New Helix Dish Feed Calculations
Dish Performance in the Antenna Range
To source the TransSystem Inc AIDC 3733 downconverter, go to www.ebay.com and search for AO-40 where Bob Seydler, K5GNA, is your one stop shop for all things TransSystem. Bob ships worldwide and is great to deal with. If you're truly lazy he even provides them ready made and tested.
The modified TransSystem used in the above article put in a very respectable NF of 1.14dB and a gain of 39dB.
K5GNA Modifications in PDF, Word, Text
I did four modifications in total:
o Convert integrated dipole & reflector to N type socket
o Change crystal to make IF 144MHz instead of 122MHz
o Cut stub: the stub notches out 2400MHz, so cutting this is important!
o Change second filter to Murata filter: without this, both the 2400MHz and the 2112MHz images appear (with Local Oscillator [LO] = 2256MHz, first image at 2400MHz, second at 2112MHz) not to mention the LO itself. This filters out the LO and the 2112MHz image improving the noise figure further.
Link to WA5VJB's Yagis used in the uplink - I used two of the 8 element 435MHz units on the same boom in an X formation (or "crossed yagi").
Updated G3RUH helix calculations
Match your dish's illumination angle to the 10dB beamwidths below.
| Turns | 10dB Beamwidth |
| 2.25 | 116 |
| 2.75 | 107 |
| 3.25 | 95 |
| 3.75 | 88 |
| 4.25 | 85 |
| 4.75 | 81 |
| 5.25 | 78 |
| 5.75 | 74 |
| 6.25 | 71 |
| 6.75 | 69 |
| 7.25 | 66 |
| 7.75 | 64 |
Dish Performance Measurements taken at the AMSAT-UK Space 2001 Colloquium
Results of the 60cm Dish on the Antenna Range with different numbers of turns on the G3RUH helix feed (note that lower Axial Ratio is better). 5.25 turns looks optimum. James Miller's new patch feed with a matching dish is shown for comparison purposes. The patch feed is quite difficult to construct compared to the helix.
| Turns | Gain (dBic) without range adjustment | Efficiency (%) without range adjustment | Gain (dBic) with -1dB range adjustment | Efficiency (%) with -1dB range adjustment | Axial Ratio (dB) |
| 3.25 | 19 | 35 | 18 | 28 | 6 |
| 5.25 | 21 | 55 | 20 | 44 | 2 |
| 6.25 | 21 | 55 | 20 | 44 | 5 |
| G3RUH Patch feed on G3RUH Dish | 22 | 72 | 21 | 57 | 0.2 |
The antenna range may well have been up to 1dB optimistic as there was some concern that at 22dB, the G3RUH patch feed & dish configuration was running at 72% efficiency which is into the upper end of what might be expected.
Be careful about using the G3RUH patch feed with any old dish: the patch is designed to be used with the G3RUH dish which has an illumination angle of 156°. The example offset fed dish used here is only 74°, so you'd be significantly over-illuminating the dish.
Axial Ratio is a way of defining the circularity of an antenna's radiation pattern. Strictly this is the difference between the maximum and minimum on-axis gains as the antenna is rotated through all angles. The above measurements were only performed at two perpendicular angles and so axial ratio may be worse (greater) than specified. If the axial ratio is 0dB then the antenna is perfectly circularly polarized. At the other extreme, a 20dB axial ratio would suggest a linearly polarized antenna.
First find the dish aperture A:
A = pi * r2 where r is the radius of the dish.
Next calculate the efficiency E:
E = 10(G/10) / [(4 * pi / w2) * A] where G is the gain in dBi and w is the wavelength (0.125m in this example).
or...
E = 10(G/10)/(pi * D/w)2 where D is the dish diameter (D = 2r)
For the above calculations the prime focus G3RUH dish has D=0.59 and the effective diameter of the TVRO offset fed dish has D=0.60 (the effective boresight).