Time for a new aerial, I’ve replaced my doublet with an OCFD

Earlier this year I decided it was time to replace my doublet with a new wire aerial.  There was nothing actually wrong with the doublet, I’ve worked plenty on it but it always galled me that I had a smarttuner connected to it which limited my transmit power.  I could only run 100W SSB or 30-40W using any of the data modes.  Apart from anything else, it had been there since 2013 so it really was time for a change.

I decided to hang either a dipole for 40m or an OCFD with 40m as the main band and make sure it was capable of taking full legal power on both SSB and data.  That meant having it resonant and with a good enough balun to handle the power.

Logically this meant that the feed point would need to be supported because a decent balun with that sort of power handling would be too heavy and cause the feed point to sag too far if it was only supported at the ends.  I also wanted to try and utilise any existing potential aerial supports in the garden and after some measuring, pacing and head scratching, I decided to go with an OCFD.

An OCFD is an off centre fed dipole where as the name suggests, the feed point is moved from being right at the centre to approximately one third of the way from one end.  Rather then present a 50R impedance to the wireless. this configuration presents around 200R.  This is transformed to the more normal 50R by the use of a 4:1 balun.

I appreciate that an OCFD is a bit of a compromise aerial and that I when I made one before, it was only there for a month before I replaced it but I wanted to try again.

My search for a suitable balun led me again to Balun Designs.  I used one of their products before when I replaced the balun on my MA-5B so I knew that they’re good.  After exchanging emails with Bob, KZ5R, I ordered a 4115ocf.

Unfortunately the package got held up in customs so not only was it slow to arrive but I got stung for import duty, VAT and the ridiculous ‘handling’ fee which is added on top of the other charges.  This turned out to be an expensive balun.

It looks good though (all the pictures in this post can be clicked for much larger versions).

Balun Designs Model 4115ocf 4:1 balun

Balun Designs Model 4115ocf 4:1 balun

It looks even better with the cover off.

Inside of the Balun Designs 4:1 Model 4115ocf balun

Inside of the Balun Designs 4:1 Model 4115ocf balun

While this was on the way, I sorted out everything else – I got myself a 20ft pole and started looking at different types of coax. It took me a while to decide what cable to buy as I’d seen a tweet mentioning a company called Messi & Paoloni who I’d never heard of before.  I found their online catalogue and spent a long time poring over the huge range of cables before ordering 25m of Airborne 10.  The claimed loss is really rather low at less than 0.3dB for 25m on 7MHz with a power handing of over 6.5kW on that band.  At 28MHz the loss for the length I’d be using is just under 0.5dB with a power handling ability of nearly 4kW.  Of course, these power levels are far more than I’m ever going to use or would want to put anywhere near a PL-259.  It’s good to over spec everything.  This is also one of the lightest high power/low loss cables they sell which was another good reason for choosing it.

Unfortunately, when the cable arrived, it wasn’t quite what I expected.  It uses an unusual material for the outer jacket, it’s not very flexible like RG213, it’s somewhat plasticy and feels like the old H100 (if anyone remembers that).  Once bent into place, it stays there.  It would work but I knew I’d have to be very careful when routing it in the shack.

When I started getting everything ready in the garden, I discovered a big problem.  I’d planned to mount the 20ft pole on an existing pole in the garden which supports a washing line but somehow, despite all my careful measuring and pacing, I realised that wouldn’t work.  One aerial end was far too long for the end supports I’d planned and the other was too short.

Luckily, we had another washing line support pole and this was perfectly located but it meant my feed point was now much further down the garden that I had originally planned.  No real problem except the coax I’d bought wasn’t long enough.  I decided that rather than buy a complete new length, I’d extend the existing run and so I bought 10m of Messi & Paoloni Ultraflex 7.  I only needed to use 5m and so I made five good patch leads with what was left over.  The power handling isn’t anywhere near as good as the Airborne 10 and it’s lossier but I didn’t really see either as a problem.  On the plus side, this is nicely flexible and works well in the shack.

