RF 3 position switch for either antenna switching or transceiver switching 0 - 150MHz. 

In order to connect multiple radios operating within the 144MHz - 148MHz band to a single antenna, a switch was needed. Upon evaluating the MK1 version of my antenna/radio switch, it was discovered that it barely met the port isolation requirement for the specified frequency range. This limitation resulted in the transmitter power being restricted to less than 25 Watts and posed a risk of inadvertently damaging the receiver of one of the radios. Consequently, a redesign of the radio switch became necessary.

Photo 1 In service 3 radio ports to single antenna port switch.

In order to address the need for improved switch port isolation, the RF switch must be designed to ensure effective isolation between the ports. While port isolation may be less critical when switching between antennas, it becomes crucial when switching between radios that transmit from one port, as the ports connected to receivers, even when turned off, require adequate isolation to prevent receiver front end damage.
 
Considering these requirements, the specifications for the switch are as follows:
 
Maximum RF power: 100 Watts (50 dBm)
Maximum frequency: 150 MHz
Minimum port isolation: -45 dB

 
Additionally, the switch design should incorporate a 12V DC switched circuit to accommodate a masthead pre-amplifier.

Assumptions

The above specification are based on Ian (W2AEW)’s presentation on issues relating to using an antenna switch to switch radio transceivers.

Refer to video link: https://www.youtube.com/watch?v=lMMql1gEORQ

The video suggests that many ham radio transceiver receivers are capable of withstanding up to +20dB (100mW), however Ian (W2AEW) decided that as a matter of caution he would base his calculation on +10dB (10mW). Even +10dB may be way too much for many receivers such as scanners etc., therefore study equipment specification to determine suitability.

Radio specification examples

·         The Icom IC-706MK2G transceiver’s service manual on page 1 states; DO NOT apply an RF signal of more than 20 dBm (100 mW) to the antenna connector. This could damage the transceiver’s front end.

·         The RSP1 SDR radio states on page 5 of the manual that; In any configuration the maximum input power to the RSP1 must not exceed 0dBm.

·         The maximum RX power of HackRF One is -5 dBm. Exceeding -5 dBm can result in permanent damage!

Calculations

A 100W transmitter would represent 50dBm

Therefore 50dBm (100W) – 10dB (safe receiver exposure) = 40dB (minimum required port isolation)

Based on the above calculation the RF switch just satisfies the required port isolation at the desired 146MHz (2m band), but would not be suitable at higher power at the 2m band and would not be suitable at higher frequencies such as the 70cm band at almost any realistic power.

Below 54MHz sufficient port isolation would allow for 200W transmitter to be used however the current rating of the relays used is 1 Amp per pole and are connected with two poles in parallel producing a rating of up to 2 Amps and would restrict power to 100W.

100W of RF at 50Ohms impedance will produce a voltage of just over 70V at current of just over 1.4Amps therefore the radio switch is really only suitable for power levels at or below 100W.

Design 

Fig 1 Schematic of the radio switch unit

Construction

The switch is a simple 3 position single pole wafer switch that activates a relay for each radio input. When power up only one relay can be activated at a time with the other relays switched to ground and when the unit is not powered up all relays will switch to ground to offer the radios some protection from the effects of lightning.

The relays are mounted to a grounded PCB with grounded PCB shield mounted between the relays. All internal coax interconnections are with RG316/U  

Photo 2 Relays mounted with grounded shielding.

C1, C2 and C3 are to ground any RF and prevent RF from travelling out on the DC supply.

The 12DC switched circuit for a masthead pre-amp presents a DC supply to a  2.1mm DC panel socket.  

Testing

Test was carried out using a NanoVNA calibrated for Mag with the switch switched to an adjacent port with a 50ohm load connected. The below results and chart show port isolation against various frequencies.  

Photo 3 Test set up for port isolation evaluation.

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