Wednesday, 30 May 2018

In The Loop! - First Impressions of the MFJ-1788

Was feeling pretty chuffed after repairing the MFJ-1788 'Super Loop' and couldn't wait to try it out! So for a couple of evenings of experimenting I put the loop in the garden on a 5ft pole held up by a heavy drive-on stand with 20m (65ft) of RG58 running into the shack.

Temporary test setup
I chose the easy option of using FT8 to do some testing, selecting the 30m FT8 frequency initially. I tuned the loop and was met with a cacophony of signals, far louder than my usual OCFD would receive. Working with around 30 Watts had a few contacts in a brief 30 minute session, including a nice one in Greece with SV1IW.

PSKReporter showing where I was spotted
What was striking was the lack of noise and just how tight the tuning was, indeed I had to tweak the tuning a couple of times during tests, a slight adjustment either way and the signals just disappeared. I checked out 40m and 20m as well with similar good results.

I had a few more sessions and a few days later I tried it out to receive the Shortwave Radiogram broadcast from Bulgaria on 9400kHz, this time as it was a broadcast band had to use my ear to do the tuning, adjusting till I heard a rise in 'noise' and signal.
I have now got the loop up on a rotator and mounted slightly higher up with a shorter length of RG213 (not on the video) it is still quite low and unfortunately is slightly shielded to the south by the neighbours metal roofed building,



I am very happy with the loop. Transmission wise it unsurprisingly doesn't seem a huge improvement over the OCFD on its resonant bands, it scores over the OCFD is on its 'non-resonant' bands such as 30m and 17m. But the massive improvement is in receiving, signals are stronger and noise is much lower, picking up some more distance signals even given the poor conditions.
The antenna cannot be said to be a pretty thing to have in the garden! The tuning is very particular, in the video I show the 'auto tuning' isn't ideal. It requires the radio to be putting out a signal into a mismatched load for what could be nearly a minute. Not good for the radio and is a source of QRM during this time, the usual technique of tuning slightly off a QSO frequency is more problematic due to the sharpness of the resonance. You can still tune off frequency and then tweak with the slow tune buttons to bring it in. I've noticed that on some of the higher bands it occasionally doesn't auto-tune because the 'dip' seems very short/sharp and the controller doesn't react in time and overshoots especially using low power settings.



I also have had issues trying to operate the radio remotely, I have tuned it up on an FT8 frequency in the morning and then later in the day logged in to try to make a few QSOs during a coffee-break to see the loop has drifted out of resonance. This can only be down to the loop getting warm in the summer sun.

I am still evaluating the antenna but am looking at making a better controller, over on AmateurRadio.com where this blog is syndicated, I have had a number of kind comments including one from Elwood Downey, WB0OEW who pointed me to his published design of a controller, using a similar method to what I was toying with. Thanks Elwood.

73 for now, more updates soon.

The antenna farm

Thursday, 17 May 2018

In The Loop! - Repairing a MFJ-1788

Over the last few years I've seen various musings about magnetic loop antennas and their supposed efficiency and performance. I had even toyed with the idea of building one considering the eye-watering prices commercial ones are sold for! Sadly that idea had been added to the big pile of 'to-do' projects.

Earlier this year I spotted a posting on social media by a member of my club who had brought a MFJ-1788 second hand who was having issues with it not working. I had offered some advice on its repair and glibly offered to take it off his hands should he want to dispose of it. A few months later I got a message asking if I was still interested? I certainly was at the price he wanted.

I collect it at the club meeting a fortnight ago still with a 5ft pole still attached as a bonus! Thankfully it fitted in the car (just).

Loop as purchased, with pvc tape covering loop


Control Box

I downloaded the manual and schematic from the MFJ website and I saw there were conspicuous warnings about using an 'isolated' power supply both in the manual and on the back of the controller, with ominous warnings of damage if you didn't. Most shack supplies have the negative/black pin connected to the chassis/ground.

