Using External Antennas with Your Eggfinder System
With the included ¼ wave stick antennas, the Eggfinder system has been tested to over 8,000’
range line-of-sight with no loss of signal at apogee. This is adequate for most hobby rocketry
sport flights, since it’s OK if you lose the signal as long as you pick it back up as the rocket is
coming down… after all, the goal is to find out where your rocket landed, not necessarily to be
able to track its flight (although it is very cool when you can do it!).
However, there may be cases in which you need to extend the range of your Eggfinder. Since
the output power is fixed at 100 mW (20dBm), the way that you’re going to do this is by
improving on the antennas. The Eggfinder is designed so that you can solder an edge-mount
RP-SMA connector (RP-SMA female, which means that the edge connector has a center pin
rather than a jack). This allows you to use a variety of other antenna option that will
significantly improve the reception, with some caveats.
First, you need to understand how antennas work. A transmitter antenna converts the electrical
output of the transmitter to electromagnetic waves, and on the receiver side converts the
electromagnetic waves to an electrical signal that can be amplified, filtered, and decoded so
that it is the same as the electrical signal that was fed into the transmitter’s antenna. The
transmitter has a fixed amount of power available, and when that power is converted to radio
waves they propagate in free space in a certain pattern that is dependent on the type of antenna.
Think of the total radio output power as being like a round balloon, the distance from the
center at any given point is the power in that direction and the air in the balloon is the total
output power. If you squish the balloon in the center, it will bulge and flatten out at the center,
so the power output is higher there at the cost of being less at the top and bottom where you
squished it. That’s basically what a dipole or “stick” antenna does… the output strength in free
space looks like a donut, about the same all around the sides but much lower at the extreme top
and bottom. The total output power is still the same, it’s just distributed differently.
Now, if we take that round balloon and squeeze it so that it’s long and narrow, you have the
same amount of power of course, but it’s much higher in one direction, at the cost of being
much less in the other directions. The harder you squish, the longer it gets, but also the
narrower it gets. An antenna with a pattern like this is called a directional antenna, they have
high gain (signal amplification power), but only in one direction. The angle in which the
signal can be propagated before it starts to drop off sharply is called the “beamwidth” of the
antenna. Typically, gain and beamwidth are opposites; the higher a directional antenna’s gain,
the narrower the beamwidth. Very high gain antennas, like satellite dishes used for TV, have a
very narrow beamwidth; you need to aim them directly at the satellite or you won’t get any
signal at all.
Since you can’t tell ahead of time which direction your rocket may be going, it doesn’t make
sense to put a high-gain directional antenna into the rocket itself. In the rocket, you’re going to
use an omnidirectional antenna, but it you’re flying relatively high (over 10,000’) you can get
some extra range by using a higher gain omnidirectional antenna, that puts out a stronger signal
over a narrower “donut”. Since you’re going to be relatively far away, the fact that the
beamwidth is narrower doesn’t matter as much as the extra gain.
On the receiver end, you can either go with an omnidirectional antenna, or if you have an idea
which direction your rocket went you can point a higher gain directional antenna at it and get
