FM amplifier hookup

audiofox · 2-Sep-2012, 10:27 PM

Hello,

I have a tv amplifier that I was considering using for my AV receiver. It is good down to 50Mhz and is 15 db. I plan to hook it up to a coax approx. 70ft long to an antenna around 25ft high. My concern is that the amp may be a bit strong for my receiver. Any one out there have any experience with this?

GroundUrMast · 3-Sep-2012, 6:35 AM

15 dB of gain is a reasonable amount when dealing with typical cable and splitter losses in a home. But a deep fringe amplifier is easy to overload which is why I ask for the following information; Your FM Fool signal report, so it's possible to tell how strong your signals are. A TV Fool report would also be helpful, given the antenna and amplifier are going to be passing the OTA TV frequencies as well, regardless of whether you connect a TV. What make and model amplifier are you considering? And finally, will there be any other receivers (FM or TV) connected.? If so, how many?

Generally speaking, an amplifier can be helpful if you have FM signal levels below -60 dBm in the air. Also, the best place for an amplifier is at the antenna, not the receiver. All of the losses between the antenna and amplifier subtract from the signal to noise ratio and can not be recovered because the amplifier will amplify both the signal and noise delivered to the input.

audiofox · 9-Sep-2012, 4:29 AM

Hey GroundUrMast,

I am planning on hooking up to just one receiver using 70ft. of rg-6 since the receiver calls out for 75 ohms. My report should be attached, KMOD_FM is what im shooting for. I made the antenna from an old tv antenna using the two elements that were closest to the middle of the FM radio band. Not sure if I will need some matching between the coax and the antenna. I have yet to solder the rg-6 to the antenna. Looks like the report shows it should be hitting the elements at around -72.4dbm. I looked up the loss on the rg-6 for 97Mhz and looks like it will be a little less than 2 db for 70ft.

Im planning on using a SVI amp. If im on the right track, Im thinking the 15db should raise the -72.4dbm to 14.928db, subtracting the 2db for the rg-6, should leave me with approx 12.928 db to the receiver as long as there are no matching problems or unforseen losses elsewhere.

Am I tracking this or am I missing something?

audiofox

GroundUrMast · 9-Sep-2012, 5:03 AM

I'm not familiar with the SVI amplifier. Perhaps you have a link to the spec's?

I think you're estimate of loss is about right. However, decibels are added, not multiplied. If you deliver -72.4 dBm to the antenna and it has 3 dB gain, you should expect -69.4 dBm at the antenna terminals. If you deliver -69.4 dBm to the input of a 15 dB gain amplifier, -54.4 dBm would be the expected output. After traveling through coax with 2 dB loss, the power to the tuner input would be -56.4 dBm.

audiofox · 9-Sep-2012, 2:52 PM

DBs are confusing to me, I was thinking the amp was in db's and the received signal was in dbm and that there were 1000 dbm in 1db

Here is the link you mentioned for the amp:

http://otsecure.net/weeks/InstallMat...Amplifiers.pdf

My model #is SV-A15PRS. Can get them pretty reasonable but Im pretty sure they are a quality amp.

GroundUrMast · 9-Sep-2012, 6:48 PM

The explanation at http://en.wikipedia.org/wiki/Decibel does a very good job of explaining the difference between dB (a ratio expressed in logarithmic form) and dBm (a ratio referenced to 1 milliwatt of power).

The SV-A15PRS is very similar to the Winegard and Channel Master distribution amplifiers. (HDA-100 & CM-3410). It will do fine, just keep water out of it.