Introduction: Ham Radio Bands 2 Metres/ 70 Cms Microstrip Pcb
Hi, this is Andy G0SFJ
I could not find anywhere in the literature any plans for microstrip antenna boards for the ham bands of 70 cm and 2 metres. They all seem to be for rfid devices or 2.4 Ghz or above.
So I set out to develop microstrip printed boards for these lower frequencies (146 Mhz and 430 Mhz in these examples) using folded copper strips onto pcbs by commercial manufacturers. These offer ten boards for 20 dollars including postage.
In developing these boards, this is how I did it (please note that I have used images which I have recorded during my learning):
Step 1: Moving From Copper Etching to Design Software
Previously I have used etching fluid and a special etching marker pen to create copper boards which have a design scribbled on them.Some of these are shown above. In these examples I had a copper ground-plane underneath.
Eventually I used a 10 *3 cm printed copper vero (strip) board, and joined 3 strips together top and bottom, to make an eighth wave length for two metres.
To calculate the total length, (v= f * lamda, where v = 300, f = 146 MHz), then divide the result by 8 to get an eighth wavelength, which fits on the board.
Without a ground plane, I found that it behaved like a "rubber duck" whip antenna, and I measured an SWR of 3.65 : the best I can say is, it's encouraging: it opened a repeater 10 km from me.
So now I have decided to try to standardise the boards.
To do this I needed to use "Gerber files". These are a suite of files generated by design software to send to a pcb manufacturer to make simple pcb boards.
I had used other peoples’ Gerber files, downloaded from sites on the github, to generate exact copies of small satellite boards such as $50 SAT and Kicksat-Sprite. The files were downloaded and sent to a manufacturing studio in Hong Kong/Shenzhen: they offered ten boards 10cm *5cm for about $20 including return postage.
It worked. I got the boards within about ten days.
Step 2: Using Gerber Files and Eagle
To design my boards, I downloaded Eagle which is free (you have to register, but that's OK): Autocad give a free licence for hobbyists, when you download Eagle you enter your email address.
There are loads of tutorials online but mostly they concentrate on transferring a circuit diagram (schematic) onto a board. My project is simple in that it just uses copper strip, but complex in that there is no tutorial for it. So in this note I am describing how I did it (so far). Those of you who know Eagle can start laughing now!
I had downloaded Eagle aeons ago - version 6 – and had half a dozen false starts with the manufacturers.
So I downloaded the latest version of Eagle. For win 10 this is 9.2.2. It’s a bit slow to setup on my pc.
Most important – I downloaded a CAM processor as well. The CAM processor is the gizmo that prepares the Gerber files from the design. Eagle 9.2.2. has a good CAM processor and I have also used one called OshPark.
Setting the frame for the board
I tripped up here at first but have now solved it. The first step is to set the layer to “20 Dimensions” then set the grid size. As you can see below, there’s a little box that allows me to set the background grid to 10 cm blocks and switch it on.
Step 3: Drawing the Simple Design in Eagle - 1
Then with the grid at 10 cms I drew a frame using the “Draw” and “Line” functions. I matched it to the 10cms * 5 cms for experimenters' boards.
the next step is to draw the track, and here I selected the maximum width.
Step 4: Drawing the Simple Design in Eagle - 2: the Track
Drawing the track
The next step is to draw the outline onto the grid. To make it easier I set the grid again to 5mm, and made it visible, zooming as necessary.
I found that the 1mm and below settings were too hard for me to see and to control.
Here I have used four components:
Via (the green dot thing) – these are at the joins of the track
Line – goes between each Via
Hole – I’ve put one off the track at each corner to mount the thing, also I have made a hole at each Via (I am not sure I need to do this).
I've made the total track length to be the length of the quarter or eighth wave I want at these frequencies.
Step 5: CAM Processor
This is the automatic clever bit. The CAM processor converts your design into the production suite of files, the Gerber files.
You can use other CAM processors but the one in Eagle 9.2.2 is fine.
As a bonus in Eagle 9.2.2, if you click on the "select zip file" box it will automatically zip up the files - and it's the zip file that yous end to the pcb manufacturers.
Step 6: Two Elements for 2 Metres
In these last images I used an online Gerber viewer to show my designs. Of course you can check them in Eagle from your original file.
This board has two separate antenna elements each slightly over 1/16th wave. My plan is to connect them in series and experiment with an inductor, either between the two boards (a centre loading) or at the feed point (base loading).
Two sets of these could make a dipole. Or a quarter wave whip.
It's all in the experimentation.
Step 7: One Element for Two Metres (times Two?)
Again using an online Gerber viewer, here is an approximate quarter wave copper strip for the 2 metre amateur band, contained on a 10cm * 5cm pcb.
Two of these could be a dipole.
Step 8: For the 430 MHz DMR Band
This one is a simple dipole designed for 430 MHz on one board .
These are DMR frequencies in the UK.
This size is easier to fit on the standard hobbyists' board size of 10cm * 5cm.
Step 9: Conclusion
I hope you'll agree that these boards are an elegant and reproducible solution to printing microstrip antennae for the 2 metre band (146 Mhz) and 70 centimetre band (430 Mhz).
These are the only designs I have seen for pcb antennae at these frequencies.
I see these boards could be suitable for applications such as small satellites (cubesats or smaller) and I'll be looking for opportunities there.
There could be more opportunities for low profile antennas.
Now you know these steps, I am sure you can improve on my designs, but i hope I have given you an insight into the potential.
73 de Andy G0SFJ
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