C Band Transverter for 5668 - 434MHz
Here in Europe, direct broadcast satellites on 10-12GHz provide a cheap source of offset fed dishes suitable for use with phase 3D. The dishes are available in a number of sizes from 0.3 to 0.8m diameter. With a suitable feed these are ideal for use with P3D on 2.4 / 5.6 and 10GHz. The size of the dish required for C band will be dependent on the output power of your amplifier. As generating power at 5668MHz is never cheap, it's likely that a dish of 0.5 - 0.8m will be necessary. An added benefit of using DBS dishes is that only the most Eagle eyed of neighbours will realise that it's really your ham radio antenna!
There are a number of choices for dish
feeds for P3D. Under ideal circumstances you should use RHCP (right hand
circular polarisation) There are several antennas which can fulfill this.
One is the circular polarised patch antenna by Freddie de Guchteneire ON6UG.
This multiband feed was detailed
in the proceedings of the AMSAT UK Colloquium 1996, operates from on 5 bands
from 1.3 to 10GHz and is suitable for short
focal length dishes. Another suitable candidate is a helical design. The helix
can be wound with as many turns as required for the focal length of the dish.
For the DBS dishes with an FD ratio of 0.6, four turns is a good number to
start. The single disadvantage of the helix is the difficulty in making the
matching section. For 5668MHz the space between the base plate and the matching
section is very small. It can be done, but for long term reliability it would
probably have to be enclosed in a Radome. - To avoid dead bugs / water /
corrosion between the matching section and the base.
Two other choices described below are a horn feed and a small log periodic. Both of these antennas have linear polarisation but are easy to build
The helical feed described here can be used with a dish with an F/D (focal length to diameter ratio) of apx 0.6.
The dish feed is based on a small 4 turn helix mounted on a die cast aluminium box of 50mm x 50mm.
An SMA socket is used to feed RF power and to terminate the helical winding. A 65mm length of 4BA brass studding is used as a support. The SMA connector is mounted using countersunk M2.5 screws. All nuts and screws are cadmium plated brass.
These two photo's show the mounting arrangements for the SMA connector and the brass studding.
The helix is wound using 1.6mm dia enameled copper wire. The important dimensions are:
4 + 0.5 turn for the impedance matching (Anti Clockwise)
Spacing between turns 12mm
Gain Apx. 10dB
Beamwidth Apx. 55 degrees
Reflected power at 5668MHz Between -10dB (worst case) and -18dB
The impedance matching from 50 to 140 Ohms is
achieved by having the first half turn of wire running parallel to the ground
plane. The wire diameter of this half turn is increased by soldering a 3mm wide
section of copper 'shim'. This can easily be cut to shape with a pair of
scissors. The matching section does not touch the ground plane but is apx 0.6mm
above the surface of the aluminium. The far end of the helix is held in place by
a small section of insulating material. In the prototype I've used a piece of
glassfibre from a PCB.
LINEAR POLARISED HORN ANTENNA
As P3D orbits, the polarisation will change depending on the angle between the antenna on the spacecraft and the earth. Consequently the disadvantage of using linear polarisation is rather less than the 3dB you may imagine.
A good feed horn can be constructed by locating a surplus C Band Coax to waveguide transition and then adding four sides to form a small horn antenna. The size of the horn can be adjusted to give the correct beamwidth for the focal length of the dish.
For sheer simplicity a dish feed doesn't
come much easier than this.
Kent, WA5VJB, has designed a small PCB based log periodic antenna that covers 2.1 - 6GHz.
The PCB has the antenna printed on it. You need to solder a small length of coax to it and fit a connector. The recommended cable is UT085 semi rigid with an SMA etc. The big advantage here is that you can use it on S band and C band.
A disadvantage is that it's efficiency will be slightly lower than optimum due to increased losses of the FR4 fibreglass dielectric.
In the USA this PCB costs just $4. In the UK it's available from the UK microwave group at £4 for members or £5 for non members including postage.