Transverters for VHF and Up: Use Your Radio on 144 MHz and More
A transverter is a box that lets a transceiver work on a band it was never designed for. You connect it to the antenna socket or to a special low-level output of your radio, set the radio to a convenient frequency, and the transverter shifts the transmit and receive frequencies up to a higher band, such as 144 MHz, 432 MHz, 1296 MHz or even 10 GHz. The name comes from “transmit converter” and “receive converter” built into one unit. Because one good radio then serves many bands, a transverter is the usual path of microwave operators, and it is also an economical way to try VHF and UHF weak-signal work without buying a new multimode radio for every band. This guide explains the principle, the numbers and the practical points that decide whether a set-up works well.
The Basic Idea: Mixing With a Local Oscillator
A transverter contains a mixer and a local oscillator, usually shortened to LO. When two signals are mixed, the output contains their sum and their difference. On receive, a signal on the high band is mixed with the LO, and the difference is a lower frequency, called the intermediate frequency, or IF, which is exactly what your transceiver can tune. On transmit, the transceiver produces a signal on the IF, the mixer adds the LO to it, and the sum is the high-band signal that goes to the antenna. The same oscillator is used in both directions, which is why the frequency relation is fixed. If you want the technical background of the process, the article on the transverter and the one on the superheterodyne receiver are good places to start.
A simple example is the 144 MHz band with a 28 MHz IF. The radio is tuned to 28.000 to 28.700 MHz, the LO runs at 116 MHz, and the sum 116 + 28 = 144 MHz gives the bottom of the 2 metre band. The operator tunes the radio from 28.000 to 28.700 MHz and covers 144.000 to 144.700 MHz. The tuning direction is the same as that of the radio, since the LO is below the signal. If a design uses an LO above the signal, the tuning direction is reversed and the sidebands are swapped, and the transverter must correct for it.
Typical Frequency Plans
The IF is chosen so that the transceiver does something useful on it. For 2 metres many people use a 28 MHz IF, because most HF transceivers cover 10 metres. For 432 MHz and higher bands, a common IF is 144 MHz, since a 2 metre all-mode radio is a convenient tuner for these bands. A few popular combinations are shown in the table. Notice how a harmonic of a reference oscillator is used to make the LO, a point that comes back later when we discuss accuracy.
| Target band | Typical IF | LO frequency | Comment |
|---|---|---|---|
| 144 MHz | 28 MHz | 116 MHz | HF radio becomes a 2 m rig |
| 432 MHz | 144 MHz | 288 MHz | Easy to build, often a multiple of 96 MHz |
| 1296 MHz | 144 MHz | 1152 MHz | Multiplied from a lower crystal |
| 10368 MHz | 144 MHz | 10224 MHz | LO accuracy becomes critical |
What Is Inside
A basic transverter has a receive path and a transmit path that share the oscillator. On the receive side there is a low-noise amplifier at the antenna input, a band-pass filter, a mixer and an IF amplifier. The LNA sets the noise figure of the whole system, which for weak-signal work is far more important than the exact value of gain. On the transmit side the IF signal from the radio passes an attenuator, then a mixer, a filter that removes the LO and the unwanted sideband, and a power amplifier. A relay or a diode switch changes the antenna between the receive and transmit paths, and a sequencer controls the timing, so that the relay does not switch while the amplifier carries power. The same sequencing ideas appear in our article about the mast-mounted preamplifier and its sequencer.
Many transverters are sold as kits or as boards, and a good number of operators build their own, since the circuits are well documented. Others buy a ready-made unit for 144, 222 or 432 MHz, and build only for the microwave bands. The filtering matters in all cases. The mixer produces not only the wanted sum, but also the LO, the IF and the image, and a transverter with weak filtering can transmit unwanted signals on neighbouring frequencies. The output filter is therefore a part of the design that should not be skipped.
Drive Level: the Most Common Mistake
A transceiver that produces 100 watts on its antenna connector is far too powerful for the input of a typical transverter, which wants somewhere between a few milliwatts and about ten watts, depending on the design. Connecting a 100 W radio to a transverter that expects 1 W is the fastest way to destroy it. The following methods are used to reduce the drive.
- Transverter port on the radio: many transceivers have a special low-level RF output, usually in the milliwatt range, for exactly this purpose. When it is available, use it and select the transverter function in the menu.
- Power setting plus attenuator: set the radio to its lowest power, often 5 or 10 watts, and add a high-power attenuator in the line. A 20 dB pad turns 10 W into 0.1 W.
- Built-in attenuator in the transverter: some units contain an adjustable input attenuator and a monitor for the drive.
Whatever you choose, check the power with a meter or a dummy load before you connect the transverter, and never rely on the display of the radio alone. Our guide to dummy loads and testing without radiating describes a safe way to do this, and the article on power and ALC on digital modes explains why a clean, undistorted drive is important, since a transverter will reproduce whatever the radio gives it, including distortion.
