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Above & beyond

VHF and Above Operation

C. L. Houghton WB6IGP San Diego Microwave Group 5345 Badger Lake Ave, San Diego CA 921J9

The winter weather and all Of its ill effects should keep you indoors awhile, leaving you more time for in-house construction projects. Continuing aEong with that theme, this month I would like to cover a few little gems to keep you and your soldering iron busy. Let's consider construction of preamplifiers for the low VHF range. This month I'll cover component selection and parts substitution, and how to modify circuits accordingly. The primary goal is lo use components you have on hand. Consider a dual-gate MOS-FET preamp for 30 MHz. See Figure 1 tor the schematic details.

The amplifier shown in Figure 1 can work well over the frequency range of 10 to 50 MHZ. The 40673 dual-gate MOBFET is capable of higher frequency operation; however, there are belter devices today for Ihose applications. If you want to build this circuit it will work; however, il is primarily used for component selection examples. With the schematic diagram (Figure 1) in mind, let's go shopping for parts. Don't go and purchase everything brand-new—a lot of retailers would appreciate that, but rather see what components you have on hand that can fill the bill to hold down costs and keep the project in a "hobby" realm.

Use the design in Figure 1 as a guide. It need not be followed exactly; most component values can be varied about 10% without changing the circuit performance. You do not have to use the exact material specified for the resonant elements (tuned circuits). Changing these components can be very cost-effective if you can use something you have on hand.

Let's take a closer look at the resonant elements, the inductors and the capacitors that form this part of the circuit The inductors used in this circuit are two variable 2.1 microhenry ^iH) coils and two 25 jjH inductors. What do we go shopping for in the coil department? Two or three RFCs whose value is 25 ^H and two 2.1 ^±H inductors for the resonant elements. The 25 ^H RFCs role on the input is not very apparent, This RFC provides a ground return for both the input tuned circuit to the ampler and a DC path for the detector diode lo ground. See Figure 4. It also matches the diode's higher impedance. (Here is my chance to slip in some microwave activity). This preamplifier is normally used in WBFM applications for a diode detector in a microwave cavity. For 10 GHzd this is a section ol waveguide, and for lower frequencies it could be a tin can called a polaptexer. It's basically a tin car or waveguide whose size/opening es the right dimension for the frequency of use. For 10 GHz, a copper pipe 1" in diameter is about right. For 1296 MHz, a one-pound coffee can is perfect. The diode detector is placed 1/4 wavelength at frequency from the back of the can and at 90 degrees in reference to the diode orientation. There are several ways this same coupling can be done but this is the most inexpensive method. Such a detector diode has an impedance of about 200 to 400 ohms, and when coupted to a preamplifier it will deliver maximum when the amplifier input circuitry is matched to this same impedance range, hence the input circuitry.

The output inductor (RFC) is used to separate RF and DC, it drives up DC power from the output coax and powers the amplifier for operation in a remote location from the main station equipment The other two coils comprise the tuned circuit and are shown as variable coils. They can be fixed if we wish to make the capacitor {15 pF) variable. The circuit will work well either way with variable coils or variable capacitors. This is one of the cost-effective choices to make by using your "Junk box."

What form can the inductor take to make the circuit work? As an example, take a toroid that is capable of working at 30 MHz. Look at Table 1. Looking at toroid cores from Ami don Associates, a popular amateur parts supplier, we determine that a T-XX-6 or T-XX-12 core is suitable.

At this point the -6 (yellow core) is the most important ingredient. The table states that a -6 core is good for 10 to 90 MHz use. A red core -2 could be used, but the frequency stated is not suitable; It's good from 1 to 14 MHz max. Alternately, a -12 core (green and white) would work, but that's kind of overkill; put this idea in the "might use" category. A -6 (yellow) core would be an easier core to locate in the junk box as it is very popuEar, more so than a -12. In either case, lets use the -6 yellow core and proceed to wind a 2A fiH inductor,

The Amidon charts list the toroid cores by core size (the XX above) and type (-2 or -6 or -12, etc,). Amidon has published a numerical value called J'AL;1 or ftiH per 100 turns}. With this "AL'h value for a selected core size we can compute the exact number of turns for our 2,1 ^H inductor, Let's select a T-25-6 core. By the way, the "25" of the part identification number refers to the size of the outer diameler of the core, in this case 174". In comparison, a T-37-X would be a core with a 0,370" diameter, Now, looking at Table 1, the UAL" value for the T-25-6 core is 27. That means that for 100 turns on a T-

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