All posts by M1GEO

1kW 144MHz Amp Lives!

Those of you who have been following my 144 MHz 1kW amplifier project (previous posts machining heatsinks, soldering transistor down and building the pallet) will, I’m sure, be delighted to hear that I have had life out of the amplifier. In excess of 1 kW, I hasten to add!

The amplifier was able to maintain in excess of 1000W for over 2 minutes.  At this point, the Bird dummy-load started to get a bit warm, so a longer test was abandoned. The amplifier pallet, however, remained cool enough to touch. As the F1JRD original design notes, the 10-Ohm coax balun does become hot (Lionel suggests around 120C at 1kW with no cooling). I, however, used a small fan running slowly to provide a gentle draft which greatly reduced the balun heat.

The next step is to add the Dallas-Maxim DS18B20 temperature sensor – the idea is to have the sensor buried into the pallet next to the transistor, to measure the copper heat spreader temperature.

BA5SBA RTL-SDR Kit

A few weeks ago I ordered a BA5SBA RTL-SDR direct sampling kit from BangGood (link here). When it arrived, I decided to put it together. The kit includes everything needed, an RTL-SDR dongle, case, PCB, enamelled wire and so on. I worked from numerous build instructions (here, here, here and here), following the clearest description of each stage.

I disassembled the original RTL-SDR dongle, removing the USB plug, IR remote receiver and Belling-Lee socket. This was easy to do. I then soldered the module into the main PCB. The SMT components were easy to solder on. I added the few remaining passives, some larger electrolytic capacitors, etc.

Two wires tack on to various voltage points to add extra smoothing, which were easy enough to connect – I used some medium thickness tinned copper wire, I guess around 0.7mm diameter. That did the trick.

Winding the two inductors was done blindly. I followed instructions to wind 8 turns around a 5mm drill; however, somewhere else said 6-9 turns around 3mm. I noticed after soldering in the coils that 300nH was the suggested inductance. In the future I will remake the coils to the correct value.

Winding the small transformer, T1, was relatively straight forward. I wound 8 turns around the ferrite core. Although I’m not entirely sure my core was ferrite. It was indistinguishable from a 2mm plastic washer. My kit had blue-red-yellow trifiliar wire in, so I followed the colour scheme in the 3rd instruction link above (page 11).

The chip has two pairs of I-Q inputs, pins 1, 2, 4 and 5. The first pair, pins 1 and 2 are connected to the E4000 front end, which mixes the higher frequency signals of VHF and UHF down to an intermediate frequency (IF). The second pair are also used in this kit to take the HF bands (on the Realtek RTL2832U, 0-24 MHz) as a second IF input. A “direct sampling” mode can be selected in the PC software to select this second input, but, there is no default wiring as this has no use inside a TV tuner dongle. By far the hardest part of this build was the soldering of hair-sized wires to the Realtek RTL2832U chip, which then go to the transformer, T1.

After a considerable struggle, these two wires were solder onto the chip. I wish I could offer some useful tips on how I did this, but I cannot – I simply struggled, and faffed around until I made the connections. I would suggest a mobile phone camera placed above the board may help, since you can use the digital zoom to see in some detail. The image above was taken as I was soldering.

Finally, I used some glue to hold the (very) fragile wires in position and soldered the other ends to the transformer. I also added a small amount of glue to the transformer, too, so as to stop it moving. It looks messy, I know, but hopefully it will add some security and stability to those otherwise poor solder connections to the Realtek chip.

My final build looked like this:

Amazingly it also works! The image at the top shows the device inside the supplied box! Excellent!