With these problems overcome, I set about getting the aerial up.  I mounted the balun to the top of the pole and also managed to extend the feed point support pole so it that it was 25ft up in the air.  It also helps that this washing pole is right by a tree so my support goes almost through it.  My two legs measured 12.14m and 8.28m and I didn’t change these at all once the aerial went up.

Feed point of my OCFD - Around 25ft AGL

Feed point of my OCFD – Around 25ft AGL

You can see from this the following two pictures that the feed point was absolutely perfect here, the leg lengths are spot on for my two end points.

Far support pole holding my OCFD

Far support pole holding my OCFD

The end of the OCFD on my main mast

The end of the OCFD on my main mast

The important question to ask after all this, is does it work?

The main band I built it for is 40m and since putting it up, I’ve worked all over Europe including the Netherlands, Germany, Finland, Sweden, Italy. Hungary, Serbia, Slovenia, Romania, Russia, Ukraine, France, Poland, Portugal, Spain, Czech Republic, Denmark and many others.  I’ve also worked Gabon, the USA, Iceland, Argentina, the Canary Islands, Canada and Brazil using it.

I think the answer to that question is “yes, it works”!

I ran 0.001W (1mW) of WSPR up the OCFD on 40m for 24 hours when conditions were absolutely dreadful and was received by the following stations.

1mW of WSPR over a 24 hour period through my OCFD on 40m

1mW of WSPR over a 24 hour period through my OCFD on 40m

Posted in Amateur radio, Construction, HF | Tagged , | Leave a comment

SCC RTTY Championship 2017

It’s been some considerable time since I put in any serious contest entry and I’ve been looking for the chance to do it for a few weeks.  I sorted out my data problems a while ago and have been happily able to operate RTTY, PSK, etc ever since.  I’ve been looking for a data contest with a section for a 15m only entry.

I’ve said before that 15m (21MHz) is my favourite band and I like the fact that I can work a contest until the band closes in the evening and then pick it up the following morning.

This weekend was the SCC RTTY Championship which is a contest that I’ve never entered although I did give away a few points back in 2011.

Bearing in mind the solar conditions recently, I expected it to be slow but I didn’t expect things to be as slow as they were.  Conditions were absolutely dreadful with just 73 QSOs made on Saturday.  I had the reverse beacon open to see where I was being received and there were very few spots being reported.  I also had the WSPR map available so I would see when any WSPR signals started crossing the Atlantic and I’d know roughly when to start beaming that way.  There were no WSPR spots whatsoever between the UK and the USA on 15m and although I did try pointing the aerial that way for a while, there were no RBN spots of my callsign either. I worked EA8PT in the Canary Islands, left my beam pointing that way for a while and was lucky enough to make a QSO with CV7S in Uruguay but that’s the only DX QSO worth reporting.

Sunday morning was livelier but still nothing to write home about with no unusual QSOs, just lots of European stations calling.  I operate pretty much exclusively by calling CQ during data contests and settled into a nice routine where the rate was slow, so I could do other stuff as well.  In the end I finished with 190 QSOs in the log of which six were duplicates.  One station called me four times but I always work duplicates in case they’ve made a logging error and got my callsign wrong – By working dupes, I can at least hope that they’ve got my call correct on one of the instances.

This contest uses “four-digit number of the year of operator’s first ever official amateur radio licence” as the exchange and the multiplier and I received them from as early as 1947 (a club station) up to 2017.  Out of a range of a possible 70 multipliers, I received 57 different years back.  I think that’s quite a decent result.

Top 10 countries worked by G6NHU in the SCC RTTY Championship 2017

Top 10 countries worked by G6NHU in the SCC RTTY Championship 2017

As this contest is run by the Slovenia Contest Club, it’s not surprising to see that Slovenia is right near the top of the list of countries I worked (click the chart for a less blurred version).

You can see other interesting graphs, charts and a full and very detailed log analysis by clicking here.

On the whole, this was an enjoyable contest and a good chance for me to get my hand back in.  It was slow, very slow and conditions were (to be blunt) bloody awful.  It was fun to operate it though and good to see that my station still works, even considering how bad things were, RF wise.

