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Powered FM antennas- Safe for older Recievers?

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DocMontana

Active Member
I was in Radio Shack the other day, and found something interesting..A powered FM Antenna. It's supposed to really boost FM reception and also looks kinda cool, sort of an obelisk-shaped thingie. I have in the past used an outdoor antenna with my stereo rig, but I've always worried about lightening, especially where I'm currently living, so all I'm using is one of the short wire indoor antennas (@ 300 ohms) which doesn't always get the best reception, of course.

The thing that kept me from buying it on the spot was a concern that it might somehow not be compatible with my early '70's solid state Sansui Receiver. Could a powered FM antenna damage my receiver or possibly cause a shock or electrical hazard? It would sure be nice to expand my radio range without having to fool with an outdoor rig.
 
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A powered FM antenna improves FM reception. It does not send power to the receiver, so no worries there.
 
It is possible for an antenna amplifier to over-drive the front end of the receiver, which may or may not damage anything but it can cause distortion. They also raise the noise floor in the process of increasing the received signal, so won't solve all reception issues. A lot of them are fairly worthless in my experience.

John
 
jdurbin1 said:
It is possible for an antenna amplifier to over-drive the front end of the receiver,
Click to expand...

They can work well on lower sensitivity receivers/tuners but usually l have found them to overdrive the front end also, (ie amplifying noise between stations). If you get one just make sure the gain is adjustable.
 
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I have bad reception in my area, even when using my Mac 7082 tuner which has excellent sensitivity. I added a powered antenna from Rat Shack and reception has increased significantly. This setup has been working well for a couple of years now.
 
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I'm not at my house now, but I can assure you the antenna was nothing special. Just something I picked out of the bunch that were available.
 
I'm curious if it actually increases signal quality or just level.

I have one station where the dial barely gets to 3/5 and it's clear monaural and mildly staticky on stereo. If I throw an amplified antenna on, will I get clear stereo, or just the same noisy signal or just swing the signal meter more?

I suspect you could do some interesting stuff with, say, a software-defined radio device-- take in the signal, process it to reject anything that doesn't look like normal FM, scale that up and output... but that seems overkill.
 
Hak Foo said:
I'm curious if it actually increases signal quality or just level.
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If a weak signal provides static noise through your speaker and a strong signal eliminates most, if not all of that static, wouldn't that be an indicator of improved signal quality?

Putting it another way, static noise is a symptom of a weak signal. A stronger signal from the station, or an antenna capable of pulling in same signal will not draw in static unrelated to the station.
 
They are very useful for pulling in stations you can't receive easily, although you need to have a unit with adjustable gain to prevent excess front end noise.
 
Basic RF theory tells us that it is the noise figure of the RF amplifier that determines the overall noise floor.

The following is a basic example.

In this example the tuner/receiver has a noise figure of 3 dB which is a fairly good number (the lower the better).

The RF amplifier in the amplified antenna might have a noise figure of 5 dB (not an unreasonable number) and a maximum gain of 15 dB.

At maximum gain the overall noise figure will be about 5 dB. If the gain of the antenna is turned down to for example 5 dB then the noise figure is about 5.4 dB. Note that the noise figure has increased and a lower number is better.

Here is the math if you want to check the numbers.

upload_2017-4-29_1-19-14.png

As long as the noise figure of the RF amplifier in the antenna is greater than the noise figure of the tuner/receiver, it will degrade the overall noise figure. Unfortunately the noise figure of amplified FM broadcast band antennas is not usually published.

There is no substitute for a full sized antenna for the FM broadcast band. If using a physically small amplified antenna improves one's reception, that means that the electronics (the FM amplifier) in the 30 dollar antenna is better than the electronics in the tuner/receiver. See the math above.

If the noise figure of the RF amplifier in the antenna is less than the noise figure of the tuner/receiver (a good thing), reducing the gain of the RF amplifier in the antenna will still result in an increase in the overall noise figure. For example.

Noise figure of antenna amplifier 3 dB. Noise figure of tuner/receiver 5 dB. These are realistic numbers.

