signal that covers a wide band, you will
essentially need a spreading function. In your case, there is no
corelation that will de-spread the signal at the receiving end, now,
regardless of the technique you employ to achieve this transmission,
one thing is sure, you will need some really heavy power.

imagine that you can cover 300 meters using a 10mW transmitter. Now,
this transmission is received by a receiver with a selectivity of
about 50KHz. If you were to cover such a signal every 50KHz for 1MHz,
you will require 1000/50 = 20 times as much power, that is, about
500mW. Correspondingly, if you wanted to transmit over a 10MHz
bandwidth, you will require 5Watts output to achieve the same result.

Now, lets move onto the next part: what kind of modulation? Frequency
modulation requires that the carrier should shift. Therefore, if you
modulated broadband noise, it wouldn't do. You will have to put out
carriers every 50KHz. This will require a comb generator. Hers is how,
pass the carrier through an XOR gate and feed the other input of the
XOR gate with a 50KHz carrier. This will keep flipping the phase by
180% and generate sidebands. The strength of the side bands will
depend upon how square your 50KHz carrier is. The carrier can be
frequency modulated as normally done.

This is a quick and dirty method. there are a number of problems with
this approach, first, there is really no way of limiting the signals
to a particular band unless you do extensive amount of filtering at
the output. second, the signals will be the strongest in the center
and taper off exponentially (not really, but actually) towards the
edges of the band.