Transmit and Receive Sequencing
When the radio starts to transmit, the transverter must switch its antenna relay and turn on the transmit path before any RF reaches the power amplifier. If the RF arrives first, the relay may switch under power and burn its contacts, and an amplifier without a load can fail. The usual solution is a PTT line from the radio to the transverter, with a delay of some milliseconds built in. Most transceivers have a separate transmit control output or a relay contact that can be used for this. After the transmission the sequence is reversed: the RF stops first, then the relay returns, and only then are the receive amplifiers re-enabled. It is a good habit to check the sequence on a dummy load and to observe it with an oscilloscope or at least by listening for relay clicks before the carrier appears.
Why the Local Oscillator Must Be Accurate
The frequency of the transmitted signal is the sum of the IF and the LO, so any error of the LO appears in full on the air. At 116 MHz, an error of one part per million is 116 Hz, which is hardly a problem on SSB. At 1152 MHz the same relative error is more than a kilohertz. For the 10 GHz band, where the LO runs near 10.2 GHz, one part per million is about 10 kHz, and the signal may be outside the passband of the other station completely. Microwave operators solve this by locking the LO to a stable reference. A GPS-disciplined 10 MHz reference, as described in our guide to the frequency calibration of a transceiver, is a common choice, and it makes the frequency of the transverter precise and also repeatable from day to day.
The noise of the oscillator matters too. Every multiplication of a reference by a factor of N raises its phase noise by 20 log N decibels, and a poor LO produces a wide skirt of noise around your signal and limits what you can hear on receive. A clean, well-filtered reference and a good multiplier chain are what separate a strong microwave station from a weak one with the same output power. A related effect on receivers is explained in our article on phase noise and reciprocal mixing.
Practical Set-Up Advice
Start by deciding what you want to work. For 144 MHz weak-signal SSB and CW, a 28 MHz IF transverter plus a 100 W amplifier and a Yagi gives a station that can work meteor scatter and tropospheric lifts. For the higher bands, a 144 MHz IF is usual, with the transverter on the mast or close behind the antenna to keep the feedline loss low. At 10 GHz, even short coax cable loses so much that the transverter must sit at the dish. The IF line to the radio then carries 144 MHz, which is easy, as well as the DC supply and the PTT.
When you first power the unit, measure the supply voltage and current, check the LO frequency with a counter, and measure the output into a dummy load. Then adjust the drive level so that the output is linear: if the power does not grow in step with drive, the stage is already compressed and the signal is distorted. Then test the receive side by listening to a beacon on the band. Beacon lists exist for almost every VHF and microwave band, and a steady beacon helps to calibrate the dial, since you know its frequency. Our guide to the 23 cm band gives examples of what to look for in a UHF station.
Choosing Between Kit, Ready-Made and Home-Built
A ready-made transverter saves time and usually comes with a tested filter and a sequencer. A kit gives a good price and the pleasure of building, but it needs measuring instruments for the alignment, at least a power meter, a frequency counter and a dummy load. A fully home-built unit is the most work, and it is realistic mostly for those who already build microwave equipment. Whichever you choose, ask the same questions: what IF and drive level does it need, what is the output power, how clean is the output, and what reference does the LO use. Those four answers tell you more than the price does.
Common Problems
If the dial reads wrongly by a fixed amount, the LO is off frequency or you mis-entered the offset. If the dial moves in the wrong direction, the LO is above the signal and you have to use the opposite sideband. If you hear strong local signals on several places of the band, the IF stage may be overloaded, and an attenuator before the mixer helps. If the transmitted signal is wide or splatters, the drive is too high or the radio is overdriven. Hum or noise on the signal can mean a supply with poor filtering. Finally, if the relay chatters, check the PTT line and make sure the sequencer is not triggered by RF pick-up.
The Bottom Line
A transverter shifts the transmit and receive frequencies of your radio by mixing with a local oscillator, so one transceiver can serve many bands. The frequency plan is simple: the band equals the IF plus the LO. The things that decide success are less obvious: a correct and clean drive level, safe sequencing of the relay and amplifier, good filtering, and an LO that is accurate and low in phase noise, especially above 1 GHz. Check each of them on a dummy load before the first antenna test, and the transverter will repay you with new bands at a modest cost.
Frequently Asked Questions
What is a transverter?
A transverter is a unit that converts the signal of a transceiver to another band for transmit and to the transceiver’s band for receive, using a mixer and a local oscillator.
How do I calculate the LO frequency?
Subtract the IF from the target frequency for a low-side LO. For example, 144 MHz with a 28 MHz IF needs a 116 MHz LO, and 432 MHz with a 144 MHz IF needs 288 MHz.
Can I connect a 100 W radio directly to a transverter?
No. Most transverters need between milliwatts and a few watts. Use the low-level transverter output of the radio or reduce the power and add an attenuator.
Why is LO accuracy so important on microwave bands?
The LO error appears in full on the transmitted frequency, and it grows with the LO frequency. At 10 GHz one part per million is about 10 kHz, so a locked reference is normally used.
Where should I place the transverter?
As close to the antenna as practical, especially above 1 GHz, where coax loss is large. Run the 144 MHz IF line, the supply and the PTT to the unit.
Do I need a linear amplifier after the transverter?
Many transverters produce a few watts and are followed by a power amplifier. For SSB and CW the amplifier must be linear, otherwise the signal is distorted.