The controller had come to me supplied with a standard fused lead, you know the ones that come to connect your ATU/SWR meter lamp to the shack supply? Mmmm.. my spider sense was tingling!

I pulled out a small double-insulated 12V plug-in supply from my collection (you can spot them as they often have a plastic earth pin) and with nothing else connected powered up the controller. The meter lamp came on and some of the LEDS briefly flickered and heard a few clicks from the internal buzzer then nothing. Pressing buttons did nothing and then I caught the unmistakable scent of burning electronics!

Opening the box up on the bench quickly spotted the source, the regular had well and truly smoked, but since nothing had been connected the short must have been in the controller itself (and it was not the dodgy wires that look like they had been victims to a wayward soldering iron)


Inspecting the rest of the board it was clear that it had been repaired (badly) once before, the two main transistors/FETs (Q1/Q5) used to control the motor clearly showed signs of being replaced (misaligned and with tell-tale scorching from an hot air gun)


a SMD diode had been swapped and one of the 'fine tune' switches had been replaced, its removal had obviously been problematic taking with it some of the through hole copper and adjacent tracks which had been lifted/damaged.

 


Before opening up the box I had expected to see 'through hole' components and DIL logic ICs not for it to be all surface mounted. Undaunted I went through the parts list and decided to get replacements for all the semiconductor parts not knowing at this stage what I would have to replace. Getting two of each device in some cases five or ten of each due to minimum order quantity, the staggering cost was £8 ($11) including postage!

While waiting for the parts to arrive I decided to check out the actual antenna head. I removed the plastic covering and attempted to extract it from the pole..


First issue I had was the mounting. One bolt had seized, the nut inserts are held in quite soft aluminium so not surprisingly the insert came out out the mounting when trying to remove it. Using grips to hold the insert the bolt still refused to turn and in the end it sheared off with very little force! Obviously quality fittings these! 



The variable capacitor was suffering a common problem due to poor quality control, the fins had gone out of alignment. Each fin is a separate assembly held on a shaft with a nut to compress them together. It is a simple fix, just realigned and tightened the bolt. The limit switches and motor connection were okay (another common problem) Applying a voltage to the coax connector the motor turned fine, reversing the voltage reversed the direction of turn, the vanes going fully in and fully out before the limit switches activated.



The components arrived prompty and so I got the hot air gun out and set about repairing the board.


I started off replacing the regulator (and the damaged power track) and Q1/Q5 and we had the flickering of life. Pressing the AUTO TUNE button the LED lit and there was 9V across the antenna connector, pressing the other AUTO TUNE button had -9V across. This is why you need an isolated power supply since it reverses the voltage to control direction. The radio doesn't see any of this voltage due to the bias-t arrangement. However if the supply shared the shack ground you would short out the supply, but the current would go via all those delicate electronics! Which is what I think had happened.

It soon became clear that most devices had suffered damage and so I ended up replacing the regulator 78L12, Q1 (SN7002 FET), Q2 (MMBT3906), Q5 (MMBT3906) all the logic ICs CMOS 4001, 4011 and 4066 and the LM324 Quad op-amp chip. I also gave the board a good clean since was covered in grime and flux residue.

Once I was happy the controller was reassembled and connected to my FT-857D on minimum power and the loop antenna propped up against the side of the shack. Success! I was able to successful tune it as per the instructions on 40m and 10m, the two extremes of operation and could here the telltale rise in receive noise as it became resonant.

I have yet to put the loop up in situ for a proper evaluation but have refurbished an old TV rotator to mount it on. I have fitted some nice new quality bolts. The black insulating tape round the loop has been removed, preferring the silver look myself.

Waiting to be put on the pole

Shiny once again

Assuming it does performs and will find out this weekend, I have paid a total of £65 ($90) (£50 for the initial purchase the rest for replacement parts) which is an absolute bargain as to buy new it is expensive.. very expensive!