Posted in Amateur radio, Contesting, Data, HF, RTTY | Tagged , , | Leave a comment


This is one of the very few non amateur radio related items on this site.  I’m posting it because it’s a useful tip and I hope that it will assist fellow Mac users.

I noticed about three years ago that a particular cache folder on my iMac was getting larger and larger.  I did some research on this and finally was able to track down where it comes from and what’s causing it.  It only affects people who are using WhatsApp on an iPhone with backup to iCloud enabled.

WhatsApp uses a cache folder in your library whenever it does a backup to iCloud and it doesn’t tidy up properly after itself.  When I bought a MacBook late last year, I forgot to do anything about this cache and when I remembered and checked, the folder had grown to over 50Gb!  It’s just wasted space and it’s perfectly OK to delete it.

Your library folder is hidden by default but you can easily check it by opening Finder, clicking on Go in the menu bar and then holding down the option (alt) key.  This will cause Library to show in the list below, click on it and then work your way into Caches, then com.apple.bird, then session and finally g.

For the last year or so, I’ve run some AppleScript to tidy this folder up on a regular basis and I’ve experienced no problems whatsoever doing it.  Everything still works properly, I just have a folder which doesn’t grow every time WhatsApp does a backup.   After I took delivery of a new iMac a few weeks ago, I decided to clean this cache in a different way (partly because I couldn’t remember how to get it working with launchd).

I’ve written a short script which I run via cron.

To do this is very easy.  I’ll show you how to create a scripts folder, change to that folder, write a simple script and then how to add a cron job to run that script on a regular basis.  I’m sure there are other ways to do this but this method is simple and it works.

Be careful when you do this.  You’re going to be using a Unix command which can be very nasty if you get it wrong.  Providing you check and double check everything, you should be OK.  I will take no responsibility for trashed systems if you make a mistake.  Always have a backup before you do this sort of thing.

Open a terminal and type the following commands:

cd documents
mkdir scripts
cd scripts
sudo nano delete-cache.sh

Your Mac will now offer you a “Password:” prompt. Type the password for your account and you’ll be presented with a blank window like this:

Nano editor window

Nano editor window

Copy the following text and paste it in the editor window, making sure you change “your_home_folder” to be the name of your home folder.

cd /users/your_home_folder/library/Caches/com.apple.bird/session
rm -rf g

Mine looks like this:

My script to delete the unwanted com apple bird session g folder

My script to delete the unwanted com apple bird session g folder

Press control-x then y and finally hit the enter key to save the file.

Now type the following:

sudo chmod +x delete-cache.sh

This is to make the script executable. You may be asked for your password again.

Finally, you need to edit cron to execute this script on a regular basis.  My iMac is never switched off so I run it once a day in the early hours of the morning.  If there’s a time you know your computer is likely to be switched on then you can change the time.  It doesn’t matter if it gets missed sometimes.  I like to run it daily just to keep that cache empty.  If you’re not sure when your Mac will be switched on, you could even run it hourly with no problems.  I’ve been deleting this cache on a daily basis for years with no ill effects.

sudo crontab -e

You may be asked for your password again, if so, then enter it.

This time, you won’t be using the nano editor, you’ll be using an editor that can be very powerful but also quite confusing so I’ll be precise.

Press ESC and then the I key.  At the bottom of the screen you should see the word –INSERT–

Type or paste the following, again replacing “your_home_folder” with the name of your home folder as before.

10 2 * * * /users/your_home_folder/documents/scripts/delete-cache.sh

This will run at 02:10 every day.  If you want it to run at a different time then change the first two numbers.  The first number is the minutes past the hour and the second number is the hour.  So if you want it to run at 09:30 every day then the first two numbers will be 30 9

When you have done this press ESC again so that –INSERT– disappears and then hold down the SHIFT key and press Z twice, so SHIFT ZZ

That’s it.  You’ve created a script, edited it, made it executable and configured your Mac to run that script on a regular basis.

If you own more than one Mac and you have your documents folder synced to iCloud then you only need to add the line into cron on the other computers as the script will be shared between them (assuming your user account is the same name on all of them).

