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Indestructible Attenuator

WARNING

This attenuator should not be used with transmitters which cannot tolerate a badly mismatched load (high VSWR). This should be unusual, but if you are using an older transistorized transmitter, it would be a good idea to check the documentation.

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This is a really neat attenuator. Once you have a few simple tools, it is easy to make. It will pass considerable RF energy when set to zero dB loss, and it can't be damaged if you do transmit with it attenuating. It will work on virtually any radio frequency up to at least 5 GHz. For you technical people, it operates on the wave guide beyond cutoff attenuation principle. (See the TAB Press book "TRANSMITTER HUNTING, Radio Direction Finding Simplified" page 61 for a write-up.)

When the two halves are done, they slide together to form a coax line of somewhere near 50 Ohms impedance. As you slide the two parts away from each other, the amount of attenuation will increase.

To build the attenuator you will need to visit a hobby shop to pick up some raw material and a very fine toothed metal saw. I bought mine at a model railroad store.

PARTS LIST

• Some scrap circuit board, without any copper on it. You will cut four small washers from it. See Figure 1
  
• Two BNC type chassis connector that look something like those shown in Figure 2. You will have to be able to solder to them on their base.

From a Hobby or Model Train Store:
Look for a display of brass tubing. These are each 1 foot long, and they come in graduated sizes, at each multiple of 1/32 of an inch. They can each telescope into the next large size. Buy one each of these sizes, given in inches. This is enough for two attenuators.

• 1/2
• 15/32
• 6/32
• 5/32
• 4/32
• 3/32
• Also buy a fine toothed saw for cutting this tubing.
• You might also want to buy a miter box for cutting straight across. You can make such a thing yourself, if you wish.

Building

The first two Figures build one half of the attenuator, with the second two building the other half. If you cut all the parts at one time, keep them separated as in figures 1 and 3. This will help cut down any confusion during building.

First Half

Cut one each of the following lengths of brass tubing. Dimensions are in inches.

Diameter	Length

15/32		1-1/2

1/2		5-1/2

6/32		1-5/16

5/32		1-5/8

4/32		2

3/32		2-1/4

Cut 4 small washers from the blank circuit board material.

ID		OD

6/32		7/16

Figure 1 shows a graphical representation of the above cutting tables. Display Figure 1. Click to download this as a GIF image. Its name is CoaxAtt1.GIF.

Assemble these into the two subassemblies shown in Figure 2. Click to download this as a GIF image. Its name is CoaxAtt2.GIF.

Before putting them together drill two or three small holes though the 1/2 inch diameter outer tubing. This will aid in soldering. When hot, feed solder in through these holes.

With a fine file, remove any excess solder, especially where the units must slide together and make contact. Also bevel the edge of the tubing at point "Y" in Figure 2. This bevel aids in the tubing's mating.

Now slide two of the fiberglass washer you have made over the center conductor, and affix with 5 minute Epoxy at the positions shown in Figure 2. Be careful to avoid getting any on surfaces that must slid together.

Second Half

Cut one each of the following lengths of brass tubing. Dimensions are in inches.

Diameter	Length

1/2		1/2

15/32		4-1/2

6/32		4-1/4

5/32		3/8

4/32		7/16

Figure 3 shows a graphical representation of the above cutting table. Display Figure 3. Click to download this as a GIF image. Its name is CoaxAtt3.GIF.

Assemble these into the two subassemblies shown in Figure 4. Display Figure 4. Click to download this as a GIF image. Its name is CoaxAtt4.GIF.

Solder. Epoxy Fiber washer on as per Figure 4.

We need to provide a hole to relieve air pressure. Without this, air pressure will push the two halves apart after we push them together! Drill all the way through one wall at point Z in Figure 4. Use the teeniest drill you have. Do this before assembly with the outer conductor so that you can remove the drill chips.

Adjustments

Now slide the two center conductor subassemblies together all the way, noting the force required. If little or no force is required, then use a sharp center punch to place a small dimple at point "X." Make it very small at first and then retry sliding together. Increase the amount of dimple slowly until a snug fit is obtained.

Lay the two mated center conductor pieces on a flat surface.

Now slide the two outer conductor pieces together all the way, and lay them on the flat surface, next to the center conductor sections. The outer conductor sections should fit nicely between the flanges of the Coax connectors. If they don't, adjust the center conductor sections by reheating some of the joints.

Final Assembly

Now assemble these parts to look like Figures 5 and 6. If not presently displayed, try; Display Figure 5... Display Figure 6. Click to download this as a GIF image. Its name is CoaxAtt5.GIF.

Click to download this as a GIF image. Its name is CoaxAtt6.GIF.

Solder together at the coax connector flanges.

The attenuator is now ready to use. You might want to scribe some fine lines on the outside of the inner tubing (Figure 6) to act as a logging or reference scale. Every 5 mm, starting at the point where the center conductors break contact (use an Ohm meter) would be good.

George R. Andrews
President

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(31 Jan 1996)

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