Construction of a Quadrifilar Helix Antenna

8/13/2019 Construction of a Quadrifilar Helix Antenna

1/5

Design and Construction of a Quadrifilar Helix Antenna for receiving NOAA

Satellites VHF transmissions

Receiving images from the NOAA series of satellites can be a very exciting and

rewarding activity. Aside from the sheer thrill of receiving images direct from

satellites moving at speeds of five miles per second at altitudes of greater thanfive-hundred miles, careful study allows the hobbyist to study their environment

independent of other sources. Whilst creating a setup that allows detailed images

from space to be received at home sounds like a near-impossible feat, it is

certainly not beyond the reach of most sensible people.

Whilst researching this topic, I found it very hard to find clear, concise

instructions. It is therefore my intention over two articles, to describe clearly

the processes involved.

This first article focuses on the hardware, whilst the second focuses more on the

software.

In both articles, I will add an appendix of sources for materials, which will be

referenced in-text. However, due to the volatile nature of the internet, I cannotpromise that these will remain true for any amount of time.

General Overview

Figure 1; the hardware layout

As can be seen from figure 1, a room-full of computers and high tech instruments is

not required to receive telemetry from the NOAA satellites. Starting with the

quadrifilar helix antenna (we will discuss this later) the signal is fed through a

balun to a length of co-axial cable, to which I attached a PL259 connector to allow

connection to my radio. I then built an adapter out of a female PL259 and a male

Belling-Lee connector to allow connection to the digital TV USB dongle. And

thats it. It would certainly be easier to omit the PL259 connection and simply

connect the coaxial straight from the balun to the dongle with only a Belling-Lee

connector.


2/5

Quadrifilar Helix Antenna

Whilst it has a complicated sounding name, the

QFH (figure 2) is a pretty simple piece of kit,

and is also incredibly good at the job it is

designed to do. Like other helical antennas, it

has a wide bandwidth and is particularly good at

receiving circularly polarised signals.

Just as visible light can be polarised through

diffraction gratings, so can radio waves (figure

3). This has the benefit of allowing them to

transmit better in a certain direction, or

receive better from a certain direction. However,

if we try to vertically or horizontally polarise

the signal from a satellite, we make it very

difficult to receive on the ground, as the

satellite is constant changing position. This can

be overcome by circularly polarising the signal

by polarising so that each piece of data is sent

during one circle we get the benefits of

polarisation with the benefits of the signal

being easy to receive.

Figure 2; a rained-on QFH

Figure 3; vertically polarised (a), horizontally polarised (b) and right hand

circularly polarised (c). Note, is the spiral were in the opposite, it would be

left hand circular polarisation.

A QFH is a balanced antenna, constructed of two loops, one name the large loop, the

other the small loop. A balanced antenna is one that is not ground referenced. Asit is connected to an unbalanced line (the co-axial cable), it will need something

called balun to connect the two.

Do not worry about calculating dimensions, each will be given to you.


3/5

Building the Antenna

Materials:

6 m copper tubing I used 15mm diameter in my first build, but this wasvery hard to bend. I plan to use 8mm diameter in the next build. I have also

been told of a type M copper piping, which is apparently much easier to

bend.

10 copper elbows 1 copper T piece 500mm of 20mm plastic waste pipe more if you intend to mount it as in

figure 2.

1 bending springMethod:

We will start by constructing the large loop first. Cut the copper pipe so

you have two one-meter lengths. These will form the helix of the large loop. Mark

one a distance of 804mm on each length,

leaving enough space either side to get a

good grip (figure 4). Then cut two pieces

175mm long and one 367mm so you have five

pieces of pipe in total.

Figure 4

Now, using the bending spring, carefully bend one of

the meter long sections into a semicircle, with the

bend finishing right on the 804mm markers (figure

5). Kinks will simply not do. Repeat for the second

meter length.

Figure 5

Now, you must offset each side of the tube. Figure 6 shows the end result after the

first bend from head on, leaving an offset from the centre of 86mm. Once this is

done, offset the other end by 86mm in the opposite direction. Repeating for the

other length. The end result from head is in figure 7.

Figure 6 Figure 7

Make sure that the pieces are quite symmetrical before cutting them at the marked

points, leaving you with two helix arms of length 804mm.

Now take four elbow joints and the T piece. After cleaning and fluxing the ends of

all five pieces, I recommend soldering the two 175mm pieces to ends of the T piece,


4/5

to form one section that is the same length as the 367mm piece. Once this is done,

assemble the loop, with the T piece section on the bottom and the 367mm section on

top before soldering the sections together

with elbow joints. Dont try to solder one

joint at a time without the loop built up,

as any slight offset will make it rather

hard to contruct a proper loop.

Now check and twist the loop. Ensure the

bottom and top sections are parallel and

that the loop is 542mm tall (figure 8)

Very well done! Now the same must be done

for the small loop. This time, however, cut

two one meter lengths and mark out 752mm

instead of 804mm. Bend it in the same

manner, but leaving it with an offset of

80mm at each end instead of 86mm. The top

and bottom sections are 343mm long.

One that is soldered, again check the top

and bottom are parallel and that this time Figure 8

the overal height of the small loop is 517mm.

Now mark the middle of the top of each loop

and measure 11mm either side. Cut this 22m

long section out of both loops. So you are

left with two loops looking similar to figure

9.

Insert the two loops into each other, so the

top and bottom sections are perpendicular.

Cut a length of the waste tube pipe about

100mm long. Mark a hole 15mm from one end and

drill it out through the other side to justover 15mm. Then from the same end, mark 50mm

down and perpendicular to the first holes.

Drill this out to about 18mm. Cut a pair of

slits to each hole from both ends (figure 10), Figure 9

and then carefully slot the pipe into place,

secure the splits with tape or cable ties.

The bottom sections should now be quite

secure,

but be careful with them yet.

Now take two elbow joints and drill through

the top and bottom of the elbows, in order to

facilitate the bolt size for you balun. I

suggest something around 4mm.

Then, in the configuration shown in figure 11,

solder the joints to join the two loops at the top. And thats the QFH finished.


5/5

Construction of the Balun

Documents

Construction of a Quadrifilar Helix Antenna