Ten meters or the 28MHz band is showing glimmers of life from sunspot cycle 25. Even so, its not an obvious choice if you want to build a portable SSB monobander and have lots of contacts. But 28MHz is the IF of choice for VHF and UHF transverters. After noticing the 6, 2 and 70cm transverters available on eBay from the workshop of UR3LMZ and the good reports from buyers, I cooked up the idea to build a 28MHz ‘transverter IF’ transceiver, to be paired with one or more VHF or UHF transverters.
The result is a homebrew single conversion superhet SSB monobander on 10 meters with a UR3LMZ two meter transverter in the case. Bringing the 28MHz receiver and transmit drive signal out to connectors on the rear panel makes this a tunable IF for an external 6m or 70cm transverter down the track.
Homebrew 28MHz SSB transceiver for a UR3LMZ 144MHz transverter (Summit Prowler 8) by VK3HNTweet
The objective of this project was to get a portable, compact 2m SSB transceiver for SOTA and portable use, something with sufficiently good performance to match some of the older 2m SSB superhet rigs. I toyed with the idea of homebrewing a dedicated 2m SSB transceiver, probably achievable with an si5351 as the VFO and a conventional IF. The 144MHz stages would need special care.
However I decided that buying the VHF module and installing it in front of a monoband HF transceiver would be much more achievable and more versatile. I’ve never built a transceiver for any higher frequency than 17m so getting good gain and overall performance on 28MHz was a challenge in itself.
Design and schematic
The design of the transceiver is conventional — a BF961 dual gate MOSFET RF amp with bandpass filtering front and back, a Minicircuits L7 diode ring mixer (JMS-1), post mixer Class A amp (2N3866), three stage IF amplifier using pairs of BF246 JFETs, SA612 Gilbert Cell product detector, audio preamp and a ULM2002 power amp.
On the transmit side, the mic amp, balanced modulator and transmit mixer use 2N5245, TL072, LM1496, LM1496 respectively. At 28MHz a 2N3904 predriver and BFU590GX driver excite a Mitsubishi RD16HHF1 for around 5 watts. More than the usual number of bandpass elements were added (easier to achieve in a monobander) to give the 28MHz signal a higher degree of spectral purity.
CW was omitted in this rig on the hunch that in VK what limited 10m/VHF/UHF activity to be found would be telephony. That eliminated some of the sockets and circuitry normally required by this mode and simplified the build.
The full kicad schematics are here.
The case is an aluminum angle and sheet creation. This one uses 32 x 32 x 1.5mm angle for the front, rear and sides, with a snug fitting 2.5mm sheet base. The top is 1.25mm sheet, cut at the supplier.
Small brackets are added around the top of the front, rear and sides, dropped by 1mm to allow a sheet aluminum top to drop in.
VFO, BFO, Controller
I built yet another Arduino Nano and si5351 VFO, BFO and Controller onto a narrow 28mm wide PCB. It had to be this narrow to fit vertically into the 32mm high case. After discovering that the si5351 breakout board was 2mm too high I took to its right side with a miniature grinder, turning 2mm of it’s edge into dust, while watching for any evidence of a copper trace. Fortunately none were found.
This module consists of a Yaesu XF-8.9HS crystal filter, found on a Hamfest table a few years back. It is from the FT-901, is on the unusual frequency of 8987.5kHz, and is 2.4kHz wide. The three stages of tuned amplification after the crystal filter are taken from EI9GQs book and use a pair of JFETs in a cascode arrangement. This module showed signs of instability, so brass plate shield was installed which made some difference. However to really get it to settle down it was necessary to add a 5k6 damping resistor across the tuned circuits of the first two stages.
Receiver front end
Nothing special here, a BF961 RF stage, JMS-1 mixer, balanced tee diplexer (see below) and 2N3866 post mixer amp. A MMBT2222 gets the VFO level well above that required by the L7 mixer (a pluggable 3dB pad sets the level on mixer VFO input).
The receiver uses a 4-pole bandpass filter on the front end (EI9GQ) and a two pole one on the output of the RF amp. This is probably excessive filtering but I chose it to get a clean signal to the mixer. The diplexor is built on a small board off to the side. The receiver front end module is quite stable, despite the fact that the signal path snakes around from one side to the other, navigating shielded compartments, slightly violating the golden rule of keeping outputs well away from inputs.
The exciter is a familiar sequence of microphone amplifier, balanced modulator, then off to the crystal filter and IF amplifier. Upon return the 9MHz SSB signal is mixed with the VFO to 28MHz using another LM1496 Gilbert Cell mixer to get to 28MHz.
28MHz driver and PA
The output device is a Mitsubishi RD16HHF1. This FET has flat gain to 50MHz and the convenience of a source (not drain) tab making it easier to mount and heatsink. From the LM1496 transmit mixer there are two gain stages, an untuned 2N3904 common emitter stage followed by 28MHz bandpass filter to a BFU590GX driver, to a broadband collector transformer and followed by a low pass filter. Output power is 3.5 watts with a bias of 150mA. Being a monoband exciter, it is easy to build in bandpass and low pass filters in place on the boards.
