Making an attic antenna for LoRa APRS & TinyGS

Recently I changed my job. 

Sadly this led to the loss of use of the wideband Discone antenna mounted for test purposes on the side of the old building, which I'd used for my TinyGS and LORA APRS igate. 

I have got an iGate at my home, however I'm disappointed by it's woeful performance on the co-linear antenna I fitted in November last year, it requires further investigation when the weather improves!

The plan is to mount a simple quarter wave ground-plane antenna in the attic. 

First things first, an antenna is made up around an SMA connector, and given a tune-up using the Nano VNA. You can get rough dimensions from this great on-line calculator. 

Next, I need to come up with a method to mount the antenna. 

The plan is to mount the antenna on the top of a piece of 20mm electrical conduit, as a simple mast. 

A slotted cylinder and locking ring is 3D printed. It's a bit on the thick side, as the first one I made was a bit thin, and split...

The slotted cylinder fits over the antenna radials, and secures it to the 20mm PVC conduit. As the cylinder is now so thick, it's a really snug fit, and the locking ring is not required. 

The 3D model for the cylinder can be found here :

https://www.tinkercad.com/things/9T2Tnnhmuzc

As I want to feed two receivers (one APRS, one TinyGS) I've fitted an LNA and a splitter (both procured from AliExpress). The LNA will overcome the losses in the splitter (especially as I ordered a 4 output splitter by mistake!). You can just see in this photo, that I coiled the coax around the PVC conduit. This type of thing is usually used as a choke balun. It's not really needed here, but was just a convenient way to tidy up the coax! 

TinyGS

 

LORA APRS

I also connected a third receiver to the output, just as an experiment. This feeds a Raspberry Pi running ionosphere, which as an APRS decoder. This one's tuned to 144.8 MHz, and yes, this antenna isn't going to be much use at that frequency, but let's see how it fares...

Ionosphere can be found at https://github.com/cceremuga/ionosphere

It supports linux, mac os, windows, freebsd, and, of course,  our Raspberry Pi. There's a handy script on the page for installing the software on a pi automatically. 

There's some config to set, but it's quite straightforward.

The Raspberry Pi and SDR are zip tied to the mast...

A supply distribution is made up, consisting of a redundant USB hub & a buck converter. There's a clip-on ferrite between the buck converter and the hub, in an attempt to keep any noise out. This will be fed from a 12v plug in supply once mounted. 

Off to the attic! 

A collegue of mine tells me there's a russian phrase for this kind of construction ... "Проводной с соплями" - "Wired with snot" Yep, it's not tidy!

The Base of the antenna can just been seen, sticking out of a redundant water tank. 

Thankfully, my wifi reaches the attic, and all devices log on to their respective servers...

The two APRS iGates appear on the map ...

and TinyGS is working very well.... This picture was taken after a few days, you can see the increase in the numbers of packets received...

... including some well out of footprint! (Tropospheric ducting, perhaps?) 

Sadly, neither my VHF or UHF APRS stations have captured anything yet, as there's not much traffic here... I'll update you on those as it goes ...

Aliexpress LoRa Amplifier AB-IOT-433

I've been experimenting with APRS using the T-Beam LoRa hardware, as described in Andreas Spiess HB9BLA Wireless' YouTube video here. (Andreas has two YouTube channels, both are very much worth subscribing to, here and here.)

Now the standard T-beam hardware outputs around 100mW, and I'm getting a range of around 9 miles, although one packet managed to make a 12 mile journey one morning!

Perhaps a little more output power would help.

AliExpress advertises a small RX/TX amplifier. A quick search for "Lora amplifier" pulls up a few, and I bought this one.

It arrives from the seller very promptly. 

It advertises a 2.3W output power at 5V supply, so you will need to have a suitable license to operate this. I have a ham radio license, so I'm good on 433 MHz.

I gave it a quick test on the bench, and it makes 2W happily at 5V, so a real improvement, however the second harmonic is not very well suppressed... 

We're going to need a simple filter. 

Using the tracking oscillator of my Spectrum analyser, I cut an open circuit length of coax to make a suitable stub filter, and once it's cut to the correct length, It's connected via an SMA T connector to the output. 

The output is measured again...


Much improved :) 

I've curled up the stub, which does not effect it's performance, and connected the amp up. It's probably best not to run the amplifier without an antenna or load connected. 

Lashed up in the car for testing... 

