AM vs. FM

Invisible Fence and nearly everyone else uses AM radio systems.  The Dogwatch Brand uses FM radio systems and promotes this as being less prone to interference.  Often our customers ask us if there is a difference. 

In our opinion the short answer is no.  Interference is rare.  We have never seen a real life example of where an FM system performed better than an AM system. 

Dogfence often uses the analogy that FM radio stations are clearer than AM and the example is disingenuous.  Radio signals have to travel 10’s of miles to get from the transmission tower to your radio often through all kinds of obstacles.  The dog fence signal only has to go a couple of feet from the boundary wire to the dog, through at most a couple of inches of soil.  In this circumstance, being FM is of no practical value.

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  1. Bp968 says:

    @Pat: Actually the issue your describing isn’t really because of the modulation differences between AM and FM, its due to the frequency differences between those used to broadcast the two standards. If you were able to broadcast AM on 100 mhz you’d find it propagated quite similar to to the FM radio you listen too (in fact you can do exactly this using HAM radio, transmit AM on 144mhz for example). AM radio is transmitted between 530 khz and 1700 khz compared to FM radio’s 88-108 *mega*herz range.

  2. Pat says:

    Actually, there is a significant difference between FM vs AM. Haven’t you ever been listening to an AM radio station and have driven under a stop light and the signal cuts out until you pull away? It is called interference due to electrical noise. An Am signal is WILL be disrupted by a strong enough electronic appliance, causing the dog’s receiver to give a false correction. This is why dogWatch converted their signal to FM and then very smartly put a patent on it so nobody else could copy it for 17 years. An FM signal isn’t prone to electrical interference because it’s signal cannot be duplicated.

    Also, if you bury any brand system next to one another with the exception to a DogWatch Hidden Fence, the signals WILL interfere with one another causing either dead spots or cause the signal to be amplified causing the dog to get shocked in the middle of the yard. Because DogWatch uses an FM signal, you can bury Any other system next to it without worry of disrupting the signal.

    Beware of people giving advice without knowing who they really work for and finding out how long they have been in the industry. I suspect the author of this original post either works for Invisible Fence (because all of these products offered on this site are owned by Invisible Fence) or the person has not been in business long enough to have learned all that there is to know.

  3. Sam Chapman says:

    I disagree have swapped out Petsafe & PAC due to false activations. One Petsafe customer had been using the system for years and then something or somebody moved in near to him and all three of his dog’s were being activated in the middle of the lawn. Petsafe could not resolve the problem – swapping to DogWatch did. Also FM is way quicker in reaction time = less dog outs!

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