SP3L's Antennas - Add a Panadapter to an Older-Generation Transceiver

Add a panadapter to an older-generation transceiver

When we compare an older-generation transceiver with a modern one, the main thing that strikes the eye is the absence of a spectrum analyzer in the former. Fortunately, any transceiver can be enhanced with such a function if you connect an external SDR receiver to it and use it as a panoramic spectrum analyzer (panadapter). It's not complicated, and you don't need to rewire anything inside the transceiver. Depending on what ports your transceiver is equipped with, you can connect the SDR receiver to it in different ways.

Method #1 - using an external T/R antenna switch

This is a universal method, suitable for virtually any shortwave transceiver. However, we must have an external transmit/receive (T/R) antenna switch in addition to the external SDR receiver. Figure 1 shows how these two devices are connected to the transceiver. The diagram uses exemplary device models, but the method is also applicable to the other types of T/R antenna switches and SDR receivers. When the transceiver is operating in receive mode, the signal from the antenna is passed through the T/R switch to both the input of the transceiver and the input of the SDR receiver. Thus, you can receive a correspondent either through the transceiver or through a PC connected to the SDR.

When the transceiver is switched to transmit mode, a strong HF signal from the transceiver's transmitter appears on its antenna port. The T/R switch detects its presence and immediately disconnects the SDR receiver from the transceiver-antenna connection. This prevents damage to the input stages of the SDR receiver. Usually, the T/R switch can also be controlled by the PTT signal coming from the transceiver. Therefore, if you do not completely trust the circuit detecting a strong HF signal inside the T/R switch, you can add such a connection (not shown in Figure 1). You will then be absolutely sure that the input of your SDR receiver will not be burned out during transmission.

Figure 1 Using an external T/R antenna switch with the FT-450D transceiver as an example.

In addition to the panadapter, Figure 1 shows other connections that you probably have if you have connected your transceiver to a computer, and you are using, for example, a logging program. You don't need to change anything in these connections because of the addition of the panadapter. By creating the circuit shown in the figure, you will be able to simultaneously use the SDR receiver application and the logging application (in this example, SDRUno and Log4OM, respectively). In order for both applications to communicate with the transceiver at the same time, you need to use one more program, used to split the computer's serial port. Probably the most popular program of this type is the free Omni-Rig. In the settings of both applications, you then choose not the number of the actual serial port of your computer, but Omni-Rig. On the other hand, in the configuration of the Omni-Rig program itself, you set the actual port number to which the transceiver is connected.

If your transceiver does not have a USB interface, but an RS-232 interface, and the computer has only USB interfaces, you additionally need an RS-232 to USB converter. In the case of Yaesu transceivers, it is good to use the SCU-17 converter of the same company. Of course, the baud rate setting of the Omni-Rig must be the same as that of the transceiver, otherwise you won't get communication.

In the circuit shown in Figure 1, the frequency and mode settings are synchronized between the transceiver and both applications. For example, if we change the operating frequency by turning the knob on the transceiver, both applications will update their frequency settings. If we change the frequency by clicking on the waterfall chart in the SDR application, the transceiver will tune to the new frequency, and also the logging application will update its frequency value. Finally, if we click on the selected DX in the logging application's DX Cluster, both the transceiver and the SDR application will tune to the correct frequency and set the correct modulation type.

Nothing prevents you from running some other application, for example, for digital modes. You just need to make sure it is compatible with Omni-Rig. It, too, will synchronize with the transceiver and the existing applications.

The synchronization described above is possible only for transceivers equipped with CAT (Computer Aided Transceiver) system, i.e. those that have an interface for connecting a computer. If your transceiver does not have such an interface, it is still possible to use a panadapter based on an SDR receiver in an arrangement as in Figure 1, but you will then have to tune the transceiver and SDR independently.

Method #2 - using the output for an external receiver

If your transceiver has a dedicated output for connecting an external receiver, you will not need an antenna T/R switch. The following older models of KF transceivers have such an output: FT-101, FT-101ZD, FT-102, FT-767GX, FT-901DM, FT-902DM, FT-920, FT-980, FT-990, FT-1000D/MP, FT-ONE, TS-520, TS-590SG, TS-870S, TS-930S, TS-940S. Models marked in bold have the CAT interface.

The socket for connecting an external receiver is usually described as: EXT RCVR, RX ANT OUT, REC or similar. However, in the TS-590SG, it is called DRV because this port may have different functions, depending on the settings in the transceiver. You should read in the manual of this transceiver how to configure this port to act as an output to an external receiver. Figure 2 shows how to connect a panadapter to this transceiver model. Reception is still possible either through the SDR and PC sound card, or through the transceiver. Given how highly regarded the TS-590SG receiver is, in this case, it's likely that any ham radio operator will prefer to listen through the transceiver.

