Building an MMDVM hotspot on a budget

I bought myself a new handheld at the Newark rally earlier this year, an Anytone AT-D878UV to use on DMR. When I went on holiday in October, I took a couple of copies of RadCom with me and in one of them there was a review of my handheld. The author mentioned that he’d used it through a hotspot and this got me thinking because although there’s a DMR repeater near me, I’d never played around or experimented with hotspots. In fact, I’d never even seen one!

I like learning about new things so I contacted the author of the review asking for recommendations about hotspots and he suggested the Zumspot as being a decent pre-built hotspot that’s easy to get going. I dutifully ordered one while I was away and it was waiting for me by the time I got home.

Setting it up was very straightforward and within a few minutes, I had it all up and running and working perfectly with my handheld. I quickly learned all about the different modes it can be used with and spent some time upgrading it to the latest version of the software and fine tuning it.

I decided that I’d like another hotspot to use with D-STAR as there are many reflectors not available through the local D-STAR repeater and I thought it might be interesting to listen to some of them.

Rather than buy another Zumspot I thought I’d try and buy all the parts needed to assemble one myself and see how cheaply I could do it.

There are lots of MMDVM modems available on eBay so I just picked this one at random and ordered it. It wasn’t quite the cheapest but it was within a few pennies of the absolute cheapest. I paid £14.07 but I see that the same module from the same supplier has now dropped to £13.39.

It arrived quicker than expected as well – I ordered on the 1st November and it was delivered on the 13th November which I thought was pretty good considering it came from China. Unusually, the online tracking worked as well.

This is what I got:

MMDVM Hotspot Support P25 DMR YSF 32 bit ARM Processor for Raspberry Pi XY from eBay
MMDVM Hotspot Support P25 DMR YSF 32 bit ARM Processor for Raspberry Pi XY from eBay

The two sets of pins are for the GPIO header on a Raspberry Pi and the SMA aerial socket needs soldering to the board. The size is perfect for mounting on a Raspberry Pi Zero W and I had one spare so it only took a few minutes to solder the header and the socket in place and I quickly had a unit ready to go.

MMDVM Hotspot attached to a Raspberry Pi Zero
MMDVM Hotspot attached to the Raspberry Pi Zero

I wrote the SD card with a Pi-Star image, put it in the Pi and booted it. I’m not going to go into the details of how to write a card or configure Pi-Star as there are many places on the internet showing how to do this. The site by Toshen, KE0FHS is the best I’ve found for Pi-Star resources and you can visit it here. It took me about ten minutes to set everything up and reboot it.

A lot of hotspots come with little screens on them. In the limited time I’ve had the Zumspot, I really don’t look at the screen at all so I didn’t include one as part of my build.

This homebrew hotspot is working as well as the Zumspot. It doesn’t come in a fancy case, it doesn’t come with a screen but neither of those are important to me as I just want functionality.

Here’s a full breakdown of the cost:
MMDVM modem from eBay – £14.07
Raspberry Pi Zero W – £9.60
Raspberry Pi PSU – £8.00
16Gb SD card – £5.99

Total cost – £37.66

I could have built four of these for less than the cost of the Zumspot.

I’m not saying that people shouldn’t buy commercial hotspots but just wanted to demonstrate that it’s very easy to get the parts separately and assemble a fully working hotspot.

Posted in Amateur radio, Construction, D-STAR, DMR | 5 Comments

My new Hexbeam is broken … and fixed

Nearly two weeks ago the UK suffered from ‘Severe storms’ with high winds across most of the country. I went outside in the rain on the Saturday morning, looked up at my aerial and was disappointed to see this.

Broken Hexbeam blowing in the wind

You can see the 40m element in the top is very loose and blowing freely in the wind.

Broken 40m element
Broken 40m element

It wasn’t immediately clear what had happened because although I could see that the 40m element was flapping around, it wasn’t obvious exactly what and where it had broken. I checked and the VSWR had gone very high so something bad had occurred.

