In microwave planning, transmit power gets a lot of attention because it is a single, easy-to-compare number on a spec sheet and is often used for comparison in product RFPs. However, transmit power is only one input to link performance, and it is often not the factor that determines availability.
Focusing on transmit power alone is like looking at your favorite baseball team’s box score, seeing a high number of runs scored, and determining them the winner. While it is important to score a lot of runs, it only really matters that they scored more than their opponent that day. Considering both sides of the box score is the only way to know if your team won or lost.
In microwave links, that comprehensive box score view is the link budget. The link budget contains factors such as:
- Radio-specific factors: transmit power, receiver threshold, diplexer losses
- Architecture factors: indoor radios vs split-mount vs outdoor mounted radios determines where the link budget begins and if waveguide losses are part of the budget
- Antenna gain: from both transmitting and receiving antennas
- Other losses: taken by power splitters, couplers, filters, and any other equipment in the path on both transmit and receive sides
- Path or free space loss: as a function of frequency and distance
- Fade margin: as an additional margin planned into link budgets to account for environmental fading
The figures in the first bullet represent the direct impact of the radio on your link budget and are combined to define system gain. System gain is the difference between the radio’s transmit power and receiver threshold at the planned channel size and modulation. Effective system gain considers any losses taken by diplexers for radios which publish output power figures before diplexing and require additional (often external) diplexers.
Below are three common myths that can lead to apples-to-oranges comparisons. Each myth is paired with a practical way to evaluate microwave systems using link-budget fundamentals.
Myth #1: High transmit power guarantees a better link
A higher “max transmit power” specification does not automatically translate into a better link. Transmit power is only one input to the link budget, and the published number is not always directly comparable across vendors or architectures.
Transmit power is not constant. As modulation increases to deliver more capacity, output power typically declines, and different radios decline at different rates. In addition, published “max output power” is typically achieved only at the lowest modulation and is rarely the operating point used to deliver meaningful capacity. “Output power” may also be measured at different points in the RF link budget, such as before or after filter and diplexer losses. As a result, two radios can advertise different power numbers yet deliver similar end-to-end performance once these losses are accounted for.
Architecture can change the effective result at the antenna. For example, an all-indoor design may introduce waveguide loss that a split-mount approach avoids. A lower-power split-mount radio can therefore deliver higher received power in a like-for-like scenario. Comparisons should also confirm whether published specifications include losses from filters and diplexers or whether those losses must be added into the link budget separately. For comparisons, look beyond transmit power to system gain (transmit power minus receiver threshold) and compare system gain at the capacity being engineered.
Myth #2: Receiver sensitivity doesn’t matter—just increase transmit power
The receive side of the link budget is as important as the transmit side. Even when a system can produce higher output power, the signal still must be decoded at the far end. That capability depends on the radio’s receiver threshold, which is often listed as receiver sensitivity.
Receiver threshold is not a fixed value. As modulation increases to deliver more capacity, the receiver threshold will become less favorable. It can also shift with channel bandwidth. Different radios have different threshold curves, so systems can behave very differently at the modulation and channel size required to meet a specific capacity target.
The impact shows up in fade margin and availability. A worse receiver threshold effectively reduces fade margin and link robustness during fading. This is why radios should be compared using system gain (transmit power minus receiver threshold) at the over-the-air capacity being engineered, rather than relying on transmit power alone.
Myth #3: Capacity is whatever the Ethernet port says
Capacity is often presented as a single headline figure, but that figure can be defined and measured in different ways. Without a shared definition, comparisons can unintentionally mix Ethernet interface throughput with true over-the-air throughput.
For microwave engineering, the key number is typically over-the-air capacity because it is what consumes licensed spectrum. That capacity varies with modulation and channel size, and it can appear different depending on where it is measured (for example, at the Ethernet port versus over the air). Ethernet framing will increase the number seen at the port, especially with smaller frame sizes, because Ethernet headers are not transmitted over the air. Furthermore, differences in a radio’s spectral efficiency will deliver different capacities at the same channel bandwidth and modulation. One radio may be able to deliver a target capacity at a lower modulation than another radio, so the system gain comparison should be done based on what each radio needs to deliver that target capacity.
Additional techniques can further complicate comparisons. For example, header compression reduces non-essential header information so the effective rate can look higher, but it can also skew comparisons if one system’s headline number includes compression gains and another’s does not. For a stronger comparison, use over-the-air capacity as fully two-way communication and without header compression.
How to apply this in your next microwave comparison
Taken together, these myths point to the same conclusion: fair comparisons do not come from a single headline specification. They come from consistent assumptions and an end-to-end view of the link budget—accounting for external losses, receiver threshold behavior, and the target capacity you are engineering.
System gain can be treated as a budget you can spend in different ways—such as reducing antenna size, extending hop distance, increasing capacity, or improving availability. As a rule of thumb, 3 dB may allow a one-step reduction in antenna size (with results varying by frequency and hop length).
Additional comparison pitfalls include inconsistent capacity measurement (Ethernet port rate vs. over-the-air capacity, with or without header compression). Finally, high output power will come at a cost as radios which publish higher output power will likely have a significantly higher power consumption, size, and weight than another radio. Therefore, it is important to assess whether the greater power consumption or tower lease cost will offset any expected gains with the higher output power.
A practical takeaway: when comparing microwave systems, focus on system gain at the capacity you are planning for, and make sure link budgets include the same end-to-end losses (such as waveguide, filters, and diplexers). For capacity, compare what is actually transmitted over the air—fully two-way and without compression—rather than based on Ethernet port throughput.
Want the full walkthrough?
Watch the complete “Transmit power trap: Why ‘more power’ doesn’t guarantee a better link” webinar for the charts and examples behind these myths—and a full breakdown of additional transmit power myths that go beyond what we covered here.
Continue the conversation with Kevin and Andre at UBBA Summit & Plugfest — October 12–15, in Fort Worth, TX.
