Doublet multi-band antenna

For many years, I've used a lazy loop antenna for the HF-bands, and it has served me well. I've used it for working the difficult zones for WAZ. This antenna has - literally speaking - worked the world!

Now I've decided to return to an old favorite: The center-fed doublet antenna. From 1991 to 1996 this antenna helped me work many Carribbean stations on CW with 20 W output. In order to complete my QRP-DXCC award, I need QRP QSOs from that part of the world. That is the main reason for reverting to this antenna type. My present lazy-loop antenna is not strong into that part of the world.

Center-feeding a wire antenna is a big advantage, because it can be forced to work on (nearly) any frequency. Even though the antenna is resonant in the 40 meter band (7.0 - 7.1 MHz), the 80 meter band (3.5-3.8 MHz) can be covered using an antenna tuner.

The antenna dimensions are those of a 40 meter dipole:

Doublet wire antenna (resonant in the 40 m band).

I'll let you know how this antenna works.

Merry Christmas and Happy new year!

OZ1BXM Lars
Homepage: oz1bxm.dk 

Connecting fldigi to a WebSDR

This post explains how I connected Fldigi to a WebSDR on a Windows 10 computer.

Fig. 1. Goonhilly logo.

Qatar-OSCAR 100 Narrowband WebSDR is an excellent receiver! It is located at Goonhilly Earth Station in Cornwall, England. This WebSDR offers different modes, variable bandwidth, fine tuning, waterfall display, spectrum display, zoom function, and so on. Why not use this WebSDR for receiving digital signals from the QO-100 satellite and decoding them in fldigi?

Fig. 2. Fldigi desktop icon.

Fldigi is the leading application in digimode. It offers a multitude of digital protocols, from CW and RTTY over PSK31 to DominoEX, Olivia, THOR, and many more. Fldigi was written by W1HJK and is maintained by him and his team. Fldigi is a digital swiss army knife!

Step 1. Enable Stereo Mix on your Windows 10 computer. This link explains how to do it:
https://www.howtogeek.com/howto/39532/how-to-enable-stereo-mix-in-windows-7-to-record-audio/

Fig. 3. Sound window.

The Sound window is here: Control Panel > Hardware and Sound > Sound

Fig. 3 shows how Stereo Mix is enabled on my Windows 10 Home computer. A music video is playing in a browser window, and the audio moves the green bar up and down. The audio level is set to 100%.

Step 2. Select "Stereo Mix" as input source in the fldigi configuration window. The input source is called "Capture" in fldigi.  

Fig. 4. Configuration window in fldigi.

Fig. 4 shows how Stereo Mix is selected in the fldigi configuration window. The configuration window is opened by selecting Configure > Sound Card.

Fldigi can now receive audio from a webSDR or another sound source.You can test the function by playing a music video in a browser, and the fldigi waterfall will look like fig. 5!

Fig 5. Audio stream displayed in fldigi

Fig. 6. MFSK16 transmission from PA1SDB received by Fldigi.

Vy 73, OZ1BXM Lars

Webpage: oz1bxm.dk

Help - fldigi won't start

Fldigi is the leading application in digimode. It offers a multitude of digital protocols, from CW and RTTY over PSK31 to DominoEX, Olivia, THOR, and many more. Fldigi was written by W1HJK and is maintained by him and his team. Fldigi is a digital swiss army knife!

Having installed fldigi on Windows 10 Home, I executed the setup wizard and typed call, QTH, locator, and other details.

Fig. 1. Short-cut on Windows desktop.

The next day, I double-clicked the fldigi short-cut on my desktop (Fig. 1). Nothing happened. I did it again, but still no response from fldigi. PC rebooted. Nothing. Cleaned the Windows registry with Glary Utilities. Nothing. Uninstalled fldigi, rebooted the PC, and installed fldigi again. Still no action.

The solution came from WO9B in this blog post. One or more files in folder fldigi.files are corrupt. Here is what to do:

1. Rename c:\users\oz1bx\fldigi.files to c:\users\oz1bx\fldigi-old.files
2. Restart fldigi. 
3. Fldigi will discover, that fldigi.files is missing, and it creates a new one. 