One of the main supplier in the UK, have it on sale for £699 ($940) but this includes their engineers inspecting and rebuilding each one before shipping! (Can only be due to poor build quality control and warranty claims) the RRP seems to be around £570 ($770)

It does amaze me how much this units costs new. While the loop construction is generally good, the mounting fittings appear to be poor and the issues with the capacitor and quality control are widely reported.

The components in the controller are not expensive, the switches, meter, box are the usual MFJ fare and the design is quite old (the PCB has copyright 1998 on the silk screen) the auto-tune process requires the radio to be putting RF into a mismatched load for up to a minute and even with low power and SWR protection this isn't good for the radio PA.

Perhaps most surprising is given the risk of damage due to incorrect use is why firstly the loop isn't supplied with a power supply anyway as they only cost a pittance and secondary why their isn't any protection built in? I intend fitting a simple 100mA fuse to offer some protection should a problem occurs.  

If the loop performs it may be another project to build a better controller, there are a few designs out there on the internet using Arduino and DDS devices to create auto-tuners. Has that pile of potential projects just grows taller?

73 for now.. and promise to post a bit more regularly 

Sunday, 27 August 2017

868MHz LoRa HAB Tracking Success

Most HAB (High Altitude Balloon) tracking in the UK involves the use of 434MHz ISM devices due to the ready availability of suitable antenna and receiver equipment due to it sharing the 70cm amateur radio bands.

868MHz ISM devices are available but are not as widely used. Back in December I posted about trying to track a flight by HAB Enthusiast Dave Akerman who is experimenting using LoRa devices on this band. I had limited success receiving that flight and another ones so when I saw that Dave was planning another flight today using 868MHz LoRa I had a rethink on how to approach tracking it.

The antenna I used was a collinear one built from cheap satellite coax, similar to that I built for ADB-S however rather than having a long coax run to the LoRa gateway I opted to put the Raspberry Pi up on the pole at the base of the antenna to limit any loss.

The antenna can be seen connected to the Raspberry Pi and LoRa add on board, the Pi has a WiFi dongle.


The antenna was inserted into a piece of conduit to keep it upright and it and the Pi were strapped to a 5m painters pole (using a plastic lid as an insulator)


A USB power pack was also strapped to the pole to keep the Pi powered, hopefully you can see it in the photo below.


The pole was put up and was about the same height as my normal dual band collinear


Well it all worked extremely well, and got a lot of decodes as can be seen by the pie-chart generated by the habitat.habhub.org system, the third best receiver only bettered by Dave himself (M0RPI) and a station nearer to the flight path.

You can see my geographical position relative to the flight below (I am the station NNE with the green line, about 160km away) and received a lot of telemetry strings even when the height of the balloon meant I was outside the 5 degree above the horizon circle (shown in green).


The Pi was connected to my network via WiFi and controlled by a Putty console on my shack PC


It was a useful experiment and I am thinking of installing a dedicated mast mounted LoRa receiver with 868 and 434MHz antennas (and possibly pre-amps and filters) following this result.

Monday, 14 August 2017

Shortwave Radiogram

In March 2013 The Voice of America (VOA) started an experiment called the VOA Radiogram which transmitted digital text and images using the powerful 50 year old analog shortwave broadcast transmitter at the Edward R. Murrow Transmitting Station near Greenville, North Carolina.

The idea was anyone could receive the radiogram on any shortwave radio, even an inexpensive portable one with no SSB capability. By feeding the audio from the radio to a computer, either by a audio lead or even using the a microphone the listener could decode the text and images using simple software.

I had seen mention and reports on social media of people receiving them but somehow never got around to trying it myself.

VOA ended the experiment back in June this year a week before the retirement of the program producer Dr. Kim Andrew Elliott.  An offer to continue the broadcasts on a contract basis was declined, so a follow-on show called Shortwave Radiogram began transmission from the WRMI Radio Miami International transmitting site in Okeechobee, Florida and Space Line in Bulgaria.

