Tucson IsoTennuator Installation and operating instructions

IsoTennuator Attenuator and UnBias Tee
for the GNSS Unfiltered Splitter (“GUS”)
Assembly and Operation anual
IsoTennuator – Revision A
Revised: 30 August 2023
©2023 Tucson Amateur Packet Radio Corporation
Introduction
The IsoTennuator is an RF attenuator that incorporates a DC bypass path so it can be used with DC voltage
present on the coaxial cable, as when a GNSS receiver is powering its antenna. The UnBias Tee is a DC
block that includes LEDs to indicate presence of DC voltage. The GUS Accessory Kit includes 3
IsoTennuator boards – one each with 3 dB, 8 dB, and 15 dB attenuation – and one UnBias Tee board.
The main purpose of the IsoTennuator is to compensate for the gain of an in-line amplifier. For example, the
LNA in the GUS antenna splitter has about 8 dB gain from input to each of the outputs. Sometimes that
amplification results in too much signal for the receiver being used. Adding an 8 dB IsoTennuator at the
antenna input compensates for the LNA gain.
You can’t use a standard attenuator for this application, because its resistors provide a DC path to ground
which will draw power from the source, resulting in wasted power as well as a loss of voltage delivered to the
antenna. The traditional way around this is to put a DC power injector (“bias tee”) on either side of the
attenuator to break the DC path but allow the RF signal to go through the attenuator. That’s kind of messy, so
the IsoTennuator combines those three elements on one small circuit board with S A connectors.
The UnBias Tee is a very simple board with S A connectors that blocks DC but passes RF. It includes
LEDs to indicate the presence of DC voltage on either port. It’s useful if you want to prevent DC traveling
upstream from a receiver or amplified splitter.
IMPORTANT NOTE: The IsoTennuator is not a precision device and shouldn’t be used for
measurements. The attenuation ma be ±0.75 dB from nominal, and frequenc response has not been
checked outside the 1 – 2 GHz range.

Circuit Description
The IsoTennuator is very simple. It is a pi-configured attenuator protected from DC by C1 and C2. DC at J1
flows via L1, C3, FB1, FB2, C4, and L2 to J2. This path passes DC but blocks RF and is essentially two
“bias tee” circuits connected back-to-back. LED D1 indicates that voltage is present. JP1 allows the LED to
be disconnected to reduce current consumption.

The UnBias Tee is even simpler. It’s simply a capacitor that blocks DC and passes RF. A bias tee on either
side of the capacitor provides a DC load and lights an LED if voltage is present on that port. The 220 ohm
resistors R1 and R4 mimic the DC load that an antenna would provide. This is to satisfy the antenna fault
circuit in some receivers that expects to see current drawn from the antenna connector. Jumpers JP1 and JP2
may be used to disable the LEDs if desired to reduce power consumption.

Assembly
IsoTennuator and UnBias Tee assembly involves only soldering the two S A connectors and optionally
using a utility knife to cut the trace at JP1 (and JP2 for the UnBias Tee) to disable the LED(s).
The small size of the boards can make it challenging to keep the connectors in place while soldering. A
clamp (I use a pair of pliers with rubber band putting pressure on the handles) or heat resistant tape can help
keep the board in place. The connector bodies will become hot when soldering the ground pins; don’t burn
your fingers!
The kit includes a piece of ½ inch clear heatshrink tubing that can be cut to length and used to protect the
circuit board after assembly.
ake sure you keep track of the attenuation of each board. Use an indelible marker to mark the appropriate
value on the back of the board before adding the heatshrink..
The resistors are 0603 size (i.e., tiny) and you’ll need a good magnifier to read their values. An easier way to
identify the boards may be to measure the resistance across R1 (the middle resistor).
Attenuation Center Resistor (R1) Top and Bottom Resistor (R2/R3) Measured Resistance
3 dB 24X 46A 16.9 ohms
8 dB 70X 07A 42.8 ohms
15 dB 14A 83X 70.0 ohms
In the table, “ easured Resistance” is the ohmmeter value between both sides of the center resistor (R1).
The resistors have 1/10 watt ratings, so the IsoTennuator is a receive-only device and should never be
connected to a transmitter.