By Bob Hawkins
Coaxial attenuators are resistive networks, pi or t networks, connector to RF / Microwave connectors. These attenuators are commonly used to adjust signal levels in military & commercial rf & microwave systems. When choosing the right coaxial attenuator for your application, one should keep in mind certain key parameters.
Connector type: SMA attenuators, BNC attenuators, & type N attenuators are very common. One can also buy fairly readily attenuators with TNC, 2.92mm, & 2.4mm connectors. There are a few manufacturers who offer QMA, 1.85, SMB, F, & Reverse Polarity (N, SMA, TNC) attenuators. Of course coaxial adapters can always be used with any attenuator connector, but we all prefer to avoid them unless necessary.
Connector material: Both Brass & Stainless Steel are commonplace. One must watch the associated torque values; a brass sma requires 3-5 in-lbs, a stainless steel sma requires 7-10 in-lbs, depending upon the manufacturer. Using a 10 in-lb torque wrench on a brass sma will usually twist the sma nut right off the part! SMA, BNC, Type N, & TNC attenuators are available in both materials, with brass being used for many commercial applications, & stainless lasting over increased mates/demates. Undertorquing certain attenuators, like the sma, can cause degraded performance at higher frequencies, often 15 ghz & above.
Power CW: For small signal applications to 18ghz 2 watts is the most commonly available attenuator, though there are a few 0.5watt & 1 watt attenuators also available. Above 18ghz the choices are less, often just 0.5watts at 50ghz. Likewise below 18ghz higher powers are available, with uncooled 500 watt units at 3ghz commonplace. Power CW is usually specified at room temperature, but derates as temperature increases; a 2 watt unit @ 25C may only handle 0.5watts @ 125 C
Frequency: As frequency increases, the resistive chips must be made with more precision, & hence cost more. Commonly available bands are 0-6ghz, 0-18ghz, 0-26ghx, 0-40ghz, & 0-50ghz, 0-65ghz. As the construction is simply a resistive network, the lower frequency range will always be 0. At the upper frequency limit is where one can expect the most ripple in passband response, & the widest deviation from desired attenuator.
Directionality: Small signal attenuators are bidirectional, either port can be used as the input. Most of the very high power attenuators are unidirectional, they have an input & output, & hooking the DUT backwards is normally fatal. This is because high power attenuators use cascaded attenuator chips; perhaps 2-3 db in the first chip, 3-5 in the second, 6-30 in the third, effectively spreading the heat to be dissipated along the length of the attenuator. Applying power to the output results in virtually all the power being dissipated in one chip which then overheats & fails.
DC Handling: Being resistive networks, coaxial attenuators aren’t intended to handle DC & will change the DC. Often the attenuator will dissipate too much heat in its resistive elements & fail. One can bypass the attenuator with dc blocks & bias-t’s. A few bias passing attenuators are available which incorporate the dc blocks & bias t’s.
Contact Materials: Almost all available attenuators use BeCu female contacts, & Brass male contacts. A few consumer applications will substitute brass for the female contacts for cost reduction, but these should be avoided for all but consumer applications.
Electrical Specifications: One can expect these specifications: VSWR, Attenuation value, Attenuation accuracy (tolerance), Frequency (upper), Power Handling CW, Power Handling Peak, Operating Temperature Range, Impedance, Connector Material, and Contact Material.
Common examples of these attenuators can be found online, such as at Sma attenuators and N attenuators.