Bulletin Board #35

I've been discussing transmitting devices on the Spyking forum and thought I'd cut some of it and see if we can get a little controversy going here.

I wrote the following in response to an inquiry regarding the difference between FM transmitters and FM wireless microphones. How about a few comments and responses and maybe we can get something going here again.

I'm hoping there may be a few disagreements or clarifications.

Tim

I've worked with and evaluated a lot of fm wireless mics (aka transmitters and/or bugs) and these are some of my observations.
  1. Most of the Radio Shack stuff is junk and is sold for kids to play with. There are exceptions, but those ARE the exceptions.
  2. If you are looking for stability and clarity but don't need long range, the best results can be obtained by purchasing a commercial wireless mic that operates on a crystal controlled frequency.
  3. I've put together a number of kits and have used them for demos at my two day seminars and they work great (for the most part). I don't think I've ever had one that didn't work when I finished putting it together (luck ?). One had a range of 910ths of a mile operating off a 9 volt battery and using my car radio as a listening source. Had I used a scanner, I would venture that it would have been even greater.
  4. All the units built could be tuned with a capacitor and/or slug. They were all relatively difficult to get to just that special point on the dial, but it could be done if you had an orange stick or plastic tuning wand. What was great about the home made devices was the ability to tune them OUTSIDE the FM range (here, you WILL need a scanner to pick up the signal. Typically, I can build the units to be tunable from approximately 50 MHz to 200MHz without any major modifications.
  5. Antenna length a problem? Decide what your operating frequency is going to be and experiment with a field strength meter to get the optimum antenna length by determining the 1/4, 1/8 and 1/16 wave length and cut the wire to slight longer than those lengths. Then, remove 1/16 inch of antenna lead until you have reached and passed the point of greatest signal strength. You then cut a new antenna to the length of the greatest signal strength and transmit away.
  6. Range a problem? That is a typical complaint, but what do you expect from something operating from 1.5 to 3.0 volts-Voice Of America? 1 1/2 to 3.0 vdc will typically get you to the next room or across the street; seldom more than 100 meters in the open. On the other hand, you'd be surprised at the range of voltages those little suckers will operate under. If you've got one you want to play around with, get a variable output DC power supply and start off at the suggested operating voltage then increase it by .1 to .5 voltage increments. You may get as high as 5 or 6 volts with some of the 1.5 vdc units before smoke occurs and you will probably have increased the range by a factor of 4 to 10. 9 vdc units will often operate up to 18 to 21 vdc with proportionate increases in range.
  7. In any situation, the bigest problem is going to be the power supply. The quality and range of the transmitter signal begins dropping off as soon as you activate it. It won't be discernable immediately, but it's happening. A rule of thumb in setting up an eavesdropping operation (whether audio or video) was to determine the distance between the target and the LP and have an output capable of reaching twice as far this was regardles whether it was battery or AC powered. To many pucker factors involved to try to operate on the fringe of a signal (And besides, you may only get one chance to get into the target area).
I hope this helps the readers understand some of the factors involved with the "FM Wireless Microphoners" and other "Spy" transmitters. If you want it done right, do it right the first time; build a crystal controlled device and install it in such a way that the target provides power (if possible).

Good reading to all and keep the comments coming Augie.

P. S. It would be fun to get together and play with locks for awhile. My feel has gone, but I still get lucky occasionally.

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