Carsten Groen's (OZ9AAR) personal place

23cm feedhorn for EME (f/D 0.4)
(design by OM6AA)

I needed a suitable feedhorn for my new 4.8M dish (with f/D of 0.4) for 23cm EME. 30+ years ago I made a couple of W2IMU feeds for my 5 meter and 8 meter dishes, these dishes were both f/D 0.5 and my new 4.8 meter is f/D 0.4, so I needed something different for the new dish.

I was recommended by many fellow EME friends to look at a feed that OM6AA have designed.

I had some trouble locating a ready made feed, so I decided to try and make my own version of it. I got a lot of valuable input from Mark 9H1BN, Graziano IK3MAC, Francesco IK2DDR, Ingebrigt LB6B, Walter ON4BCB and of course Rasto OM6AA during the process.

Early on, I decided to document everything and also make available all design files and measurements (see comment below!) for the feed, this way others are able to take ideas from it and maybe make their own feed based on this information.

As most of the parts was ordered specifically for the feed, I had to order "more than one" of everything. Because of that, I have a few extra feeds available (parts or possibly finished feeds) for anyone interested.

I don't have (anywhere near!) a "full mechanical workshop" myself, so most of the stuff I got manufactured at a couple of professional workshops (CNC parts, welded parts etc.). If you are able to machine the parts yourself, a good amount of money can of course be saved!

The hardest part to source in the project was the main tube for the feed, this is a 190x3mm aluminium tube (740 mm long), a fellow EME friend pointed me to a company that could extrude this in 6 meter lengths for me (at a reasonable cost) in Austria (Blecha). The tube was ordered, delivery is 8 tubes each cut to exact length.

The sheet metal (aluminium) parts for the choke (baseplate and rim) were produced at Scandcut, they are always my "go to" provider for all kinds of laser cut sheet metal (and other materials) when I need that for my projects. I highly recommend to look at Scandcut, especially if you are in Europe, prices are very good and service is exceptional from them.

According to Rasto OM6AA the dimensions are not to be publicly shared! Please contact Rasto for further information about dimensions etc. as I'm not allowed to share them directly! I can then probably share the STEP files etc. if you need.

Background information

Rasto OM6AA had two very interesting articles about the feed in the DUBUS Magazine. I got permission from Joachim DL8HCZ, the guy behind DUBUS, for permission to make the two articles available here.

If you don't already subscribe to the DUBUS magazine, I can only recommend you do so, lots of interesting articles by some of the brightest minds we have in the Ham community, lots of EME related stuff also.

Circular Polarization and Polarization Losses, DUBUS magazine 4-2006, PDF file

Prime-focus circular waveguide feed with septum polarization transformer, DUBUS magazine 1-2007, PDF file

The design

As usual, I designed all the parts in CAD. Some of the details are based on the dimensions from Rasto OM6AA and input from fellow EME friends, and some are from my own ideas (mostly the choke ring and the TX/RX adapters). 

The feed uses a DIN 7/16 connector for the TX port and a N female for the RX port.

The TX DIN 7/16 connector is a Radiall type R185403547.

RX connector is a Telegärtner type 100024052 (old type J+1+21B0008).

The choke needs to be electrically "well connected" to the main tube and people uses different ways to achieve this. I chose to do a mix of 12 x M6 set screws and a stainless steel spring that is clamped between the main tube and the "ring" that holds the choke assembly. Time will tell if the spring is really needed or not. The ring for the choke is made so it sits very tight/close to the main tube.

Below follows some screenshots from the design process as well as pictures form the actual produced parts.

Septum

Initially, the Septum was sketched out based on dimensions from Rasto OM6AA. The Septum is a crucial element that requires precise cutting and manufacturing. The primary tube measures 184.0mm in inside diameter, with the Septum featuring 0.1mm space at both the top and bottom to facilitate insertion into the main tube. Securing the Septum involves 10 x M3 screws at the top, 24 x M3 screws at the bottom, and 9 x M3 screws at the rear end (attached to the backplate). The Septum has a total of 43 x M3 threaded holes, each hole is threaded to a depth of minimum 8mm.

