Last Updated on May 10, 2026 by John Berry
There are many reasons why a ham radio station may, on one band or other, suffer threshold degradation (TD).
Mostly, TD occurs through added presence of noise.
Simply, there should be a favourable receiver threshold, but because of noise, it’s degraded.
TD, and the similar parameter, threshold impairment, TI, are parameters from professional communications systems. They are specifically from the world of coexistence analysis between services.
Defining threshold degradation
Threshold degradation is simple to understand. Take a station operating moonbounce/EME with a Yagi and a long feeder. The feeder loss will effectively degrade the receiver sensitivity. The receiver sensitivity might be -159dBW for 0dB S/N (in a 2.5kHz bandwidth). That’s about -187dBW for a Q65 transmission. A 3dB feeder loss will degrade the receiver threshold sensitivity to -156/-184dBW respectively. More signal will be needed to overcome the noise caused by the feeder. And instead of seeing a signal report of -25, the report will reduce to -28, for example. Those previously reporting -28 will not be seen.
In receiver systems, loss causes noise. Noise causes threshold degradation.
The bulk of the gains and losses on the path from the transmitter are greyed to focus on the TD.
Typically, a radio amateur would employ a low noise pre-amp at the antenna to overcome the feeder loss. The LNA would be used to mitigate against the feeder loss. A cascade calculation would reveal that a 0.6dB noise figure LNA (with 20dB of gain) would recover much of the degradation. The result might be a TD of 0.5dB and a real threshold sensitivity of -158.5/-186.5dBW when the LNA is installed. Note my comments elsewhere about the decibel. Effectively, when we’re discussing points of a decibel, we’d round to the nearest dB.
Threshold degradation at HF
TD is also common on the HF bands where noise in the local environment degrades the receiver threshold. On HF, for example, mitigation might be to use a closed loop antenna and good earthing. In that case, the TD is recovered somewhat, perhaps from a 20dB degradation to perhaps a 6dB degradation. That’s three S-points down to one S-point. There are so many examples of threshold degradation – equipment losses, ground noise, Sun noise, antenna response and many more. The design aim is to identify the cause of the degradation and implement mitigation.
Typically, TD is a useful way of discussing the hit on performance that a ham station takes because of poor design. As a decibel value, it’s relative. It’s relative to the threshold that could be achieved or was achieved before the action that caused the degradation.
Threshold impairment
Threshold impairment is also simple to understand. If a service enjoys a particular level of performance, and stations from another service set up on the same frequency, it may suffer interference. It may suffer interference even if the new stations are some distance away. That interference impairs the first service’s performance. In modern systems, different services use different transmission schemes and hence interference appears as noise. At the November 2023 WRC meeting, regulators argued that the amateur service would impair stations in the radio navigation service. The regulators’ argument prevailed. As a result, the amateur service will need to re-plan its use of the 23cm band. The mitigation is to have us move frequency, thereby reducing the threshold impairment (TI) of the primary user’s stations to acceptable levels.
Typically, TI is referenced to the performance of thousands of stations without the noise or interference source.
The TI is then reported in those stations once the noise or interference source is switched on. In the spectrum management world, TI is difficult to determine on a national or regional scale. Some stations will be impaired more than others and statistics will be employed in descriptions. Typically, modelling is used to determine the TI levels and propose mitigation.
A typical acceptable impairment is that the victim service shall experience a TI of not more than 0.5dB in 10% of its stations (across the region, or whatever). So, we must all accept some interference, but not too much, to live together in harmony.
Application
TD, as I’ve used it here, is principally about the degradation in performance of a station because of the local environment or poor design. TI talks more about one station impairing the performance of another.
TD is also useful in ham radio because it’s use avoids getting into first principles calculations. These calculations can be inaccurate, with inaccurate and errored assumptions. In the end, ham radio equipment manufacturers specify receiver threshold.
Developers like Joe Taylor, K1JT, specify their (software) thresholds based on those from the (hardware) manufacturers. And they are typically based on the 2.4kHz SSB bandwidth.
So threshold is a useful point of departure and TD, a useful way of defining modified system performance.
When the front-end stages in a ham receivers are designed to a price, they may of course have poor threshold sensitivities. In that case, a good LNA working with a poor receiver threshold can yield threshold improvement (acting positively on the threshold). Another TI, not to be confused with the TI in impairment!