Fig. 2. Connection of the SDR to the connector for the external receiver in the transceiver, using the TS-590SG as an example.

Method No. 3 - use of RECEIVER ANT IN and RECEIVER ANT OUT inputs/outputs

Some transceivers have not one, but two connectors relating to an external receiver and an external receiving antenna. Such connectors are usually described as: RX ANT IN and RX ANT OUT, but in some Yaesu transceivers you will find them named: TO u-Tune and FROM u-Tune. This is because a tunable bandpass filter (antenna preselector) can be connected to them, which is called by Yaesu: u-Tune Kit.

Figure 3 shows a simplified diagram, showing how the aforementioned inputs/outputs are connected inside the transceiver. When neither an external receiver nor an external receive antenna is connected to the transceiver, the RX ANT OUT and RX ANT IN are shorted to each other either by an external jumper on the back panel (see Figure 3A) or by internal relays (see Figure 3B). In order to use them to connect a panadapter, we have to disconnect them. In older transceivers, we simply remove the jumper. In the newer ones, we set the appropriate option in the menu and/or press the appropriate button on the front panel to activate the relays. You need to check in the user's manual how to perform this operation for a given transceiver model.

Fig. 3. Simplified block diagram of a transceiver equipped with RX ANT IN and RX ANT OUTPUT connectors.

And here are the models of KF transceivers having the mentioned inputs and outputs (whatever they are called): FT-950, FT-2000, FTdx1200, FTdx3000, FTdx9000D/MP, IC-701, IC-720, IC-735, IC-740, IC-741, IC-751, IC-761, IC-765, IC-775, IC-780, IC-781, IC-7600, TS-950S. Models marked in bold additionally have the CAT interface.

In method No. 3, a two-way signal splitter is needed. As you can see in Figure 4, its input is connected to the TO output of u-Tune. On Icom and Kenwood transceivers, this jack will be described as RX ANT OUT. The splitter passes the input signal both to the external SDR receiver and back to the transceiver through the FROM u-Tune jack. Ideally, the splitter should have an operating frequency range of at least that of the transceiver and have an input and output impedance of 50 ohms. Such splitters are not popular, hence their rather high price.

Fig. 4. Using the TO m-Tune and FROM m-Tune ports in FTdx1200 transceiver to connect a panadapter.

A compromise solution is to use a popular, inexpensive TV splitter for the 5-1000 MHz or 5-2450 MHz range and an impedance of 75 ohms. This introduces some mismatch, but its impact is hardly noticeable. As my reception tests showed, such a splitter introduced an insertion loss of 3 dB for frequencies above 5 MHz (as expected) and 3–6 dB for the 80 m and 160 m bands. At the lowest frequencies, we usually switch on an attenuator at the input of the receiver anyway, so the additional 6 dB attenuation is not very important. And, of course, the splitter attenuates not only the signal itself, but also the background noise to the same degree. The splitter I used, along with the accompanying cables, is shown in Figure 5.

Fig. 5. Two-way splitter with associated cables for connecting the panadapter to FTdx1200.

I soldered RCA plugs at the ends of short sections of RG174 coaxial cable and additionally used RCA to F-type connector adapters. An SMA-type male plug is mounted on the end of the cable attached to the SDR receiver.

With a panadapter connected to the transceiver, we have a better chance of calling a DX station. In the spectrum of received frequencies, you will immediately notice on which frequencies the strongest DX calling stations have positioned themselves. All you then have to do is tune in somewhere between them, and thus increase your chances of getting through the pile-up. This is especially useful when working with telegraphic emissions.

The panadapter also has the advantage of being able to quickly determine whether there is high or low activity on the band you have just entered. With just a glance, you don't even need to turn the tuning knob.

If you are using an aged transceiver, you may find that listening through an SDR receiver will give you better signal quality. The software of many SDRs (including the SDRUno) has built-in digital signal processing which allows a dramatic reduction in the noise and interference so typical, especially on the lower shortwave bands. Investing in an SDR receiver and connecting it as a panadapter can therefore significantly increase our capabilities and comfort.

Fig. 6. Radio station with a panadapter connected to FTdx1200.

The photo in Figure 6 shows a radio station retrofitted with a panadapter. Don't you think that after such a change, the old FTdx1200 can compete with modern transceivers?