Today was the first combination of a calm and dry day we’ve had since so I took the opportunity to lower the mast and check. Once it was lowered, it was very clear what’s actually happened. The loop at the end of the element has simply slipped off the centre pole and the element has threaded itself back through the first spreader.

The loop at the end of the element
The loop at the end of the element

All I had to do was hook it back over the top of the centre support above the balun.

The loop hooked back over the top
The loop hooked back over the top

To stop this happening again, I used a couple of cable ties, one through each loop and securely fixed in place.

Two cable ties holding the loops in place
Two cable ties holding the loops in place

This also gave me the opportunity to make sure I can reach the top of the aerial without any difficulty by using my new ladder. I’m going to install an ADS-B aerial right on the top and now I’m confident that I can get to it safely. Note that when I climbed the ladder to work on this that I had people supporting the base and it’s also well pushed into the garden.

Working position for the top of the Hexbeam
Working position for the top of the Hexbeam

Posted in Amateur radio, Broken aerial | Tagged , | Leave a comment

Installing a dipole for 30m

I’ve never had a decent aerial for the 30m (10 MHz) band and have struggled to work much on the band either with my old doublet or the inverted L that I’ve had up for the last couple of years.

About a month ago, I realised that I could hang a single band 30m dipole in the space that I was previously using for an off centre fed dipole. I’ve tried an OCFD a couple of times now but they don’t seem to work well for me.

I make no secret that I still consider baluns a magic art and I’ve not taken the time to get my mind around balun construction yet. In theory, I should be able to do this as I’ve wound so many wires around ferrite cores when I’ve been making filters and QRSS/WSPR transmitters that it’s become second nature. I’m just not comfortable with the idea of building baluns.

The largest rally in the UK is the Newark Hamfest which takes place in September every year and I was hoping to pick either a commercial 30m dipole or a suitable balun when I went but sadly I didn’t see any of either of them. Shortly after returning from the rally, I ordered a pre built dipole which as delivered a couple of weeks ago.

Although it’s not been particularly cold here recently, we’ve had a fair amount of wind and rain and this weekend was the first chance I’ve had to do anything.

The first thing I had to do was clear the bushes from the support pole at the end of the garden. I generally lower and raise my wire aerial a few times throughout the year but it’s been two years since the last time I did that and there was about three yards of undergrowth to clear (note to self: a hedge trimmer really isn’t suitable for this kind of job, a chainsaw would be far more useful). This took a good hour to clear and has made it obvious to me that I need to make the effort over the winter and clear the end of the garden properly.

Cleared space to the support pole
Cleared space to the support pole

With that clear, I was able to lower the paracord I use to hold the end of my wire aerials and then get the centre support pole down. I don’t like leaving the centre of my wire aerials just dangling so I have a 25ft pole in the garden which is strapped to a washing line end post.

It was straightforward enough to fix the new dipole to the centre support pole and lift it back up again.

Here you can see the centre pole, the dipole and the far support pole. Note that it’s well clear of the overhead power cables which are behind the pole.

30m dipole
30m dipole – If you can’t see it clearly, click the image for a larger picture.

I didn’t get a chance to properly test it straight away but I did make a couple of quick QSOs using FT8 and although the internal SWR meter on my TS-590SG showed below 1.7:1, I had to use the built in ATU to persuade it to transmit full power.

This morning I put my Mini VNA Tiny on the dipole and was surprised to see that the SWR was a lot worse than the rig had showed me at around 2.4:1 in the centre of the band (2). It was most resonant at 9.9 MHz (1) but wasn’t particularly good there at nearly 2:1.

30m dipole swr after installation
30m dipole SWR after installation

To feed the dipole, I used the length of RG-213 I previously had connected to my inverted L and added a run of Westflex 103 up the pole. To save me having to run backwards and forwards to the shack to check after each adjustment, I took my laptop outside, disconnected the Westflex 103 from the RG-213 and made my measurements there.

I had to lower the ends and chop three lengths off before I got the aerial tuned correctly. In total, I took just over six inches off from each side which brought it bang into the middle of the band.

