Nuts and Volts - February 2008

appreciate a schematic if at all possible.

— Nathanial C. Blacker

AIf you have a roof mounted community antenna, then the reception is probably as good as you can get. If you are using a rabbit ears type of antenna, then you need a better antenna. In order for an RF amp to do any good, it must have a lower noise figure than the TV RF amp.

For a better antenna, consider the Terk TV55. It is a helical antenna with an amp and covers VHF/UHF. I don’t have any experience with it, but reviews are good. The cost is about $100 and you can order from: Crutchfield, P.O. Box 9032, Charlottesville, VA 22906-9032. The antenna is less than four feet long and about three inches in diameter, so it should fit in your space.

The RF amp schematic that you requested is shown in Figure 2. It is not real wide band — it only covers the VHF band — but the gain is nearly 20 dB (see Figure 3). The transistor, 2SC3356/ NE85633, is an SOT-23 surface mount and the resistors should be surface mount as well for best performance. The input impedance of the amplifier is probably not 75 ohms, so the cable from the antenna to the amp should be short to avoid reflection problems.

PARTS LIST

PART DESCRIPTION

• S1-S5 SPDT Mini Toggle

• R1, R5 4 x 2. 21 MEG +160K

• R2, R4 4 x 22 MEG + 2 MEG POT

• RG1 500 OHM, 0.1%

• RG2 10 OHM, 1%

• RG3 10. 2 OHM, 1%

• IC1 INA128P

• C1, C2 10 μF, 16V, 10%

• C3, C4 0.1 μF, 50V, X7R

MOUSER PART NUMBER
108-1AS1T1171-EVX
71-CCF55-2.21M, 71-CCF55-162K
291-22M-RC, 81-PV37W205C01B00
71-PTF56500R00BYBF
271-10-RC
271-10.2-RC
595-INA128P
581-TAP106K016SRW
80-C320C104K5R

spectrum is wider than the old analog signal so there would be some degradation if the unmodified RF sections of the radio were used. The detector would have to be redesigned and all the processing after the detector would have to be designed.

Presently, reception of HD radio is free but it is possible that in the future, you will be able to purchase specific channels (rock and roll, oldies, classical, etc.). European countries have adopted a different standard, so your USA radio won’t work over there and vice versa.

prefer the output to light one of four LEDs, or perhaps trigger a variety of voice alerts. How would I construct a circuit to convert a sequential series of pulses to switch four separate outputs?

— Rob Gordon

AA shift register seems the logical device for this application. The relay becomes the clock; the

■ FIGURE 2

HD RADIO

QThe question I have is about the HD radio broadcasts. Is there a way to modify a regular FM radio to receive these HD stations?

CONVERT SEQUENTIAL PULSES TO FOUR OUTPUTS

QI have a stand-alone wireless motion detection unit that reports alerts from four detectors. The system sounds one to four beeps to distinguish between the detectors, and also conveniently closes a relay in sync with the beeps. I would

— Bryan Fischer

AThe short answer is no. Broadcast stations now have the ability to send all analog, hybrid digital (including analog), extended hybrid digital with analog, or all digital (no analog). There are presently no receivers available for all digital FM and no all digital stations, as far as I know. The digital signal can be subdivided into multiple channels, each carrying a different signal. The digital signal is lower power than the analog and if the digital signal is too weak, the receiver is designed to switch to the analog signal.