Simple Inexpensive 10 GHz

The 10 GHz band is a popular amateur microwave band. (Is it the most popular above the 1296 MHz band?) This may be due to the history of available surplus parts. Sizeable antenna gain is likely also a factor. It is a busy band during the ARRL 10 GHz and Up contest that takes place over one weekend in August and another weekend in September.

There are several options to get on this band.

There are assembled and kit transverters from several sources including W1GHZ. Transverters do require an IF radio and IF radio interfacing. They are not difficult to use and can provide great performance. The W1GHZ transverter is quite fun barefoot, and performance add-ons (LNA, PA, T/R switch, antenna) can be added to reach high end performance. It is not a difficult build, but it does have a number of steps.

Gunn diode oscillator modules were popular for wideband FM years ago partially due to the inexpensive surplus units that were available. (These were used for motion detection to open doors, etc.) These are still available today new and for bands 10 times as high in frequency, but they are a bit pricey. There may still be surplus sources available. And perhaps an old-timer in your area has an old WBFM rig and can get on the air with you.

The HB100 type DRO doppler radar modules are inexpensive purchased new. These can be tuned from their stock 10.525 GHz frequency down into the amateur radio band with the tuning slug hidden under the QA sticker. For the price they are fun. DRO TX to DRO RX can do some distance. With a high performance receiver, they can do miles. See the Walter Clark DROplexor article where he uses these in place of the old Gunn diode oscillator modules for wideband FM.

N6IZW has explored a bit using a HackRF (maximum frequency of 6 GHz) with some external bits to get onto 10 GHz. It can certainly generate 10 GHz if it drives a frequency doubler for example.

A simple and inexpensive approach

N6IZW and I started exploring simple and inexpensive approaches for the higher bands.

Refer to the parts page for more info.

RX

The approach described here has been used for quite some time. N6IZW for example recorded the loud (!) DL0SHF EME (earth-moon-earth) beacon a number of years ago. (He did use a small dish antenna to get enough gain.)

Parts list:

• RX as described on the 1296 page except for the 1296 antenna
  • RTL-SDR (basic or improved)
  • PC or laptop or other compute device
  • USB extension cable (not mandatory, but convenient)
• satellite TV LNB
• bias tee
• CATV coax (75 ohms, F connector plugs)
• F plug to SMA plug adapter - use anything; for example:
  • F plug to BNC jack adapter
  • BNC plug to SMA jack adapter
  • SMA plug to SMA plug adapter
• F plug to DC power adapter - use anything; for example:
  • F plug to BNC jack adapter
  • BNC plug to banana jack adapter
• 12 V power source (power supply or battery)

Assembly

The satellite TV LNB is nifty. It is a receive converter with a built in antenna and LNA. It's LO is 9.75 GHz, so 10.368 GHz is converted down to 618 MHz which the RTL-SDR RX can receive.

The integrated antenna has directionality that is reminiscent of a flashlight. It is also linearly polarized. With a linearly polarized source, you'll easily be able to figure out which is the right way to orient the antenna.

We've used the inexpensive Avenger LNB. (TODO: price check, was about $10)

And we've used the Bullseye LNB. (TODO: price check, was about $30) It is more expensive, but it has less dial frequency error. (TODO: look at how it compares in frequency stability)

The LNB needs power applied through its IF output jack. This is done with a bias tee. Yes, the RTL-SDR has an internal bias tee that can supply some power. However, that is suitable only for some LNAs and not suitable for the LNB.

A bias tee is an RF through connection. It has an extra DC power port that is connected to one RF port (through an RF choke) and blocked from the other RF port by a capacitor. An ohmmeter is useful to check a bias tee.

Because the LNB is designed for using CATV coax, it has an F connector jack. This makes using a bias tee with F connectors convenient, as an inexpensive CATV cable can be used to connect the LNB to the bias tee.

Adapters are needed to get from the bias tee to the RTL-SDR. Because this bias tee has an F connector for the DC power port, adapters are convenient to get from F to something you can hook up power with.

An F plug to BNC jack adapter is useful.

So is a BNC plug to banana jack adapter.

Here's a bias tee connected to an RTL-SDR with adapters. It is ready for connecting an LNB and LNB power.

Here it is hooked up with an Avenger LNB.

Here it is hooked up with a Bullseye LNB.

Note: the Bullseye LNB has two connectors. Use the green one. (The red one has a reference frequency output which we are not using. There may be some interesting applications for it, though.)

Testing

On 1296 MHz, the RTL-SDR LNA gain should be turned up to maximum or near maximum. That is not the case when the LNB is installed! Turn the LNA gain down to minimum or near minimum.

Connect and turn on 12 V to power the LNB.

Your SDR RX software may be used to tune in 10368.1 MHz (the SSB/CW calling frequency) by tuning to 618.1 MHz. Your SDR RX software may have an option (gqrx does) to enter the LO frequency of a downconverter. If it does, enter 9.75 GHz. Then, you can tune directly to 10368.1 MHz in the software (which will correctly tune the RTL-SDR to 618.1 MHz.)

If you have a test signal, see if you can tune it in. A DRO module works. An HT tuned to 144.0 MHz works, either by tiny harmonic output itself, or by creating harmonics in the LNB. (144 x 72 = 10368) Or run an ADF4350 module at 3456 MHz and listen for its 3rd harmonic.

