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

Needing an RF switch to switch a single antenna between three 2m band radios, it was essential to design a system that achieved high switch port isolation. While switch port isolation is not as crucial for switching between antennas, it becomes important when transmitting from one port, as the ports that have receivers connected, even when turned off, need to be well isolated from RF leakage.

With this in mind, the specifications for the switch include a maximum RF power of 25 watts at a maximum frequency of 150 MHz. Additionally, a 12V DC switched circuit is included for a masthead pre-amp.

The specific requirements for the switch are as follows:

Maximum RF power: 25 watts (44 dBm)
Maximum frequency: 150 MHz
Minimum port isolation: -36 dB

It is important to note that these specifications are based on Ian (W2AEW)'s presentation on issues related 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 +20 dB (100 mW). However, as a matter of caution, Ian (W2AEW) decided to base his calculation on +10 dB (10 mW). It is important to note that even +10 dB may still be too much for certain receivers, such as scanners. Therefore, it is advisable to study the equipment specifications to determine their 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, as mentioned on page 5 of its manual, specifies that "In any configuration, the maximum input power to the RSP1 must not exceed 0 dBm."

Furthermore, the HackRF One has a maximum RX power of -5 dBm. It is crucial not to exceed this limit as doing so can result in permanent damage.

It is essential to carefully adhere to these power limitations specified in the respective equipment manuals to avoid any potential harm or damage to the devices.

Calculations

A 25W transmitter would represent 44 dB. Therefore, the calculation would be 44 dB (25W) - 10 dB (safe receiver exposure) = 34 dB (minimum required port isolation). Based on this calculation, the RF switch meets the required port isolation at the desired 146 MHz (2m band). However, it would not be suitable for higher power levels in the 2m band or for higher frequencies such as the 70cm band, even at realistic power levels.

Below 54 MHz, sufficient port isolation would allow for a 100W transmitter to be used. However, it's important to note that the current rating of the wafer switch used would likely not support power levels much beyond approximately 100W.

It's crucial to consider these factors when assessing the suitability of the RF switch for specific power levels and frequency ranges.

Construction

The switch is a simple 3-position, single-pole wafer switch. All lead connections between the wafer switch and BNC bulkhead mounts are made using RG58 coax. The coax shield braid is soldered together at the wafer switch, and the center core connections are kept as short as possible, around 6mm (1/4").

The 12V DC switched circuit for a masthead pre-amp has a short tail with a 30A Anderson connector and 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.  

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