I think it all depends on the features you're looking for and what you value more, time or money. I work full time and I'm pretty busy even on my off hours so time is valuable to me. That antenna might just be 5 dollars but if I added up the hours and gas spent finding a junk yard, looking for the part, and then looking for the other parts to make it work (power source, wire nuts, wire, timer, all of which I'd probably just buy new at the hardware store to save time) we're getting way past 5 bucks. Then there's the time and lumber or sheet metal to actually build a door to use it on. Plus the time to put it all together and rig it up so it looks tidy (a must for me).
After all that it still doesn't have some of the features that I'm looking for like running on a battery, the option to add a remote switch outside the coop, or automatically adjusting the open/close time with the changing of the seasons.
This isn't to knock it or say it's not a good idea, because frugal options are always appreciated and I love DIY projects. Everyone's coop and scheduling needs are unique, so the antenna door might be perfect for some (money over time) but the ADOR1 is perfect for me (time over money).
I'm looking forward to cutting a hole in the coop, screwing it on, throwing a battery in it and having a glass of tea.