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, 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

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/
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

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/

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 , , , , | 6 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.

Posted in ADS-B, Construction, Data, Raspberry Pi | Tagged , , , | Leave a comment

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

I’ve broken my Hexbeam – Again!

With the title to this entry, I can imagine a collective sigh from my readers, followed by “Oh no, not again”.

This time is different though, this time it was nothing to do with RF or with melted insulators.  A few weeks ago I decided to add some anti-bird measures to my aerial consisting of six bird repellant diverter discs.

I’ve changed the method of mounting these a few times and I think I’ve sort of got it right now.  I had problems with the discs getting caught on the elements but by moving the mounting position, I’ve resolved that.

Two weeks ago, after some high winds, I noticed that there were only two discs left out of the six.  I didn’t know what had broken, whether it was the clamps, the discs themselves or the cable ties holding them in place and it saddened me.

Last week, while out for a walk, I spotted one of my missing discs on the grass area at the end of my road, around 100m away.  When I checked it, it all looked intact so it appeared the cable ties must have broken.
The next day, two more of the discs appeared on our doormat.  Some kind neighbour found them and dropped them back to me.

How does this relate to my broken aerial?

Last weekend, I lowered the mast in order to replace the discs and also to modify the way the Hexbeam was mounted.  I lowered everything before Tony, G0MBA came along to help and I accidentally dropped the mast too far and too quickly and managed to split the end section of one of the spreaders.

Broken end spreader section on my Hexbeam

Broken end spreader section on my Hexbeam

These sections are under tension so Tony and I wound a number of thick cable ties around it to try and take some of the pressure off but as soon as it was back in the air, I realised I’d have to get it replaced.

The broken end section of my Hexbeam with the aerial in the sky

The broken end section of my Hexbeam with the aerial in the sky

I emailed Ant, the original supplier of my G3TXQ Hexbeam and he put a replacement end section in the post to me straight away.

This morning I lowered the mast, replaced the broken section and raised it again, all in about half an hour.

New end spreader piece on my G3TXQ Hexbeam

New end spreader piece on my G3TXQ Hexbeam

As you can see, I’ve gone from using cable ties for the bird repeller to nice bright paracord.  I use the same paracord (although in grey) to support my doublet.

As an extra point, when I put the mast back up last weekend with five of the six discs on, the kids from next door knocked on my front door with the remaining disc.  It had landed in their garden but they hadn’t realised where it came from until they saw the aerial go back up again with five discs on top.  I have nice neighbours.

I really can’t recommend the G3TXQ Hexbeam as made by Anthony, MW0JZE enough.  It’s a superb design, an incredibly well made aerial and he continues to support it, even though I’ve had this for nearly six years.  I know I’ve broken it many times (see the ‘broken aerial‘ category on this blog) but each time the design has been changed and the aerial has just got better and better.  It’s larger than my old MA-5B but I still think the visible impact is lower because it just looks more pleasing than the spiky beast that is the MA-5B.

If this aerial broke completely, I’d replace it with the same again without hesitation.

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Are you transmitting illegally on 70cms?

An interesting subject came up on our 70cms voice repeater earlier today following an incident on a local DMR box yesterday and it got me thinking.

There are a lot of 70cms repeaters in the UK with input frequencies between 430 MHz and 431 MHz and many people don’t realise that our licence only allows reduced power in that part of the band.  I’ll put my hands up and say that although I’m well aware of the 100km Charing Cross rule, I wasn’t fully up to speed on what I’m about to talk about here.  Incidentally, I live within that 100km radius by less than a quarter of a mile!

Specifically, the maximum power for Full and Intermediate licensees is 40W ERP and that goes down to just 10W ERP for Foundation licence holders.
Power limits by ERP are unusual as apart from this small section of 70cms, only the two VLF bands have ERP power limits.

