Deer get hit by semi trucks on the highway all the time. In other words, a really loud sound within their hearing range is not enough to get them off the road. I doubt anything you build will be louder than a semi, regardless of frequency.

I have deer whistles on my vehicle but I don't think they do anything. (Have you guessed, that I might be married?)

 

Wayne - first of all Merry Christmas and you're right the deer whistles have been proven thoroughly ineffective through numerous studies.
High powered ultrasonics are still worthy of testing according to a few other studies like the one I cited earlier. I'm hopeful this will be an effective deterrent, and I'd rather have it than not, I had a deer cross my path on my way to work Wednesday morning, so this IS getting built and I will find a way to test it. My current Hornet ultrasonic alert will get the attention of squirrels at least.

 

I had a look at the Hornet website. What a crock of kuhscheiße! There's no proof, AT ALL, just the word "proven" repeated without backup. And "latest military technology" whatever that is (I spent 43 years doing military R&D, and don't see it). Then there's the dubious technobabble, like
"In certain areas of some states, your odds of having a deer/vehicle accident may be less than 1 in 3 within a year..." . Yeah, I guess so.

About the inductor: it low pass filters your square wave so you get more of the lower frequency fundamental. They can't hear the harmonics, so why waste power up there, and it might just burn out your tweeter.

I've hit 2 deer (actually one hit me!) and they had a path through my yard last winter and ate my bushes. I might just try some experiments on them if they keep it up. I think an intermittent tone might startle them to run away rather than a persistent squeal, which would have to be really loud to repel them.