Table of Contents
Crystal NOAA Weather Radio (NWR)

David Wagner 2007/11/21 16:37

In the US, the National Oceanic and Atmospheric Administration (NOAA) broadcasts FM1) weather information in the VHF range.

162.400 162.425 162.450 162.475 162.500 162.525 162.550MHz
1.84601 1.84573 1.84544 1.84516 1.84488 1.84459 1.84431m
72.6777 72.6665 72.6553 72.6441 72.6329 72.6218 72.6106in

San Antonio weather is broadcast at 162.550 MHz, WXK-67, 1000 Watts.2)

Station         Transmitter location Call     Frq. Power Latitude  Longitude  Antenna Elev.
San Antonio     San Antonio          WXK-67   550  1000  29 30 25  98 34 29   1168 / 356.1

Map of Transmitter Location (May be out of date.)

http://www.weatherradio.info/usa/techlist.html: This page is archaic.[…]The National Weather Service has discontinued giving out the latitude/longitude information for NWR transmitters[…]

Antenna

An AM broadcast band crystal radio is a high-impedance 3)and seems to work best with a high-impedance antenna, usually a simple random wire antenna for the AM broadcast band. A [FIXME even multiple of?] one-half wavelength end-fed wire should provide the highest impedance, and this is practical for the FM broadcast and weather radio bands.

It seems odd that NOAA recommends using a folded dipole, an antenna with a wide bandwidth4). Also, the dimensions given do not seem to account for the velocity factor of twinlead. The standard formula, 468/MHz for the length (in feet) for a specific frequency5) gives, and 34.5” for weather radio frequencies.

The standard AM crystal radio operates from an unbalanced (one-wire) feedline/antenna. See A Simple 75-ohm Coax Balun for 88-108 MHz for how to make an FM balun.

Yagis

Tuning

It should be possible to make a crystal set to tune these stations, though the Q necessary is 162.550/0.025=6502, and the frequency seems to be beyond what an LC circuit can do.

  • Consider using a VHF resonant cavity, as used successfully for some crystal radio designs.
  • Consider using a quartz or ceramic filter.
  • Design a PCB with the FIXME” antenna, coil, and half the capacitor printed on it.
  • Use FIXME/FIXME Litz wire (size for frequency).
  • Micrometals Materials: 17, 12, 0

Start with a basic FM crystal radio circuit, and add output impedance improvements. The basic circuit appears to have something like these coil and capacitor combinations.

  • 0.12 μH coil/50 pf air variable capacitor, tapped in the middle at 0.03 μH
  • 0.15 μH coil/80 pf air variable capacitor, tapped in the middle at 0.04 μH
           \|/                 \  /       
            |                +--[]---||--+
            |                |  S1   C3  |
     L1     |         /      |         / |
+----UU+UU--+--|<---VV---UU--+-------VV--+
|      |    |      /R1   L2  |      /R2  |
|    / |    |                |           |
+--||--+-||-+                +-------||--+
| /C1    C2                          C4  |
|                                        |
+----------------------------------------+
                           

Or, try an even more basic circuit.

  o----------------+
 A                 |
  o--+-----UU------+
     |     220n    |
     |       /     |
     +-----||------+
     |    /5-30p   |
     |             |
     +--|<--+--||--+---o    
            |  100p     )
            +----------o
  • 0.22 uH: 1 turn, d=4”

Antenna

299 792 458 m / s / 162475000 = 1.845 m

7” x 98000000/162475000 = 4.22” FIXME should decrease by the square root?

Coil

Tuning Capacitor

Other Capacitor

Choke

The LCRA also rebroadcasts our weather radio information an the AM Dial 1610, for a radius of 10 to 20 miles. Their transmitters are located at Lakes Buchanan, Marble Falls, Travis, Bastrop, and Fayette.WFO, Austin/San Antonio

1) ”…voice program audio is transmitted over the NWR transmitter network using standard pre-emphasis for narrow band VHF FM of 6 dB per octave increasing slope from 300 Hz to 3,000 Hz applied to the modulator.”–NATIONAL WEATHER SERVICE NOAA WEATHER RADIO (NWR) TRANSMITTERS NWR SPECIFIC AREA MESSAGE ENCODING NWR SAME
3) FIXME: calculate tank impedance

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