The Septum is constructed from 8mm thick aluminum and is fully CNC machined. All the 43 x M3 holes are threaded in the CNC process, there is no need for any manual labor in the construction of the Septum.

I did some test measurements on the final CNC machined part, the largest deviation from the specified dimensions I could find was +/- 0.05 mm.

The top and bottom edges of the Septum are machined so the curve follows the internal diameter of the main tube:

Main tube

The primary tube, or main tube, has dimensions of 184.0 mm internal diameter and 190.0 mm external diameter, which was challenging to source. Fortunately, I was recommended a company willing to extrude a 6-meter length for me (Blecha in Austria), resulting in 8 finished main tubes. The tube is supplied in specified lengths, eliminating the need for additional cutting.

To accommodate the required holes for the screws and RX/TX connectors, I created a 3D printable "drilling jig" to assist with drilling the holes in the main tube. This jig enables me to drill the 24 holes (in a circle) at the tube's end, as well as the holes for the TX and RX ports, and at least some of the 10+24 holes for the Septum. The remaining of the 10+24 Septum holes are drilled using a longer jig that covers all necessary holes.

The two large holes in the main tube (18 and 20 mm) for the RX/TX connectors and their adapter plates are not too critical in dimensions. I designed the CNC machined adapters so that they would be responsible for keeping the 50 Ohm impedance to the center pin and the RX/TX probes. This makes it less critical to make the 18 and 20 mm holes in the main tube for the connectors. The two large holes have to be done manually (all holes in the main tube are done manually with a drill press) so it was important that it would be as easy as possible to do these.

The TX and RX probes and their adapter plates. The adapter plates will go into the 18.0 and 20.0 mm holes in the main tube.

Due to limitations in what is possible with CNC machining of the RX/TX adapter plates (there is a need of a 1 mm radius fillet at the edge of the structure that passes thru the main tube), the two 18 and 20 mm holes in the main tube needs to be rounded a bit on the outside for the adapter plates to fit. This is shown in the two green circles below.

3D printed drilling jig.

End plate

The endplate is also CNC machined. It is made of an 8 mm thick piece of aluminium. The 24 x M3 threaded (around the edge) holes connects it to the main tube, it also have holes for the 9 x M3 screws that goes into the end of the Septum. The outer diameter is 190.0 mm and the inner diameter is 183.8 mm to leave a small gap between it and the 184.0 mm inner diameter main tube.

TX and RX probes

Note: The pictures of the TX and RX probe adapter plates below are of "Revision A" of the adapters. The screenshots are however of revision B (that will be used in the feed)

The TX and RX ports both needs a small "adapter" between the connectors and the main tube. The connectors are meant to be mounted on a flat surface but the main tube is round! One way to handle this would be do machine a flat area around the TX and RX port holes on the main tube. Instead of this, to keep the manual work at a minimum, I designed two small adapters (also in aluminium). These adapters makes it possible to mount the N and DIN 7/16 connectors to the main tube. Both connectors are held in place using four M3 screws, the main tube have threaded holes for these.

The holes in the main tube for the two adapters needs to be manually drilled. To make this less critical (to keep the 50 Ohm impedance) both adapters have milled profiles that go thru the large hole in the main tube. The hole needed for these profile are 18.0 mm (for RX probe) and 20.0 mm (for TX probe) in diameter. It is not super critical if this is 18.0/20.0 or 18.1/20.1 as the impedance for the connectors are controlled by the internal diameter of these profile (which both are CNC milled).

The TX and RX probes are made so that they interface with the DIN 7/16 and N connectors. The N connector has a M3 threaded rod for the center pin. A M3 threaded hole are simply made in the RX probe allowing it to be screwed onto the N connector. The TX probe have a M3 threaded hole. The DIN 7/16 connector have a "solder cup" with a 3 mm hole (7 mm deep). A suitable length of M3 threaded brass rod (I used a long brass set screws) are soldered into the DIN 7/16 connector. The TX probe is then screwed onto this threaded rod. The holes in the main tube and in the adapter plates are made so that the center part of the probes will be close to 50 ohm (6.95/7.0 mm and 16.00/16.1 mm). Both  probes are made of aluminium.

