Directional antenna (e.g. satellite dish) broadcasts power in some specific directions (space angles) around the antenna (which are called main-lobes). This is to increase the chance of picking up the signals at the receiver (which is also pointed at the transmitter antenna).

Omnidirectional antenna on the other hand, transmits information in all directions (kind of like a sphere around the antenna). The transmitted power per surface is identical at a certain distance, and for the same input power, power density of an omnidirectional antenna is less than that of a directional one.

Complete omnidirectional antenna doesn't exist in practice, but sources like the Sun and wireless modems (some of them) come close to this definition. BTS towers on the other hand, make use of directivity methods to further cover the area, which is why you see something called SAR (Specific Absorption Rate) on your smartphone's box. In layman's term, SAR measures how much EM power your body-flesh absorbs because of your phone, and as you might guess, the less SAR, the better.

It's the same as the difference between a laser/spotlight and a point source/diffuse light - aka absolutely nothing biologically or physically if the intensities are the same at the point of measurement.

The power level makes a difference.

Please see my comment below; sneering about this topic isn't helpful or fair.

If you mean power, then talk about power, not unidirectional/omnidirectional, unless there is some aspect of the unidirectional emission that you believe, or have evidence to suggest is different from an omnidirectional emission.

I have Omnidirectional antenna's that put out power at about 46 dB, and directional antennas that do likewise at -24 dB.

Sure, that's a good clarification.

You could have replied to that effect to begin. HN is a better place when people are charitable in their interpretations of comments and kind in their replies.

Sure - I was genuinely curious. I'm not an RF engineer, and haven't really studied this topic, other than to know my company has Omnidirectional and directional antennas. I was curious as to whether there might be some sort of phase-alignment or other type of behavior that might be happening with the directional antennas, unrelated to the power/duty cycle. My question with regards to education was truly genuine.

It was too, I'm so sorry.

I can get a little indignant (I'm working on it, I really am), over the tendency of some HN commenters to exhibit belligerent skepticism in response to people sharing sincerely-considered views or insights regarding yet-to-be-settled matters in health/medicine/science.

In that state, I mistook your comment as being sarcastic, which I now realise it wasn't. Please accept my sincere apologies.