Nagoya NA-771 Antenna X-Ray

Some weeks ago I brought a Nagoya NA-771 antenna from BangGood. I had previously brought such an antenna before, but noticed this was slightly different in design. Upon use, I noticed this antenna was inferior in terms of both transmitting and receiving performance. When compared to the standard flexible whip antenna supplied with the radio (Icom ID-51E+), the Nagoya antenna performed significantly poorer. Using a local digital voice repeater as a test end-point, it possible to explore the transmitter bit error rate (BER) and the receive signal strength indication (RSSI) meter reading. Both results indicated the Icom antenna was better (with a TX BER of 3.5%) compared to the Nagoya antenna (with a TX BER of 5.9%). The RSSI meter confirmed those results on reception, too.

I decided to give the antenna to David Mills G7UVW to X-ray the antenna base, and give an insight to what’s inside! This post just has a few pictures.

Below left you see a cross section of the antenna, with a double spiralled steel spring whip, inserted into a metal pin, which in turn links to the centre pin of the SMA antenna connector. Right is the physical appearance of the antenna.

Below, a different rendering of the data.

Thanks to Dr David Mills (G7UVW) for the X-Rays. Dave has lots of X-Ray fun on his Twitter feed, @DTL.

The total antenna height is 386mm, base to tip, of which 325mm is the whip section. Below shows the antenna dimensions entered into simulation environment. The left column shows the radiation pattern in both the horizontal and vertical planes, and the right column shows the VSWR plot over a large range, centred on the band. Finally, a 207 MHz row is included since this is where the antenna appears to be resonant.

Band Radiation Pattern VSWR Plot
145 MHz
435 MHz
207 MHz

Some time after this was originally published, I was contacted by Jeff WB7AHT who explained that he had found similar issue with cheap antennas, and that he was able to carefully trim the whip length, resulting in a VSWR of below 1.4:1 on both bands. Jeff also notes that old speedometer cable makes an ideal whip since it is brass wire strands wrapped around a steel core.

160m WSPR

Over the night of 14-15 April 2017, I decided to run WSPR on the 160 metre doublet at GB0SNB. The transceiver was the Icom IC7100 with a transmit power of 3W.

The map of both transmitted and received signals was reasonably interesting:

We received 18 unique spots and was spotted by 47 unique stations. Our best DX was into the United States.

Dave M0TAZ also operated on 40 metres over night.

1kW 144MHz Amp Pallet

Those of you who have been following this project evolve will have seen how I soldered the transistor to the heat-spreader and before that how I machined the heat-spreader & heat-sink after their initial use. Most recently, I have been building the new W6PQL pallet, based on the revision 4d schematic, found here.

This pallet offered several design changes compared to the original F1JRD design. The first is temperature tracked biasing for the FET. The F1JRD pallet didn’t have temperature tracking, but the W6PQL design uses a combination of 10kOhm and 22 kOhm NTC thermistors to track the temperature change of the pallet. A 6V Zener diode is used to clamp the bias supply and to also limit the maximum gate voltage the FET can see. A small 200 Ohm pot allows the bias to be adjusted to get the correct quiescent current. This is the next task.

The story continues with the initial power-up testing! First I need to commission my new General-Electric 50V/40A PSU I brought at the Rosmalen Hamfest back in early March.

BNOS LPM144-10-100 Repair

I have had a B.N.O.S LPM144-10-100 solid state linear amplifier for some time. It brought it at a ham fest and it worked fine. However, when I tried to use it recently, I noticed that sometimes the amplifier would work, but other times there was no output. Due to the intermittent nature of the fault, I knew it couldn’t be the main power transistor (MRF247). The most likely cause was one of the 3 relays. There were also 5 electrolytic capacitors. I decided to change all 8 parts.

The first thing I did was cross correlate what I had with the circuit diagram (click for full size image/download).

The PCB 

Using a desoldering station to melt and vacuum extract the solder, the 3 relays are easily removed from the PCB with no board damage.

Closeup of the 3 removed relays:

Comparing the original PCB photo with the one below, you can see the 3 replacement relays and 5 capacitors.

