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. Resistor values (and their part numbers) for other power levels are listed at the end of the technical article, and can be purchased from any of the major distributors.

Assembly time is about 1 hour and 15 minutes, 25 minutes of which is spent winding the two transformers. Following the assembly guide in the recommended order will provide the easiest assembly experience.

 

The first part to install is the 10pf capacitor. The footprint on the board is for an 1808 size SMD 3KV chip capacitor; however, your kit may also be supplied with the larger metal mica shown in the photo. If supplied with the metal mica, position the part to leave enough room for r1 and r2.
 

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

R6 in this photo should be 180 ohms for the standard setup of 1kw/100w...the part shown here is actually two 360 ohm resistors in parallel (I had just run out of the 180 ohm parts when I assembled this board, and had to improvise).

It should be noted that 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 upside-down.
 

Time to make the transformers. The T2 core will be slightly larger than the T1 core. Both cores will have a length of magnet wire packaged with them, 25 inches for T1 and 26.5 inches for T2.

The magnet wire will be used to wind 33 turns on each core. Pass the wire through the bottom of a core until each end is the same length. Begin winding by passing the first turn through the top 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 on one side, turn the core over and wind the rest of the turns on the other side.
 

 

Your completed transformer should look like this. Make the second transformer the same way as the first, and count the number of turns on each one to make sure each has 33.
 

 

These next two photos show 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.

 

 

Place 3 cable ties on the remaining transformer as in this photo. One of these cable ties should be placed in the bare spot of the core between the two ends of the coil. The other two cable ties will capture some of the turns on the transformer, but this time it's OK. Do not trim away the excess yet.

The reason we're doing this is to create some spacing between the board and the transformer; the lock portions of the cable ties will serve as the spacers.

If you make an error and get a tie-wrap on incorrectly, there are 3 extra in the kit; just cut away the bad one and replace it.
 

 

Insert the end of the cable tie covering the bare spot on the core into the hole near the T2 marking, and the other two into their respective holes. Do not trim yet.

Place the lock of another cable tie onto the end of each of the 3 ties exiting the back side of the board.

Pull all of the locks up tight against the board, and trim away all 6 ends.

Because the locks protrude out the bottom of the board, whenever you get around to installing it, it will need to be mounted using 1/4" spacers, or by using another method allowing the locks to be clear.
 

 

The wire ends of the transformer will need to be trimmed to length and tinned as pictured here.

On T2, the wire coming from the bottom of the core will be soldered into the hole under the T2 marking. The wire coming from the top of the core will be soldered to the center of the large trace above it.

On T1, the wire coming out the right side of the core above the large trace will be routed around the outside of the core and soldered into the hole just to the upper left of T1. The wire coming from the left side of the core will be soldered to the trace at R1,R2,R3.
 

Here's a closer look at connections for both transformers.
 

Prepare the T1 transformer coupling loop by cutting a 1 inch (25mm) piece of the white Teflon wire. Remove 1/8" (3mm) insulation from both ends, tin them, and form approximately as shown.

If you accidentally cut away some of the wire strands, there are an extra 2 inches supplied in the kit, just tray again.
 

 

Pass the wire through the center of T1 and solder onto the large traces as you see it here. The wire should pass through the center of the core just above the tie-wrap band.
 

 

Cut a 2 inch (51mm) piece of the Teflon wire for the T2 coupling loop; remove 1/8" (3mm) insulation from both ends, tin them, and insert one end through the top of T2. Route this end to the pad at R5,R6 and solder it there.

The end coming out of the top of the core should be routed through the middle of the core, just over the top of the tie-wrap band on the left, and soldered to the pad at R1,R2,R3.
 

 

You're all done, but I want to leave you with a little 'black magic' trick here.

I've never seen a problem with directivity from 1.8 through 30 MHz, but once in a while it will measure a bit low at 6 meters. The symptom will be a slight reverse power reading when there is no reverse power present. To correct this, I've had to crowd the turns on T1 over to the right side as you see it done here, leaving about 8 turns widely spaced on the left side, and then move the T2 coupling loop back and forth a bit as well. Once the adjustment is optimized, put a small amount of silicone sealant across the top half of the T1 core to keep the turns in place (you can see it in the photo if you look closely).

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, but the transmitter/dummy load method will work for those not having access to such an instrument, though it does take a lot longer to do that way.
 

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.

The easiest transition for making connections into and out of the board is to use the two provided coax fasteners. When properly installed, each connection should look like this photo. The recommended coax to use is RG142, it is widely available, flexible, and will handle in excess of 2 kw through 6 meters. Here's how to use the fasteners to make your connections:
 

Trim 20mm of the outer insulation from the end of the coax. Using a pointed tool (sharp tweezers will do), separate the last 10mm of shield and spread to the sides.
 

Form the shield to the underside of the coax and trim away all but about 6mm of shield. Tin the end of this shield with solder to keep all the strands together.
 

bend the shield back under the untrimmed section. This will form a mounting foot, and will space the coax at the proper height for connecting to the board.
 

Remove 5mm of insulation from the center conductor. While holding the tin fastener in a vice, solder the shield foot to the fastener in the position shown.
 

Place the top of the fastener against the underside of the board and align to the mounting holes. Fasten into place using 4-40 screws and locking nuts. Solder the center conductor to the board trace, and repeat the process for the connection on the other side.