Posted in Not amateur radio | Tagged | Leave a comment

A detailed look at three different methods of operating the low power WSPR mode

There seems to have been an upsurge in people operating WSPR recently.  Barely a day goes by (sometimes barely an hour) without someone posting a map of where their signals have been received on Twitter.  When I first started experimenting with WSPR and QRSS some years ago, I did exactly the same.

I’m quite sure that this increased interest in WSPR is largely down to a product which was released in late 2016, the “WSPRlite Antenna Performance Analysis System” from SOTAbeams.

Over the years, I’ve used three different methods of transmitting WSPR, I’m going to describe all of them, starting with the aforementioned WSPRlite.

SOTAbeams WSPRlite

The WSPRlite is a very simple to use unit, measuring roughly 55x50x17mm (excluding screws and connectors), all you so is plug a micro USB cable in one end, an aerial in the other and push the single button on top to start the transmission.  It has a built in real-time clock which will keep it running accurately for up to 30 days without any further intervention.  At the end of that thirty days, you have to push the button again. The WSPRlite is configured by a simple piece of software which will work on any version of Windows from XP onwards (XP isn’t officially supported but it works).  A version for OS X is available from the WSPRlite Facebook group.

WSPRlite configuration utility

WSPRlite configuration utility

Here you can see the WSPRlite configuration utility running on Mac OS X.  My WSPRlite is configured on 20m, running 50mW with a 20% chance of transmitting and configured to run for up to 30 days without any further intervention from me.  Output power can be configured at 5mW, 10mW, 20mW, 50mW, 100mW or 200mW and each unit is calibrated to be +/- 0.3dB of the selected level.  The software will automatically pick a random frequency on the band you’ve chosen.  It has a built in low pass filter which is good for 14MHz but if you’re going to operate on any band below that, it is strongly recommended you add an additional filter.  You can’t operate on any band above 20m using the WSPRlite.

The WSPRlite itself is a nicely produced unit which looks surprisingly good.

SOTAbeams WSPRlite

SOTAbeams WSPRlite

It’s small, it’s light and it’s easy to handle.  It’s very portable so you can quite easily take it out and about with you.  Just remember that if your locator square changes, you’ll need a computer to reconfigure it.  Not many amateur radio aerials are fitted with an SMA connector though so you’ll need an adaptor to plug your aerial in.

Of course, I couldn’t have one of these in my hands without wanting to have a look inside.  With no idea whether or not it would invalidate my warranty, I opened it up.  The nuts on the bottom are all nyloc style to prevent them from rattling their way off so they were tight to open.  I’ve opened mine so you don’t have to!

SOTAbeams WSPRlite with the cover off showing the circuit board and construction

SOTAbeams WSPRlite with the cover off showing the circuit board and construction

What you’ve got here is a very professional unit.  The construction quality is excellent.  You can click the image above for a full sized version.

Hans Summers Ultimate 3S

The next unit I use for transmitting WSPR has been mentioned in this blog before – The Hans Summers Ultimate 3S (U3S).  This is “the latest edition of the third version in the “Ultimate” QRSS/WSPR kit trilogy.

This is supplied in kit form and therefore requires construction.  As far as kits goes, it’s not complicated and the basic unit can be built in an afternoon.   When you order it, you specify one low pass filter for the band you want to transmit on.  Additional filters are available and there is also an option for an additional relay board so you can program the unit to switch bands and aerials automatically.

Whereas the WSPRlite requires a computer to configure it, the U3S is completely controlled via a fairly complicated menu system.  It has a backlit LCD screen and all the options are configured using a pair of buttons to step through and select/configure.  Once you’ve done it a couple of times, it becomes second nature.

For frequency stability and clock accuracy, the U3S has a GPS input so once it’s set, you can leave it running forever.  It will run through a calibration cycle at regular intervals to keep everything accurate.  With the OCXO version, frequency control is incredibly accurate and Hans sells a couple of different versions of this kit, one as a VFO/Signal generator and one as a programmable crystal replacement.  The GPS also provides location details so if you want to use this portable, there is no reconfiguration to do, it will detect your new location automatically.

The U3S will do a lot more than just WSPR.  It supports many other low power, low signal modes and I encourage anyone who has one of these to look into QRSS and consider configuring their transmitter to run QRSS as well as WSPR.

