FM signal amplifiers: A benefit to receiving weak radio stations

credit: stock.xchng

credit: stock.xchng

Could you be receiving more radio or television stations and not even know it?

Adding a signal amplifier, commonly called a ‘preamp’ to your setup may be the answer you need if you believe your radio or televisions could be doing more.  It can also be a cheap solution to combat local signal attenuation when your listening post is located in a less-than-preferred location.

What is a preamp?

The Radio Shack 15-2507 preamp that I use.

I use the Radio Shack 15-2507 preamp, pictured above.

A preamp is a device, typically used in-line with coaxial cable, to improve broadcast reception and bring signals above the noise level to be heard and logged.  I use the Radio Shack 15-2507 amplifier, which is pictured to the left.  It can boost any FM and TV signal by 30 dB.  It also has a knob on the device itself that can control 10 dB of the attenuation, allowing listeners to bring down the amplification to 20 dB, if necessary.  The amplifier consists of an indoor unit (pictured left) and a roof-mounted device which, together, amplifies broadcast signals.   Read the 15-2507 amplifier’s manual, which has more details about installation and specifications.

Although I know it isn’t the best amplifier out there, it has served its purpose and I’d recommend it to others.  I first purchased it in 2004 and replaced it with an identical unit nine years later after the original unit died.  It appears Radio Shack no longer sells this exact device, but sells similar models.   I purchased my replacement preamp with the original in 2004 and it went unused until needed.  I have heard Channel Master-brand amplifiers are the best in terms of performance and quality.

Will a preamp work for me?

I’ll admit, preamps don’t work for everyone.  As the device, by design, amplifies signals up to 30 dB (more or less, depending on model), I’d predict that it could possibly cause overload if a DXer is very close (within 10 miles) to a strong FM transmitter.  These devices will amplify everything–including IBOC hash on adjacent-to-local frequencies, as well as radio frequency interference, if present.  I’ve been in many locales where all that is needed to receive FM signals over 500 miles away was a thin wire antenna without an amplifier.  A preamp is not always necessary to achieve good DX.

But there are some exceptions.  Listeners in rural areas who have relatively empty FM bands, or those with overall weak signals would likely benefit from the use of a preamp, especially if they are far from broadcasting transmitters.  It could mean the difference from receiving a listenable stereo signal or nothing.

credit: stock.xchng

credit: stock.xchng

DXers who have localized reception problems, regardless of distance from local transmitters, would also likely see an increased level of reception with a signal amplifier.  In my case, I’m 15-25 miles away from the majority of my local Washington, DC and Fredericksburg, VA radio transmitters.  Most would argue that an amplifier is not necessary, especially since I live close to the northeast corridor where FM signals are aplenty.  However, I live at the bottom of a steep hill which severely attenuates local signals, regardless of direction.  My home is 49 feet above sea level, while several other hills in all directions nearby are 60 feet or higher.  Indoors, stereo signals from my locals are difficult to achieve.  For example, using a clock radio in a basement bathroom at my home, 95.5 WPGC Morningside, MD, a 50kw blowtorch signal at 23 miles away that dominates its frequency on any area radio, is staticy and often fades out into oblivion.  My local signals are listenable throughout other floors of my house but are often not strong enough to bring in stereo reception, RDS or IBOC decodes.  These same signals come in strong stereo with RDS/IBOC with my roof antenna, but signals outside of a 30 mile radius are very difficult to receive without an external antenna and/or an amplifier.

To combat this severe local attenuation, a preamp is a required to receive anything other than local FM signals.

DXers who have lots of splitters and cable runs in their setup may also benefit from a preamp to combat the RF loss that splitters and long cable runs can introduce.

What I’d suggest is get a preamp regardless of your distance to local FM transmitters, but be sure you can return it for a full refund before purchasing.  This way, you can easily get your money back if you find that a preamp, in practice, makes DXing worse.

Understand that an amplifier can’t perform magic.  If deadband conditions are present where you’d normally not receive any signals, you aren’t going to suddenly pick up exotic signals.  All the preamp does is amplify the RF that your radio receives and it makes signals which are already there stronger so you can more easily hear it.  But during good DX conditions, it could mean the difference of picking up a signal or not receiving it at all.

Examples of how a preamp boosts signals

Click on image to enlarge.

