I recently inherited a never-used television antenna, mast and rotor–all still boxed and crying to be used. Extended, the antenna “boom” measures 8 to 10 feet long, IIRC. I’m guessing the whole setup is about 15 years old, but looks showroom new.
With cable and satellite TV, these things have become anachronisms, but I’m wondering if I could mount them in my attic (don’t want them on my roof) to improve my FM radio reception.
Is there an aftermarket for this stuff today?
Of course it will help. It’s overkill, but it will help immensely.
Channel 6 on yout TV is approximately 87 MHz. I used to have a tuner with the old slide rule tuning that I could slide down and pick up audio from the local Channel 6 TV station.
FM is from 88 to 108 MHz. Most TV antenna cover this range - and do it quite well.
Anything up high is better than what most receivers have internally.
Just trying to find a use for this antenna. 
The property that counts on an antenna for getting the most possible signal is the capture area. This increases as the directivity of the antenna increases. The TV antenna is quite directional, hence the rotator so you can point it toward the station, so it will capture more power from the incident signal than just a straight wire or rod.
If you are currently just using the built in FM antenna on your receiver you should get an increase in signal from the TV antenna.
Does your boom come in two pieces? For FM you’ll only need the half with the long arms, in case that makes it easier to mount in your attic. Note that it will help reception only in the direction it’s pointed, which is towards the end of the boom with the shorter arms. If you need to receive stations from multiple directions, you’ll need to use the rotator, assuming your attic is open enough for it to rotate freely.
Way back when, I had just that setup – antenna in the attic hooked to an FM receiver. I can testify that it works, but it doesn’t work miracles. FM is still a line-of-sight wavelength, so you won’t be able to pull in stations from South America or Europe.
However, the directional nature of the TV antenna eliminates a lot of echo and ghost signals that can screw up reception even for a strong station.
The rear boom on a tv antenna is called a reflector and is made to block signal from coming in on the back side. That way channel 7 from the north doesn’t interfere with channel 7 from the south.
Cut off the back boom (the longest one) and your antenna will have twice the reception.It will still be directional though.
Yeah but the directive gain and thus the capture area is reduced by a factor or 2 which means the received voltage will be down by a factor of 1/(2)[sup]1/2[/sup].
This probably won’t make much difference.
One thing to remember is that your receiver has automatic gain control (AGC). If the setup you have now provides enough signal to reach the level where the AGC is fully on then increasing the received signal isn’t going to improve your reception any. The reason for this is that if the received signal doubles, the AGC reduces the gain by almost a factor of 2 leaving you back where you started as far as output is concerned.
Can you translate that into “low tech”?
The cable from the antenna is also worthy of mention. Coax, typically RG-6 is better than 300 ohm twinlead. Some antennas don’t accept the 75 ohm cable directly, meaning you need a balun to perform the 300/75 conversion. At the receiver end, you’ll need a splitter if using this for more than just FM. As to what you can pull in, that’s also a function of the transmitter power. My elevation is ~700’ ASL and with an antenna/rotor, my TV can pull in Phila, Baltimore, and DC stations, and I’m just south of Harrisburg.
The receiver has circuitry that that converts the frequency of the incoming signal to a lower and standard frequency. It is at this frequency that all the amplification of the radio signal takes place. There is additional circuitry that detects how strong the signal is and applies a signal to the amplifier circuits so as to maintain a constant output amplitude for all input signal amplitudes within its range of control. This is the automatic gain control (AGC) circuit.
For a very weak signal that is barely detectable the receiver amplification (gain) is at the maximum. the gain stays at the maximum until some threshold output amplitude is reached at which point the AGC circuit starts reducing the gain of the amplifiers as the signal gets stronger. That threshold is usually referred to as the knee of the AGC curve. Once that input signal exceeds the threshold every input increase results in an gain decrease so as to keep the amplifier output constant.
If there are further questions about this ask away. If I can’t make it understandable to you I’m sure there is someone around who can.
What he’s saying is that there is automatic circuitry that will automatically adjust the audio incrementally. If it’s too low it will boost it until it passes a preset threshhold, whereupon it will reduce the audio output. After that any input increase will result in the AGC circuitry reducing the output. Remember back in the day when you used to be blown out of your seat by commercials on cable TV? Upgraded AGC circuits have pretty much taken care of that. AGC is designed to keep the output at a consistent level.
Not quite. The AGC works on the radio frequency amplifiers (actually the intermediate frequency amplifiers but let’s leave that distinction for another day).
The point is that if Carnac’s present setup provides adequate input signal for the stations that he can receive now, then an antenna that increases the input signal will not improve the performance on those stations.
Nor will it allow receiving a signal from stations from further away than line-of-sight, as kinilou stated. If you want to get additional stations from a greater distance you have to raise your antenna higher.
I know. I was trying to make it “simple”, but what the hell. 
[QUOTE=David Simmons]
Yeah but the directive gain and thus the capture area is reduced by a factor or 2 which means the received voltage will be down by a factor of 1/(2)[sup]1/2[/sup].
This probably won’t make much difference.
Well I guess I was sick that day
There is a couple of TV antenna manufacturing plants in this area that make a special antenna just for this area.
Long time ago we found that by cutting off 2" of the reflector we could recieve channel 7 from Quincy on the backside of the 4,6, 8,antenna.
Funny thing though. 15 miles west of here it doesn’t work because channel 7 Cedar Rapids comes in instead.
Anyway removing the reflector would increase Carnac’s reception.
[QUOTE=justwannano]
BTW so would just throwing a spool of flat lead over the roof.
Of course, for those stations for which there is already an adequate signal no improvement is needed. I’m just trying to get across the idea that a miraculous improvement shouldn’t be expected. For line of sight stations most commercial FM receivers are already close enough to optimum that improving their performance requires more effort than most are able to put out and more detailed knowledge than most of us have.