Test the sensitivity. The LNB has a good NF (noise figure) and a lot of gain to ensure it establishes the overall NF for the RX. This can be used to see the difference in the thermal radiation (the temperature!) of the sky (brrr cold!) and the ground (toasty, around 300 K). This is just like those IR thermal cameras. Point the LNB at the sky and look at the noise floor with your SDR RX software. Now point it at the ground or a wall or anything at room temperature. Look at the noise floor again. It should have gone up noticeably. Satellite system antennas are designed to try to not see things on the ground, because that would raise the noise floor.

If there is a beacon in your area, try to hear it. With just an LNB, I went to a high spot to see how many of the 4 SoCal beacons on 10 GHz I could hear. I easily heard the San Diego beacon on 10368.360 MHz at about 20 miles and the Palos Verdes beacon on 10368.300 MHz at about 85 miles.

TX

For the transmitter, N6IZW and I originally planned to use the ADF4350 to drive W1GHZ's 10 GHz personal beacon. This should work quite well. W1GHZ reports that keying the last two stages gave good results.

N6IZW recalled that the ADF4350 has fairly good harmonic content. We programmed the ADF4350 for 10368.1 / 3 MHz. He measured a unit as having about -11 dBm output at 10 GHz. I measured one myself at about -19 dBm. That's not loud, but it is simple! It's loud enough to be heard at some distance and lots (!) of antenna gain is easily had at 10 GHz.

Parts list:

• TX as described on the 1296 page except for the 1296 low pass filter
  • ADF4350 module
  • Arduino Uno clone with 3.3 V I/O
  • Arduino Uno shield breadboard
  • SMA 50 ohm terminator (for unused output)
  • RCA jack
• coax to waveguide transition

Assemble same as the 1296 TX except for the 1296 low pass filter.

We want to transmit the -10 dBm 10 GHz harmonic but not the +5 dBm 3 GHz fundamental. A high pass or band pass filter could be used. This is doable as a DIY project. But wait...

An open ended waveguide works quite nicely as an antenna with about 5 or 6 dBi of antenna gain. So, a coax-to-waveguide transition works well as a simple antenna.

As a bonus, an open ended waveguide antenna will suppress the 3 GHz fundamental. WA5VJB's 2 to 11 GHz log periodic PCB antenna would not; it would require a separate filter.

TODO: how much would WA5VJB's 10-25 GHz or 5-10 GHz Vivaldi PCB antenna suppress 3 GHz?

Coax-to-waveguide transitions are not cheap even on the used market. (Best bet is to check with your local microwave community.) However, they can be quite easy to make.

Simple coax-to-waveguide transition

This is the simplest we know of so far. It requires two parts:

• 3/4 inch copper pipe cap
• panel mount SMA teflon probe or SMA panel mount with a soldered on wire probe

A hole is drilled in the side of the pipe cap. The SMA panel mount with probe is inserted into the hole and brazed to the pipecap. Adequate performance should be possible with rough dimensions.

TODO: insert dimensions for reproduction

Best is if you (perhaps with help from your local microwave community) can measure return loss to trim this for the best performance. Alternatively, use your 10 GHz RX and trim for maximum signal.

TODO: insert pictures of construction and finished transition

Two papers from W1GHZ provide great notes to help with this:

• Wade, "Rectanglular Waveguide to Coax Transition Design", QEX, NOV/DEC 2006, pp. 10-17.
• Wade, "Understanding Circular Waveguide -- Experimentally", QEX, JAN/FEB 2001, pp. 37-48.

A similar but more complicated transition built by a WB6TFC is shown here. It was designed to fit a specific repurposed circular waveguide dish feed hence the mounting flange. [WB6TFC and KK6AOX, Proceedings of Microwave Update, 2015, pp. 127-128, WB6TFC and KK6AOX] Shown here until details added on the simpler version.

Testing

Testing can start as soon as the ADF4350 assembly is done and the microcontroller is programmed. At this point, even with no antenna, with a sensitive receiver nearby you should be able to hear some output on 10 GHz. The ADF4350's built in module's lack of accuracy and stability is 10 times as bad on 10 GHz, so you may have to look a little more to find it.

With the open ended waveguide antenna installed, the signal should be detectable from much farther away. If you think of the probe as a 1/4 wave vertical, the polarization is obvious. For fun, try measuring how much cross polarization rejection you can get.

Upgrades

See the 1296 page for some ideas. N6IZW's FM upgrade works on 10 GHz, too.

At this point, the biggest impediment to increasing range is the frequency accuracy and stability. Even a decent TCXO would be a helpful improvement.

At 10 GHz with transverter systems, OCXOs, GPSDOs, and Rubidium references are all popular. The OCXO is the simplest and is more than adequate for SSB and CW operation. Even a decent TCXO would be helpful. Check with the microwave community. N6IZW has found used ones to be available for reasonable prices.

TODO: add OCXO source information

Antenna gain is generous at 10 GHz. A sheet metal horn isn't big but can easily provide about 20 dB of gain. (price check: N6IZW says brass sheet is expensive)

Dishes provide lots of gain, even in the still easy to wrangle 1 to 3 foot range. An 18 inch dish provides 30 dBi. A 3 foot dish provides 6 dB more. It is a bit more mechanical to deal with however.

Lens work at microwave. Rubber balls or glass lenses work. How about a 3D printed lens design with integrated mounting bracket?

We are thinking about this and will share details about our future antenna experiments.