How do you use a tennis speed gun?

Question

I purchased a Bushnell hand-held radar speed gun for the purpose of determining the speed of my tennis serve, however the results so far have been somewhat confusing. I and my practice partner took turns using it while the other served and the results were all over the place. At one point he hit three serves that were visibly of equal speed and the results were as follows.- 68mph, 96mph, and 17 (yes, 17) mph. As you can imagine, I finished with no more idea of how fast I serve than before I started. We even tried standing in a few different places, but the results were much the same. Is there a specific place I should stand and way I should aim that would help me get an accurate reading? Your help would be much appreciated.

Answer 1

Its been a long time since I've tried using a radar, is it a stand type?
I think I recall standing on the other side of the net near the service line, pointed at the server (not directly in the way of the serve) off to either side.
Not sure if that will help, I do recall the radar we used being pretty fickle with speeds as well.
Take it with a grain of salt.

Hope this helps!