These notes have been written to provide more detailed constructional
information to augment the paper presented by Zdenek,
These notes also apply specifically to the implementation that I decided to use there are other ways of fabricating the metalwork, and comments made here may or may not be applicable!
I decided to fabricate the square outer tube in two equal U sections, with a short(13mm) flanges on the top of each leg of the U, so that the two halves can be bolted together. The septum plate was to be cut from a single sheet, and 10mm wide spacing pieces of the same thickness were to be inserted between two casing halves in front of the septum plate. A thick back plate would be used, for two reasons:
All parts were fabricated from aluminium, with the exception of the tuning screws and feed monopoles. All dimensions were taken from the Excel spreadsheet provided by Zdenek. 3mm stainless steel machine screws would be used for all fixings. For other constructional dimensions see the appropriate tables below.
1. First take the two halves of the casing and check the bending dimensions. Check that this is consistent at each end of each half, and decide how the two halves will be aligned with each other small errors in the distance and the degree of bend can be compensated for by deciding the orientation of the two sections against each other - once you have decided the best fit, mark the two sections carefully to identify the mating points. Also decide which will be the back of the feed I decided to make this the end that presented the most accurate set of angles and distances.
2. Mark a line along each flange 6mm from the edge, for the whole length of the flange. Measuring from the back of the feed, mark out the holes for securing the sections together.
3. Drill all points 3mm
4. Using 4 screws and nuts, fasten the two sections together at the back and front most holes only. Carefully correct any slight mis-alignment of the remaining holes with a needle file.
5. Now take the septum plate. Mark out the drilling lines, allowing for the overlap on each side of the plate.
6. Allowing for the thickness of the back-plate, use the data from the table to mark the drilling positions for the holes.
7. Separate the two halves of the cavity. Lay the drilled septum plate on upturned half and check the hole alignment. Again, put the second half in place, and fix with two screws at the back one each side. Put another screw into the hole at the front of the septum, and then re-check the alignment of all the holes, adjusting as necessary.
8. Similarly, mark out the spacing pieces and fit.
9. Using the assembled cavity as a guide, offer up the back plate and check whether any fine shaping is required to get a good fit into the back of the cavity. Once a good push fit is obtained, mark out and drill the plate a 5mm plate will take carefully centred and drilled 3mm threads use a 2.5mm drill. I used 4 screws along each side, equally spaced about the centre line. Tap all holes.
10. Also mark out the holes for the two tuning screws in the back plate. Drill these and tap M6.
11. Decide on the connectors you will use for the Tx and Rx ports I used SMA for Rx (to minimize the no of adaptors etc in connecting a relay and preamp) and 7/16 for Tx, to handle the power.
12. Referring to the diagrams in Zdeneks presentation, mark the sides of the cavity for the Tx and Rx connectors. Remove the sides of the cavity and drill for the connectors.
13. Make up the tuning screws I used M6 x 50mm brass screws, and soldered a 15mm brass disc to the head. A slot was cut in the other end to aid adjustment.
14. Once re-assembled, the feed is ready for setting up!
|
WG thickness |
2 |
Screw length backplate fixing |
6 |
Hole spacing for back plate mounting |
40 |
|
Septum plate and spacer thickness |
1.5 |
Spacing across WG - internal |
144.9 |
Tuning screw spacing from side of cavity |
30 |
|
Back Plate thickness |
5 |
Offset from back of WG to first screw |
15 |
Diameter of tuning disc |
15 |
|
Screw length- WG sections |
10 |
Subsequent hole spacing |
60 |
|
|
Table 1 -23
cm dimensions -all dimensions in mm
|
WG thickness |
1 |
Screw length backplate fixing |
6 |
Hole spacing for back plate mounting |
20 |
|
Septum plate and spacer thickness |
1 |
Spacing across WG - internal |
82 |
Tuning screw spacing from side of cavity |
15 |
|
Back Plate thickness |
5 |
Offset from back of WG to first screw |
25 |
Diameter of tuning disc |
screw head only |
|
Screw length- WG sections |
6 |
Subsequent hole spacing |
30 |
|
|
Table 2 -13
cm dimensions -all dimensions in mm
The following comments apply to 1296MHz for 13cm band measurements follow the same procedures; results should be similar.
I used a sweeper and network analyzer for my initial measurements, although a signal source and an SWR bridge will do! Mount the feed so that there are no obstructions in front of it, and connect the signal via the bridge to one of the ports. Carefully adjust the tuning screw for minimum SWR with care an SWR better than 1.1:1 should obtained. Now repeat on the other port. On both ports I have been able to establish a VSWR of better than 1.07:1 a return loss in excess of 30dB. Remember to lock the tuning screw in position with an M6 nut once the best match is obtained.
There are no more adjustments just checks to confirm that the feed is working correctly:
Isolation between the ports should be around 27dB. Feed a signal of a known level into one port, and measure the power level present on the other port.
Check the circularity of the feed by using a linear antenna (dipole) and a power meter. Feeding low power 1296MHz signal to either port of the feed, measure the power level received via the dipole. Now rotate the dipole through 90 degrees to the other polarisation, and measure the power level it should be within 1.1dB of the first measurement. Now slowly rotate it back to the first position, and watch that the level stays within about 1 dB of the orthogonal power measurements.
Remember that the feed has a beamwidth of about 130 degrees so is best suited for dishes of around 0.35 f/D. You will need to experiment with choke collars and /or flaring of the mouth of the feed to properly illuminate other sized dishes.
[1] Feed for Parabolic
Dish with Circular Polarisation Zdenek Samek OK1DFC: 10th International EME Conference,
[2] Analysis of the OK1DFC Septum Feed Paul Wade W1GHZ; DUBUS 1/03