What this actually means is that our Effective Radiated Power is limited when working any of the nearly 150 repeaters on 70cms with an input frequency from 430-431 MHz.  These are almost exclusively digital repeaters, either DMR or D-STAR.  You can see the most recent list of 70cms repeaters by clicking here (hint: click on the [receive] column to sort by repeater receive frequency).

ERP is calculated by taking into account the output power of your wireless, the loss of your coaxial cable and the gain of your aerial.  Remember that aerial manufacturers generally quote the gain of their aerials in dBi which is a theoretical figure and is impossible to achieve.  All my calculations below are using dBd which is dBi minus 2.15.

Let’s look at a fairly typical mobile setup first and by coincidence, this will be exactly what I’ve got in the car.
An Icom ID-5100 has three power settings, it runs 50W, 15W or 5W on 70cms.

If I have the radio on the High power setting of 50W and am using a very common type of aerial, a 2 x 5/8 whip aerial with 10ft of RG-58 coax, my actual power going into the aerial will be about 40W.  The aerial gain is approximately 3dBd so my ERP will be 80W.  That’s illegal.  By reducing the power to Medium, I’ll have around 12W going into the aerial and so my ERP will be 24W which is legal.  With this setup, a Foundation licence holder can only run Low power of 5W to remain legal as that will give around 4W into the aerial and so be 8W ERP.

I think most people operating DMR use handheld radios – I’m not aware of anyone locally with an actual mobile radio transmitting DMR so if used in a car with an external aerial then the 5W output from a typical DMR handbag will still be legal.

It’s when you get indoors that things start getting a little more tricky because of the wide range of available aerials and radios.  I’ve tried to select common equipment.

At home, I have another Icom ID-5100, my aerial is a Diamond X510 and I have around 15m of EcoFlex 15 coax.  My coax loss is 0.9dB on 70cms and the aerial gain is 9.55dBd.  With the radio set to 5W output I’ll be getting around 4W going into the aerial and the 9.55dBd gain gives me an ERP of 36W.  Providing I only use the radio on the Low power setting of 5W, I’m legal.  Just.

Many people have smaller aerials such as the Diamond X50 or equivalent.  The gain on the X50 on 70cms is a fraction over 5dBd but let’s round it down to 5dBd for ease of calculations.

The MD-380 is a common UHF DMR wireless with 5W output. The Icom ID-51 runs the same amount of power on D-STAR so let’s go with 5W as our power for these calculations.

If you’ve got your dual band aerial up at a decent height, you may have 20m of coaxial cable.  I hope you’re using RG-213 as an absolute minimum on UHF.  If this is your setup then 5W output fed through 20m of RG-213 gives you 2.4W at the aerial.  Take into account the 5dBd gain from the X50 and you’ve got 7.5W ERP. You’re legal, no matter what your licence level is.

The same setup with a Diamond X200 gives you around 9W ERP but if you go to a Diamond X300 (or equivalent) then your ERP goes up to 12W.  You’re now transmitting an illegal power level on 70cms if you’re a Foundation licence holder with the 10W ERP limit.

The above examples are just that, examples.  You should know how much power you’re running out of your transmitter, you should know how much loss there is in the cable and you should know the gain of your aerial. With those figures, it’s fairly straightforward to calculate your ERP.  You could use this as a starting point for working out your coax loss (input cable type, cable length, frequency and power) and then this to calculate your ERP (using the calculated power from the previous link and aerial gain in dBd).

It’s worth being aware of this if you operate on 70cms digital repeaters as I suspect a lot of amateurs won’t have realised this power limit is in place.  I know that everyone is supposed to know how much power they can run on what frequency but this ERP limit is quite unusual, we’re more used to the limits of 10W, 50W and 400W that our Foundation/Intermediate and Full licences allow.

If you do transmit on 70cms between 431 and 432 MHz then you really do need to take a close look at your station to make sure you’re transmitting legally.  Remember that we are secondary users of the 70cms band and you don’t want to upset the primary users.

Posted in Amateur radio, D-STAR, DMR, FM, UHF | Tagged , , , , | 2 Comments