Setup data must be typed again. Move important files from fldigi-old.files to fldigi.files

Vy 73, OZ1BXM
Webpage: oz1bxm.dk

Lower SWR on Helix13 from Wimo

The QO-100 satellite uplink is in the 13-cm band. I'm using a 21-turns helix antenna (Helix13 from Wimo) for uplink antenna to the NB transponder. This RHCP helix antenna works fine except for bad SWR.

Fig. 1. Helix13 antenna from Wimo (picture from 2009).

Fig. 2. Remove this alu-tube.

First I removed the alu-tube from the antenne. This tube is intended for impedance matching. It is not needed any more, since I'll use a different matching method.

An N-connector is fitted where the alu-tube was before. You must drill holes in the helix reflector for the flange of the N-connector.

Fig. 3. Dimensions of the impedance matching plate.

I made an impedance matching plate for the antenna. The plate is cut from thin copper sheet. The length is 1/4 turn and it is mounted above the helix reflector (see figure 6). This matching plate converts the helix impedance to 50 ohms.
  

Fig. 4. Purpose of the holes.

The impedance matching plate extends the helix wire. The plate is soldered to the N-connector's center pin (green hole). The red hole is used for fastening the alu wire. You can use a short bolt for this. I soldered the helix wire to the matching plate using a short brass tube. Before soldering, the brass tube was pressed with a pipe wrench to make it fit the helix alu-wire.

Fig. 5. The new N connector.

Fig. 6. The impedance matching plate.

Fig. 7. Click picture to enlarge.  

Notes to fig. 7: Distance A is 3 mm above the reflector. Distance B is 7 mm above the reflector.

Fig. 8. Measuring SWR.

The graph in fig. 8 begins at 2.0 GHz and ends at 2.7 GHz. The marker (yellow triangle) is at 2.4 GHz.

The SWR at 2.4 GHz was measured to 1.08 which is an excellent value. All SWR values between 2.0 GHz and 2.55 GHz were below 1.2.

My antenna analyzer is a hand-held instrument N2201. It covers 137.5 MHz to 2.7 GHz. The N2201 has a smaller brother: N1201. The difference is that N2201 is equipped with an additional power meter. The less expensive N1201 would be sufficient for this SWR adjustment.

Vy 73 from OZ1BXM Lars
Webpage: oz1bxm.dk

The GPSDO makes my LNB rock-stable

A consumer-type LNB for Ku-band can receive the QO-100 satellite transponder. A LNB with PLL-oscillator has an acceptable frequency stability after warm-up, but it still drifts. The reason is outdoor temperature changes and a simple oscillator using a cheap crystal.

Adding a GPSDO will greatly improve the frequency stability in the receive chain. The block diagram below shows how the GPSDO injects a clock signal into the LNB. The clock signal from the GPSDO has superior stability compared to the LNB oscillator. 

GPSDO generates a 25 MHz clock signal for the LNB.

I decided to buy a GPSDO developed by 4 danish radio amateurs. The project is named RFzero. The price of the RFzero board was less than 50% of the Leo Bodnar mini-GPSDO. Another advantage of the RFzero is the possibility of writing your own software using the Arduino IDE. The RFzero board is assembled, and only a few optional pins are soldered by the end-user.

The RFzero board.

I've mounted the RFzero board in an aluminium box. Power (5 V DC) is supplied via the USB interface. An external PSU can also power the board.

GPSDO front view.

GPSDO rear view.

GPSDO with lid removed.

Conclusion: Adding a GPSDO has greatly improved my receive chain. The Narrow-Band beacons are now rock-stable regardless of the LNB temperature. 

Vy 73 OZ1BXM Lars

My webpage: http://oz1bxm.dk/ 

Transmitting via Es'hail-2/QO-100 geostationary satellite

Transmitting via QO-100 was easier than I thought. The transmit chain is shown below.

TX chain for satellite QO-100.

A 70-cm transceiver was already in the shack. My trusty old Yaesu FT-847 is still doing well. The FT-847 output was reduced to 3 W on 432 MHz.

Yaesu FT-847 transceiver.

