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Related Article: meshabout.com/converting-a-144-430-vhf-uhf-antenna-to-868mhz-for-use-with-meshtastic/
Let's convert a 144/430 VHF UHF antenna to 868MHz to use with Meshtastic
To do this you will need a few simple tools, such as a pair of cutters and a craft knife, as well as a Nano VNA or comparable analyser and a SMA male to SMA male coupler
This will allow you to modify antennas designed to be used on VHF and UHF handheld transceivers such as the popular Baofeng radios on a Meshtastic node. In this particular example, I will be modifying a Nagoya 144 and 430 MHz antenna. The antenna as standard is not designed to be used on 868MHz, so before we can use it we must try and reduce the SWR measurement. To achieve this we can use a device such as this Nano VNA that can measure and visually plot the resonance of an antenna, as demonstrated here using a tuned, stock antenna
Now let's check the SWR of the Nagoya antenna as its stands, but first, we will need to connect the SMA adapter, luckily as we will also be using the same adapter when fitted to the Meshtastic node we won't need to calibrate the Nano VNA to account for it, however, if that were not the case, the Nano VNA should be calibrated each time, the same goes for using a feeder or cable between the antenna and the Nano VNA
As you can see the SWR is currently measuring between 5 and 7, when it comes to SWR lower is better with 1 being perfect, and anything above 2 is considered less than ideal. So let's see how much better we can get this Antenna to resonate on 868MHz. First, we need to remove the plastic end cap, this may be glued on and require a little heat to slide off cleanly
As you can see the internal construction of this Antenna is twisted brass wire. The idea with this method is simply to ever so slightly reduce the length of the wire and reanalyse its SWR measurement in an attempt to get as close to 1 as possible
The cutter on these combination pliers eventually got the job done but a more substantial pair of cutters would have been beneficial for this task. So since I'm not monetized I'll speed this bit up
Now let's see what difference those few millimetres made and test the Antenna again on the Nano VNA. As you can see the reading is now between 4 and 6 so we are moving in the right direction. Now we need to repeat the process again and remove a little more. The next attempt and it's down to between 3 and 4, let's repeat. As you can see the Antenna is now reading a SWR of around 3.1, 3.2
While the SWR is still not ideal, you have to bear in mind that at some point as the antenna becomes shorter and shorter, the SWR will begin to rise as you take more and more off, and the results you get with this method will vary depending on the antenna
Now just carefully remove a little of the outer sleeving of the antenna with a craft knife and fit the plastic cap. At this point, it is important to retest the Antenna as the nib and sleeving is enough to affect it Although with this Antenna I couldn't get it below 2, I did reduce the SWR of this particular antenna by around 50%
I will be testing this antenna and comparing it against a few others in an upcoming video so please press like and subscribe if you have a spare second or two
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