You can see the results of my trimming below – The SWR is flat over the entire band (it’s only 50 kHz wide) at a fraction over 1.6:1.

30m dipole SWR after trimming
30m dipole SWR after trimming

The two markers are the band limits.

When I connected the aerial back to the wireless, everything was much better than before. The rig transmits full power without needing the ATU and the internal SWR meter reads unity.

I had a quick session this evening and have already worked a couple of new countries on 30m so it’s having the desired effect straight away.

The downside is that I now don’t have any aerial up for 160m, 80m and 60m but my SG-237 smartuner is now free and I’m wondering if I can string a loop around the garden somehow.

Here’s the dipole from the other side – Note again that it’s well clear of the cables you can see running across the picture.

Dipole at sunset
Dipole at sunset

Posted in Amateur radio, Construction, FT8, HF | Tagged | Leave a comment

Anderson Powerpoles

Some time ago back in 2012 I first heard about the Anderson Powerpole in episode 53.5 of the Practical Amateur Radio Podcast and I immediately bought a load of them to use in the shack. I have a number of tails hanging off my PSU, all terminated in Powerpoles.

I’ve always soldered the metal contact to the wires but last year at the Newark radio rally, I bought a Powerpole crimp tool from Richard at SOTAbeams and finally had a reason to use it as I needed to make a new tail.

Powerpole tail all ready for my PSU
Powerpole tail all ready for my PSU

When I’ve fitted Powerpoles before, I’ve always found them a bit fiddly to work on but the crimping tool made a big difference. If you don’t use Powerpoles in your shack then you’re really missing out on the convenience. I thoroughly recommend them.

Powerpole crimping tool
Powerpole crimping tool

Posted in Amateur radio, Construction | Leave a comment

Replacing my hexbeam with… another hexbeam!

At the end of July, I happened to glance up at my aerial and noticed the 15m element hanging down. It was broken. It didn’t look serious though and I was sure that it was just that the connector on the end of the element had snapped off.

Around the same time, I was offered a second hand G3TXQ hexbeam, less than a year old which came with the 40m section. Since I took down my delta loop back towards the end of 2011, I’ve struggled to get a decent 40m aerial here so I jumped at the chance.

I took my hexbeam down on the 26th August and I was correct, all it needed was a new connector crimping on so I did that and sold it locally.

Today, Martello Tower Group members, Paul 2E1PAW, Colin M1EAK and Terry G4GHU all descended on my QTH and between us we put the replacement hexbeam up. I did a few hours work yesterday getting things prepared and so today we were all done within three hours.

The first job was to assemble the aerial and we managed it without stepping on too many elements or tripping over them. Anyone who has built a hexbeam will appreciate how tricky this can be and the 40m section adds a new level of awkwardness.

Terry, Paul and Colin working on the aerial
Terry, Paul and Colin working on the aerial

Once the aerial was assembled, we decided that we’d need to use scaffolding to get the aerial fixed onto the stub mast. Fortunately Paul was already prepared for this and had brought some with him so we unloaded it from his van and assembled it. I had considered using a different, longer stub but decided against it because I knew what I had worked and was loathe to change it. It’s been through a lot of bad weather and survived well.

Colin and Paul planning the next stage
Colin and Paul planning the next stage

Here you see Paul on the scaffold tower getting things in place for lifting the aerial.

Paul making preparations for mounting the hexbeam
Paul making preparations for mounting the hexbeam

We managed to lift the aerial onto a 10ft aluminium pole and offer it up to Paul. Colin climbed another ladder and helped Paul turn and mount the aerial onto the stub mast on the top of the rotator. This was the ‘hairiest’ moment of the entire procedure and it went relatively well with no major drama.

Paul standing triumphant with the hexbeam on the mast
Paul standing triumphant with the hexbeam on the mast

With that done, it was a simple matter to finish everything off, attach the coaxial cables, feed them down the mast (making sure to leave a loop for the rotator) and finally raise the mast back to vertical.