Output power on the U3S isn’t configurable via the menu but it is possible to set up a separate power supply to the output stage and configure the power that way.  This is what I’ve done, I have two voltage regulators in the case, one supplies 5v to the U3S and the other powers the output stage with a slightly higher voltage.

Hans Summers Ultimate 3S transmitter

Hans Summers Ultimate 3S transmitter

Another view of the Hans Summers Ultimate 3S

Another view of the Hans Summers Ultimate 3S

Here you can see one of my Ultimate 3S built into a custom perspex box complete with relay switch board.  I have a couple of these transmitters, one has been continually running on 10m since early April.

The U3S will transmit on all amateur HF bands and higher.

WSJT-X software and your own transceiver

The final method of transmitting WSPR is using your existing computer and transceiver, it’s the WSJT-X package from Joe Taylor, K1JT and is currently on version 1.8.0-rc1.  This software handles multiple different modes including JT65, JT9 and the new FT8 as well as WSPR.

You connect your radio to your computer using whatever method you like and if you employ full CAT control, you can configure the software to automatically switch bands at different times of the day.  For example you could switch to the lower bands overnight when you expect there to be more DX and the higher bands during the day when 80m and 40m are closed to any DX.  This is a very nice feature, not currently implemented on either of the other devices (although I understand something similar will be added to the U3S).

The software is very easy to use and as long as you leave your computer on, you can have WSPR pickling away in the background all the time.  One thing to be aware of is to ensure that the time is configured very accurately on your computer.  If it’s more than a couple of seconds out, nobody will decode your WSPR transmissions and you won’t receive anyone.

Output power is controlled by your transceiver.  Many modern transmitters won’t go below 5W which is really far too much for WSPR but that’s easily resolved either by winding down the output audio from the software or injecting an ALC voltage into the back of the wireless to limit the output power.  See your transceiver operating instructions for details on how to do this.

Out of all three methods listed here, running software on your computer is the only way to receive WSPR as well as transmit.  Without stations receiving WSPR transmissions, the mode would be utterly useless.

A few minutes monitoring WSPR on 20m using WSJT-X 1.8.0-rc1

A few minutes monitoring WSPR on 20m using WSJT-X 1.8.0-rc1

You can see here (click the image for a full size version) that even though conditions aren’t great, I’m starting to receive some north American stations as well as Europeans.

Reasons to run WSPR

People say that they run WSPR to see if the bands are open.  I think this is a bit of a false reason really with a few exceptions.  Most amateur radio stations are capable of transmitting a signal which under the right conditions, will go around the world.  By its very nature, WSPR creates those conditions.  WSPR is carefully crafted to be received when signals are very weak, down to nearly 30dB below the noise level and there are receivers dotted around the world specifically looking for those really weak signals.  Stick a few hundred milliwatts up an aerial running WSPR and someone, somewhere is going to hear it.  Increase that power to the base level of a modern transceiver (5W) and it would be very disappointing if that transmission didn’t travel many thousands of miles at some point during the day.

Just because a WSPR signal can be received in VK land on 20m, it doesn’t mean that you can fire up your wireless, slap 100W up the same aerial and work all the Bruces!

However there are some exceptions to this.  A good example is 10m and Sporadic E.  I mentioned earlier that I’ve had one of my U3S transmitters running on 10m since early April.  Most of the time I get very few WSPR spots but as soon as Es starts appearing on 10m, that number of spots increases massively.  When that happens, it’s a really good indication that the band is opening and I’ve seen for myself time and time again that shortly after the spots went up, stations started appearing on 10m SSB.

Another reason for WSPR is for “aerial testing”.  My comments above apply to this as well.  WSPR by nature is designed to receive weak signals.  You could hang a bit of metaphorical damp string out the window and throw WSPR up it – It will be received somewhere so this doesn’t really prove anything.

Again though, there are exceptions to this.  If you have two aerials and two WSPR transmitters running at the same time then you can look at the signals received for the two transmissions and see how the aerials compare against each other.  This is a very good use for WSPR, one which I sadly haven’t used yet because although I have multiple WSPR transmitters, they’re not all running the same power so the results would be skewed.