And the range now?

Almost no change! I'm not sure what I was expecting. UHF at these frequencies is very line-of-sight, and the range in this case is ultimately defined by terrain. A testament to the robustness of the LoRa protocol at just 100mW, and an interesting, but ultimately pointless experiment! 

Thanks to Jayne, M0JNE, for her help and thoughts on this project.

Ham radio digital modes interface.

Many moons ago I built an interface to connect my computer to my IC-706IIG transceiver so I could operate a digital mode, PSK31.
I don't do much operating really, but quite fancy coupling my new Raspberry Pi400 up to a radio and having a go with another digital mode, FT8 (I know, I'm a bit late to the party.)

So, the plan is to re-jig my interface and up-grade it a bit. 

First things first, is to work out what I did originally !

It's all a bit of a lash up of parts. 

There's a stipped out USB hub, which has 2 USB to serial adaptors plugged in, and a really cheap USB sound card connected. 

The sound card mic and headphone sockets feed two pots on the front panel to control RX and TX level..

... and onto this PCB, where the audio is isolated by two transformers and connected to the transceiver via a 9-Pin D.

One of the USB to serial converters is just gunged to the back panel. It's the socket on the right. This was originally interfaced to the transceiver to control it's frequency. It's redundant now, as I have USB CAT control leads. 

The other USB to serial converter feeds two optoisolators on the PCB, which are used as KEY or PTT to tell the transmitter to go into transmit. Why two lines to do the same task? Practically speaking, there's no difference between KEY and PTT, except KEY is traditionally meant to mean a morse code or telegraph key, and PTT or "Push to talk" for voice modes. Software generally requires one or the other, rarely (if ever) both, and is generally configurable in your chosen software package anyway, so the RTS or DTR of the serial port can be specified to trigger transmit, as either PTT or KEY. 

A decision is made to rebuild. I want to upgrade the little USB soundcard in there, as it's not a great performer. After doing a bit of research, it seems the Sabrant USB soundcard rates well, and is only £9.99 from Amazon. As pointed out above, there's little point in having two PTT and or KEY isolators, so I'll make one, and make it switchable between a DTR and RTS, and output as either PTT or KEY to two separate jacks.

Both the serial converters in the box can go. I will use an FTDI board to control the PTT, which has the added benefit of only having a 5v output, rather than +/-15V (although I somehow doubt the original RS232 converters actually did this, I may be wrong!). Also it will provide convenient access to a +5V supply to run a level monitoring circuit.

The FTDI board is wired up to give a 5V supply, GND, RTS and DTR outputs.

After a couple of false starts, it turns out that the RTS and DTR signals are inverted, active low, but this was resolved by re-wiring the opto isolator, using the FTDI to sink the current. Whilst this seemed like a good idea at the time, something rang alarm bells with me... a check of the datasheet shows I can only sink 6mA, so I use a couple of BC547 transistors to invert the output and drive the optocoupler. 

More false starts... lord only knows what the isolation transformers I have are, but they're not the right thing at all  (did the original box ever work at all? I'm damned if I can remember!). 

I order some small 1:1 600 ohm isolation transformers from eBay

and conjure up a schematic in Kicad ...

I've changed the BC547 transistors to their smd brothers, to maximise the available space. It actually turned out a bit smaller than needed, but that's no bad thing...

Because (this time) our transformers are 1:1, we may well need to add a bit of attenuation into the signal. This is provided by the four SIT (suit in test) resistors R1, R5 ( transmit audio) and R11, R12 for received audio. RV2 is going to be mounted on the front panel, so it connects via a three pin header. I also plan to connect another front panel pot before the input at J1, it's the same deal as RV2. You may notice the RTS and DTR lines are coupled together. I'll feed one or the other from a change over switch. 

May as well design a board ...

I do love Kicad's 3d modelling. It's not just pretty, its great for polishing the layout of labels etc..

The gerbers are sent to JLC PCB for manufacture, and after a little wait for delivery, arrive. 

A little while is spent mounting the components. 

The four SIT resistors are set for no attenuation to start with. (R11 & R1 S/C, R12 & R5 O/C)

... and as usual, an error is spotted... thankfully it's just on the legend for the FTDI, the 5V and GND are reversed! 

It's all wired up into the enclosure... I'll tidy it up later, after we're happy with the performance...

WSJT-X is loaded up onto the Raspberry Pi400, and set up, the interface connected to my FT-817, and we're decoding!