The Shortwave Radiogram transmission schedule is (at the time of writing and all times UTC)
Saturday 1600-1630 9400 kHz
Sunday 0600-0630 7730 kHz
Sunday 2030-2100 11580 kHz
Sunday 2330-2400 11580 kHz
All via WRMI except for 9400 kHz, which is via Space Line in Bulgaria.

I spotted a tweet a few weeks back (can't remember who from) mentioning the @SWRadiogram so my interest was piqued, I wasn't around for this weekends Saturday transmission in Europe but had a go on Sunday for the one from America. I wasn't expecting great things due to the levels of noise and poor conditions of late.

I have the decoding program FLdigi already installed on my computer for other datamodes and for information on how to decode the radiograms (sent using MFSK32) visit this page

On the 0600UTC transmission I managed just one poor image


 but during the Sunday 2030UTC transmission I got four decent images, the fifth was lost to QRM





I also made two short videos (on hand held mobile phone so a bit shaky) which show the incoming text and images.

Monday, 24 July 2017

ARISS 20th Anniversary SSTV

This weekend has seen another SSTV event from the International Space Station, this time in commemoration of the 20th Anniversary of ARISS (Amateur Radio on the International Space Station)

The 20 year history of ARISS was displayed through a collection of 12 images highlighting the accomplishments of the project over the last two decades.

As the ISS has orbited the world it has been transmitting the SSTV signals using FM on the usual downlink of 145.800 MHz, here at my QTH in the UK the passes have occurred late at night into the early morning, averaging 3 - 4 reasonable passes each day.

The signal has been very strong and so some excellent low noise images have been received by many people using just modest equipment. While not the greatest technical achievement in the world it nonetheless generates much needed interest in ARISS and amateur space communication.

My own system consisted of the Yaesu FT-857D and MMSSTV running on the shack PC and was left on automatic receive (I was tucked up in bed) and managed to get decent copies of all the images.

Image 8 reminded me of the fun I had back in 2011-2012 of receiving the ARRISAT-1 and was one of the key things that convinced me to finally get off my backside and actually get licensed, even if it took me another 12 months and to this day haven't really cracked satellites myself! My previous blog posts on that can be found at http://nerdsville.blogspot.co.uk/search/label/arissat-1

Here are the best of my images, for a full description of what each one depicts visit http://ariss-sstv.blogspot.co.uk/2017/07/anniversary-image-descriptions.html















Monday, 24 April 2017

First SOTA activation - Normanby Top G/TW-005

We ventured out to the Lincolnshire Wolds again on Saturday and spent most of the morning taking in a fabulous little 5 mile circular walk from the village of Wold Newton with the wife finding over twenty geocaches along the way.


After last weekends disappointment I had purchased a new Baofeng UV-82L dual band for the princely sum of £27 from eBay to replace the suspect UV-5R. It certainly seems more sensitive when receiving and the transmit audio is loud and clear.

With the 'flower-pot' antenna in the rucksack I spent the walk monitoring the local repeaters (and could hear the regular idents) as well as the calling channels. I was putting out the odd call (with little success) when we stopped to allow the wife to search for the geocaches.

One man and his dogs
However I wasn't disheartened as the plan was to spend the afternoon on Normanby Top, the highest point on the Lincolnshire Wolds trying to activate the summit for SOTA.

I had been spurned on to have a go at an activation after watching a video of  Michael Sansom's (G0POT) presentation on the SOTA scheme for the Chertsey Radio Club. The video is available online here and Michael has written an excellent companion introduction to SOTA on his website http://peanutpower.co.uk/sota

View across to radar station on Normanby Top
After finishing the walk we drove the short distance stopping to collect more geocaches along the way and as we neared Normanby Top we stumbled across Thoresway Motoparc where they were busy practising their motorbike scrambling skills and I spent a while taking some action photographs from the side of the road.

