Increase Effective Distance on 'ebay' Flash Remote Trigger Transmitter With Antenna

Introduction: Increase Effective Distance on 'ebay' Flash Remote Trigger Transmitter With Antenna

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 <strong> TOTAL </strong> 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.



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20 Discussions

The antenna length is not so simply determined as implied here.

In fact, the impedance presented to the RF amplifier will be extremely different for odd number of quarter-waves (low impedance, about 30 ohms) as it will be for even number of quarter waves (high impedance, several thousand ohms). The optimal reception pattern will be different, too, tending toward broadside of the wire direction for odd quarter wavelengths to the end of the wire for even quarter wavelengths.

So the best thing to do is to make it an odd number of quarter wavelengths for maximum power transfer and for optimal reception pattern.

This article in fact recommends a single quarter wavelength -- which is great! But don't fall into the trap of thinking "longer is better" by trying a 1/2 wavelength or even a full wavelength.

For optimum range, you should leave the antenna straight.

Coiling it like this creates an inductor that effectively makes the antenna longer than its physical length. It may only be nanohenrys, but at these frequencies, a little inductance goes a long way.

great instructable, mine has little longer antena on pcb (13cm) but it was not enough, I cut it and use 17,3cm wire.

before: reciever with fresh batteries max 7m, reciever with old batteries max 4m
after: 10++m both of them


11 years ago

Strange. I did this mod yesterday and afterwards the range was decreased to only some millimeters. After I removed the wire-antenna the range was original again. Of course I cut the length of the antenna as suggested here. I suspect the heat shrink tube could shield the electromagnetic waves. I'll try it tomorrow again but without the heat shrink stuff.

4 replies

I ran into the same problem when I done the mod, but I found a fix. Like yourself I used a thin multi-strand wire that was 75mm long. After trying it on both holes I could only get 100mm of range. So I tried a solid copper wire about 260mm long, that I got from a coaxial cable. I stripped it bear and soldiered it on. It worked! I went from from getting 10m of range to 30m of reliable range and up to 50m of range but it is inconsistent. I thought I would then cut the cable down to 75mm to match the wave length and back came the problem. So I got a new wire about 270mm long and it's working again. I have since wrapped the wire in a electrical shirk and haven't had a problem. I can only put it down to an earthing or resistance problem. I have checked it with a multi-meter and can't find anything.

So once again. If you find your range has been drastically reduced by the mod. Use a sold wire like from coaxial cable and make it at least 260mm (10.2 inch) long.

NOTE: I have also done a AA battery mod to the receivers.

I like your process, for testing and experimenting, but I think your results are somehow wrong. Let us know what happens. Heatshrink is made of a type of thermoplastic and has no shielding properties at all that I know of. Unless you've located a crazy sheilding type! In any case, the length might have to be adjusted a little, or some damage might have occurred during the mod. I had very good luck with my extension to this model.

Still no luck. I started with a wire length of approx. 7" and tried to fire via the test button. After each unsuccessfull test I cut approx. 1mm off and tried again. Most of the time it didn't even work for distances of one inch. There were some lengths (shorter tha 5") which worked for some meters but I never got a longer distance than with the on circuit antenna. Stopped when the antenna got as short as approx. 2"... After I desoldered the wire it worked again. Funnily enough I have to wait a second or two after a test shot before trying again or it won't work even with the original antenna. This was even before I altered something at all. Battery voltages ar 12,2 and 3,1V which should be fine. I have no clue at all anymore although I once learned electronic technician. Even with the worst antenna/wavelength match it should work for some inches I suppose.

I had the same experience. Completely dead after adding the wire as described. However, I kept cutting off 5mm pieces until I had a small stub of about 2.5" when I managed to trigger at a foot or two. Further cutting gave me a range of about 50 yards (5 times what I had before) with a wire length of about 2". So don't give up! Keep shaving off bits of wire.

Thank you for the instructions. I proceeded as described and my range seems to be improved significantly (I will try to do the better measures later).

However, I had to do one change: I had to use the second hole in the other front corner of the PCB. If I connected the antenna to the hole pictured in th Step 2, not only that the signal won't get to the receiver, but also the red light ON THE TRANSMITTER almost stopped flashing.
I don't know if it was some short circuit or something (I am not into electro thingies), but it worked after touching the second hole. I should note that this was not my idea - I have read somewhere else (on Flickr?) about someone who used another hole too.

1 reply

Strangely enough, today's tests showed that: 1) it didn't matter which of the two holes in the PCB is used 2) the range was extended to about 20 meters from the original 5 - 8 3) the exact antenna length didn't make the difference... 14,1 cm or 15 cm or something around 40 - 45 cm... the results were the same (the long antenna had to be in the U shape though (don't know why)) 4) The no-fires of the flashes are gone. 5) Receivers work even close together, which was very unreliable before the modification. Anyway, I am satisfied even though some people managed to obtain much greater ranges.

