A Simple Receiver Protection Device

Matt Roberts - matt-at-kk5jy-dot-net

Published: 2019-09-28

Updated: 2023-04-03

When operating on HF from home, I almost always use separate antennas for transmission and reception. Sometimes, it is also advantageous to run an independent receiver, that is not part of the transceiver at all, such as an SDR for a panadapter, or a separate transmiter and receiver for a low-budget or low-power setup. When using a configuration with independent TX and RX antennas, it is often advantageous to insert other devices into the signal path of the receiving antenna, such as preamplifiers or attenuators. In most of these cases, the receiving antenna system needs to be able to protect its connected devices from strong signals from the transmitting antenna, to avoid damaging sensitive small-signal components.

There are a number of commercial products on the market for protecting receive-only components, but some of these tend to be a bit pricey for my taste. In fact, spending $100 for a device to protect a $30 SDR dongle borders on the absurd, at least to me.

Figure 1
(click to enlarge)

As it turns out, even the pricey commercial protection products tend to employ fairly simple circuits that are easy to build from inexpensive components. Figure 1 shows a schematic of such a device, which I have been using for quite some time to protect a variety of receiver-related components. It is little more than a clipper circuit designed to pass small-signal RF, while clipping extremely strong signals.

The design is simplicity itself, but there are a couple of notes about part selection. My preferred diode for protection purposes is the Schottky. Because of the way Schottky diodes are constructed, they have essentially zero switching and recovery time. This means that their ability to shunt excessive voltage comes with very little overshoot, and they can be operated into the VHF range, if needed.

The diodes I use are very inexpensive parts in the 1N5817 family. These have maximum working voltages in the range of 20V to 40V, a V_F in the range of 0.3 to 0.9V, and an average current capacity of 1A. In a 50Ω or high-impedance recieve antenna system, these should be more than adequate for protection against power levels common in the Amateur service. They can also be readily obtained for less than $0.10 each in small quantities.

One design decision that should be considered when building such a device is what power or voltage level should be considered the "maximum safe" level for the receiver signal chain. This determines how many diodes need to be stacked in each leg of the "ladder" so that higher voltages are clipped, but lower voltages are passed unchanged.

E.g., in a typical stack of four diodes per leg (eight diodes total), in a clipper device using 1N5817 diodes with V_F of 0.32V, the point where the device starts to clip signals is 0.32V x 4 diodes = 1.28V. In a 50Ω system, that corresponds to a signal level of about 33dBm, give or take variation in the diodes. If a higher or lower clipping level is desired, the chain of diodes in each leg should be longer or shorter, respectively. Alternatively, diodes with higher or lower V_F can also be selected to set the clipping point as desired.

For low-gain antennas (such as small loops), a much lower threshold can be used, since the strongest expected signal levels from the antenna are lower, as is the pick-up from the transmitting antenna. For higher-gain antennas such as beams, verticals, dipoles, etc., a higher threshold is usually desired. If the option is available to place the protection device either before or after an outboard preamplifier, it should almost always be placed in front of the preamplifier, both to protect the preamplifier, and to allow the device to use a lower threshold without clipping.

The units I have built range from one to four diodes in each leg of the ladder. Some commercial units are more conservative, and use a dozen or more diodes in each leg. The selection of leg length depends on the intended usage of the device.

The capacitor isn't a required component, but I added it for some extra isolation from static charges. I normally connect the radio to the connector nearest the capacitor, so that any static charge from the antenna can drain through the diodes. If a capacitor is included on one or both sides of the diode ladder, it should be a ceramic disc, MLCC, mica, or other RF-appropriate low-loss type.

For best isolation and shielding performance, the circuit should be enclosed in an all-metal case of some kind, using panel-mount RF connectors. I use molded aluminum enclosures that are roughly 4"x2"x1". These are very durable project boxes, small enough to produce no noticeable insertion effects, but large enough to accommodate typical project boards for larger diode ladders. They are also large enough to easily accommodate bulk-head BNC or F connectors on the ends or sides.

While there are other ways to protect a receiver from strong transmitter signals, these simple devices are a very inexpensive way to get basic protection.

Some cautions are in order, however, when using the receiver protector described above:

If a receiver installation has issues with strong nearby broadcast transmitters, the appropriate filter should be placed in front of any diode-based receiver protection device, to prevent intermodulation distortion from being generated in the diodes.

Similarly, if you use DC bias voltages on receiving antenna lines, e.g., for driving remote preamplifiers, the DC bias should not be passed through this type of device. The DC bias voltage injector should have its unbiased receiver port facing the protection circuit.

This is not a switching device, which means that it cannot be used to isolate a transmitter from a reciever on a shared feedline. This type of device is only appropriate for protecting a receiver from overload from a transmitter whose antenna system is isolated from the receiving antenna system by an appropriate distance.

Perhaps most importantly, this type of protector should not not be relied upon as a lightning or EMP protection device. While diode ladders may provide limited protection against lower voltage spikes from nearby lightning strikes, the little 1-amp diodes will quickly fail if substantial impulse energy is presented to the antenna port. Lightning protection devices have a very different clamping range and method of action, and should be placed at an appropriate location between the receiver protector and the antenna system. Both types of devices are appropriate for a receiving antenna system, and they each play a role in protecting receiving equipment.

Copyright (C) 2019,2021 by Matt Roberts, KK5JY. All Rights Reserved.