While you can use a standard VHF scanner or communications
receiver to pick up weather satellite signals, the results are often
disappointing. The reason for this is that most scanners and communications receivers only provide a choice of two bandwidth settings for VHF FM reception: "narrow" and "wide". The narrow setting gives a bandwidth of ±15kHz or less, which is fine for NBFM reception. However, it is too narrow for undistorted
reception of the weather satellite signals which need a bandwidth of at least ±25kHz.
By contrast, the wide bandwidth setting usually gives a
bandwidth of about ±100kHz, so this is the setting that must be used. Unfortunately, this is really too wide for weather satellite signals and, as a result, the demodulated audio level is relatively low. At the same time, the wider reception bandwidth allows more noise through, so the signal-to-noise ratio can become quite poor.
In short, for best results you really need a receiver with an
effective bandwidth of ±30kHz, or not much more. This type of specialised VHF receiver is available but they are not very thick on the ground and those that are available are fairly pricey. Hence the motivation for developing the low-cost weather satellite receiver described here.
As you can see from the photos, the receiver is built into a
very compact plastic instrument box. All of the circuitry is mounted on a double-sided PC board, so it's quite easy to build. It has switch tuning between two preset frequency channels, for ease of use. There are RF Gain, Audio Muting and Audio Gain controls and the receiver can drive a small monitor speaker or headphones, as well as providing a line level signal to feed into your PC for recording and decoding.
The sensitivity is quite respectable, at about 0.7μV for 12dB of quieting. At the same time, the effective bandwidth is approximately ±35kHz, which is quite suitable for weather satellite reception.
Bear in mind though that for good reception of these signals,
you really need to use a masthead preamp as well. The receiver provides 12V DC at the antenna connector, for "phantom powering" such a preamp. We'll describe a matching preamp in the third of these articles, along with an easy to build turnstile/reflector antenna for 137.5/137.62MHz.
- Two presettable channels in the 137-138MHz band
- Sensitivity: 0.7μV for 12dB of quieting
- Bandwidth: ±35kHz (approx.)
- Plugpack or battery powered
- Provides 12V DC phantom power to power a masthead amplifier