How to build a satellite yagi antenna out of an old umbrella
Introduction
Several amateur radio satellites transmit around 436MHz. The antenna described here has successfully been used to receive AO-27, LO-19, FO-29, AO-51 and GO-32. These satellites transmit several types of information around 436MHz including telemetry, digital packet and voice comms. Coupled with a 70cm receiver, the satellites can be heard with good strength. One of the important points about these satellites is that they are in approximately 1000km high orbits, and take about 90 minutes to orbit the earth. This means that they are not always above the horizon, and also that when they are above the horizon, they move across the sky. This requires the antenna to track the satellite motion in order to get the best signal from it. The antenna described here is best for mobile operation, as it is small and light, and is intended to be hand held. Every single part, with the exception of the cable and fixings was made from an old umbrella I found in some bushes.
Parts needed
- Suitable umbrella
- Tape
- Short piece of coax with suitable plug
The image below shows the completed antenna
The antenna is about 80cm long, and is a yagi type with 5 directors and a folded dipole feed. The antenna should give a gain of at least 10dBi in its best direction, and is linearly polarised.
Construction
The first step is actually destruction. The umbrella needs to be disassembled into its component parts; specifically, the arms need to be detached from the central shaft. The umbrella I found in the bushes has a central shaft 90cm long, including handle and was made of metal. The supporting arms are made of 3mm iron piano wire. These can be used for the antenna, but only if they are conductive! The picture shows a selection of bits from a disassembled umbrella. I found two umbrellas, one was made of some fibrous material, the other entirely out of metal. I used the fibrous shaft and the metal arms.
The next step is to cut the arms to length. This step should be done carefully, as the lengths are critical.
| Element |
Length (mm) |
Spacing from reflector (mm) |
| Reflector |
340 |
0 |
| Feed |
640 |
70 |
| Director 1 |
310 |
160 |
| Director 2 |
310 |
280 |
| Director 3 |
300 |
415 |
| Director 4 |
300 |
580 |
| Director 5 |
300 |
790 |
Next, pairs of holes need to be made in the shaft using the spacings given in the table. It is important to make the holes accurately, otherwise the elements will be either poorly spaced, or wonky. All the elements must be parallel, which means all the holes for the elements must be parallel and perpendicular to the axis of the shaft.
The feed is the most fiddly part to make, as it needs to be folded, and since it fits through the shaft, it must be folded in situ. The feed should be folded so that it forms a hoop 320mm long, and 10mm wide. Note, the wire is not long enough to close the hoop, and there should be a gap about 5mm wide. Folding the wire is quite tricky as it is springy and quite stiff. Arrange the hoop so that the gap is accessible on top of the shaft (see picture)
Poke all the remaining elements through the shaft (in the correct sequence) and ensuring that they are exactly central, and all parallel, fix them in place. I was not able to satisfactorily fix them in place, so they are currently held in with tape. However they are held, ensure that they are tight enough to not slip or slide about.
The yagi antenna has an interesting fact that all of the elements are actually at 0V potential in the middle (including the feed). This means that they can all touch the metal boom in the middle as there is no potential here anyway. This only holds for a very skinny boom, and in practice the umbrella shaft will touch the metal elements at about 7.5mm distance from their middle on either side. I wouldn't expect this to make much difference. I used a non conducting boom anyway!
Finally attach the coax feed to the feeder as shown in the picture. The antenna should match quite nicely to 50 ohm, and show better than 3:1 VSWR over a few MHz bandwidth. A balum might help at the feed point, but my antenna worked OK without, perhaps because I used a short feed cable.
In use
The antenna is lightweight and easy to point. Using computer prediction software and up to date orbital elements find a satellite above the horizon and simply aim the antenna at it. If the radio is tuned correctly, it should easily be heard. The signal can be improved by rotating the antenna or sweeping about the sky for the best signal. Note that the doppler shift of the satellites radio signal will be shifted by upto +/-9kHz depending on the relative velocity between you and the satellite. The table below gives a starting point for some satellites I have received near London, UK. You will need to use prediction software to work out when the satellite is above the horizon, and note that not all satellites transmit continuously!
| Satellite |
Downlink frequency (MHz) |
Type |
| AO-27 |
434.797 |
FM voice / packet |
| AO-16 |
437.026 |
Data |
| LO-19 |
437.125 |
CW telemetry |
| FO-29 |
435.85 / 435.795 |
CW comms / CW telemetry |
| AO-51 (Echo) |
435.3 |
Data |
| GO-32 |
435.225 |
Data? |
Useful software
Wxtrack satellite tracking software
EZNEC antenna simulator (demo is insufficient to simulate this antenna)
Quick Yagi 4. Old DOS software but seems to work a treat for designing yagi antennas
Have fun, Hoppy