I needed a 13-cm transverter for the uplink. I choose to buy a ready-made one from SG Labs in Bulgaria. It was delivered assembled and tested from the factory. Time from order/payment to delivery was 6 weeks. The transverter provides 2 W output at 2400 MHz which is sufficient for making contacts on the satellite.

13-cm transverter from SG Labs.

The QO-100 uplink signal should be circularly polarized. 3 dB is lost if a linear signal is transmitted. I decided to employ my 10 year old helix antenna with 21 turns. I had to adjust the bracket so the antenna was elevated 24 degrees (this is the elevation of a geostationary satellite seen from central Denmark).

Helix antenna for 2400 MHz. Elevation is 24 degrees.

I was happy to learn that my transmit signal was visible (and audible) on the waterfall display. I had my first QSO via QO-100 on April 24th. 

The QO-100 narrow-band transponder can be monitored via this web-SDR: https://eshail.batc.org.uk/nb/

73 from OZ1BXM Lars
Homepage: http://oz1bxm.dk/

Receiving the Es'Hail-2/QO-100 geostationary satellite

Es'Hail-2 geostationary satellite (credit: Es'hailSat)

The main purpose of the new geostationary satellite Es'Hail-2 is to provide television services to the Middle East and North Africa. In addition to commercial services, Es'hail 2 includes two linear transponders for amateur radio; one with a bandwidth of 250 kHz and another one with 8 MHz. The uplink for both transponders is 2.4 GHz; and downlink is on 10.45 GHz.

Es'Hail-2 is called QO-100 in the radio amateur world (i.e. Qatar Oscar 100). Service for radio amateurs was opened on 14-February-2019. The linear narrow-band transponder has been busy since then. Two beacons indicate the upper and lower limit of the passband.

Es'Hail-2 Narrowband Linear transponder:

•  2400.050 -  2400.300 MHz Uplink
• 10489.550 - 10489.800 MHz Downlink

Es'hail-2 footprint.

The footprint of Es'Hail-2 covers half of the world. That huge pattern offers new opportunities for many hams: DX can now be worked without erecting big HF-antennas. A satellite dish on the balcony is all you need.

My RX set-up is shown below. It is simple and cheap while still providing a taste of satellite DX. The Win 10 computer runs SDR#

Receive chain at OZ1BXM

The satellite dish is Triax TD78 (offset, 70 cm wide, 78 cm high). The LNB is Maximum ST-11 (single port) with PLL and 25 MHz crystal oscillator.

Offset dish with LNB.

Bias T for 12 V DC.

RTL-SDR dongle V.3

The narrow-band transponder can be monitored via this web-SDR: https://eshail.batc.org.uk/nb/

73 from OZ1BXM Lars
Webpage: oz1bxm.dk

uBITX 10 W SSB-transceiver for 10-80 m

I've just acquired an uBITX. It is a 10 W SSB-transceiver operating from 10 meters down to 80 meters. The uBITX is made in India. The transceiver is not a kit - it is delivered as two assembled PCBs, an LCD-display, and wires.

The main board is the transceiver module and the smaller PCB contains the VFO. The user's task is to mount the circuit boards into an enclosure and fit terminals and controls.

The uBITX block diagram.

The block diagram above shows the RX-part which is a superheterodyn receiver with two IF stages at 45 MHz and 12 MHz. The TX-part is the same concept, but the transmit signal flows "backwards".

Main board and two digital PCBs mounted vertically. 

The main circuit board measures 15 cm x 14 cm and contains most of the components. There are no adjustments to make as they are set at the factory.

The VFO comprises an Arduino Nano and a programable oscillator Si5351. The oscillator frequencies are set with an encoder, and the operating frequency is displayed on an LCD. The factory delivered display is a 2 x 16 LCD; however, a larger color touch display can be added.

I found a cabinet at amateurradiokits.in. It has all holes punched and makes the transceiver look "factory made".

Support group for uBITX: groups.io/g/BITX20
Facebook group for uBITX: fb.com/groups/uBITX/

Vy 73 from OZ1BXM Lars
Homepage: oz1bxm.dk