My new hexbeam with 40m
My new hexbeam with 40m

I’ve not put the analyser on the aerial yet but the radio SWR meter isn’t reporting problems on any bands. As expected, I need to use the built in ATU at the bottom end of 40m but it copes admirably.

For quite some time, I’ve been chasing DXCC on individual bands and have been stuck on 99 entities worked and confirmed on 40m. I was hoping to be able to get myself to 100 quite quickly, especially with a couple of large contests coming next month.

This evening, I had a tune around the SSB section of 40m and it’s absolutely heaving with stations working a contest but I didn’t hear any new countries. I fired up FT8 and within a few minutes had worked Mauritania for the first time on 40m. He’s already confirmed on LoTW so I’ve now completed my 40m DXCC. I didn’t expect it to happen that quickly.

Posted in Amateur radio, Broken aerial, Construction, FT8, HF | Tagged , | 2 Comments

Installing a radio into my car

About a year ago I changed my car and it’s taken a while to get around to fitting the radio into it. I’ve missed having a wireless in the car and recently resorted to keeping a handheld in the glove box so at least I could natter on the local repeater while driving around town.

Although I could probably have done it myself, when it comes to installing radios in my cars, I always prefer to have someone do it for me, normally someone who does this as part of their job and will have all the tools and know all the tricks of hiding the wires out of the way and making it all nice and tidy. I just don’t have the patience!

Last week I travelled to Chelmsford and met up with Nigel, G6ZVV who installed my Icom ID-5100E into my Mazda 6. He did a great good job, it’s neat and tidy and the control head is easily useable and visible without blocking my view out of the windscreen at all. Icom don’t produce a proper mobile mounting bracket for the control head so I have their MBA-2 paired with a RAM Mount RAM-B-102U-A which I imported specially from the USA. For what it’s worth, I think it’s pretty crazy that the only bracket Icom produce is a suction mount.

Mazda 6 with Watson W-770HB
Mazda 6 with Watson W-770HB

The installation itself was pretty straightforward, Nigel ran the cables through and took a feed direct from the battery while I mounted the aerial on the boot. I can’t remember the make of the boot mount, it’s either Diamond, Watson or Sirio but it did me well on my previous car and although it looks a little tattered, it’s perfectly good.

The aerial I use is a Watson W-770HB which I’ve had for years. There are many similarly specced aerials on the market (it’s a half wave on 2m and 2 x 5/8 on 70cms) but out of all I’ve tried, this has been about the best. I tried replacing it with a Diamond aerial a couple of years ago but that didn’t perform as well so I returned it and put this old one back on.

The main body of the radio is mounted in the boot, underneath the parcel shelf. It’s accessible enough that I can get to the SD card slot but positioned well enough that it won’t get in the way of anything I put in the boot. It’s neat and tidy.

Icom ID-5100E mounted in the boot
Icom ID-5100E mounted in the boot

The important part is how well I can see it while I’m sitting in the driving seat and how accessible the radio is. I wanted it to be easily viewable and in a position where I can operate it without having to move my hands far from the wheel or my eyes far from the road, without actually blocking any of the view out of the window. I appreciate this is a tall order for modern cars but I think that I found a good place. This meets all my requirements and was surprisingly easy to fit.

Icom ID-5100E control head
Icom ID-5100E control head

I mentioned the mount before, here’s how it looks from behind.

The back of the control head mount
The back of the control head mount

When I get out of the car, it’s a very simple matter to remove the control head, disconnect the cable and take it away for security.

The mount without the control head
The mount without the control head

I was a little concerned as to whether I’d suffer from any electrical interference from the car or whether transmitting high power on 2m or 70cms would interfere with anything within the car but it all seems to be OK. I had a long drive from home to Manchester and back just a couple of days after installing the radio and it worked very well.

It’s nice to be mobile again after over a year with no radio in the car. I’ve missed it!

Additional – I’ve been asked how I ran the power from the battery through the bulkhead and into the car so here’s how it was done.