I used a Finder 12V miniature DPDT 8A relay as my replacements sourced from Rapid Electronics in the UK, but these relays are universally available from different manufacturers. You will need a DPDT relay with 12V coil (not SPDT as this article previously stated [thanks to Keith GM4YXI for spotting this issue]). My current suggestion would be the TE Connectivity / Schrack RT424012 (datasheet).

Below is the amplifier working! Yay!

Soldering Expensive Transistors

This morning, Royal Mail delivered me a parcel from Jim W6PQL all the way from California, USA. It took a couple of days to clear customs, but it arrived within about 5 days of being ordered. If you followed my previous post on this subject, about machining heatsinks, you’ll know that the last transistor I had failed on the testbench. You’ll also know that the copper heat-spreader was re-machined to suit the new PCB. This is why the heat-spreader has a few extra holes. Seeking advice from veteran microwave DXers & constructor (G4BAO, G4DDK, G8KBV, et al.) I was instructed to solder the device down. I watched a few of Jim W6PQL’s videos on soldering LDMOS parts to the copper heat-spreaders and replicated his instruction as closely as possible. You can see Jim’s instruction video here.

A small length of thin leaded 60/40 solder was made into a wiggle for the length of the transistor and placed in the groove previously machined in the head-spreader. I liberally applied flux to the bottom of the groove and the underside of the transistor and then sandwiched  the solder in between.

The copper heat-spreader was placed on the electric infrared hotplate and heat applied. The black dot is used to allow a laser thermometer to monitor the copper temperature. NB: this method didn’t work well.

The next two images show the solder has melted and the excess squidged out the sides. It’s clear to see when the solder has melted, since the the transistor drops. It is advised to move/slide the transistor in the molten solder to remove any voids and any excess solder. I immediately killed the heat and removed the spreader from the hotplate and placed it on a heatsink. It only took a couple of minutes to cool to a temperature I could handle, and I checked the location of the transistor against the PCB mounting holes.

The PCBs were finally mounted as a test fit. I will populate the boards before mounting them. Unlike the original jrd1 boards, these PCBs do not need to be soldered down. This means the boards can be soldered up and then mounted.

Stay tuned for more updates…

MMDVM Efficiency & GB7KH Usage

As many of you know, I run the GB7KH multimode digital voice repeater on the north east side of London. The project is based around the MMDVM project. When installing the RF hardware, I ensured that the repeater had a calibrated RSSI (received signal strength indicator) output, which I did using a communications test-set. When the MMDVM host program is running, it also output’s an average BER (bit error rate) at the end of each reception. Since the repeater had been operational with the RSSI output for some time, I decided plot some graphs showing how the RSSI effects BER. These graphs appear below:

I also decided to look at the average transmission length on the repeater. Most of the transmissions are for less than 1 second. This is typical of “kerchunking” (to key up the repeater to see if it is there and the user is in range). Speaking with other repeater keepers, it seems this behavior is quite common.-

Some simple Perl scripts running on the repeater controller are able to provide user information. These homebrew scrips consider use since new year’s day 2017.

  • D-STAR: A total of 2115 transmissions were made by 38 callsigns.
  • DMR: A total of 525 transmissions were made by 23 callsigns.
  • YSF: A total of 1294 transmissions were made by 14 callsigns.
  • P25: A total of 0 transmissions were made by 0 callsigns.

Earth Fault on Yaesu G-5400B

I brought a Yaesu G-5400B azimuth and elevation rotator & controller system from a friend at a local radio club about 6 months ago. I brought the rotator as faulty. When I powered the rotator up on the bench, I couldn’t find any fault. I built a PC-Rotator controller interface similar to the Yaesu GS-232 interface to accompany the G-5400B controller, and while doing extensive testing, no fault with the rotator became apparent.

