Simple Inexpensive 1296

The 1296 MHz band is the lowest frequency amateur microwave band. Maybe this is a good first amateur microwave band? How? There are several options.

Handheld FM transceivers are available. A useful gain antenna is small. This has been popular recently for SOTA in several areas.

All-mode (FM, SSB, CW) transceivers are also available.

Transverters and transverter kits are available from several sources including W1GHZ. Transverters do require an IF radio and IF radio interfacing. They are not difficult and can provide great performance. The W1GHZ 1296 transverter would be quite fun barefoot, and performance add-ons (LNA, PA, T/R switch, antenna) can be added to reach high end performance.

HackRF is an interesting SDR transceiver front end that covers 1296. Clones of this open hardware design are quite inexpensive. (price check ~ $100 even spring 2022) A filter is required to cleanup transmit and will help the RX in the presence of strong out-of-band signals. Again, performance add-ons can be added.

A simple and inexpensive approach

N6IZW and I started exploring additional simple and inexpensive approaches for the higher bands. So, we tried an RTL-SDR RX paired with an ADF4350 transmitter.

Refer to the parts page for more info.

RX

An RTL-SDR does require appropriate compute HW and SDR SW. Simply plug and play.

Parts list:

• RTL-SDR (basic or improved)
• PC or laptop
• antenna or piece of wire
• USB extension cable is nice to have

Testing

Start out with the simplest antenna you have, even if it is just a 1/4 wave wire stuck in a coax jack. (~ 5.8 cm or 2.25 in)

If you have a local beacon, try tuning it in. Our beacon in San Diego is so loud, that a whip antenna can easily pick it up inside a building 20 miles away.

If you don't have a beacon, then setup an ADF4350 module to generate a weak test signal on 1296. (See below.) That will work even if you don't add an antenna or filter to it. (Separate TX and RX versus a transceiver is useful for testing.)

Our local San Diego beacon is GPS locked, so it can be used to correct for tuning dial error. I have also used a local broadcast TV channel ATSC pilot tone to correct for dial error. The new ~ 1 ppm RTL-SDRs make it easy to find a signal. With the non-upgraded RTL-SDRs, you should do an initial correction. Thereafter, it should be close, but there may be some temperature drift. Calibrating a tuning dial and using a tuning dial are two basic radio skills.

SDR offers performance, but it also offers latency. This can be a problem for using the RX for CW sidetone. Or it can be a problem when trying to peak up a signal with a directional antenna.

With gqrx, counter-intuitively, I found that the lower HW sampling rate settings increased latency. Setup poorly, latency was ~ 1.6 s, which makes it difficult to peak up an antenna bearing. Setup well, it was reduced to ~ 0.3 s which is OK for antenna peaking. TODO: see what can be done, perhaps GNU Radio?

Upgrades

A bandpass filter may be helpful in some situations to avoid problems with out-of-band signals.

Upgrading the antenna from a whip directly increases the sensitivity of the receiver and the range you can cover.

TX

We've found the inexpensive ADF4350 synthesizer modules great for transverter LOs. The synthesizer has a register controlled output buffer which can be used for OOK (on-off keying). OOK means we can do CW/Morse. TODO: try Hellschreiber. Perhaps there are some interesting FSK modes or something with coding gain, like QRSS, that could be explored.

Parts list:

• ADF4350 module
• Arduino Uno clone with 3.3 V I/O
• Arduino Uno shield breadboard
• SMA 50 ohm terminator (for unused output)
• SMA f-f adapter
• RCA jack

The ADF4350 can be used with a number of microcontrollers. We continued with our Arduino clone stack and added some features. (Almost any microcontroller should work.)

Make sure the Uno clone jumper is set to 3.3 V I/O!

The Uno, shield, and ADF4350 module was assembled as we did for transverter LO work. The Uno provides power to the ADF4350 module. These notes were used for a SDMG transverter group build. Only the following on the ADF4350 is wired up to the Arduino:

| ADF4350 module | Arduino |
+----------------+---------+
| GND            | GND     |
| VCC            | +5V     |
| LE             | D10     |
| CLK            | D11     |
| DATA           | D12     |

My adf4350-examples repository holds the FW (called a sketch in the Arduino world) we used for transverter work. A new example used that starting point and adds this simple transmitter capability.

Register values were modified to get 1296.1 MHz.

A key jack input was added on D5. Unos traditionally have an LED built in on D13, so I added a visual keying indicator.

We realized sidetone output could be useful for two reasons. The first is that SDR RX latency can be too much for sidetone use. (I have found that I can tolerate some latency, but there is a limit.) The other reason is that if there is some dial error on the transmitters, we end up working split and so can't use the RX for sidetone, anyway.