Power cable from the battery
Power cable from the battery

The power cable comes from the battery and is fused, then there’s a section of plastic held in place by a number of clips similar to the one marked in yellow. Those clips simply prize off and leave space for the cable to run underneath.

Power cable comes out from the plastic tray
Power cable comes out from the plastic tray

The cable comes out from the tray at the other end of the car (note, this is a right-hand drive car) and drops into the space down by the door – The picture below should help explain this.

Power cable dropping down into the space by the door
Power cable dropping down into the space by the door

Once it’s dropped down the gap, it’s a simple matter to bring it in at the bottom of the door, loop it over the door seal (held in place with a cable tie and drop it under the trim and into the cable run to the back of the car along with the cables for the control head, microphone and speaker. This is the only place that the power cable is visible. As you can see, it doesn’t actually go through the bulkhead as we couldn’t find an easy and available place to run it.

Power cable coming in through the bottom of the door
Power cable coming in through the bottom of the door

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

Essex Radar

I’ve been running an instance of Virtual Radar Server (VRS) for some time and recently decided that this is something I can easily share to anyone who is interested.

What is VRS though?

VRS is a server running on my network that can take a feed from multiple ADS-B receivers, consolidate them together and provide a decent graphical interface showing all the aircraft tracked. Unlike most of the commercial services, this is unfiltered and real-time. Of course, it only shows aircraft that I’m actually receiving signals from but I do have a pretty decent setup here so the coverage is good.

Essex Radar

A project like this needed its own website and so Essex Radar was born at

I have two receivers feeding aircraft to Essex Radar – One is in my loft and is using a Raspberry Pi 3 Model B, a FlightAware Pro Stick Plus SDR dongle and a homebrew two element J-Pole collinear. The second is outside on top of my mast and is a Pi 3 Model B+, an Airspy Mini SDR dongle, LNA and FlightAware 26″ aerial.

Essex Radar was used to create this 24 hour timelapse video of Boeing 747s.

Monitoring aircraft is something I find quite fascinating and it combines two of my interests, radio and aviation and is something you can do quite cheaply. Although I’ve briefly described my two receivers above, I’d like to give a few more details and options as to what you need.

Be aware, this is going to contain a lot of links to various products.

No matter what you choose to do, you’ll need to start with a single board computer and in my opinion the best one to use will be a Raspberry Pi. There are others on the market but the Pi is the simplest to get hold of and appears to have the best support.

At time of writing, the current Pi is the Raspberry Pi 3 Model B+ and if you don’t own a Pi, this is the version I recommend you buy. If you already have a Pi then it will almost certainly work as an ADS-B receiver although I’m not sure I’d want to use anything older than a 2B. There are many different suppliers and they’re all around the same price.

You’ll need a power supply for your Pi. This must be a good, stable PSU capable of at least a couple of amps at 5 volts. My recommendation is the official Raspberry Pi Universal Power Supply but another option is to run a less stable PSU at higher voltage and drop it down using an LM2596 DC-DC converter. I know from bitter experience that some power sources are particularly bad. My ADS-B receiver which is mounted on top of my mast was originally running from PoE but continually gave me under voltage warnings. To resolve this, I switched the output from my PoE device to 9 volts and used an LM2596 converter to drop it to 5 volts. I no longer have any power problems.

The other essential item is a memory card. Not all SD cards are built equally and some are significantly slower than others. I’ve had great success with SanDisk Ultra 16 GB MicroSDHC UHS-I cards and use them in all my Pis.

A Pi, PSU and memory card will cost around £40-£45 depending on where you buy them from. I’m not going to recommend anywhere specific for these three items because prices change regularly so just shop around for the best deal.

The next thing you need is a receiver. If you want to do this the cheapest way possible then you can get an RTL SDR dongle from eBay for less than a tenner and it will work but it won’t be very good.

There are other receivers available that cost a little more but in my opinion, the one that will give you the best value for money is the FlightAware Pro Stick Plus. This dongle comes with a built in amplifier (LNA) and 1090MHz filter and is what I’m currently using for the receiver located in my loft. It’s available from various UK suppliers for around £30, works really well and is perfectly good for most people.

