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 Previous: Rhode Island PRB-1 Bill Put into Legislative Hopper |  Next: Wounded Boy Transported to Dallas Hospital |
By Jack McElwain, W5SVZ
April 1, 2000
Looking for a quick and easy way to perk up your receiver? Here's a unique approach we had not encountered before. It's easier and cheaper than replacing the old rig and doesn't involve that hard-to-fathom DSP stuff they'll be asking about in future amateur exams.
Through the years of searching semi-dead CW bands, I have always been puzzled by two things: First, I've never heard an RTTY signal less than S9. Second, as I "swished" up and down the band, a signal that was very loud when I went by it, was barely readable when I came back to it and tuned in on it. Very strange!
Then one day it dawned on me. Of course! (the proper scientific term is Eureka!--Ed). Something I was taught many years ago: "The faster a conductor cuts a field, the higher the induced voltage" (or words to that effect).
By changing the frequency, as the signal was being received, I was, in effect, cutting the electrical field faster by moving the frequency of the signal in the tuned circuit. The real effect was to increase the slope of the signal waveform; thereby increasing the conductor cutting frequency (ie, the speed) of the signal.
The RTTY signal already accomplishes the same effect as it shifts the frequency between "mark" and "space" using FSK (frequency shift keying). But, how could I use this to my advantage? If I "dithered" the front tuning inductor it would make the signal unreadable (ie, the same as "swishing" the tuning control)?
Figure 1--The schematic of the breadboarded circuit. |
To speed up my experiment I used a gear-reduction direct current motor to drive a variable air capacitor and cancel out the X1 with a varying Xa.
Since the IF inductor can is made of aluminum, I can simply wrap the link around the outside of the IF can without disturbing the receiver circuits in any way. In fact, the aluminum can will allow magnetic coupling yet act as a Faraday shield to keep noise from being introduced into the IF stage.
A rheostat in series with the 12 V dc motor and its supply will allow me to change the dither rate for optimum performance. A three-turn link connected by a twisted pair was used for both the IF can and the external inductance.
Figure 2--A clever chart that shows how an increase in motor speed affects signal strength. The optimal motor speed appears to be just shy of 500 RPM, but your mileage may vary (and your hair may fall out--Ed). |
A graph of the motor speed versus signal strength is shown in Figure 2.
As you can see, there is nothing critical about the speed. I tried coupling to each of the three 455 kHz IF cans in my receiver, but I could detect no difference in signal strength. Apparently, the circuit is extremely non-critical.
For those of you who wish to conduct this experiment: I used a 12 uH inductor (49 turns on a powdered-iron toroid with an A of 49). A 40 pf air variable across the inductor cancels it out very nicely. I used a borrowed strobe-tachometer to determine the gear motor speed, but you can just vary the speed for maximum signal strength.
I used this circuit with my Kenwood TS-850S, but I see no reason why it would not work just as well with any receiver as it did with mine.
A breadboard layout of the completed and, as-yet, unnamed receiver-enhancement device by W5SVZ. [Photo by Jack McElwain, W5SVZ] |
Editor's note: Jack McElwain, W5SVZ, is an ARRL member. He lives in Dallas, Texas. He acknowledges and expresses his appreciation to Larson E. Rapp, WIOU, for the technical inspiration. "This article could never have been written without him. Thank you, Larson, wherever you are" he said. Don't try to contact the author via e-mail. He doesn't have e-mail. No, not even Juno.
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