1296MHz Activities

A description of my equipment for Earth-Moon-Earth communications at the higher frequencies of 1296MHz.

1296MHz EME Update

Over the last 6 months I have been building up my 1296MHz equipment. My plans have changed somewhat from my initial ideas, not only as equipment and funds have become available but also as my knowledge and understanding of the requirements of this particular band has grown.
Here is my equipment for 1296MHz, as it currently stands.

Block Diagram


1296MHz EME Station Concept Diagram.

The use of separate transmit and receive coaxial lines is to allow extra gain blocks and filtering to be added as required, and also to ease the relay isolation requirements. The first RX preamp will be mounted at the feed (to minimise system noise figure). The directional coupler is used to pick off some signal to send to a Fun Cube Dongle SDR, to allow processing of the full 90KHz segment of the 23cm band, whist the main line signal is demodulated by the conventional receiver in the TS2000X radio.
The rest of this page details the construction of some of the components needed to realise the EME system.

Antenna and Feed

I started out with a home-made 1.8m dish, but have been able to obtain a second-hand TVRO dish (2.3m diameter).
It's a little weather beaten, but still quite serviceable.

2.3m TVRO dish

Dish prior to mounting. You can see my old 1.8m dish in the background.

Dish tracking

A test of the automatic tracking mechanics. Note the shadow of the feed cage should be cast on the exact centre of the dish.

Feed Alignment

Aligning the feed is critical for good performance. Note the LNA in a sealed box, mounted as close as possible to the RX antenna probe.


I encourage anyone who is contemplating a parabolic reflector antenna to have a look at the excellent work done by Paul Wade, W1GHz. His on-line antenna book (see Useful Links) proved to be an invaluable resource for me.
I decided to build the OK1DFC septum feed, as it would be the easiest, quickest and cheapest for me to build.

OK1DFC parts

All parts are made from 1.6mm thick Aluminium sheet (scrap from another job).

Septum Attachment

My version of the backing plate is made from 2 sections of folded Aluminium sheets.

Backing Plate

This shows my the backing plate bolted to the septum. Another trip to the hardware store is needed to buy more M3 nuts and bolts.

CAD View

CAD cut-away view.

Backing Plate

More of the Backing Plate.

Exploded CAD View

Here's how it all fits together.

Front View

The view looking into the feed. It never looks quite as good as the CAD model.

Measured S11 & S22

Measured Return Loss of both ports.

Measured S21

Isolation between ports.


Performance measurements

The best way to measure receive performance is to define an overall "figure of merit" for the system. In that way, any improvements made can be easily compared to a baseline. The Gain over Temperature (G/T) is such a measurement. It is a relatively easy measurement to make provided some obvious caveats are observed. The SETI League has an excellent website which details the procedure (Useful Links).
It starts with a simple sun noise vs cold sky measurement (usually called Sun-Y, in the literature). This Y figure, together with the interpolated quiet sun flux measurement of the day (Useful Links) is used to compute an overall G/T measurement. This represents an overall system performance metric. It is possible to delve further into system performance. By measuring (or calculating) antenna gain, receiver noise figure, LNA noise figure it is possible to estimate System temperature, dish spillover, or dish efficiency.

My initial measurements are summarised below. They are made using the excellent Linrad software running from the TS2000 radio's Intermediate Frequency through a Funcube Dongle SDR. Some effort has been made to verify the measurement system is linear so that the calculations are therefore valid.

Parameter Measurements
Sun (dBm)-90.2-90.4-90.9
Cold Sky (dBm)-98.7-98.8-98.8
Sun Y (dB)8.5dB8.48.0
SFU (10^-22W/m^2/Hz)585849.3
G/T (dB)8.38.28.4

Given a dish diameter of 2.3m and an estimated feed efficiency of η=0.6, I calculate a gain of 28.3dBi. This yields an overall system temperature Te of 107K. This is in the ballpark, but not yet optimised. Clearly there are some improvements to be made in my system.

System Improvements

The first step to improve performance will be to try adding a kumar choke to the feed, so as to improve dish illumination and minimise spillover.

Simple Kumar Choke

Made from some scrap aluminium, as per Paul Wade's W1GHZ dimensions.

Simulation output

3-D EM simulation of the E-field around the feed.

Simulated Directivity

Radiation pattern of the feed. See text for further discussion.


