Akademisk Radioklubb


Month: October 2017 (page 1 of 2)

A quick sleeve metal dipole for 23 cm

We have been looking for a horizontally polarized omnidirectional antenna that could be used on a future 23 cm beacon. While a perfectly omnidirectional antenna would be best we are able to cope with some nulls in the antenna-pattern.  A quick and easy antenna that does this is a half-wave dipole, which is largely omnidirectional, but may have some nulls at certain elevations.

The half-wave dipole has a feed impedance of around 72 Ohms. To get even better performance the dipole can be matched to 50 Ohms with a balun. We chose to use a quarter wave impedance transformer, which is often called a sleeve or bazooka match. The sleeve also prevents unintentional radiation from the coaxial  feed cable.

The rest of this post is a tutorial on how to build the 23 cm sleeve dipole. For outdoors mounting it is made extra waterproof, this adds some additional design considerations.

Required materials:

  • 10 cm or longer coaxial cable (we use RG-223 for this guide)
  • 20 cm of 6 mm inner diameter 7 mm outer diameter copper tubing. If you use a thicker cable than RG-223 you can use a wider copper tube. This will increase the bandwidth of the antenna but lower the efficiency somewhat.
  • 20 cm adhesive lined shrink tubing 5-6 mm shrunk diameter
  • 5cm self-amalgamating tape (heat activated)

Required tools:

  • Hacksaw
  • Cable cutter
  • Knife
  • Soldering iron
  • Heat gun
  • Measuring tape or caliper
  • SWR-meter or network analyzer covering 23 cm (up to 1300 MHz, preferably higher)

Start by cutting three 55 mm long sections of copper tubing. I used 6 mm inner diameter and 7 mm outer diameter for my copper tubing in order to fit around the cable I decided to use. If you decide to make the antenna for a different frequency the cut length can be calculated roughly as (where f is the frequency in Hz):

The copper tubing is used for each leg of the dipole, as well as the quarter wavelength sleeve.

The next step is to prepare the coaxial cable. Three cuts should be made, one to remove 5 mm from the end of the jacket, where the dipole legs should be soldered. The two remaining cuts should be made at 55 mm and 60 mm, where the sleeve will be soldered to the coaxial shield.

The next step is to prepare the end of the coaxial cable for soldering to the copper tubing. If you are using a coaxial cable that is already terminated you must make sure to thread the sleeve tube and a piece of heat shrink that is 7-10 mm long over the coax before soldering the dipole legs.

Take extra care to avoid solder bridges between the left and right leg of the dipole, they should be electrically separate. When soldering the sleeve in the next step, make sure to avoid contact between either leg and the open end of the sleeve.

Next up is tuning the resonance. This is done by cutting the dipole legs until the antenna resonates at the desired frequency, for me this is 1296 MHz. One very interesting thing to note when adding heat shrink tubing on the outside of a radiator is that the resonant frequency will shift down. This is due to the increased dielectric constant of the heat shrink plastic compared to air. To mimic this behavior during measurements I place the heat shrink tubing over the copper tubing, but I don’t shrink it until I am satisfied with the resonance. Keeping in mind the dielectric effect, I tune the antenna a little bit too high to compensate for the dielectric constant, which gets a little larger when the plastic is fully shrunken.

Now all that remains is to add heat shrink to the dipole legs. In order to make it waterproof at the middle and at the ends I use a type of self-amalgamating rubber that melts into a solid mass when heat is applied.

The end result is a cheap and simple antenna that performs well. In total the antenna took about one hour to make.

Kuhne amplifiers and 1 to 10 GHz stage three

ARK is currently working on a project that will allow us to work Earth-Moon-Earth, satellites and various scattering modes on the amateur bands between 1 and 10 GHz. Our solution uses a 3 m parabolic dish together with a set of discrete amplifiers, the entire system is excited by a USRP SDR.

