...uses a device to emit a strong radio signal to block any cell phones within a desired radius in order to silence the conversation of one annoying valley girl...
You used to be able to do this with a ham radio using some quick math. Find the transmit frequency of the desired cell phone with optimally a frequency counter, a nearfield receiver or the ham radio itself (discussed below). Take that frequency, add 45 MHz and divide by 2. 45MHz because the tower transmits 45MHz above what the portables do. When you divide by 2, you should have a frequency that most dual band (or 70cm monoband) radios can transmit on with a mod. When transmitting, the first harmonic will be on the same frequency the cell phone is receiving on and if you're close enough you could easily overcome the cell tower and cause the phone to drop call. Of course, that's not so easy now with most phones digital and many in the PCS block at that. A first order harmonic is weak enough as it is and each multiple beyond this gets increasingly weaker thus much more difficult to overcome a transmission on a primary frequency.
You might ask how can you listen to an analog cell phone on a ham radio when the frequencies are blocked. A lot of ham radios, although the cell frequencies are blocked, you could still receive cell phone signals imaging onto another frequency. For example, let's say the ham radio had an IF of 41.85MHz and the cell phone was transmitting on 833.1, the ham radio's LO is at 791.25 mixed with the IF to make 833.1, but the radio's CPU blocks 833.1 from being entered on the keypad. But a superhet receiver "receives" at the IF frequency above and below [abs(f − fLO)] so if you could tune to 833.1 you'd actually be listening to 833.1 AND 749.4 and the 749.4 would be filtered out with some filters. With that in mind, you can tune your radio to 916.8 (which isn't blocked) making the LO tune to 874.95 and you'll receive both 916.8 and 833.1. Now 833.1 would be filtered out with some bandpass filters but usually a person within an earshot with a cell phone will give a plenty strong enough signal to overcome those filters. On top of this, many ham radios have poor filtering outside of the ham bands anyway.
As for interfering with radar, a strong signal could very well overload a radar gun's receiver, especially if it was very powerful and broadbanded but many radar guns have an audio out where they can hear when something is interfering. Plus, if you are watching a bunch of cars and see what their speeds are, it wouldn't be hard to visually determine that someone is driving over the speed limit, especially if they were going 15+ over. Also, you would lose your ability to detect radar. You would have to transmit like this on every radar band used. You would also set off every other persons radar detector around you which could slow down the flow of traffic. It's also probably not too safe to sit around higher powered microwave transmitters for extended periods of time unless you plan on growing a third testicle.