I just tried it and had basically the same results as Sacha, although I did not test quite as thoroughly. The only thing that seemed to work was a full-wavelength antenna hanging straight - after twisting it into a more manageable form it no longer seemed to help at all. An 1/4 wavelength or thereabouts antenna effectively reduced the working distance to a couple of centimeters. I'm wondering what could explain this major difference in results... Mine is a 433MHz unit as well and has the same FS-616 board. How close should the antenna length really be to 1/4 for this to actually work? Are we talking millimeters or centimeters of tolerance here?

1 reply

I can't explain the differences in your results. One way to try this for maximum value is as follows: - dissasemble the transmitter for the antenna - get a buddy to grab your receiver and walk down the street watching the receiver LED until you see a loss of signal; do some experiments to gauge the natural distance of the unmodified transmitter - make the antenna connection, but make it longer than desired by one-quarter inch... so if the suggested length is 3.2" then make it approximately 3.45" - leave the transmitter case unassembled and rerun the distance test... it might be slightly better, but perhaps not - begin by cutting small amounts off the end of the antenna wire - make these very small, perhaps 1/16 in or less... each time, retest the distance where the receiver fails to operate, and each time it should get better - note when the distance gets a lot better, and when this distance is at least 4 times the unmodified distance, then begin cutting tiny amounts each time - when the distance doesn't appear to change much on a pair of micro-cuttings, then stop. This technique allows you to reach a reasonable improvement at a possible unknown frequency 'close' to your base frequency, without resoldering. If you overcut and lose great distance, or never hit the sweet spot, simply unsolder and start with a larger wire. NOTE: there is no downside to continuing to cut until you get an improvement, even past the suggested distance... just keep going until an obvious improvement occurs and then switch to micro-cut mode. Even if you cut off way more than you expected in this experiment, the worst you have to do is unsolder the stub and resolder a longer antenna to start over. Finally, I'm sorry if any of you have not had good luck with this mod, as it has worked gangbusters for several transmitters of the RF-04 variety quite happily. If anyone has any thoughts or improvements, please email me here.

Btw has anyone seen a solution to make the recever/trigger set off flashes that won't work ? (I have two cullmann 42's and one Pentax af360fgz that refuses to go off). Goffen

I can offer a much simpler solution. Especially for thoose not comfortable with a soldering iron. Find any cable with a 3.5 mm minijack in one end. (mono or stereo, the stereo might even give you twice the range :-) Cut it to 1/2 or 1/4 wavelength (or use as it is, it seems any length is an approvement). Of course a 90 degree plug and a bit stiff cable to keep it erect is preferable. Plug into the connector at the side. This works in the same way as connecting a headset to an mp3 player with a radio acts as an antenna as well. I discovered this when playing with using a set of transmitter/receiver connected together as a relay unit. I got far greater range FROM the relay than from the first unit to the relay. The cable between the trans/recv worked as an antenna... I have not modified any units accd to your recipee, so i cannot offer any clue as to how much less range (if any) my solution gives compared to yours. Would be nice to know though. Goffen

Hi. I just got two of these and rearing to make the mods. One guy though soldered the antenna to the big hole on the right?

antenna mod by blp1979

Which is which and which is better?


how did you decide to add the antenna to the transmitter instead of the receiver? Is one more efficient than the other? It seems as the transmitter will put out the same power no matter what, and the improved receiver antenna would tune the signal more effectively. Can someone confirm/deny this thinking?

1 reply

Well, that's an interesting question.

In a perfect situation, transmitters and receivers accomplish symmetrical and equal parts of the remote transmission of the signal. The steps are: modulation, amplification (including the antenna), carrying the signal through the air, amplification/attenuation (again including the antenna), then demodulation and detection. In reality, there are other considerations that make one side better to modify than the other.

Here's two reasons I decided to amplify the transmitter with a better antenna, rather than amplifying the receiver with a better antenna:

- In this sort of system with a single master and many slaves, I just have to make the mod one time <- best reason... ever. No matter how many receivers I add... and I will typically run a portrait setup with at least three external flashes, maybe four. In other situations you could have a lot more.

- I'm not completely sure of the sensitivity of the receiver and perhaps any interference from other frequencies on the receiver board, so I didn't want to modify the receiver circuit. By modifying the transmitter only, I increase only the amplitude of the transmitted signal seen at the receiver - not all the other signals or noise in the area.

In actuality you are right; I could have modded the receivers, instead. If had been a lot easier, I might have done so.

As to your comment about the transmitter power - note that the transmitter power is the product of the available power of the transmitter and the efficacy of the antenna. That is: the antenna is like a passive amplifier, and the better a match of the antenna to the transmitter circuit, the more powerful a signal. Any further antenna/transmitter theory goes well beyond the scope of this article!

I'll correct the original article with these comments where appropriate:
- the blue wire was the antenna, but I later applied black heatshrink.
- the trace is the antenna on the board, and the added one extends it

I'm not convinced it was actually designed for an extra antenna - I believe there is a twin trace underneath, and the two are attached by the plated-through holes.

Many thanks for the comments!


11 years ago

Since I've now got three of these transmitters and only need one (It was easier buying the kits than extra receivers) I guess I'll give this a try ;) I presume the blue wire in step 2 is the same as the black wire in the following steps? The existing antenna is just that trace on the PCB ending in that hole I take it? Looks like this PCB was designed for taking an extra antenna anyway ;)