TODO: find out if SDR RX SW can implement a mute input

The ADF4350 buffer, besides off and on, also has -3, -6, and -9 dB settings. I experimented and found that using them to do crude waveform shaping reduced the sideband content in a useful way. When the microwave bands become crowded enough to worry about key clicks, we'll recommend leaving the synthesizer running and do the OOK with good shaping with an external buffer. And celebrate.

The ADF4350 is harmonic rich, so output filtering should be applied when an antenna is added. We added a Mini-Circuits LFCN-1525+ low pass filter module.

Testing

Try out the transmitter on the bench before adding an antenna or even filter.

On powerup, the FW configures the PLL. If the register settings are configured to drive a lock LED, ours are, you should see the lock LED turn on within a moment.

The key input can be verified with the visual indicator LED or sidetone output as well as observing RF output.

Even with the key up, the output buffer off state has limited isolation, so it may be possible to hear the backwave with a nearby RX. Fortunately, the backwave is down far more than enough for pleasant listening. TODO: add measurement

Key the transmitter and see the full output power. Our samples, with the low pass filter installed, measure +5 dBm output per the data sheet.

Pair with the antenna of your choice.

Upgrades

Other useful functionality could be implemented with the microcontroller. Some ideas:

• PTT output for external amplifier
• T/R relay control with appropriate sequencing
• beacon features
• QRSS, Hellshreiber, or FSK modes
• iambic keying

External amplifiers are an obvious upgrade both for higher conducted output power as well as for cleaner OOK waveform shaping.

Antennas can provide useful gain for EIRP for increased range. We have used a Vivaldi PCB antenna as well as a WA5VJB cheap yagi.

The reference XO included with the ADF4350 modules does not have great initial accuracy nor temperature stability. It is good enough to make two way QSOs if both sides are patient. (At least on 1296. 10 GHz, not so much.)

TODO: try styrofoam over included XO

It may be possible to use a temperature sensor and a heating element (resistor) controlled by the Arduino to ovenize the included crystal. It won't be as good as a crystal specified for OCXO use. TODO: try and find out

There are decent TCXOs available for not too much money that will do better. N6IZW and I have started to try some out. TODO: add suggested TCXOs

The ADF4350 board can be modified to use an external reference. It is common on higher microwave bands to use OCXO modules (which work well even at 10 GHz), GPSDOs, or even Rubidium sources. To modify for external LO, there is a 3 pad resistor selection jumper footprint. The board in the photo below had a solder bridge instead of a discrete zero ohm resistor.

If using an external reference, it is a good idea to also cut power to the on board XO. Lift the series power inductor shown in the photo below (but keep, perhaps soldered to one pad, to restore later if desired).

N6IZW added a microphone and microphone amplifier module to his ADF4350 module coupling the audio signal into the PLL feedback loop's external loop filter. This produced great sounding narrowband FM which the RTL-SDR can also receive. This opens the door to all sorts of soundcard modes. TODO: add details TODO: make a FM repeater QSO

Separates and transceiving

Separate TX and RX is useful for testing and measurement. Transceiver integration makes for pleasant operating or contesting efficiency. Start with separates for testing capability if you have no other test equipment. Start with separates for simplicity. The simple inexpensive setup above has independent tuning for TX and RX, and so, even with integration, will continue to have some user interface characteristics of separates.

TODO: measure absolute amplitude with RTL-SDR?

TODO: measure antenna with return loss bridge

Operating separates

Use independent TX and RX antennas. Isolation will be enough for the simple setup. If directional, that is two antennas to point.

TODO: explore compactly packaged stack of separate TX and RX antennas

It can be easier to find each other, even if frequencies aren't netted, if one side can put up a carrier while the other looks for it in direction and frequency. Trade directions. This is a great way to get that first QSO done with the most simple setup.

Make things easier by adding better frequency references or otherwise netting your frequencies on the bench, before you go to the field.

T/R switching

Manual T/R switching would allow for a single antenna to be shared. With a big antenna, this can be a useful reduction in the gear carried. Is there a simple and inexpensive manual T/R switch available?

Automatic T/R switching provides more convenience. There are a number of options which we are exploring. Sequencing can become important.

TODO: add notes describing options

TODO: does T/R switching allow for sharing of a common, useful for both TX and RX, bandpass filter?

Automatic T/R switching allows for LNA and/or PA use with a common antenna if upgrading performance. That can certainly be done with this gear, but it may also be time to consider a transverter based setup.