Friday, 23 August 2013

Weather Satellites - Antennas

Summary: two meters of wire and a meter of coax for a good antenna, 5 meters of wire and
RTL-SDR, SDRSharp, WXtoImg, QFH, Software Defined radio, weather satellite image
a wood frame for an excellent antenna.
Cost is less than 10 euros, construction time is maximum one hour for the example here.
Read on to find out how.

Your antenna choices for Weather Satellites are:

1. Monopole with 4 radials. Construction detailed in the AIS antenna guide, elements are 54.5 cm long. Not discussed here.
2. Turnstile. Four wires plus two coax cables. Portable version discussed here, takes 30 min to build.
2a. Eggbeater - add more wire to the turnstile. Tested, better signal strength. Option if don't want to build a QFH.
3. Double Cross / Lindenblad: Complicated to build, not suitable for marine use.
4. QFH, short for quadrifilar helix. A support with four arms, holding 5 metres of wire. Weather satellites use this type - build this.
5. Yagi-Uda: the TV antenna type found on chimneys, requires manual tracking. Not discussed.


Wire - the thicker the better. Center conductor from cables work, better results with house grounding wire. I use 3mm (No 6-10 AWG for metric readers) solid copper wire with insulation left on. Some guides use 1/4" or 8mm copper tubing, needs special tools.
Support - your choice. I use wood as it's easy to work with, cheap and readily available.
Connectors - screw terminal blocks are great to hold wires in place.
Grounding wire comes as two or three 3mm cables combined in one cable. When uncoiled for measuring / cutting, it can hit you with sufficient force to take your eye out. Pieces being cut also fly to surprising distances.

I've built and tested different versions of the following antennas, best performance with easiest construction is discussed here. Form follows function, so the results might not be aesthetically pleasing, but they work.

Portable Turnstile

Built and tested six different versions, received the image on the right with the version below.
Officially called a crossed dipole, basically four horizontal wires with a phasing harness (more on that later).
Wire lengths are either 1.09m (half-wave) or 54.5 cm (quarter wave). Longer length gave no visible improvement and a pain to mount / carry around.
Wire diameter can be either center of coax cable (1mm) or grounding wire (3mm). Theoretically thicker is better, this is true, thicker cable gave better signal strength.
Phasing harness is coax cable cut to a specific length. Some guides recommend 50 and 75 Ohm cables, tested multiple versions, could not see any difference in received image. If googling turnstile antenna for images, you might notice that sometimes only one cable is used on top of the assembly.
I found that having two 75 Ohm cables, four terminal blocks, 3 mm grounding wire and a wood support is a quick, easy and portable solution.
Wires are removable with a screwdriver, making for a compact and transportable package.
RTL-SDR, SDRSharp, WXtoImg, Turnstile, Software Defined radio, weather satellite image


Wire - Cut four (4) 55 cm wire. Remove the insulation for 5mm on only one end.
Cable - You need two cables of a specific length. The length required depends on the insulation type around the center conductor. Strip away the braid and check the material surrounding the center wire.
If it is plastic, you need 36 and 72 cm long cables.
RTL-SDR, SDRSharp, WXtoImg, Turnstile, Software Defined radio, weather satellite imageIf it is foam (same material as a coffee cup, soft to the touch) you need 43 and 86 cm long cables.
Leave space on each end of the cable so you'll have enough cable to work with. Separate braid and center wire. Folding the center wire onto itself makes for a better connection at the block.
Screws - self-tapping screws (they make their own hole as you turn them) are great.
Wood / mounting block - mount the components on a piece of wood, or inside an enclosure.
Terminal blocks - one end holds the wire, other end connects the cable. Screw one of the screws fully in. This will be a stop for the radials, e.g. the 54.5 cm wire. Also test that the wire fits into the terminal end tightly.

RTL-SDR, SDRSharp, WXtoImg, Turnstile, Software Defined radio, weather satellite image

Mount the four terminals at 90 degrees to each other, like North, East, South and West on the compass rose. Leave enough space in the middle for the cables.
Opposing ends of the four terminals are connected, so they form a North-South and East-West pair for each of the coax cables. Push the center of the braid into the hole of the screw connector, and tighten the screw. I find it easier to do the braid first, then the center.
Connect the shorter cable: center to West, braid to East. Insulate braid and center as the other cable might come into contact with it.
Connect the longer cable: center to North, braid to South. By leaving enough center and braid to work with assembly is much easier.
Connect the other end of the shorter and longer cable with the cable going to the RTL stick - connect the three center to center and braid to braid. Insulate braid and center separately.
Locate the four 54.5 cm wires, and one by one push into the terminals, fixing them down with the screw.

Congratulations, now you have a turnstile antenna!

Transform this to an eggbeater antenna:

The eggbeater antenna is an improvement on the turnstile, using two loops, each around 220 cm long, so they form a  71 cm diameter circle. The commercial version is used by radio amateurs to communicate via satellite. If you have enough wire give it a go.
Connect the ends of the 220 cm loops into the terminal connections. That's it, the phasing harness you made will be fine, same principle.
I found that even thought the signal is better, it is too large and does not look nearly as cool as the spiral QFH.
Besides, why not build the antenna type actually used on the satellite itself?

RTL-SDR, SDRSharp, WXtoImg, Turnstile, Software Defined radio, weather satellite imageQFH, the Spiral antenna

Built eight versions, easiest to construct with best image quality described here.
The image on the right was made with chopsticks taped to a brush to form the frame, with coax cable for antenna elements. Length and distances are critical, materials are not. 
The antenna is made from a smaller and longer piece of wire, fixed to a frame, connected at the top. Google the term QFH - and a huge number of websites, guides and images come up. Recommended ones are included at the end. However, most guides require the use of power and heating tools and/or soldering.This guide shows you how to build a working version quickly with basic hand tools.