TX probe

The opening in the profile in the adapter that goes thru the main tube for the TX probe is 16.1 mm. Together with the 7.0 mm "neck" of the probe (and the 7.0 mm center "cup" of the connector), this will give an impedance very close to 50 Ohm. (The 7.0 mm and 16.1 mm are both dictated by the "solder cup" diameter of the 7/16 connector).

A M3 x 16mm brass setscrew are soldered into the DIN 7/16 connector, the TX probe is then screwed onto this setscrew.

The nominal length of the probe is 39 mm, I have ordered different sizes of 37,38,39,40 and 41 mm and will test these in the finished feed.

RX probe

The opening in the adapter that goes thru the main tube for the RX probe is 16.0 mm. Together with the 6.95 mm "neck" of the probe, this gives an impedance very close to 50 Ohm. 

The nominal length of the probe is 39 mm, I have ordered different sizes of 37,38,39,40 and 41 mm and will test these in the finished feed.

Choke

The choke is made of 3 parts. The base plate, the rim and the ring (that holds it onto the main tube).

The base plate and the rim are made of 2 mm aluminium and TIG welded together. The ring is CNC machined aluminium. The choke must be electrically connected (very effectively!) to the main tube. The ring is fastened to the main tube using 12 x M6 setscrews and a stainless steel spring (not sure if the spring is needed or not at this point in time).

The base plate is attached to the ring using 24 x M4 button head screws.

The height of the rim is 110.0 mm and the distance from baseplate to the front edge of the main tube is 132.0 mm.

Parts for choke, baseplate and the 110mm high rim.

Hardware used

A number of screws are needed to assemble the feed. I chose to use A4 stainless steel screws for every thing (except for the TX probe).

The list of screws used:

1. 4 pcs DIN 912 M3x10 socket head - For RX probe N connector mounting
2. 4 pcs DIN 912 M3x12 socket head - For TX probe 7/16 connector mounting
3. 9 pcs DIN 7991 M3x12 countersunk - Endplate to Septum 
4. 58 pcs DIN 7991 M3x10 countersunk - Septum to main tube
5. 12 pcs DIN 916 M6x10 setscrews - Choke ring to main tube
6. 24 pcs ISO 7380-1 M4x10 button head - Choke baseplate to choke ring
7. 1 pcs DIN 551 M3x16 brass screw - DIN 7/16 connector to TX probe (soldered on DIN 7/16 connector)

Measurement results

After I assembled the first feed, I did some quick testing. This was done using the nominal probes of 39mm in length. In the very near future I will repeat testing with the different probe lengths.

The picture below of the LibreVNA has S1 connected to the TX port of the feed and S2 to the RX port.
Return loss is -29.2 dB on the TX port (this is with the TX probe unscrewed 3/4 of a turn, simulates a little longer than the 39mm probe). Isolation between TX and RX port is 25.8 dB (both measured at 1296.1 MHz).

Preamp

On the feed I mounted a water tight enclosure for the preamp and isolation relay. The box has a female N connector for the RF output and a 7 pin connector for control voltages. The 7 pin connector and the N connector both connects to the 23cm PA box on my 4.8 meter dish system.

The box contains a preamp made by SM5DGX. This preamp has a measured noise figure of 0.15 dB and 33.6 dB gain when measured separately.

The system was assembled including all adapters needed, coax relay etc., basically from (and including) the RX port connector (N female) to the N female output connector on the box.

The total noise figure and gain was measured (N8973A and N4000A noise source), the resulting NF was 0.35 dB and gain 33.2 dB.

The SMA coax relay needs 28VDC for the coil, the control box in the shack delivers a constant 28VDC supply, this is then controlled using a separate 12VDC relay that switches the 28VDC to the coax relay when the system goes into RX mode.

Below is a screenshot from the noise figure meter of the complete system.