Hands-on construction projects have been popular with Cray Valley RS members, giving members a chance to learn, experiment, and expand their skills while creating useful tools for their amateur radio activities. For the past two years the Cray Valley RS club construction projects have had a Morse Code theme. This year, we’re excited to offer two entirely new projects, catering to different interests and skill levels.
Pocket Transmatch
QRP Antenna Matching Unit from Kanga Products
Those who attended the club meeting on 5th December 2024 had an opportunity to see a fully built and working example of this pocket-sized QRP ATU. The Pocket Transmatch is a compact QRP antenna tuner capable of handling up to 10W. It covers bands from 10m to 80m, making it an excellent match for popular QRP rigs such as the (tr)uSDX, FT-817/8, or QRP Labs kits. Designed for portability, it features a sturdy plastic case and uses BNC connectors for external connections. Externally the unit is of high build quality with a very solid and professional look and feel. The internal build-quality will be in your hands of course, since this is a kit! The kit is beginner-friendly, with no surface-mounted components and just one toroid to wind. It’s also practical — its Tune mode protects delicate QRP rigs from high SWR by absorbing reflected power, while a bright LED indicator simplifies tuning for optimal performance.
Absorption bridge and reflected power indicator
The Pocket Transmatch has a switch on the front that selects between Operate and Tune modes. In Operate mode, the absorption bridge and reflected power indicator circuit is bypassed, and the transceiver is connected directly to the matching circuit. When Tune mode is selected, the absorption bridge ensures that most of the reflected power from a mismatched antenna is absorbed (and radiated from resistors), which means that in this mode, your transceiver is protected and never sees an SWR worse than about 2:1 even when the antenna is completely unmatched. This is particularly useful for simple QRP transceivers like the (tr)uSDX that have delicate finals and no protection.
The other feature of this Tune mode of operation is the reflected power LED indicator, whose brightness depends on how much reflected power is seen. The higher the reflected power (corresponding to high SWR) the brighter the LED lights up. The best match is achieved when the LED is extinguished.
“Z-match” design, selectable between balanced and unbalanced feeder
Whether you’re fine-tuning a dipole or operating a doublet with balanced feeder, the Z-match circuit ensures efficient matching for impedances between about 20 Ω and 300 Ω. It works well with resonant antennas like end-fed half-waves when small corrections are needed if adjusting the length is difficult. Direct connection to a long wire (which has an impedance of several hundred ohms or more at the feed-point) is not recommended. However, from my own experience I can say that it works reasonably well when used with a random wire antenna with a 9:1 UnUn. This unit is also great to use with a resonant antenna (e.g. a dipole, or an end-fed-half-wave with 49:1 UnUn) when you need to correct any mismatch. So for example if you have cut your antenna for resonance on the phone portion of a band but you want to operate CW, the Pocket Transmatch will allow you to operate safely at the lower frequency if the SWR is otherwise too high.
The kv4p HT
Turn Your Android Mobile Phone Into a 2m FM Pocket Transceiver
This fun and easy-to-build kit is perfect for experimenting on the go and offers surprising performance for its size. Its compact size makes it perfect for portable use, quick tests in the field, or as an emergency backup transceiver.
Once built, this unit sticks to the back of your phone using a removable sticky-gel pad, and takes its power from the phone’s battery through the USB connector. The HT uses your phone’s built-in microphone and speaker, with a button on the phone’s touchscreen functioning as the PTT.
The kit is suitable for beginner constructors and is very easy to put together. It does require a bit of soldering, but nothing difficult and there are no toroids to wind.
The designer of this kit, Vance KV4P, has made the whole project open source. Both the hardware and software designs are free to use, copy and even adapt or modify.
The unit delivers just under 1 Watt of power, and includes a low-pass filter that ensures the output is spectrally clean with good harmonic suppression. It’s powered from the phone’s battery via the USB-C port on the phone.
At the heart of the design is the DRA818V radio module (the same module can also be found inside popular HTs from Baofeng and other manufacturers). Controlling the radio module is an ESP-32 microcontroller board which communicates with a custom App running on the Android phone via the USB port. The kit includes an SMA female connector, suitable for connecting SMA male VHF antennas.
This kit is not meant to replace your existing 2m transceiver. The kv4p HT is experimental, but it’s inexpensive, easy to build, and lots of fun. It’s so small that you could keep it in your pocket so that you can get on 2m at any time.
Compatibility
- Requires an Android phone with a USB-C port running Android 8.0 or later.
- Not compatible with iPhones.
The basic kv4p HT kit offered to members includes the case and USB-C connectors (a U-shaped connector and a short USB-C cable). An antenna is not included in the basic kit, but can be ordered with the kit.
Antenna options (SMA male antenna):
- Bingfu VHF/UHF pocket-sized antenna (shown in the photo)
- Retevis RHD771 VHF/UHF flexible 40cm antenna
Club Construction Project Workshop
The club will hold a construction project workshop evening so that participants in the club project can bring along their kits for building, testing, troubleshooting or just showing off! There will be more details in the next edition of QUA.
Further Information & Links
There are further details in the January 2025 edition of the club newsletter QUA.
- Using the Pocket Transmatch with a Doublet antenna:
https://youtu.be/ZI2NyR-foW8?si=cBqEEIn_uFEatnLw
- kv4P HT : https://www.kv4p.com/index.html
How to participate
Interested in building one or both of these kits? Please email Dav M0WDV directly at [email protected] by February 28th, 2025 to reserve your kits.