Frame support: Your choice. I recommend wood, with self-tapping screws only a screwdriver required.
Coax cable: due to the wide variety of cable around, their different properties and the pain of connection on the top and bottom NOT recommended.
1mm wire: the required circular shape is easily lost, signal strength is barely adeguate.
3mm grounding wire: RECOMMENDED, keeps its shape quite well.
Copper rods: you need heating and soldering equipment, pipe cutters etc. Theoretically the thicker the better, comes down to tools and personal choice.
Build one with smaller wire first, then progress to thicker materials.
Connectors, screws: as for the turnstile.


Use the calculator here: John Coppen's Online calculator.
For 3mm grounding wire, use bending Radius: 5mm, Conductor Diameter: 3mm.
Press Calculate - do not press Enter, you'll get an error message.

RTL-SDR, SDRSharp, WXtoImg, Turnstile, Software Defined radio, weather satellite image

Note the values for both the small and the larger loop, especially lengths and heights. Adjusting the width at the cross arms can be done later.

Support / Frame

Cut six pieces of wood, 360 mm in length - these will form the cross arms.
Screw the arms together so they are 90 degrees from each other - use other pieces of wood to get 90 degrees.
Cut the middle section to around chest-high - so you can work on the top connections comfortably and the final antenna will be off the ground.
Screw the top arm on the top, measure the larger loop height and screw the bottom arm at that distance from the top.
Screw the middle arm halfway between the top and bottom.

Antenna elements - wires
RTL-SDR, SDRSharp, WXtoImg, Turnstile, Software Defined radio, weather satellite image

Cut the required length of wire (WEAR EYE PROTECTION!) plus 5 mm on each end. You'll have two wires, one longer and one shorter. Labeling which is which is a good idea. Mark the midpoint of each wire with tape.
Start with the longer loop - tape the end on the top arm, then make a half turn counterclockwise as viewed from the top. If you think of the arms as directions, start at North, make a turn counter-clockwise (to the left) so the same wire ends up on the bottom arm, facing South.
By this stage the other end of the wire probably destroyed half the room, a helper pays dividends. Hint: involve your significant other or kids.
The midpoint of the wire goes in the center of the bottom arm, tape it securely on both sides.
Run the remaining wire up to the top arm, same thing again, half-turn counterclockwise so the two wires will face each other.
Repeat the same for the smaller wire, so you'll have four wires coming to the center on the four arms.
Measure top and bottom arm distances, remember, top and bottom center must be under each other.
Bend the wires into shape, so when you look down on the antenna wires look like a circle.

Top connections and finishing up.

RTL-SDR, SDRSharp, WXtoImg, Turnstile, Software Defined radio, weather satellite image

Coax cable (the one used to get the signal to the RTL stick): a small distance below the top, wrap it around the mast 5-6 times - this will help with electrical noise suppression (coiling excess cable has the same effect),
Separate braid and center on one end - leave sufficient length, 5-6 cm (2") to make connections easier.
The center of the coax connects to one side of the small and big loop, the braid connects to one side of the small and the big loop. When viewed from the top:
Center: connects North and West
Braid: connects to East and South.

Using values from the online calculator, measure top and bottom arm distances, center support - arm distances for both bottom and top. Fix the wire in place with cable ties.

Congratulations, you've just made a QFH antenna!

Antenna placement

- View of the horizon: In a city, as high as possible. Trees and buildings block the signal from the satellite.
- On a yacht, mount the antenna at a convenient location, there's no need for height. It's an interesting question how a sail, especially a wet sail influences reception. Also, place the top electronics in a weatherproof box, and pay special attention to mounting due to windage.
- Away from electrics: try not to have electrical devices nearby.
- Preamplifier: if you use one, mount it as close to the top as possible. Cable from the top picks up electrical noise, so go for the shortest length possible. (Amplifiers in a separate post)

Optional to read: for the turnstile you're making a phasing harness, a quarter-wave and a half-wave for 137.5 Mhz center frequency. Divide 300 with 137.5 to get wavelength 2.18m, half-wave is 1.09m, quarter wave is 54.5cm. If you have foam cable, google the writing on the shield and "velocity factor", such as  "RG 8/U velocity factor".
This gives you a number, in this case 0.78. You need to multiply quarter and half wave values with this to get the required length in centimetres.
In practice a few mm either way will not make a difference.

References, credits and further reading information

QFH from coax cables, an excellent read.
QFH from plastic pipes and copper tubing, one of the better manuals. Have the tools? Build one.
If you wonder if a double cross might be better, read this pdf file.
Eggbeater details and construction here, lots of diagrams.
Browse around on this page, a variation of the eggbeater, called a potato masher. What's this fixation with kitchen utensils?
Commercial eggbeater antenna for USD 221 + shipping at

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  1. Absolutely agree with you on the superiority of the QFH. I built one out of PVC and RG-6U and the results have been excellent. I ended up going with G4ILO's variation of Chris van Lint's portable QFH; as with your design, no copper tubing was required.

  2. Plain wire is easier to work with than coax, bottom and top connections are a pain with coax.
    Foam coax is best as it holds its shape, but wire is cheaper.
    Seen better results with wire, reception seems to come down to shape - even a small kink causes difference in images (birds landing on antenna squash the top section :-)