The picture above shows a waveform of a signal in Wavepad from 97.9 WIYY Baltimore, MD @ 56 miles away from my home.  My antenna was aimed NW toward WIYY’s transmitter to get maximum signal levels.  Listen to the audio clip that corresponds to the picture above below:

As you can hear in the beginning of the clip and see in the waveform above, WIYY pumps in a local-grade signal to my Northern Virginia home.  At about 3 seconds into the clip, I unplugged the cord from my Radio Shack 15-2507 amplifier.  When the amplifier is unplugged, the radio gets whatever it’d pick up if the amplifier was not installed.  As you can see and hear, WIYY disappeared without the amplifier plugged in.  Yet when I plugged in the amplifier again at about 6 seconds into the clip, WIYY came back clearly like before.

This is proof that the Radio Shack preamp can bring in signals that listeners can’t often pick up without it.  This is not to say WIYY can’t be received without a preamp at my home, but it may take a lot of regional enhancement to overcome the 30dB ‘loss’ exhibited without the amplifier plugged in.  In fact, I’ve received FM signals up to 210 miles away while the preamp was unplugged, so it is still possible.  But the same weak, distant signals were suddenly coming in loud and clear, with scrolling RDS, once I plugged the amplifier back in.

Click on image to enlarge.

However, the night-and-day results seen with the WIYY signal is not always observed while using a preamp.  The above waveform was from 98.7 WMZQ Washington, DC, a local station at 15 miles away.  Listen to the corresponding clip below:

My unplugging of the amplifier while tuned to local WMZQ had no effect on the signal quality.  WMZQ even decoded its IBOC stream while the amplifier was unplugged.  The ‘quiet’ portion of the waveform was the IBOC decoding and not a drop in signal strength or volume as seen in the WIYY clip.

Downsides of using a preamp

I’ve seen several DXers express distaste for these devices, often to the point of not recommending them to others.  There’s nothing wrong with that.  They have good reason to do so.  I’ve already mentioned overload as a potential pitfall of amplifiers.  Preamps can also introduce radio frequency interference into a setup, ruining reception.  I have not, however, experienced RFI with my 15-2507 amplifier.  I do have some RFI on my radio dial, but it is also received with the 15-2507 unplugged.

I’ve found my preamp is perfect for FM reception but not so good for digital TV reception.  Although I’ve received analog TV signals up to 1431 miles away and digital signals 218 miles away with the preamp, the Radio Shack 15-2507 amplifier does seem to negatively affect digital TV reception on occasion.

Click to enlarge. The Zenith DTT-901 receiver receives an unknown digital signal on channel 5 via Sporadic E on 7/5/13.

Click to enlarge. The Zenith DTT-901 receiver receives an unknown digital signal on channel 5 via Sporadic E on 7/5/13.

For some unknown reason, the preamp often causes local digital TV signals to drop to strength levels inadequate for a decode.  For example, most of my local Washington, DC television stations at 15-25 miles away typically come in with a 90% signal quality as noted on the Zenith DTT-901 digital converter box (this appears to be the highest signal quality it’ll display, based on internet forum discussions).  The same signal strength would be noted as ‘Good’ in the picture on the left of a reception using the device.

On occasion, the amplifier would cause the same 90% signal to drop to the 33% level, as seen in the picture to the left, causing the station to not decode.  When I turn the auto-gain knob on the 15-2507–the same knob that can adjust the attenuation from 20 to 30 dB–to 20 dB, the local signal returns at 90% strength.  Incidentally, when the amplifier is acting up, the weakened signal levels also prohibit any distant TV DX signals from coming in.  The problem itself may be present for a few hours and then it ‘corrects’ itself on its own, allowing signals to be received at full 30 dB-amplified levels.  Nothing I can do would fix the problem, including unplugging the device for a while.  It seems to have a mind of its own.

This defect only seems to affect TV reception.  Distant FM signals come in the same regardless of if this TV problem is present.  The problem is causing me to look into other preamps in the future, possibly a Channel Master brand, as I’ve received many strong FM signals from a distant city but no trace of TV signals and I know if FM signals are coming in strong, then TV signals are likely coming in with comparable strengths.


Even with some downsides, I still recommend all DXers to test out a preamp, as long as they can return it if it doesn’t work out for them.  I have proven that amplifiers can greatly improve reception conditions and bring in signals that otherwise would not be received.  TV DXers may want to look into other brands, such as Channel Master, to achieve positive results.