Another good reason to run WSPR is that it’s actually quite a lot of fun!  Transmitting low power and seeing it be received around the world.  It gives you a nice warm and tingly feeling inside to see your 200mW on 40m being received in ZL.  What’s not to enjoy about that?

If the above reads as though I’m knocking WSPR and don’t appreciate it, it’s not meant that way. I’ve tried to be factual as much as possible.  I really enjoy transmitting WSPR and seeing the dots appear on the map.

A reason not to run WSPR

It’s not a QSO mode.  Don’t set up a WSPR transmitter, see via the internet that your signal has been received in an exotic country and write it in your log book as a QSO.  Worse, don’t post a QSL card to the station who heard you.  It’s not a QSO, don’t treat it as such.
Yes, I’ve had QSL cards in the post for WSPR spots.  They’ve all been ripped up and chucked in the bin.

What’s best, WSPRlite, Ultimate 3S or WSJT-X?

None of them are ‘best’, they all serve different purposes.

If you want a basic self contained unit you can unbox and put on air with practically no effort, get a WSPRlite.

If you want a unit you can build, experiment with, play with different modes, expand, adjust and learn, get a U3S.

If you want to be able to transmit and receive WSPR, band hop, work other data modes etc, use WSJT-X.

I use all three methods on a regular basis.  If I want to run a quick WSPR session, I’ll fire up the WSPRlite, if I want to squirt some QRSS, Slow Hell, JT9, CW in a ten minute frame, I’ll use my U3S.  If I fancy receiving some WSPR as well as transmitting it, I’ll kick the software into life.

Where are these all available from and how much do they cost?

The WSPRlite is available from SOTAbeams and sells at £59.95

The Hans Summers Ultimate 3S is sold by QRP Labs.  The base transmitter with one low pass filter including shipping to the UK is $38 (just under £30 at time of writing).  There are many options available such as the relay board, case, additional filters, etc.  They also sell a deluxe six band version which comes with many of the options for around £110 (depending on exchange rate).

WSJT-X is a free download, you can get it from here.

This is not a sponsored review, I have written it independently with no input from SOTAbeams, QRP Labs or K1JT.  I have paid full retail price for all the units mentioned in this piece, nothing has been supplied by the manufacturers.
Posted in Amateur radio, Construction, Data, HF, WSPR | Tagged , , , , | 7 Comments

A new challenge – DXCC on 17m using FT8

It’s been a while since I set myself a challenge and so it’s time for a new one.

Despite my initial misgivings, I’ve decided that I do actually quite enjoy operating FT8.

I’m attempting to work DXCC on 17m using the new FT8 data mode.  Considering that in nearly 1,000 QSOs using JT65, I’ve only worked 76 separate DXCC entities, this could be quite difficult.

I’ll post updates on a semi-regular basis to show how well I’m doing.  I only decided to do this a week or so ago and I’ve already got 28 countries in the log but of course, the first ones are easy.

If you see me calling CQ on 17m FT8, please give me a shout.  You never know, you could be a new country for me.


Posted in Amateur radio, Data, DXCC-17m-FT8, FT8, HF | Tagged , , | Leave a comment

Installing a new external ADS-B receiver

Not long after my last post on this subject, back in January, the preamp I had mounted in the box at the top of 30ft of pole strapped to the side of my house failed and I was forced to removed that particular ADS-B aircraft tracker.  I still had my loft unit running and working very well with my home brew two element J-pole collinear.

Tony, G0MBA and I have been talking about the things we can do with an ADS-B receiver – We considered mounting one at the Martello Tower Group site but unfortunately the required bandwidth to upload is far higher than our monthly allowance on the 4G dongle we use for GB7CL so we quickly abandoned that idea. That didn’t stop Tony from building a receiver in a box though and before very long, he’d assembled this and put it up at a site he has access to near his home.

ADS-B receiver built by Tony, G0MBA

ADS-B receiver built by Tony, G0MBA

The problem was that it didn’t work anywhere near as well as was expected and after some investigation, we realised that it was because he was using an older version SDR dongle with no preamp or filtering.  By this time I’d replaced my Uputronics preamp with the upgraded version containing the ceramic filter so we did a deal which meant he’d get the preamp/filter and I’d have a new receiver in a box.