Saw some spectacular jumping skills
But I digress, Normanby Top, as I've said is the highest point on the Lincolnshire Wolds and the actual summit has an Ordnance Survey triangulation pillar as indicated on the map but it on private land. The SOTA rules allow someone to activate a summit by being within 25m vertically of the high point, this means the activation area for Normanby Top is potentially huge as it is more of a plateau than a steeply defined hill.

Section of the 1:25000 Ordnance Survey Map

To the South of the summit is a road with an air-traffic control radar station. We parked up near this impressive structure to have some very late lunch (time had flown by)

Radar Station on Normanby Top
I could have activated from this road as others have done but I decided to use the bridleway to the North which leads off the Viking Way long distance footpath. The wife dropped me off at Acre House Farm and she went off to hunt for yet more geocaches and I set off down the track to find a suitable place to set up. I was a little hampered by the proximity of a power line which followed the bridleway but as luck would have it just as I had finished setting up at my chosen spot the landowner came down the farm track in his Land Rover.

I had a friendly chat with him and he was fully aware of what I was doing and had no issues having met other activators in the past. He also gave me permission to walk across his land up to the trig-point if I wanted to. As I've mentioned it isn't on a public right of way but said he is more than happy for people to go to the trig-point providing they ask at the farm before hand and take home all their rubbish, unfortunately he has some recent problems with inconsiderate litterers.

I decided to decamped and following his instructions to a gap in the fence and set off to the trig-point which was a little bit further away and a little steeper than I thought, but it didn't help I was carrying a half made up antenna, poles and a bag full of kit and my not inconsiderate bulk! Anyway I got there but due to us running already behind schedule and my change of location it was getting late in the day and was nearly 5pm.

Trig-point and radar station behind

I had taken my Yaesu FT-857D, two 7Ahr SLA batteries, fibreglass pole, antennas for 20m/40m HF, 2m SSB and 2m/70cm FM. Yes I'd travelled light! Having some lovely clear views all around I decided to try some 2m 'line of sight' contacts.

I put up the Sandpiper Delta Beam for SSB (horizontal) and my 'flower-pot' dual band for FM (vertical) on a fishing pole. The wind had started to pick up and I'd not got any proper bungee cords (doh!) so I lashed it up using guy ropes around the pillar.

It was straighter than this before I went to take photo
The radio was perched on top of the pillar and I put some calls on 2m SSB with no luck and switched to FM with more success. I was using between 5-10W and got nothing exceptional distance wise but racked up 8 very nice clear QSOs to claim an activation. The wife texted me to say she had returned from her searching and was parked up patiently at the farm awaiting my return, not hurrying me by any means. It was now nearly 6pm and the wind was getting up and the temperature was starting to drop. It had been a long day for us and the dogs so I decided to packed up and went back down the hill to the bridleway admiring the view across to the Humber and the Humber crossing.

View North (Humber to the top left)

I really enjoyed myself but wished I'd allowed myself more time, this was in part because it was a late decision in the week to tack the 'summit' on the end of a day out walking and changing my mind to actually go to the summit given the chance but it was nice to do that for my first SOTA but I really need to get the hang of travelling light.

Back down and met by the wife who wanted her shopping bag back
After my last post (which is also syndicated to AmateurRadio.com) I had some questions about Geocaching - Dave, WD8CIV left this useful comment which I shall repeat as summed it succinctly - thanks Dave.

"Geocaching is like a high tech Easter egg hunt. People hide caches – waterproof containers containing a logbook and maybe some little trinkets – and then post the GPS coordinates to a Web site like geocaching.com. Others will search the site for caches in an area of interest, then use their own GPS (and intuition and woodcraft skills) to try to find them. Anyone who finds a cache signs the logbook, may take one of the trinkets and/or leave a new one, then goes back to the Web site and reports that they found that cache. There’s quite a community of geocachers, and it’s been going on for over a decade."