Shortwave antenna ideas

 

The type of antenna you’ll want to use for your shortwave receiver depends on the degree of interest you have in shortwave listening, on whether you are limited to an indoor or balcony antenna. If you cannot use an outdoor antenna, you can make yourself a whip antenna from telescoping radio antenna from your local Radio Shack store. A better antenna would be to throw a 10 to 20 foot piece of insulated wire out the window, secured to a pole or tree away from your window.

You could also place a length of wire in your attic to act as an antenna. An even better antenna would be a dipole antenna “cut” for the desired radio band of most interest. In order to construct your own dipole antenna, you can refer to the diagram depicted in Figure 8-5. Simply use the formula: 468 divided by the frequency in Megahertz for example. If you wanted to construct a dipole for 7.410 MHz, you would divide 468 by 7.410. The results would be 63.157 feet, so one-half of that number is 31.5 feet. Each leg of the dipole antenna

would then be 31.5 feet. If you do construct your own dipole antenna, you should keep the antenna away from metal objects and use insulated wire and be sure to keep away from all power lines. The lead-in wire for your dipole antenna to your new receiver can be a length of RG8X min-coaxial cable.

The passive air-band receiver

The passive air-band receiver is a type of “crystal” radio which contains no local oscillator which might interferes with the on-board aircraft’s sensitive electronics. The passive aircraft receiver broadly tunes from 118 MHz to 136 MHz, and was designed to listenin

to in-flight communications between your pilot and the control tower. The passive aircraft receiver is shown in Figure . The passive aircraft receiver can be built small enough to place inside your vest pocket and it operates from an ordinary 9 volt battery. The passive aircraft receiver is basically an amplified type of “crystal radio” designed to receive AM aircraft transmissions. The “passive” design uses no oscillators or other RF circuitry capable of interfering with aircraft communications. This receiver utilizes a coil/capacitor

tuned “front-end,” which feeds an RF signal to a detector diode. The tuning capacitor may be any small variable with a range from about 5 pF to about 15 or 20 pF. The 0.15 ìH inductor may be a molded choke or a few turns wound with a small diameter. Experiment with the coil

to get the desired tuning range. The aircraft frequencies are directly above the FM band so a proper inductor will tune FM stations with the capacitor set near maximum capacity. (The FM stations will sound distorted since they are being slope detected.) A 1N34 germanium detector diode or a Schottky diode like the 1N5711 or HP2835 should be used as the detector diode in this receiver circuit. The 10megohm resistors provide a small diode bias current for better detector efficiency.

The LM358 dual op-amp amplifier draws under 1 ma so the battery life should be quite long. Potentiometer R3 is used to adjust the gain to the second stage of the dual op-amp. The second op-amp stage drives a 100 ohm resistor in series with a 100 ìF capacitor. You must use a high impedance “crystal” headphone, since the op-amp output will not drive a speaker directly.

A speaker amplifier may be added to drive a speaker or low-z earphone, if desired, but the power consumption will increase sharply, as will the size of the receiver. The passive aircraft receiver is powered from an ordinary 9 volt transistor radio battery. The entire aircraft band receiver can be built inside a small plastic box. Simply mount an SPST slide switch on the side of the case to apply power to the circuit. If you elect, you can install a 1⁄8′′ phone jack for the “crystal”headphone. The antenna can be a 6′′ piece of #20 ga.

stiff solid copper wire or a small telescoping aluminum antenna connected to capacitor C1. Radio Shack has a good selection of whip antennas. You could elect to solder the wire antenna directly to C1 and just bring the antenna out the top of the enclosure, or you could install a small 1⁄2 ′′ mini jack for the antenna on the rear of the enclosure. The selectivity is reduce as the antenna length is increased so best performance is achieved with the shortest acceptable antenna. Try increasing the 1.8 pF capacitor value when using very short antennas and decreasing it for long antennas. In order to “tune” the passive aircraft receiver you can either leave a small hole on the side of the plastic enclosure to adjust the frequency using a plastic tuning tool, or you need to find a way to attach a plastic knob onto the tuning screw; you will have to drill a larger hole in the case to bring the knob outside the case for easy tuning.

Switchable 10 dB transmining inline attenuator

 

Any QRPer knows how much fun operating with low power can be. A decrease of power from 50 Watts to 5 Wans is -10 dB, or about 2 S-units. The increase in fun is also about an order of magnitude. Usually, decreasing power to a tenth of normal power can be effected simply by turning the transmitter's drive control down, if you have a commercial rig. But there's a point of diminishing returns, and it's frequently impossible to lower the power much less than a fav Watts, and even harder to know what that level is.

With simple QRP rigs, the drive is often fixed, and the power thereby set to 2-5 Watts. Wouldn't it he great to have the means to readily and easily decrease that QRP power by a factor of ten, thus making QW even more fun? And for about 5 or 6 dollars? Well, that's the basis for this project, a switchable 10 dB transmining inline attenuator. Simply place this device inline between your transmitter and antenna tuner (or other 50-Ohm load).

With the switch in the "Bypass" position, full power goes to your antenna normally. A flip of the mini-toggle switch to "-10 dB", and your output goes to one-tenth ofits original power. In either position (assuming your antenna tuner is properly adjusted with high-power applied), your transmitter sees 50-Ohms.

The attenuator schematic is shown above. I used eight 100- Ohm 1-Watt 5% metal-oxide resistors, available from Radio Shack for a mere 25-cents each (RS 271- 152). The DPDT micro-toggle switch (RS 275-626, $3) was selected for it's small form factor and it's excellent ratings. Two UG- 1094 BNC jacks provide my favorite means of connecting RE Ifyou do the math, you'll discover that the attenuator isn't really 10 dB (it's 9.6). This means that for 5 Watts input, you'll get about 550 mW out

FM crystal receiver

 

The FM crystal receiver circuit, shown in Figure,looks almost identical to a classic AM crystal circuit;but look closely, there are a few major changes.The FM crystal radio is relatively easy to build andcan be built on a small block of wood as shown. The

components values for the “tank” or resonant circuitwere reduced to resonate at higher frequencies in theFM band. This was done by experimenting with smallerand smaller coils and capacitors values.

The antenna is also much reduced in size (from that of AM crystal radio)to resonate at higher frequencies. The antenna is actuallya 7′′ long bare copper wire. The coil at L1 is just four

turns #18 copper or silver wire, 12 mm inside diameter,tapped at 2.5 turns, The coil was wrapped around a “magic marker,” then slipped off and expanded. As you can see, this is far different than the 150 turn coil for the AM crystal radio. The detector diode, once again, is a 1N34 germanium diode and not a silicon diode, or you could use a galena crystal as a detector if you desire. Capacitor C3 is 18 pF, but is not critical and can be 10 to 50 pF. Resistor R1 is a 47k ohm, 1⁄4 watt, 5% carbon composition type. The air variable capacitor, used in the prototype, was an 80 pF variable type and it had two trimmers in it which should be adjusted for best reception.