Two weeks later and I’ve got the receiver here and in the air.  I’m using a Raspberry Pi to run the software which uploads to Flight Aware, Flight Radar24 and PlaneFinder.  The receiver is a Flight Aware Pro Stick Plus dongle and I also added an additional Flight Aware 1090 MHz bandpass filter as well.  It’s being fed with a length of CAT6 cable with Power over Ethernet.  I chose to do this so that I’d have the absolute minimum signal loss in the coax and of course, network cable is significantly cheaper than coaxial cable.  I’m using the Flight Aware 26″ vertical.

Tony built this all for me without the filter so I had to make some tweaks inside.  It’s not as neat and tidy as when I got it but the important thing is that it all fits and works.

External ADS-B receiver using a Raspberry Pi and PoE to power it

External ADS-B receiver using a Raspberry Pi and PoE to power it

ADS-B receiver mounted at the top of my mast, just below the Hexbeam

ADS-B receiver mounted at the top of my mast, just below the Hexbeam

Some readers may have noticed that the receiver already has a built in bandpass filter and wonder why I felt the need to add an external one.  The reason is quite straightforward – With a relatively high gain aerial up in the clear, with no feeder loss, it’s quite common to overload the front end of these receivers and for them to be swamped by external signals.  I knew I’d have plenty of gain so I decided that it wouldn’t be a problem to add a little insertion loss through the filter and I’d get the benefits of additional filtering.

I did test the bandpass filter with my miniVNA Tiny and was pleasantly surprised by the results.  I was expecting the insertion loss to be around 3dB but in fact it measured around 1.15dB on 1090 MHz.  Far better than anticipated.

Flight Aware 1090 MHz Mode S Filter insertion loss

Flight Aware 1090 MHz Mode S Filter insertion loss

The main reason for a filter is to attenuate out of band signals and this seems good.  The actual bandpass part was wider than I expected but it still provides decent attenuation at cellphone frequencies as you can see.

Flight Aware 1090 MHz Mode S Filter performance from 700 to 1300 MHz

Flight Aware 1090 MHz Mode S Filter performance from 700 to 1300 MHz

The important thing is the performance – How well does it work compared to my indoor receiver?

The answer is ‘significantly better’.  I’m receiving around 25% more position reports and 4% more aircraft using the external receiver. This might seem odd as the number of aircraft isn’t that much higher but there are a finite number of birds up there and if I’m already receiving signals from most of them, then it’s just not possible to hear that many more.  The position reports is the important figure because it indicates that I’m receiving more transmissions from the existing aircraft.

One thing I was concerned about was the risk of interference when I’m transmitting using my Hexbeam.  I’ve separated the network cable and the coax as much as possible and I deliberately chose shielded CAT6 network cable.  I’m happy to report that even running maximum power, I don’t appear to cause any problems with the ADS-B receiver.

I upload the data I receive to FlightRadar24, FlightAware and PlaneFinder.  The reason for using PlaneFinder is because it generates a rather nice heatmap of where I’m receiving traffic from.

Heat map of aircraft received with my new external receiver

Heat map of aircraft received with my new external receiver

Earlier this week I captured this image which shows me tracking over 375 aircraft.

Over 375 aircraft being tracked simultaneously

Over 375 aircraft being tracked simultaneously

It’s working incredibly well which really isn’t too surprising.  It’s high, it’s in the clear and it has an almost unobstructed view to the horizon in all directions.  This is a setup I’m very satisfied with.

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FT8 – The latest data mode

I’ve posted a lot about the low power, low signal modes, JT65 and JT9 in the past and now there’s a new kid on the block. This one is called FT8 which stands for ‘Franke and Taylor, 8-FSK modulation’, developed by Steven Franke (K9AN) and Joe Taylor (K1JT).

