While I do not disagree with the problem with franks box, I do disagree that an analog cassetterecorder will not pick up stray radio signals rather easily, just drive up to any radio station tower and you will see what I am talking about. I am a ham operator of 20 years + and own several radio transmitters which cover a wide range of frequencies. Any of these transmitters can be picked up on an analog cassette recorder regardless of what frequency I am transmitting on (AM, FM, or even fast scan television). All analog recording devices incorporate an amplification circuit which can easily act as a mini radio receiver, though the amplifier is only designed to amplify the signal coming from the microphone, any stray signals can easily "infect" the circuit with extra noise which will get amplified along with the microphone signal. Obviously the quality of the recorder will make a big difference as to whether it will pick up radio signals easily or not, cheaper recorders have less buffer circuits which will most likely pick up stray signals if the signal is strong enough. You may be out in the field recording EVP and know that you are no where near a radio tower, but that does not mean a strong radio signal can not bounce your way. At certain times of the year I am able to communicate with people in places like North Carolina and Virginia from Pennsylvania where I live using very little power due to a phenomenon called ducting which carries my radio signal quite a bit of distance. This phenomenon can happen at any time and easily send a stray transmission your way when you are least expecting...
If you are receiving intelligent responses to your questions via an EVP session, I personally consider that a scientific result, but if you are receiving random words not based on any logic, it may be a stray radio signal. Just remember there are tons of radio transmitters all around you and not just the broadcast stations that you can pick up on your AM or FM radio, they are a very small percentage of the total radio transmissions happening around you all the time.
Also keep in mind that most recorders use an "Automatic Level Control" to keep the audio at a constant level. When you are standing in total silence, this ALC will turn itself up to maximum since there is no noise. This is considered "gain", increasing the gain is increasing the amplification process by ten fold and also increasing your chances of picking up a stray radio signal by 10 fold.
Digital is a different animal, and though its not impossible for them to get infected with stray radio signals as they also incorporate amplification circuits, its going to be less likely.
Read part 15 of the FCC rules :-)
One of the best quotes I have seen in awhile
"Electronics doesn't pick and choose who it works for, it either does or it doesn't work." ~ ghost_hunter_1954
In your case I have to agree. Near a tower you get what is called the Proximity effect, which in essence, means the signal is of sufficient magnetude to overmodulate the input of the recorder. When this occurs, the input transistor or chip in the mic preamp acts as a diode junction, effectively demodulating the audio from the RF carrier. If the house you are investigating is adjacent to a broadcast tower, you may as well leave your recorders at home, as you will never know for sure if you have interference or EVP. My earlier statement was based on the location not being adjacent to a high powered transmitter. We use RF field strength meters prior to our investigation to determine if the RF is at too high a level. This can also, strangely enough, cause "voices" to come out of the tub in a bathroom. When the water reaches a certain level in the tub, a resonant tank circuit exists, and the radio program comes right out of the tub! This has also occured with fillings in people's teeth, and other cavity forming proceedures performed on the body. When someone says they hear voices in their head, they very well could be!