Ditch the T/R Switch: How to Build a Smarter Panadapter for the Icom IC-735 Using an RSP1 and Raspberry Pi
If you're still using an MFJ or eBay-style T/R switch to feed your SDR from your transceiver, it's time for an upgrade. In this guide, I’ll explain why manually tracking your SDR is both clunky and risky, and how to get a true panadapter view by tapping the intermediate frequency (IF) in your Icom IC-735. We'll use a KD2S buffer board, an RSP1 SDR, and a Raspberry Pi with a 7-inch touchscreen for an elegant, modern solution.
The Pitfalls of MFJ/eBay T/R Switches and Manual SDR Tracking
1. RF Leakage and SDR Damage Risk
MFJ-style and generic T/R switches rely on relays or RF sensing to switch between transmit and receive. Unfortunately, these setups often allow high-level transmit RF to leak into the receive port. This is especially dangerous for sensitive SDRs like the SDRPlay RSP1, which can be permanently damaged by stray RF energy.
2. Switching Latency and Relay Bounce
Relays introduce delay—sometimes 10 to 30 milliseconds—when switching between TX and RX. This not only causes clicks and pops in your headphones but can cause SDR desynchronization or desense on receive.
3. Manual Frequency Tracking is Inexact
If your SDR isn’t connected to your rig’s control bus, you’re tuning two devices manually. For CW, digital modes, or contesting, this lack of synchronization is frustrating and leads to missed decodes and wasted time.
4. Messy Cabling and EMI Issues
Adding extra relay boxes, attenuators, and patch cables increases the chance of ground loops, noise pickup, and overall RF interference. Not exactly ideal if you're chasing clean digital signals.
The Better Path: Tapping the IF with a KD2S Buffer Board
Rather than intercepting the RF signal chain externally, the superior approach is to tap the IF signal internally. In the case of the IC-735, the 10.7 MHz first IF is available and makes an excellent source for a high-quality panadapter.
What’s in the Setup:
Icom IC-735 with an SMA connector installed on the rear panel for the IF output
KD2S IF Buffer Board, switchable for on/off control
SDRPlay RSP1 as the panadapter SDR
Raspberry Pi with a 7-inch touchscreen
FLRig to provide CAT control over CI-V
This combination gives you a rock-solid, frequency-locked, low-noise spectrum display centered exactly on your transceiver’s VFO.
Key Advantages:
1. No Risk of SDR Damage The KD2S buffer board provides RF isolation from the TX path, so your SDR remains protected even during transmit.
2. True Panadapter View You’re looking directly at the IF signal before any demodulation or filtering. You get a full, real-time waterfall aligned to your radio’s operating frequency.
3. Automatic Frequency and Mode Tracking Using the CI-V interface with FLRig, your Raspberry Pi keeps the SDR and rig in sync. Tune the IC-735, and the spectrum display follows. No more manual tuning of the SDR.
4. Cleaner Installation One SMA cable from the rear of the rig into your SDRPlay. No switching relays, no external RF boxes, and no added noise sources.
5. Built-In On/Off Control The KD2S board can be switched off when needed for troubleshooting or removing the SDR. It's clean and maintenance-friendly.
Where to Tap Power and the First IF in the IC-735
To install the KD2S buffer board inside your IC-735, you'll need to carefully access the main IF board and choose safe, appropriate points for signal and power.
Tapping the 70.4515 MHz First IF Signal:
The ideal tap point for a wideband panadapter is R327, located on the RF Unit and part of the 70.4515 MHz IF chain.
Connect the KD2S signal input (preferably via a short coaxial cable) to the junction of R327 using a high-impedance tap to avoid disturbing the signal path.
This location provides a clean, broad-spectrum view prior to any IF filtering.
Tapping 13.8V DC Power:
Use J19 pin 1 as your power source. This pin provides switched 13.8V DC and is easily accessible.
Always verify voltage levels with a multimeter before soldering, and include a fuse or series resistor for protection if space allows.
Ground Reference:
Solder the KD2S board’s ground wire to a nearby analog ground on the same board, such as a ground trace or a capacitor lead tied to chassis ground.
Note: If you mount the SMA connector directly to the rear case of the IC-735, the chassis itself provides ground. In that case, a separate ground wire from the KD2S board may not be necessary.
Avoid using a loose pigtail-style output cable that can snag or stress the delicate buffer board. A properly mounted SMA connector helps minimize strain and increases reliability.
Once installed, the output of the KD2S board goes to the SMA connector mounted on the rear panel, and your SDR connects there via a short RG-174 patch cable.
Raspberry Pi Integration for Full Control
The Raspberry Pi becomes your panadapter hub:
FLRig manages CAT control to the IC-735 over CI-V
SDR Software (e.g., CubicSDR, GQRX, or SoapySDR-based apps) displays the real-time spectrum
Touchscreen Interface allows for simple frequency control, band switching, and even macro buttons
You can even use hamlib’s rigctl server to route frequency information from FLRig to your SDR software for perfectly synchronized tuning.
If you’ve been living with an unreliable, failure-prone T/R switch and blowing out an SDR, it’s time to modernize. Tapping the IF output with a KD2S buffer board and routing it into an RSP1 on a Raspberry Pi not only protects your equipment, it unlocks the full visual potential of your IC-735. Add CI-V control via FLRig, and you have a powerful, flexible, and safe panadapter system with touchscreen control.
Need help building yours? Drop your questions in the comments, or follow along as I publish part two: a full installation and software configuration walkthrough for this setup.
—N5TTT