Band (MHz)EIRP (Watts) USA
902 - 9284
2400 - 2483.54
5725 - 58754

The passive radar tags will create a useful emission at the second harmonic of the radar frequency.

Free space loss, the major limiting factor in maximum standoff distance between tag and radar, increases ~ f2 ~ (1/lambda)2

We obtain this fact by inspection of the Friis free space loss equation:

path loss dB = 20 log10((4 π d f)/c)

Therefore, all else being equal (antenna gains, power, etc.) if 915 MHz is used as the radar transmit frequency instead of 5.8 GHz, we expect

20log10(5800915) + 20log10(116001830)= 32

dB less loss on the two-way path. So assuming I can use bigger antennas (radar and tags) on 915MHz to keep the same gain as on 5.8 GHz, I can use a 1 Watt radar at 915 MHz as effectively as a 1600 Watt radar at 5.8 GHz, with regard to maximum range. This may seem fantastic, but one would probably select an antenna for 5.8 GHz with more gain than at 915MHz, making the EIRP higher without a gigantic transmitter.

This is a strictly back-of-envelope approach, there are numerous factors to consider before selecting a frequency range for you application–contact us to discuss further.

We generally think only about the radar transmitter EIRP for unbiased tags, as the typical license-free emission limit (in the United States) of 500 uV/m @ 3 m = -41.3 dBm is easily met by the harmonic radar tag, since unbiased tags have 20-30 dB loss. Additionally, for the United States, the FCC has determined that such tags (like RFID) are passive devices and the certification is on the radar.
For the case of biased tags, the battery is separate from the RF (there is no RF interaction due to the battery, the battery just reduces losses of the diode) and a similar FCC argument holds.

Here is an incomplete listing of countries–note this doesn’t mean you can blast your radar across the whole band necessarily, you will need to examine the particulars for each country.

CountryFrequency [MHz] EIRP [Watts]
USA902 - 928 4
Mexico902 - 928 4
Japan 916.7 - 923.50.5 (4W licensed)
China 920.5 - 924.52
Hong Kong920 - 925 4
South Korea917 - 920.8 4
Australia920 - 9264
New Zealand 915 - 925
United Kingdom915 - 9214
South Africa915.1 - 9214

and very many more not listed here.

reference

With only 3 to 26 MHz  bandwidth depending on the country, by

Δ R > c/(2B)

we expect 40 to 6 meters range resolution. Depending on the application, one can make a workable system from that.

Since it takes a very strong signal, typically -30 dBm or more from the radar into the tag to generate a useful return, we don’t usually find interference on the radar transmit frequency to be a large concern. On the radar receive frequency, at the second harmonic of the transmit frequency, interference could be a concern for unmodulated radars using the 9151830 MHz pair, since 1830 MHz is in LTE band 3. One can select a radar modulation scheme such that uncorrelated transmissions such as LTE have a minimal impact. For other frequency pairings, consider the types of RF environments your end users will be in. Is is a large dock with high power X-band radars on the container ships? Is it in the vicinity of fixed microwave or satellite links?

Conclusion

Many times, we select what is convenient at hand for preliminary experiments, or follow on what a previous paper used. It can be easier to prototype at lower frequencies and longer range can be achieved, but ensure your application can support the required tag antenna size. Consider the regulations and interference environments of your target markets. Don’t expect more than 10’s of meters range with unbiased diode tags, unless you have some specific experiments or other evidence supporting.