Extending the range of my off-camera flash radio trigger

[Peabody]

I have a radio trigger for off-camera flash, and would like
to extend its range. It's the CowboyStudio NPT-04, which
is dirt cheap and works very well, but the range is only
about 25 feet.

I found a blog post showing a mod to a similar 433 MHz
device:

http://jerryphpics.blogspot.com/2008/07/004-cactus-v2s-modifications.html

He added a 1/2 wavelength antenna, but it was wound into a
coil. Apparently the mod was very successful. I don't
understand why the coil works.

Mine is a little different. It appears that part of the
antenna is formed of circuit board traces, and then there's
a 1.5" length of wire soldered to that.

I've posted photographs of this in:

alt.binaries.schematics.electronic

with Subjects beginning "CowboyStudio". But note that I
screwed up the post the first time, so download the
articles with the usual yEnc format.

The device broadcasts at 433 MHz, and I can't make sense of
the measurements I found - shown in the pictures.

Item A in the pictures appears to be a capacitor (no
continuity with my meter).

Item B is just a plated-through hole with nothing but
solder.

Item C appears to just be a jumper (near zero ohms).

Then as you'll see in the bottom pic, the C-to-D trace is
duplicated on the bottom of the board. I assume that's
significant.

Then at D the wire is soldered.

So I was thinking of replacing the wire with a coil like the
other guy used. I could just try different lengths, but I'd
like to understand what's going on - what ought to work.

Also, I wonder what complications might be introduced if I
do succeed in extending the range. Will the transmitter
use more power from the battery, or would the extended range
be "free"? Might there be impedance mismatches that would
tend to overload the output transistor and burn it out?

Well, this isn't really important, but if anyone would like
to look at it and offer comments, I would appreciate it. As
must be obvious, RF is not my strong point, particularly
antennas.

 

[TTman]

He added a 1/2 wavelength antenna, but it was wound into a
coil. Apparently the mod was very successful. I don't
understand why the coil works.

Because the total length of aerial wire, including what is on the pcb is an
exact fractional multiple of the wavelength, in this case 1/2. Normally one
would get by with 1/4 wavelength...
To get the correct length, you will need to know the transmitter
frequency...
The length is only approximate as there are other complex factors that are
involved. Clearly in this case, the original aerial is way off being a
fractional length, but adequate for most people to be fit for purpose.What
use is a flash at 300 feet ?
Battery consumption will go up, but by a fraction of the overall current as
most will be taken by the actual flash device.

 

[Peabody]

TTman says...

Because the total length of aerial wire, including what
is on the pcb is an exact fractional multiple of the
wavelength, in this case 1/2. Normally one would get by
with 1/4 wavelength...

Thanks very much for your reply.

Yes, but I would have expected that it would be the length
of the coil, after it's wound, that acted as the antenna,
not the length of the wire that made up the coil. Actually,
I just wouldn't have expected the coil to work well at all
as an antenna.

To get the correct length, you will need to know the
transmitter frequency...

The frequency is 433 MHz for his trigger and mine, which
would be a 1/2 wavelength of 13.64 inches. He took a length
of wire 12.39 inches long and wound it into a coil and
soldered it to the end of the antenna trace on the board.

In my case it's more complicated because the trace goes
1.25", then goes back the same distance in the opposite
direction, and that leg is duplicated on the bottom of the
board, then the external wire is attached.

See the pics, but in a non-proportional font it would look
like this:

1.25"
|--------------------------------------- <start
|
|--------------------------------------|
|------ wire
--------------------------------------| (1.5")
(parallel trace on bottom of board)

I assume I should be looking for 1/4 wavelength, which would
be 3.41", or 1/2 wavelength, which would be 6.82", but can't
come up with either one from this arrangement. One problem
is I don't know how the parallel trace on the bottom figures
in.

What use is a flash at 300 feet ?

Well remember that that's the camera-to-flash distance. The
flash could still be fairly close to the subject. Or, if
you had good range, you could position flashes at the
corners of a gymnasium, for example, and get some well-lit
basketball shots. They can also be used in the opposite
direction to trigger the shutter. That kind of range isn't
needed often, but more modern 2.4 GHz triggers, even the
cheap ones ($30), go out to 100 meters with ease, and even
have multiple channels. Off-camera flash has become dirt
cheap in the last few years.

 

[John S]

TTman says...



Thanks very much for your reply.

Yes, but I would have expected that it would be the length
of the coil, after it's wound, that acted as the antenna,
not the length of the wire that made up the coil. Actually,
I just wouldn't have expected the coil to work well at all
as an antenna.


The frequency is 433 MHz for his trigger and mine, which
would be a 1/2 wavelength of 13.64 inches. He took a length
of wire 12.39 inches long and wound it into a coil and
soldered it to the end of the antenna trace on the board.

In my case it's more complicated because the trace goes
1.25", then goes back the same distance in the opposite
direction, and that leg is duplicated on the bottom of the
board, then the external wire is attached.

See the pics, but in a non-proportional font it would look
like this:

1.25"
|---------------------------------------<start
|
|--------------------------------------|
|------ wire
--------------------------------------| (1.5")
(parallel trace on bottom of board)

I assume I should be looking for 1/4 wavelength, which would
be 3.41", or 1/2 wavelength, which would be 6.82", but can't
come up with either one from this arrangement. One problem
is I don't know how the parallel trace on the bottom figures
in.


Well remember that that's the camera-to-flash distance. The
flash could still be fairly close to the subject. Or, if
you had good range, you could position flashes at the
corners of a gymnasium, for example, and get some well-lit
basketball shots. They can also be used in the opposite
direction to trigger the shutter. That kind of range isn't
needed often, but more modern 2.4 GHz triggers, even the
cheap ones ($30), go out to 100 meters with ease, and even
have multiple channels. Off-camera flash has become dirt
cheap in the last few years.

Try this at your own risk:

1. Disconnect the Start point from the 1.25" trace at the top.
2. Connect a 6" or so wire to the Start point.

If you must cut a trace, try to do it such that you can recover from
modification.

John S