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Simple LC Series Receiver Preselector
Notes:
1.Depending on your preference, the receiver may be grounded with a separate wire to earth, as above, or through your home’s electrical system (not as good, but no separate wire required.)
2. Air variable capacitor is wired in series with inductoras shown here to form a peaking circuit. That same L/C combination can simply be wired ACROSS the antenna and ground to form a notching circuit. (shunting signal to ground.) These components can be mounted on the board using screws, glue, etc.
If you would like to play with simple preselector circuits without spending lots of money, here is an idea that will cost you less than $20…far less if you can scrounge an air variable capacitor from an old tube radio. And you get to experiment and find the best values for your set up and listening preferences.
Simple construction. Interrupt the coax or twinlead from your antenna and clip it to a pine board on which you’ve mounted 4 Fahnestock clips. The two clips on one side lead to the antenna and/or ground. The other two hook to the receiver. Insert leads from your L/C (inductor and capacitor) lash-up between the clips as shown above and short the two bottom clips with a wire for the ground circuit. Short out BOTH pairs of clips to bypass the breadboard, eliminating the preselector. Experiment with the capacitor and individual inductors or solder a number of inductors to a rotary switch so that you can quickly select the appropriate inductance for the desired frequency.
For more permanent preselector construction, you can mount the air variable and the rotary switch, along with input and output jacks in a “project box.” Get a couple of knobs. Also, go to Bob Bett’s excellent pages at <http://www.geocities.com/amdxlog/rich1.html>
Click on “preselector” to see articles and more advanced circuits. Here you will find theory as well as simple circuits for wiring up both peak and notch circuits together using mini isolation transformers, etc. These are really great pages filled with a lot of information and ideas.
Here’s a list of the components I used for my preselector. My theory has been to have many inductors available via a rotary switch. In this way I can get the sharpest notches and peaks, thereby suppressing signals on adjacent frequencies. * See footnote below.
Notes:
1. Other inductance values are available, Check the Mouser catalog. Note that the coils above are High Q, over a 100. Experiment and find which inductances are best for your antenna and radio. At these prices, you can order a large selection of different inductors and play around with them. Use 1 and 10 MILLI henry coils for LF frequencies.
2. I find the inductance values listed above best for 0.5 to 16 Mhz.
3. A tip o’ the hat to Bob Betts and Richard Gagnon of the yahoo group, Shortwave-SWL-Antenna.
Dave Griffin, N2chi
11/2005
Footnote:
I have chosen the above inductor values because I have found that the narrowest bandwidth and the most symmetrical steep-sided resonance curve will occur when the capacitance within the L/C ratio is low for the given frequency. ( Ala f=1/((2pi)(SQRLC)). I assume this is because Q=Xc/R, but also I note that Xc can never quite get up to zero while XL can increase forever, which I assume causes the asymmetry.) I therefore use more inductors than usual to cover the frequency spectrum so that I can more often be using a lower capacitance (and therefore higher capacitive reactance and Q) when peaking or notching a signal. Experiment bears this out. You can resonate these circuits with other inductances, but if for a given frequency the capacitance is on the high side, say 250 pf or more, you will notice a definite decrease in the Q and symmetry of the resonance curve. So, at 50 cents each, selectivity…the objective of a preselector…. is cheap.