The directivity plot above shows RHCP patterns for 4 principle cuts (Φ 0°,45°,90° and 135°) in blue. The bold red plot shows the opposite polarization LHCP, for Φ45°. Ideally this should be very small, but due to the nature of the feed, it is significant.

There is some debate as to whether adding a choke to such a small dish ultimately defeats the purpose: an improvement in spillover comes at the cost of increased dish blockage. The easiest way to confirm this is to build one and measure the improvement (if any) in G/T.

Parameter Measurements
Sun (dBm)-90.8-90.7-90.7-90.5-90.6
Cold Sky (dBm)-99.0-99.2-99.3-99.3-99.3
Sun Y (dB)8.38.58.68.88.7
SFU (10^-22W/m^2/Hz)49.349.349.35151
G/T (dB)8.89.09.19.29.1

From the above, the average G/T measurement is around 9.1dB/K, which corresponds to a Te of 91K.
So adding the choke has reduced my system Temperature by around 15K.

Further improvements in the feed must focus on reducing the cross polarisation ratio, which is significant in this type of feed.
This is due to the fact that the square septum feed is.... well, square. The lack of rotational symmetry in the Φ axis will always be a limiting factor on both the circularity and polarisation ratio of the feed.
A circular feed should improve matters, and a design by SM6FHZ (Useful Links) promises just that.
I am still in the process of building this feed, but the results of my simulation look promising:

SM6FHZ Feed

Made from SM6FHZ dimensions, using scrap Aluminium sheet.

Simulation output

3-D EM simulation of the E-field around the improved feed.

Simulated Directivity

Radiation pattern of the feed. Note the improved polarisation ratio.

Power Amplifier

I next pondered what to do for a power amplifier for 1296MHz. Again, looking at some of the prices being asked for some commercial units, I decided to see if there was anything that I could build that would give me some reasonable power.
Eventually I came across some obsolete devices for sale on-line (MRF9060) for not too many dollars. The datasheet claims 950MHz max operating frequency, but as these FETs are not internally matched I felt it worth the gamble to see if I could produce some power at 1296MHz.
And it looks like I'm not covering new ground here: John G4BAO has an excellent article detailing the design of a 23cm amp using the similar MRF9045 (see Useful Links). This gave me some confidence (possibly misplaced) to forge ahead.
Of course, I would be using FR4 PCB instead of the nicer (and more expensive) Taconic PCB material. And I have no idea about the '9060 impedances at 1296MHz.
But that's part of the fun, really.
So, some reasonable guesses from the datasheet, and a few minutes with APPCAD and a smithchart (Useful Links) led me to a circuit design as seen below.

Initial testing was not so encouraging. But some time with a scalpel and some copper shim enabled me tune tune the input Return Loss to better than -15dB, and an output power of around 42W (Gain around 13dB). Psat around 50W.
Clearly not as much as I would expect from the device, but given my somewhat slipshod approach, I am not overly displeased with the result.
A box around the PCB and heatsink, together with a fan and bias control circuit complete this amp.

When I get around to it, I'll re-calculate the device impedances based on the amount of copper shim I added to the prototype PCB, and I may even consider another PCB manufacture run.

PA Innards

The prototype PA build.

The Front Panel

External bias switching is used, with a front panel indicator.

Output

40W out for around 1.8W input.

Useful Links


News

1296MHz Activities
26/06/16 Antenna Improvements.
Moon Tracker Ver 2
23/06/16 A new version.
1296MHz Activities
29/09/15 Build a small dish.
LNA
26/07/15 Low-noise Preamplifier Design.
1296MHz Activities
19/04/15 Power Amplifier Design.
1296MHz Activities
13/01/15 Design and build a tower for 3m parabolic dish.
Thunderbolt
30/09/14 LCD monitor for my Thunderbolt GPSDO.
PANFI
02/08/14 A home-made Noise Figure Meter.
Moon Tracker Update
09/04/14 New rotator and better position feedback sensors added.
1296MHz Antenna
09/02/14 Begin construction of a 1296MHz feed horn for the 3m dish.
1296MHz Activities
28/01/14 After success at 144MHz, operation at 1296MHz is planned.
Moon Tracker Update
11/09/13 Added a Background Sky Temperature calculation into the Moon Tracker.
Moon Tracker Update
28/08/13 Just Added some software limits to rotation of motors.
Roger Beep installed
09/08/13 Added a Roger Beep to my radio.
Andy's Website
06/05/13 Start build of website.

Contact

Andy, VK3ANX
Yarra Valley, Australia
Andy