— David Wagner 2007/11/21 16:37

• Crystal NOAA Weather Radio (NWR) Notes

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

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

• http://www.pollenpurge.com/ has a webcast of WXK-67.

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

–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[…]

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 [ 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.

• A collinear J-pole looks promising. Perhaps eliminating the hook will leave a high-impedance collinear random wire.
• The Twinlead J-pole is popular for operation at two meters (150 MHz). (Also described here.)

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.

Perhaps a better solution is a directional yagi antenna.

• http://bfn.org/~bn589/antenna.html
• http://www.csgnetwork.com/antennae3ycalc.html
• http://www.csgnetwork.com/antennae7ycalc.html

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 ” antenna, coil, and half the capacitor printed on it.
• Use / 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”

• How a crystal radio decodes FM signals

Antenna

299 792 458 m / s / 162475000 = 1.845 m

7” x 98000000/162475000 = 4.22” 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