If the gain of the antenna amplifier is 15 dB, the overall noise figure is about 3.2 dB and an improvement over the basic noise figure of the tuner/receiver alone (5 dB). Remember the lower the number the better. This means that there may be an improvement in the reception of a weaker station.

If the gain of the amplifier in the antenna is reduced to 5 dB the overall noise figure raises to about 4.3 dB, still slightly better than the tuner/receiver alone, but it is still an increase and remember the lower the number the better.

Another variable that may come into play is the quieting slope of the FM tuner/receiver, but that is another can of worms.

The bottom line is that is not easy to accurately predict the result of an amplified antenna with unknown specifications when used with tuners/receivers with unknown performance. These antennas are cheap enough to give one a try, although it may or may not be an improvement. They will not usually cause any damage.
 
It's probably just an extra stage of RF (radio signal) amplification. As a practical matter, it's not likely to harm your tuner unless you are tuned to an already very strong (local) signal. If you start hearing weird things - like multiple stations, whistling, or howling - take it out and see if that fixes things. The biggest problem with these powered antennas is poor design that increases noise, thus making weak signals harder, not easier, to hear.
 
Don't get your hopes up too much. It won't remove the noise from a FM signal.

They don't differentiate between the FM signal you want and the noise that comes with it you don't want. If you have a weak, noisy signal to begin with, a tiny piece of metal with an amplifier will just get you a strong, noisy signal going to your receiver.

I lived right outside of NYC, a signal rich area and found them no better than a simple "T" dipole antenna, Now, I live essentially in what's a an FM desert, 90 miles away from a major city, and I find the same is still true.
 
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I LIKE MUSIC said:
If using a physically small amplified antenna improves one's reception, that means that the electronics (the FM amplifier) in the 30 dollar antenna is better than the electronics in the tuner/receiver. See the math above.
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What he said. ;)

The reception on my Mac 7082 tuner increased significantly with static virtually eliminated using a Rat Shack powered antenna. With the store closing its doors soon, you need to run and see if any remain. They'll be at a significant price cut (if any remain), and you won't be at too big a loss if it doesn't work to your satisfaction.
 
When the Terk powered antennas came out they got universal raves from the commercial audio mags. We sold them and many people bought them expecting the wonderful results they had read about.

They pretty much all came back for refunds. As mentioned above, they amplified the noise, and added a bit of their own, as well as the signal so most people were no better off. There are some situations where they may help but may sure you have a money back guarantee before buying.
 
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I have a Recoton FM100 and it made a noticeable improvement to the reception here, especially stations that I could barely pull in with an indoor dipole antenna.
 
It's a Terk, and I went ahead and picked it up yesterday. It does help reception a bit, but unfortunately it doesn't help with the one station I was hoping to pick up..Story of my life, no matter where I've lived it's always seemed that my favorite station was either totally out of range, or didn't come in worth a flip! :rflmao:

Something else I was thinking about was putting the outdoor antenna in my attic, but there would still be some tricky logistics, such as grounding issues, and I'm not sure how well such a setup would work anyhoo..Might be a heckuva lot of trouble to go to for no big improvement in reception.
 
There are coaxial fuses designed to be used with antennas, then you would need the dual prong adapter and ground one of the prongs. I've used television antennas for years....cause they are free, can be rotated and in many cases already mounted to the house.
 
DocMontana said:
It's a Terk, and I went ahead and picked it up yesterday. It does help reception a bit, but unfortunately it doesn't help with the one station I was hoping to pick up..Story of my life, no matter where I've lived it's always seemed that my favorite station was either totally out of range, or didn't come in worth a flip! :rflmao:

Something else I was thinking about was putting the outdoor antenna in my attic, but there would still be some tricky logistics, such as grounding issues, and I'm not sure how well such a setup would work anyhoo..Might be a heckuva lot of trouble to go to for no big improvement in reception.
Click to expand...

If you put a yagi antenna in your attic and feed it with coax to a grounded tuner, you really don't have to worry about grounding. You could permanently aim it at your target station or use a rotor. You don't have a metal roof, do you?

Anyway you cut it, indoor antennas are a crapshoot.
 
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