— 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.

A crystal radio is considered a high-impedance load, 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. One-half the wavelength of 162.550 MHz is 36.3” (0.922 m).

• A collinear J-pole looks promising. Perhaps eliminating the hook will leave a high-impedance collinear random wire.

[ The following notes need to be reorganized and tested.]

Use 4'9” total length for the center of the FM broadcast band, or 468/MHz for the length (in feet) for a specific frequency.²⁾ By this, 63” (5'3”) should be best for 89.1 MHz, and 34.5” may work better for weather radio.

See A Simple 75-ohm Coax Balun for 88-108 MHz for how to make an FM balun.

The Twinlead J-pole is popular for operation at two meters (150 MHz). (Also described here.)

The simplest antenna to construct is called a 1/2 wavelength folded dipole antenna and is shown as number (1) near the end of this page. The antenna is constructed from a piece of flat type parallel TV antenna lead-in. About 1” of insulation is stripped from the two wires at both ends of a 36” piece of lead-in wire. The ends are twisted together and soldered. Midway between the ends, one wire is cut and the insulation stripped back a sufficient length (1”) to attach the leads from a balun. A balun is a device that matches the type of antenna or lead-in to the input of the receiver. Once the connections are made the exposed wires should be taped. A home made balun is shown in (2) and an inexpensive commercial model is shown in (3). The finished antenna should be mounted so the two twisted ends are straight up and down as shown in (1). It could be attached to an inside window sill on a window on the side of the building from which the signal is transmitted; or better, it should be placed outside, possibly next to the window or some other convenient location to easily reach the radio in its most desired location. The lead from the antenna, if it is the parallel flat TV type, should be twisted once for each foot of lead and extended at a right angle from the antenna for approximately 18 inches prior to any vertical run.

–Radio Receivers and Methods to Improve Reception of Weather Radio Broadcasts

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

This page is archaic.[…]The National Weather Service has discontinued giving out the latitude/longitude information for NWR transmitters[…]

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

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.

San Antonio weather is broadcast at 162.550 MHz, WXK-67, 1000 Watts.³⁾

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

• Design a PCB with the ” antenna, coil, and half the capacitor printed on it.
• Use / Litz wire (size for frequency).

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