We have split the 1 to 10 GHz project into four sections. Up until now we have completed stage one and two, while stage three and four are still remaining. The stages are roughly:

Stage 1: Literature study, ordering of components – Further details in “An update on the 1 to 10 GHz project”

Stage 2: Construction of the parabolic dish and mast – Further details in “3m parabole dish ready”

Stage 3: PCB development and integration onto parabolic dish – Further details in this blogpost 😀

Stage 4: Long term projects with the dish, software, amplifiers, new antenna feed – Further details in the future.

This is a good time to elaborate more on our plans for stage 3 as the amplifiers that will be used for the project just arrived! We have purchased:

144/432 MHz IF to 10.5 GHz mixer – MKU 10 G4, 3 cm Transverter

To keep the work more organised we have split stage three into four sections. The sections are three PCB-design projects and one final assembly of all the components at the back of the parabolic dish.

1: Wideband driver amplifier
The Kuhne amplifiers and transverter seen in the previous section, will bring the output power to the level that is required to achieve Earth-Moon-Earth communications in the amateur radio bands. In order to be able to excite the Kuhne amplifiers and transverter from a USRP SDR with 10 mW max output power, an intermediate stage is required. The next step is to design and construct this amplified.

Very simplified schematic of 1 W driver.

We have conducted a study of available parts, and concluded that it is indeed possible to create an amplifier that will deliver 1 W across the frequency band from 0.1 GHz to 6 GHz.

In the figure above an example schematic using Guerilla RFs GRF4001 together with Analog Devices HMC637LP5 is shown. The device will deliver 1 W across 0.1 GHz to 6 GHz. This will allow exitation of all amplifiers, as well as the transverter that enables 10 GHz coverage. The gain should be on the order of 30 dB in order to avoid operating the USRP at its saturation power, where it is known to be quite noisy.

2: Wideband low noise amplifier
Another component that we want to develop ourselves is the low noise amplifier (LNA). There are not many good and cheap LNAs available for the amateur radio market, despite there being integrated circuits that boast very good performance for this application. If we are able to make an LNA and provide the design notes as open source, it will likely be beneficial for many people.

The LNA is one of the more challenging circuits. It needs to work using relatively cheap equipment while being largely immune to electromagnetic noise. A lot of work will likely be spent on making the supply-lines that power the HMC753 LNA circuit noise free, as well as ensuring that the metallic shielding is sufficiently tight. Another consideration is that the amplifier must be able to sustain relatively high input powers that will leak through the coaxial relays during transmit.

Outline of LNA.

The figure above shows a draft for a test assembly for performance testing of HMC753, which is a device that could be used in our LNA.

3: Controller board
Interfacing with the amplifiers will be handled through a controller board that communicates with either a computer or the USRP directly over the serial protocol RS232. The interface board is responsible for managing power supply states for all amplifiers as well as startup sequencing.

A set of RF relays are used to select which of the discrete amplifiers should be connected to the different points along the circuit. These are available as surplus devices on auction sites such as eBay.

Essentially, the interface board is responsible for ensuring that all connections and devices in the figure below are connected and powered correctly for a given configuration. It should also be able to alter the configuration in a rapid way.

Relays, amplifiers, transverters and SDR connection diagram.


4: Mechanical integration

After the three sections above are complete, the mechanical integration of the RF system onto the dish can start. This is an extensive effort as there are many concerns to deal with. Thermal management and waterproofing are two likely issues. So far we have an idea revolving around a gutter heater solution to keep the system from freezing during the winter. To keep the system cool enough we are experimenting with different heatsinks and weatherproof fans (IP68).

We hope to have the first three stages finished by the end of november, and the mechanical work started and delivered some time early next year. The time it takes to develop the PCBs gives us a good chance to secure the final funds that are required for the mechanical work (cabinets, fans, heatsinks). Overall we are really excited to see the project taking shape.

Before we finish all the sub-projects in stage three we might try to work some contacts using the 23cm module on our IC-9100, a MKU 131 AH 23 cm LNA, coaxial relays and the 200 W 23 cm PA we just bought. More on that in a later post.

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