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Spektrum DSM Radio
system Part 2.
At the end of the first part of this article I said I would go into the
use of the 3 stick type radios and setting up with a SmartWinch. Rather
than that I’m going to concentrate on the range issue. First thing that
should be noted is that the 3000 ft range stated in the specs is, well,
er, let’s be polite and just say not likely to be achieved. Best range I
have been able to get across perfectly flat ground with perfect line of
sight is about 300m which is about 1/3rd of the spec. Never the less,
while 300 m is well below the specified range it is well above the range
needed for rc boats and cars which is what the system is intended for.
However it’s very difficult to get even 300m on the water. In my own
case I get about 130 to 150 m with the standard rx antenna setup. So no
probs for me but I have sold a few systems and about half of those
customers can’t even get 100 m. Less than 100m certainly is inadequate.
Out of the box, it has to be said that the system is not good enough to
be promoted as a serious alternative to the existing AM, FM, etc.
systems. There needs to be an improvement in range. To that end, I’ve
done several experiments with alternative tx antennas with little
improvement so it appears the solution is not to be found at that end.
However, I have found a solution with the rx antenna. It appears to me
that the reason the range is so short when in a yacht compared to on
land is because the rx antenna is quite close to the water level and it
appears that the water attenuates the signal.
My solution has been to get the antenna well above the deck by using a
coaxial cable extension. This extension is done by removing the standard
antenna from the rx and soldering a shielded coaxial cable in its place.
The standard rx antenna (or an equivalent length of wire) is placed at
the other end of said coax. The cable used is RG178 which is suited to
the 2.4GHz band and is small enough in diameter (1.8mm). There will in
theory be a loss of signal compared to the standard antenna but this has
proven to be insignificant. Any loss due to the extension certainly is
negated by the increase in range that having the antenna well above the
deck has proven to achieve. By placing the rx antenna about ½ m above
the deck I’ve increased the range for my IOM by almost an extra 100m. I
now get about 220 to 230 m. The extension used was 1m long which allowed
the rx to remain inside the pot and the antenna can be run up the
backstay ½ m or so. Theoretically the antenna should not be placed hard
against the backstay. But I have tried mounting the antenna off the
backstay and also wrapping it tightly against the backstay with no
noticeable difference in range.
So after some fiddling and experimenting, I now believe the answer is
quite simple. Anyone with a fine soldering iron tip and reasonable
soldering skills can do this extension. Although the RG178 cable is not
easy to come by. Contact me directly if you want this done for your
Spektrum DSM rx for a small fee (he says while rubbing his hands
together and sneering like Scrooge).
This shows the extended
antenna wrapped around the backstay. Range is about 230 to 250 m. No
benefit was seen by keeping the antenna away from the wire backstay.
This is the same
antenna as above but all the coax is stuffed inside the boat. If range
is sufficient this saves the hassle of installing removing the antenna
from the backstay. Range is marginally better than the setup below.
This is the short
(3.6") antenna version fully enclosed in the pot. Range is about 130 to
150 m in this boat.
All of the range
distances were measured using the same tx, standing in the same position
and sailing away in the same direction.
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