Camera buffs can purchase inexpensive versions of remote triggers for external flash units, controlling either hot-shoe or 'studio' type flash units. These triggers suffer from low transmitter power and thus small effective control distance. This mod adds a correctly sized antenna, effectively increasing applied power and thus allowing operation from much greater distances. I have seen increases of tenfold in control distances after this mod. This mod will show how to open the transmitter (RF-04/RF604), create the antenna wire, locate the place to attach the antenna, mod the case, and the final construction. Enjoy. Note: this instructable was derived from material presented in the Strobist blog, here, with the actual article here. The original description is here.I found this model on ebay, from a great supplier: Magic_Trigger. There are others on ebay and other locations, but I can't speak to the exact design they use or how to modify them, but similar techniques can be used.
Step 1: Open the Transmitter
This works for the RF-04 and RF-604 transmitter types. Start by opening the transmitter body: remove the single screw at the front of the bottom of the case. It's a phillips head and small. Once this screw is removed, pull the top off at the front. Don't worry about loose pieces, because there aren't any (except for the top and the screw).
Step 2: Cut an antenna wire and attach it
First, let's talk about the antenna length. Every radio transmitter best transmits using an antenna that is optimized in length for the transmitting frequency. This length is related to the wavelength associated with that frequency. To figure the wavelength from any frequency, divide the speed of light by the frequency, and the result is a length. Here's a reference to help you.The very best antenna for any frequency would be a integer multiple of the wavelength, but this can many times be way too large - an example is this project, utilizing a 433MHz transmitter frequency where the wavelength is .69 meters, or about 27 inches! An acceptable antenna size is 1/2 or 1/4 the wavelength; note that 1/4 of 27.16 inches is about 6.82 inches, and thus within an acceptable range for our needs. Therefore, we will create a final TOTAL antenna length of 6.82".In this particular design, there already is a very small antenna formed on the circuit board (note the length of straight trace running along the front), approximately 1.25" long - this length doesn't really make complete sense as it's approximately 1/20 of a wavelength, so we will add our antenna to the end, to create the correct 1/4 wavelength. To figure the added wire length, we subtract the 1.25" on PCB trace length from the total antenna length needed, we get 5.57" for our added wire. This is the length of the wire we need to cut. Cut a length of insulated 18 or 20gauge wire to 5.57", as close as you can (the exact length is not critical, as my experiments show that two different designs which vary a bit still work very well). Strip a very small bit of the insulation from one end of wire, perhaps 1/16" left visible. Note that our antenna is a length of blue 20ga. wire I had. Later, I cover the wire in heatshrink tubing, to protect it and also to make it black, like the rest of the case.Look to the far left edge of the FS-616 board (near the far '6') and you will find a plated through-hole at the end of the PCB antenna. It is into this hole we will place the stripped end of the copper wire. Solder the wire in place in this hole, as shown in the picture.
Step 3: Make a hole for the antenna
To make a hole, you could use anything. I chose a soldering iron for melting the plastic, in opposition to a drill bit. I was concerned that a drill bit might stress the plastic and break it, where the heat method would smoothly open the hole with low stress. Use your hot soldering iron tip to carefully make a small hole in the top piece. I selected the middle of the top, reasonably near to the front of the unit near where the antenna wire is soldered (at least when the top is back in place) simply for aesthetic reasons - you could make the hole anywhere on the top or the nearest side. In fact, it is possible that a reasonable performance could be had by leaving the antenna inside the case, by looping the wire around the interior and not making a hole at all - but, I chose to make a better antenna by routing the antenna wire outside the case, and orienting it vertically for a better horizontal radiation pattern Start the process by locating the approximate middle point of the case, left to right. Use the hot tip on the inside of the case piece, slowly working into the plastic in stages. This allows the plastic to flow outward and minimize stress on the case piece, and also allow you to gauge the progression of the tip, from the outside. Keep pressing forward, and make sure that the hole never gets much larger than the copper wire thickness. The second photo shows where the wire will come out - just in front of the button and transmit indicator LED. I recommend that you stop when you can see a small hole formed, then turn to the use of a small knife tip. Use the knife edge to very lightly scrape away a bit of plastic until the wire (with insulation) can fit through the hole, but snuggly. This also allows the hole to be smoothed and to look better trimmed.
Step 4: Attach the top to the bottom
Bend the antenna wire (18ga. is stiff but bendable) from the PCB soldering point, up into a sloping angle to the place where the hole in the case piece will fit, and push the antenna wire through the hole in the case piece. As the top case piece gets close to seating on the bottom half, you may have to bend the antenna straight a bit so the pieces can fit. Press the top onto the bottom, starting at the back, where the attachment tabs are. Hold the top on, and then attach the single screw from where it was removed. The photo shows the final view, and note that the wire is now covered in heatshrink tubing. I chose an appropriate diameter to cover the wire and seal the edge, but this is not needed in general - i just wanted a bit of protection and to make the wire black, like the case. For this transmitter, I also bent the antenna back at an angle a bit to minimize the total height above the camera, which didn't appear to affect the operation much (it does a bit, I'm sure).
Step 5: Modify the antenna shape?
You could even choose to make the antenna shorter by twisting it a bit into a curlicue.
I used a pen cap and twisted the antenna around that. It looks nice, but seems to have little effect on the design.
Step 6: Results: Much longer distance
Before the modification, I could barely get 20 feet of distance from the transmitter to it's receiver. After the mod, I was able to exceed 200ft. in line-of-sight. Such improvements also help with transmitting through walls or around corners, but certainly to a lesser extent. Please come to visit Strobist - a great site that celebrates the use of off-camera strobes. If you are modifying your flash remote control, you are most probably a 'strobist'.Good luck. Please leave comments if you were successful.