The biggest change from the slow, plodding JT65 and JT9 modes is the speed.  Rather than each transmission period lasting around 50 seconds, each transmission made using FT8 is a mere 15 seconds which means a QSO can be completed in around a minute and a half.  In fact, things happen so fast that the software can now control the transmission sequence.  You reply to a CQ call, if the person you are calling responds to you, the software takes over and makes the exchanges automatically.  It’s so quick that it’s actually difficult to do it manually.  You can switch the automation off and operate manually but I don’t think many people will do that.

I made my first QSO using FT8 just ten days ago and I have to say that my initial impression wasn’t positive.  I was a little frustrated that I wasn’t able to do it manually, like I’ve done with the other data modes in the past but then after a few QSOs, my opinion has changed a bit.  It reminds me of AMTOR ARQ where two stations sync together and switch between transmit and receive very quickly.  I’m actually quite enjoying operating FT8 now.

My impressions after ten days of using FT8 is that it doesn’t handle signals as weak as JT65 or JT9 (the protocol notes below confirm this is the case) and I’m convinced by the signals that I’m seeing on the bands that many stations are running significantly higher power than was common on JT65/JT9.  FT8 reports signals into the + range as well as – and I’m regularly receiving stations up to +9dB.  That’s very loud.

I think that FT8 will be picked up by big stations and I suspect that amateurs with low powered stations and small aerials are going to suffer.  They’re going to struggle to work FT8 with anywhere near as much success as with JT65 and JT9

Thirty seconds of received stations on 20m FT8

Thirty seconds of received stations on 20m FT8

Five minutes of received FT8 waterfall on 20m

Five minutes of received FT8 waterfall on 20m

In the ten days I’ve been operating FT8, I’ve made nearly 50 QSOs and worked 33 DXCC entities.  This is very different to the early days of JT65 and JT9.  I remember calling and listening to both those modes and struggling to find anyone to work.  Every time I’ve had a session on FT8, the band segments have been very busy so the takeup of this new mode has been massive.

Here’s the list of DXCC entities I’ve worked using FT8.

Countries worked using the new FT8 mode

All these have been worked on either 40m using my OCFD or 20m using my Hexbeam.  I’ve never turned the power above 10 Watts.

This is a map of the stations I’ve received here on 20m using FT8 over the previous 24 hours.

Stations received using FT8 on 20m at G6NHU

Stations received using FT8 on 20m at G6NHU

A nice feature of WSJT-X is the ability to upload ‘spots’ to the PSK Reporter site and strangely this isn’t enabled by default.  I recommend you enable this option through Preferences, Reporting and then tick the Enable PSK Reporter Spotting box.

Enable WSJT-X for PSK Reporter

Enable WSJT-X for PSK Reporter

Here’s the official ‘brief’ description of the FT8 Protocol:

WSJT-X Version 1.8.0 includes a new mode called FT8, developed by K9AN
and K1JT.  FT8 uses 15-second T/R sequences, provides 50% or
better decoding probability down to -20 dB on an AWGN channel, and
maintains good performance on Doppler-spread fading channels. An
auto-sequencing facility includes an option to respond automatically
to the first decoded reply to your CQ. FT8 QSOs are 4 times faster
than those made with JT65 or JT9. FT8 is an excellent mode for HF
DXing and for situations like multi-hop E_s on 6 meters, where deep
QSB may make fast and reliable completion of QSOs desirable.

Some important characteristics of FT8:

– T/R sequence length: 15 s
– Message length: 75 bits + 12-bit CRC
– FEC code: LDPC(174,87)
– Modulation: 8-FSK, tone spacing 6.25 Hz
– Constant-envelope waveform
– Occupied bandwidth: 50 Hz
– Synchronization: 7×7 Costas arrays at start, middle, and end
– Transmission duration: 79*1920/12000 = 12.64 s
– Decoding threshold: -20 dB; several dB lower with AP decoding
– Multi-decoder finds and decodes all FT8 signals in passband
– Optional auto-sequencing and auto-reply to a CQ response
– Operational behavior similar to JT9, JT65

FT8 is included in RC1 of WSJT-X v.1.8.0 and can be downloaded for Windows, Linux and OS X from here.

Posted in Amateur radio, Data, FT8, HF | Tagged | 20 Comments