HF Receiver Isolation Transformer

Description

It’s hard to keep noise out of HF radio receivers, particularly broadband SDRs used with wide-frequency antenna systems.  One of the most frequent problems is common-mode coupling, which means that unwanted signals are present on the outer shield of the antenna cable and get into the receiver that way.  Common-mode coupling is often the way noise from computers or switching power supplies gets into the radio.

A way to reduce this interference mechanism is to isolate the antenna shield from the radio chassis.  And one of the best ways to do that is to use an isolation transformer — a 1:1 transformer with its primary terminals connected to the antenna center conductor and shield, and its secondary terminals connected to the radio antenna input and ground.  The isolation between the two windings blocks the common mode current and keeps it out of the radio.

The TAPR Receiver Isolation Transformer is a small PCB with SMA connectors (male on one end, female on the other) and a MiniCircuits T1-1 transformer.  It does a good job removing common mode interference from the antenna system.  TAPR provides the circuit board with connectors installed and the through-hole transformer which  is user-installed (six solder connections).  A piece of clear heatshrink tubing acts as an enclosure.

Note that you can buy similar looking transformer boards from eBay and other Chinese sources.  The ones we’ve seen use a different, less expensive, type of transformer that is not as effective at blocking common mode noise.  The T1-1’s design ensures minimum capacitance between the windings, maximizing common mode rejection.

The T1-1 transformer has less than 1 dB loss from 1 MHz to well over 100 MHz.  It has less than 3 dB loss down to about 100 kHz.

NOTE:   This product is for use with receivers only.  It is not intended to handle any level of transmitted signal.

Buy this item, AND the RX888 Clock Kit, and we will ship them free in the USA via USPS Ground Advantage (equal numbers of each).  It’s rather than give a combined product discount, as we can ship anywhere in the mainland 48 states USA, USPS Ground Advantage, for under $6)