This weekend, following the acquisition of some fibreglass poles at the Rosmalen hamfest, I decided to set up my bayed 144 MHz beams with the azimuth/elevation rotator. After mounting the antennas on the beam, fixing the phasing harness and the mast-head preamp and connecting the cabling, I noticed that the rotator was no-longer working correctly. Although both of the rotators would turn, the azimuth display on the control box failed as soon as the coax was connected to the radio (or more specifically, the coax screen connected to anything in the shack that was earthed).

Using a multimeter to inspect what was going on, it was clear that the coax ground was sinking current sent to the potentiometer inside the azimuth rotator. Looking at the schematic, the cause would appear to be that the +6V side of the feedback potentiometer was somehow becoming shorted by the connection of the coax screen.

I decided to pop the cover and see what was going on

From inspection, you can see that the original hypothesis was correct and that one side of the potentiometer was shorting to the casting – the brown wire had been caught between the plate visible and mounting point. Since the antenna metal is grounded via the coax, this effectively shorted out via the broken insulation on the brown wire.

The repair was the simple process of snipping the broken wire, and soldering a new one in. I also used two tiny cable ties to bundle the wires to the potentiometer and to ensure they were kept away from the mounting hole, too.

The rotator goes back together easily assuming you have followed the usual advice when dismantling these rotators; marking the case and internal gear such that it can be reassembled with the same aligning.

After finishing the reassembly of the the G-5400 rotator, being sure to grease the bearings, I was ready to mount the antennas and try again.

This time around, the rotator functioned perfectly. The total repair took around an hour. Now I need to finish the PC interface to make use of the fancy graphics LCD!

Machining Heatsinks for QRO Amplifiers

Back at the 2012 Friedrichshafen Hamfest I brought a 1.25 kiloWatt VHF amplifier kit for 144 MHz from F1JRD and F5CYS. These devices were fairly new at the time. It took me a year to pluck up the courage to build the pallet, but I went about it all wrong. With the help of Dad and the kitchen hob, we soldered the jrd1 Teflon PCB to the C110 copper heat-spreader as suggested in the Dubus article (see here). I had the pallet working at the time, giving around 600W of RF, which was about the maximum my 1000W 50V PSU was capable of sustaining. When I came to boxing the device up into an amplifier to use with EME and Meteor Scatter in late 2016, the part failed under test.

After much deliberation, I have ordered parts to repair the amplifier project. I found Jim W6PQL‘s website (see here) a wealth of information, and Jim also offers to supply parts and designs to help others. I ordered a set of PCBs to replace the original jrd1 board, a NXP/Ampleon BLF188XR 1400W part to replace the failed the Freescale/NXP MRFE6VP61K25H 1250W part, and some other accessories that Jim sells. The parts were posted by Jim today, so I decided it was time to recover parts from the old PCB and recondition the heatsink and heat-spreader.

The first step was to remove the jrd1 board from the copper heat-spreader. I used the kitchen hob to heat the copper heat-spreader, since the old board was soldered to the copper block. The board damage was sustained to enable the removal of the more expensive components.

Below, the heat-spreader with the jrd1 board removed. I used a solder sucker and scraper to remove as much of the molten solder.

Once the heat-spreader had cooled down, I mounted the copper spreader up in the milling machine read to re-machine the top and bottom surfaces. Great care was taken to level the block using parallels. Below you can see the fly-cutting process on the first cut, removing just 0.05mm from the surface.

With the top and bottom of the head-spreader machined flat, a small end-mill cutter was used to machine the transistor slot to the correct depth following the skimming of the top surface. Then the heatsink mating surface was machined. Below you see the first cut on the heatsink.

The finished parts. A few machining marks, but the surfaces are perfectly good enough. Some dents on the copper block, but it’s not worth removing all of the material to eliminate these.  Using a few drips of water as a substitute for thermal compound, the two mating surfaces stick together very well (with a good vacuum forming). That’s more than good enough for my needs!

Now I just need to wait for the parts to arrive before I can finalise the PCB and transistor mounting! This story continues here: Soldering Expensive Transistors.