Assembly guide for the HF dual directional detector

The attenuator resistors supplied in the kit are for the standard 1kw forward, 100w reverse full-scale power levels. Load resistance for the detectors should be 5k for best linearity. Decreasing this to 650 ohms will range the coupler and detectors for 2kw.

Assembly time is about 1 hour and 10 minutes, 20 minutes of which is spent winding the two transformers. Following the assembly guide in the recommended order will provide the easiest assembly experience.



Install all of the parts except for the transformers and their coupling loops.

It should be noted the default output polarity of the diodes used as detectors is negative (an old commercial standard). If positive output is desired, the diodes should be mounted in their alternate positions (D1alt and D2alt)

Time to make the transformers. Magnet wire will be used to wind 22 turns onto each core. Cut the 64 inches of #28 magnet wire in half, making two 32 inch pieces.

Using 32 inches of #28 magnet wire for each, wind the T1 and T2 transformers.

Leave about 1 inch of the wire outside of the core and wind each successive turn through the middle of the core to the right of the previous one.

There are two turns on the core shown here (the turns are counted by the number of times the wire passes through the center of the core).

Pull the wire up against the outside of the core, and pass it through the center to the right of the previous turns. Keep the turns close to one another so you'll have room for all of them. When you have about an inch of wire left you should have 22 turns through the core.

Your completed transformer should look like this.


This next photo shows how to mount transformer T1.

Pass a 4 inch cable tie through the bottom of the board into one of the two holes at the T1 location. Capture the bare center of the transformer between the two ends of the wiring, taking care not to capture any of the turns.

Pass the cable tie through the top of the board into the second hole, into the tie lock, and pull it snug. Trim away the excess tie material from the lock.

The transformer will be held spaced slightly above the board.

Tin and solder the two wire ends, one to the board pad at the left foreground and the other through the ground hole on the other side of the transformer to the left (view blocked by the core on that one).


Using another cable tie and the same method, fasten the T2 transformer into position as shown in this photo. This transformer will pull snug against the PC board.

Trim and tin the magnet wire on T2. the ground lead will come from the side of the transformer closest to the top right of the photo (as you see it here). Solder it at the ground via just to the right of the transformer. The other lead should be soldered to the main trace...take care to route this lead low and away from the side of the core.


Prepare the transformer coupling loops by cutting the 3" piece of Teflon wire into two 1.5 inch pieces. Remove 1/8" (3mm) insulation from both ends of the wires. Tin the ends.


Pass the wire through the center of the transformer cores and solder onto the traces as you see it done here. The wire should pass through the center of the core just above the tie-wrap band.



You're all done, but I want to leave you with a little 'black magic' trick here. You probably noticed the test leads connected to various places on this board.

I've never seen a problem with directivity from 1.8 through 30 MHz, but it will almost always measure too low at 6 meters and above. The symptom will be a slight reverse power reading when there is no reverse power present. To correct this, I had to crowd the turns on T2 over to one side as you see it done here. The position of the wire passing through this core also has a large effect on the directivity. Once the adjustments are optimized, put a small amount of silicone sealant across the top of the core to keep the turns in place.

The easiest way to measure the directivity is to connect your transmitter to the input, a good dummy load to the output, and measure the voltage at the fwd and rev pads. With 750mv or so on the fwd pad, there should be no more than 25mv on the rev pad, assuming your dummy load has an swr less than 1.1 to 1.

This is more easily measured with a network analyzer connected to TP1, but the transmitter/dummy load method will work for those not having access to such an instrument, it's just more difficult and time-consuming to do that way.


As far as directivity goes...once you have things optimized, this is typical performance. The coupler is actually useable with good specs all the way through 4m.

Directivity is the plot shown here.


Note: the following instructions for connecting into and out of the dual directional detector board show how to do it on the HF low pass filter assembly, which uses the same method.

Secure two solder lugs as shown here using the 4-40 machine screws provided; leftmost photo is top side, rightmost photo shows the two locking nuts on the other side securing the lugs.

Using a pair of needle-nose pliers, bend the ends of the lugs vertical as shown. With the lugs formed in this way, there will be considerable strength in both the vertical and horizontal planes.

Shown here is RG402 coax, though you can use any coax (such as RG142) capable of handling the power.

Prepare your coax by removing 20mm insulation covering the outer conductor.

Next, remove 10mm insulation covering the center conductor.

With the center conductor laying on top of the board and soldered to the board trace, taking care not to move the coax, position the lugs against the sides of the outer conductor and solder them to the outer.

Repeat this procedure for the other RF connection.