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So online, I found this 555 timer FM modulator circuit. But To make this a working FM transmitter, is all I have to do is attach an antenna to the output modulated signal? Or do I have to also have a series LC resonant circuit tuned to the carrier frequency?

Link: FM Modulator Circuit Using 555 Timer IC

Circuit:

The original article states

The carrier frequency is set using the 5 kΩ potentiometer behaving as a potential divider with one end at Vcc and the other at ground. The frequency of the free running oscillator is set to approximately 455.50 kHz.

This is not in the FM radio band which extends from about 88 to 108 MHz so you will not be able to pick this up on your radio - unless you somehow manage to generate the right harmonics.

The preceding paragraph says

Here is a frequency modulation (FM) circuit that uses a 555 timer in astable mode to generate a sine wave carrier. GCSE students might find it interesting as they are often playing with the 555 timer IC.

GCSE is a UK second level school standard so I suspect that this might be part of a course studying basic electronics and that a matching receiver would be required.

The North America FM band is around 100MHz, which is a couple orders of magnitude higher frequency than a 555 can manage. You would also want a sinusoidal output so as not to splatter harmonics all over the spectrum (at odd integer multiples of the base frequency).

The author is confusing you with free running oscillator set to approximately 455.50 kHz which is the down-converted intermediate freq. , or IF frequency used for an AM radio.

FM radio's use 10.7 MHz for the IF filter.

This 555 timer circuit can generate FM square waves as a voltage controlled Astable from 3.3kHz to 4.4kHz but neither useful for AM or FM radios. 555's are never used in radio designs or any serious design for that matter.

Mr. Peter J. Vis appears to have good Windows/Network/Router skills but weak on Electronic design. I would look elsewhere for better basic electronics books such as on my profile.

The circuit does demonstrate a FM modulator. It is a little wanting though.

It you wanted to use it as a transmitter well:
First of all it does not transmit at a very usable frequency, as in finding a cheap receiver that can be used to validate if it works.

I would need a rather unwieldy antenna (for a class room/ lab demo).

I'm not sure, but I a pretty sure it would not be legal to use it, wrong frequency band and lots of extra noise (from the over tones that you get with a square wave).

That being said, if you added a LC filter network tuned to an overtone, in the 88-108 range it might work.

Square waves (like what a 555 IC generates) are composed of a series of odd harmonic sine waves, of the fundamental frequency. So with a good filter and antenna, to remove the junk, you could get a nice-ish FM modulated sign wave out that (big sh-maybe) be able to be picked up by a FM radio sitting right next to the '555 transmitter'.

It would be horribly inefficient, but may demonstrate a basic working FM modulator; in a high school lab setting.