The two trimmers are adjusted on the PCB, to give an indication of level on the LED indicators...

Incidentally, my friend Pyers 3D printed the front and rear panels for me.

There's an issue though... the FT-817 won't transmit :( 

The PTT is working just fine, but there's no modulation.

A fair amount of time went into checking and re-checking cables, and audio levels, then caution was thrown to the wind, and the FT-817 gets it's first ever factory reset in the many years I've owned it... after resetting menu 26 to USER-U , it bursts into life and transmits. 

A few minutes later and our first QSO on 2m (144MHz) is complete, with Stuart, G0LGS a few miles down the road in Cheltenham.  

PSK reporter also reports my tiny signal is being heard just outside of Cork, in Ireland, and, although I can't hear him, that's not bad for 5W into a discone antenna!

I've got a number of boards left over... if you'd be interested in one, drop me a line.

The Mirfield Electronics ME-64270 Quad-band 6m, 4m, 2m and 70cm antenna review.

Now I'm not usually one to review things, but this is a bit unusual...

It's a 4 band ham radio antenna. It covers 6m (50 MHz), 4m (70MHz), 2m (145 MHz) and 70cm (430 MHz)

The reason I want to give it some publicity, is the fact that it's small, at only 1.2m in length, covers 4m, and I had some good service from the supplier....

I'd been contemplating sticking something on the roof for a while, to get better access into the local repeaters for a natter on 2m and 70cm every so often. It's got to be fairly small so as not to aggravate the neighbours or the wife....

Then I spotted a small ad in the back of RadCom magazine for a quad-band co-linear antenna. It wasn't too pricey either, having 4m is really good too. Most quad-bands are 10m, 6m,2m and 70cm. I have a magnetic loop that works well on 10m, so that doesn't really interest me.

I placed an order on the website at www.mirfield-electronics.co.uk, and very soon a cardboard tube arrived.

So, it's Sunday. The weather was looking promising, I had half a reel of RG-8 co-ax cable that would do the job, only to be thwarted. One of the radial elements didn't have a thread, and the mounting brackets seemed to be missing... :(

I telephoned the number on Mirfield's website and left a message...

... and a super chap called Martin rang me back about half an hour later (on a Sunday too!). I explained the situation.










He was great, "What WAS I thinking, must have had a bad day!" He promised to post next day, and good to his word, a nice new radial, and the missing mounting brackets turned up on Tuesday.

So, It's Saturday, and once again the weather looks favourable.

I gather the parts and assemble the antenna....













The antenna is fitted with a SO259 "UHF" connector at it's base. Quite why anyone would call it UHF is beyond me. It's a pretty poor connector at VHF, let alone UHF.... anyway, it seemed good enough for Icom to put on the back of their UHF radios, so maybe it's just me... an N-type connector would have been preferable, but at this price point, I'm more than happy.





Having soldered a PL259 on to the RG-8 co-ax, slide the coax and connector through the support tube, making sure the screw hole is pointing towards the antenna. I also slid a piece of heat-shrink tubing on too... Don't do what I did, and forget to slide the mounting clamps onto the mounting tube first!

A couple of laps of self-amalgamating tape are used to give the connector some protection against the elements, and the heat-shrink makes a tidy job ....









... slide the mounting tube onto the bottom of the antenna, and do up the screw. The mounting tube is steel, and I wonder if it could have been made a little thicker for strength. Seems to work OK though...

When doing up the mounting clamps, I found it helpful to keep them flat against the floor, as when you do up the screws, they tend to move about a bit....

I've left the radials off until I get the thing up on the roof...












So, a couple of hours later, and it's up....

(Glad I did it too, the TV co-ax was in an awful state!)








So how's it perform?

Gain figures are quoted as 2.5dBi on 6m, 2.5dBi on 4m, 3.5dBi on 2m, and 5.5dBi on 70cm. I've got no way of checking those, but the antenna opens up repeaters on 2m I haven't heard for years, and fully quietening on local repeaters at just 5 watts. The antenna is rated to 60 watts.

VSWR is better than 1.5:1 on all the frequencies I'm interested in.

Good points...

The service from Mirfield was superb.
It's short.
It's cheap.
It's largely well made, and looks like it will stand the test of time.
70 MHz!

Minor grumbles...

SO259 "good buddy" connector.
Mounting tube could be more robust.