FM trap: how do you know if antenna has one?

JRhoBurton

All roads lead to music
Our rooftop TV antenna is fine for TV, but works no better for FM than a little dipole stuck on the basement joists. Does that mean there is a trap? Is the trap a device, or a feature of the element arrangement?

Thanks,

John
 
Okay , this is one of my favorite topics...:biggrin::biggrin::biggrin:

Although there are those that claim that TV antennas will work just fine for FM reception, not all TV antennas are created equal when it comes to FM broadcast band reception.

Some may have an FM trap that is a separate feature that may or may not be switchable and most do not even have aluminum elements to cover the FM broadcast band. If you have a distribution amplifier for the antenna signal it may or may not have an FM trap that may or may not be switchable.

There are many varieties of antennas, passive and amplified. Newer antennas since the switch to digital TV signals may start their reception at 216 MHz, TV channel 7, because most of the digital TV signals are above this frequency.

Do you know the brand and model of your TV antenna?
 
An old three tier aluminum antenna decades old from the sixties and seventies works great after you take it apart and scub it with Comet. But, unless you find one still sitting on a house in an old residential neighborhood on top of the house.....your not going to find one. No fancy attachments or amps required other than a fuse for lightning and what a range, I could pick up the entire San Francisco’s Bay Area and South Bay to include Salinas, Monterey and Santa Cruz in full strength signal. Using a Marantz receiver.....
 
other than a fuse for lightning

A fuse is not likely to provide much lightning protection. The bare minimum is to ground the antenna installation to NEC requirements then apply addition surge protection as desired.

Again, three tier antenna as a description for the evaluation of antenna performance can be somewhat lacking.

Here is the thing, not all antennas are created equal. Again not all antennas are created equal. Some are very poor at FM broadcast band frequencies above about 92 MHz for a number of well known technical reasons.

Depending on what was needed for a given location, a three tier antenna could be three VHF high band ( that work at 216 MHz and above) antennas that would be quite poor for FM broadcast band reception.

I'm talking three stacks of antenna with about two feet separating the layers and angled in different directions.

Again, three tier is not a technical description of antenna performance. What you are describing is just the use of multiple antennas, in this case three, to be able to receive stations from different directions. This is done because as you posted they are pointed in different directions. This is done so that an antenna rotator is not needed. That is all.

To be clear, stacked antennas can be used to increase gain if they are all pointed in the same direction and implemented correctly. Adding a second identical antenna will gain about 3 dB and adding a third antenna will gain about another 1.5 dB. at the antennas design frequency range.

And whether or not an amplifier is needed depends on the amount of signal as seem by the antenna, the feed line loss and loss due to splitting the signal if it is split.

Again, I am not trying to be the TV antenna use for FM broadcast band reception police, but making broad blanket statements with references using a technical descriptor such as three tier leaves a lot to be desired because it means nothing in terms of how well it will meet the particular reception needs in a given situation.

As rcs16 mentioned, a picture may help if you do not not the brand and model number.






















 
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