The UR3LMZ 2m transverter was ordered mid lockdown via eBay and took around 10 weeks to arrive due to backed up freight around the world — no fault of the seller. Once it arrived in a small but exotic world-weary looking parcel with Ukrainian stamps and travel scuff marks, I was impressed by the quality and construction of the unit.
The transverter is conservatively designed and uses a Minicircuits ADE mixer, BF series MOSFETs and solid state T/R switching. I fired it up on the bench with care and heard the local beacons solidly. So far so good.
On transmit, the required drive is just 50mW, so I was fastidious about working out how to drive the thing. If you are using an FT817 you have no choice but to dissipate your 5 watts of RF power and sniff a bit off as drive. But when you build the exciter from scratch you can take the drive from wherever you want. So I set up a relay switching arrangement to route drive from the BFU590GX stage, via a pi attenuator, to the transverter.
On the bench, with the ability to vary drive, and with judicious access to the transverter’s onboard RF drive and PA bias trimpots, I had full control, and quickly saw 10 watts of 2m SSB output. A YouTube video of this transverter on a test bench with some serious test gear indicated that spectral purity is considerably better if it is throttled back to around 3 to 5 watts RF output.
Band switching turned out to be a challenge in routing the low level exciter drive and the outputs from the two separate PAs around. This was achieved using miniature telecom relays.
On 28MHz the single conversion superhet worked very well, delivering a sensitive receiver and a clean 4 to 5 watts SSB. A 28MHz quarter-wave vertical was erected on the house roof as a resonant 10m antenna. This allowed reliable reception of a local VK3 beacon for receiver testing. Although the 10m band is usually empty here in Melbourne, digital modes can be heard down the bottom end, and more recently, a mid afternoon sporadic E opening brought in VK4s peaking at 5 by 7. Ten meters is an amazing band when it opens.
The UR3LMZ two meter transverter also worked well on the bench. Once mounted snugly in the SP-8 case, some RF feedback became apparent. Three gain controls had to be judiciously adjusted, the microphone amp gain, and on the transverter board, the 28MHz drive level and the RD16HHF1 FET bias. After extensive monitoring using a digital power meter, oscilloscope and a local 2m receiver, an acceptable settings were found. The RF power on 2m needed to be wound back to around 5 watts to keep things under control.
Before going portrable with SP-8 a 2m antenna was required. I started with a homemade J-pole on a 6m squid pole. This got me on air from a nearby hill top, about a dozen contacts were made with stations all over Melbourne (most with high gain antennas), but the cross polarization penalty had to be addressed.
In QSO with Rob VK3MQ and Chris VK3KIH one Sunday morning, Rob mentioned the Hentenna, a wire loop with horizontal polarization. I knocked one up and immediately got reports from Melbourne stations around two S-points better.
After a few months, David VK3KR shared plans for his 4-element DK7ZB collapsible yagi. It is constructed from 20mm PVC drilled for 6mm aluminium elements. The driven element is configured as a dipole and is quite cunning. The inside dimension of 6mm aluminum tube can be tapped for an M5 bolt. The PVC pipe is drilled for a pair of M5 bolts, retained by one nut. Each driven element then twists easily onto the protuding M5 bolt’s shaft. A picture illustrates this much better than words. A coax matching transformer is added, wound conveniently around the PVC pipe. I’ll post on this sometime soon. Now with 6dBi gain antenna, I was a 2m QRP portable Big Gun!
SP8 was a most enjoyable lock-down project. Use of the UR3LMZ transverter was a good choice. It’s performance is good on both receive and transmit. Special care must be taken to limit RF drive to a low level. Also, others results and my experience confirmed that the transmitted SSB is cleaner at 5 watts than 10. The lost 3dB is more then compensated for by a lightweight 3 or 4 element yagi.
One interesting thing I wasn’t used to was a few hundred hertz of receiver drift in the first few minutes after switch-on. This is the 116MHz overtone oscillator in the transverter. I’m not used to drift after adopting si5351 clocks as VFO and BFO. In some images you may notice the display sometimes shows 100Hz low (For example, 144,099.900).
Most of my portable operation has been on two meters with this rig. Being able to carry a 4 element beam, rig, small pole and battery in a light bike pack is fantastic and brings local hill-top operations into range. Once up on a hill, elevation and line of sight is everything on 144MHz. Two meters SSB has turned out to be an excellent band for postable QRP operation. Signals can travel hundreds of kilometers and are usually strong at both ends. And the portable QRP operator benefits from the high gain DX-catching antennas of the serious 2m base stations. just make sure you are horizontally polarised. Two meter portable operation is highly recommended!