If you want more performance then you’re starting to get a bit more expensive and the law of diminishing returns comes into effect. The next step up is to buy a separate receiver and amplifier. If you want to do this then my recommendation is the RTL-SDR V3 dongle and the RTL-SDR 1090MHz LNA at $21.95 and $26.95 respectively and available from the RTL-SDR shop. These will take a couple of weeks to be delivered to the UK and you’ll also need to buy a bias-tee module as well which will add another tenner or so to the cost and further complicate the build.

I think that the next upgrade from this setup would be to replace the receiver with an Airspy Mini SDR dongle but at around £120, that’s not going to appeal to many people. If you really want to squeeze the maximum performance then you should consider a Mode-S Beast receiver but that’s getting extravagant as one of those will cost you around €240 in kit form.

Next, you’ll need an aerial. There have been many aerial designs posted on the internet and I’m not going to link to all of them but I do suggest looking at this thread on the FlightAware forums for some simple and easy to build antennas. The first aerial I built was the spider which is effectively a quarter wave groundplane and you can see a picture of it here. It works but it doesn’t work as well as the next aerial I built which is a two element J-Pole collinear. That collinear is still in use on my receiver in the loft. It’s a superb aerial and it works far better than I ever expected. It was easy to build and as I already had the wire and the connector, it didn’t cost me a penny.

If you would rather buy a cheap aerial then look for an ADS-B PCB aerial but be warned, it may arrive unsoldered and you’ll have to fix it yourself.

If you want a good solid weatherproof aerial to use outside, then I have experience of two different types. Search eBay for COL1090/5-H and you’ll find the same aerial we use for the receiver at the Martello Tower Group site. This isn’t always available but when it’s there, it’s a good buy at around £30 including shipping. These aerials are all built by hand so be aware that performance between them may vary slightly (I know this from experience).

A little more expensive at around £54 but of more consistent quality is the FlightAware 26″ aerial. I use one of these on my receiver here and I can’t fault it. The build quality is very good and mine has been mounted outside at 10m AGL in all weather conditions for nearly two years with no noticeable problems. Apart from being a bit faded, it’s just as good as when it was new.

I will mention one more commercial aerial but I have no experience of using it and can only go by the reports I’ve read. The ADS-B Vertical Outdoor Base Antenna is $149 from DPD Productions and is very highly rated.

Finally – Coax. Buy the best you can afford and use as short a run as possible. Cheap cable is very lossy and this is one area where you mustn’t cut corners. For example, 10m of RG58 will give you around 5.5dB loss, 10m of RG213 will give you roughly 2.8dB loss and 10m of Messi & Paoloni HyperFlex 10 has around 1.4dB loss. Don’t skimp on your coaxial cable.

Once you’ve collected all your hardware, you’ll need to set up the software. That’s pretty straightforward as there are a number of pre-built images available which do all the hard work for you so I’m not going to go into long winded instructions.
I suggest using the FlightAware image and full details can be found here. Because I’ve already gone through the hardware, you just need to jump forward to the second section “Install PiAware on your SD card”. You should be able to set the software up from start to finish in about half an hour. The guide linked above covers absolutely everything.

I appreciate that there’s a lot to take in here and I congratulate you if you’ve read this far! If you have an interest in radio and aviation then setting up your own aircraft tracker is neither difficult or expensive and it’s really fun to be able to see aircraft in the sky near you. If you do decide to do this and would like to be included as part of Essex Radar then please contact me.

The day after I put Essex Radar online was the day of the historic D-Day Flypast where a number of DC-3 aircraft flew from Duxford, over Colchester and Southend on their way to Normandy. Essex Ham were monitoring the flight and they used Essex Radar to track it. You can read more about this and see a video including Essex Radar here. I’m very pleased to have been able to provide this service.

Posted in ADS-B, Construction, Data, Not amateur radio, Raspberry Pi | Tagged , , , | Leave a comment