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ANTENNA / RADIO SWITCH

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 and therefore a design that achieved a high switch port isolation was essential. Switch port isolation is not so important for switch between  antennas, however when transmitting from one port, the ports that have receivers connected, even when turned off need to well isolated from RF leakage.

 

With this in mind the specification for the switch are for a maximum of 25W RF at maximum frequency of 150MHz and also included is a 12DC switched circuit for a masthead pre-amp.

 

Maximum RF power: 25Watts (44dBm)

Maximum frequency: 150MHz

Minimum port Isolation: -36dB

 

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 25W transmitter would represent 44dB

Therefore 44dB (25W) – 10dB (safe receiver exposure) = 34dB (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 a 100W transmitter to be used however the current rating of the wafer switch used would likely not allow for power much beyond about 100W

 

Construction

The switch is a simple 3 position single pole wafer switch with all lead connections between the wafer switch and BNC bulkhead mounts are RG58 coax. All coax shield braid soldered together at the wafer switch and the centre core connections kept as sort as possible at around 6mm (1/4”).

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

Frequency MHz

Port Isolation dB

1.000

-70.00

3.500

-65.00

10.000

-59.50

15.000

-56.00

30.000

-51.50

54.000

-46.50

146.000

-36.70

435.000

-25.00

 

 

 

 

 

 

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Page last revised 30 June, 2022
 

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