**Measuring HF (2-30 MHz) Radars using ITU-R M.1177 and Time Sampling
Techniques**

**1. Background**

International Telecommunications Union Radiocommunication Sector (ITU-R)
Recommendation M.1177 has been developed within Working Party 8B as a
technique to measure the emissions of primary radar systems. The
measurement techniques in M 1177 have been used by the United States
administration for over 20 years, in various configurations of hardware
and software equipment to support its regulatory aspects of the Radar
Spectrum Engineering Criteria (RSEC). The recommendation uses a stepped
frequency technique, where the computer controlled spectrum analyzer is
set to a zero span mode and, using a peak detector, the emissions are
measured in a resolution bandwidth consummate with the radar system
parameters to obtain a precise power measurement. A complete radar
emission measurement is made by stepping the analyzer through many
frequencies to make a plot of frequency versus power in dBm. The
measurement system frequency stepping interval is normally equal to or
less than the measurement bandwidth. The stepping time interval is
determined empirically for individual radar modes, but is always be set
long enough to ensure that the radar fundamental tuned frequency visits
the measurement frequency (for the center part of the spectrum; the same
interval will be used for the out-of-band and spurious parts of the
spectrum).

The measurements that the United States administration has performed
over the years using the stepped frequency methodology have concentrated
on radars operating at frequencies above 500 MHz. These have been radars
with pulsed and FM pulsed and phase coded modulations. The measurement
technique had not been used for frequency modulated continuous wave
(FMCW) radars or for radars operating at High Frequency (HF) (2-30 MHz).

At the November meeting of International Telecommunications Union-
Radiocommunciation Sector (ITU-R) JRG-1A/1C/8B in Mainz, Germany,
questions were raised as to the applicability of the measurements
techniques in M1177 to FMCW waveforms, and whether or not a direct
frequency measurement could be made at HF for radar systems. Some
delegates suggested that M1177 could not be used with FMCW radars, and
that environmental factors (sky noise, power lines, ignition noise,.etc)
would corrupt the measurement system itself not permit a direct
measurement in the frequency domain. They suggested that a time-sampled
waveform transformed to the frequency domain using a Fast Fourier
Transform (FFT) was more appropriate.

### To investigate and resolve these issues, the United States agreed at the JRG meeting to undertake a set of measurements on HF FMCW and FM pulsed radars to test the different measurement techniques. A group of HF radars were identified for measurement. The radars are pulsed/gated or FMCW radars, and operate at one of three center frequencies in the HF band known as COastal raDAR (CODAR). CODAR radars are used to measure the surface currents of the coastal ocean. A transmitter sends out a radio frequency that scatters off the ocean surface and back to a receive antenna. Using this information and the principles of the Doppler shift, CODAR radars are able to calculate the speed and direction of the surface current**.** These radars operate on an experimental basis, since there is no allocation for radars operating in the HF band. In addition, they are low power and would be exempt from the US RSEC regulations.

The measurements were conducted by the United States administration
in/around Sandy Hook, New Jersey, at three CODAR sites operated by
Rutgers University, and at the Rutgers University Campus, also in New
Jersey.

**2. Objective**

The objectives of these measurements were to:

1)  Perform emission measurements in various ways to determine to best
    way to accurately collect the spectrum data of an HF FMCW and FM
    pulsed radars;

2)  Perform emission measurement on an FMCW radar to determine the
    correct slope of the spectrum roll-off from the -40 dB bandwidth;

3)  Perform emission measurements on HF ocean observing radars for use
    in future work on frequency allocation and regulatory status of
    CODARS.

**3. CODAR Characteristics**

The radar that was measured is an ocean observing HF radar. It operates
in 2 modes, a FMCW mode, and a gated mode. The radar has three different
operating frequencies, 4.53 MHz, 13.46 MHz, and 24.65 MHz. A list of the
parameters of each waveform is shown in Table 1. All FMCW waveforms have
the same structure, with the difference being in the waveform
parameters. The same is true for the gated waveforms. These structures
are shown in Figure 1, with the FMCW waveform being the top picture, and
the gated waveform being the bottom picture. At 4.53 MHz, the FMCW
waveform has a 25.6 kHz bandwidth, and a sweep time (T~sweep~ of
Figure 1) of 1 second. This leads to a chirp rate of 25.6 kHz/second.
When operating in the gated mode, this waveform uses a pulse period
(T~pulseperiod~ from Figure 1) of 1946 us with a 50 % duty cycle. At the
13.46 MHz center frequency, the FMCW waveform has 49.4 kHz bandwidth,
and a sweep time of 0.5 seconds. This leads to a chirp rate of 98.8
kHz/second. When operating in the gated mode, this waveform uses a pulse
period of 669 us, with a 50 % duty cycle. At the 24.65 MHz center
frequency, the bandwidth is 101 kHz, with a sweep time of 0.5 seconds.
This waveform has a chirp rate of 202 kHz/second. When in the gated
mode, the pulse period of 486 us, with a 50 % duty cycle.

Table 1

List of Parameters of the HF Waveforms

+--------------+--------------+-----------+-------------+-------------+
| Center       | Sweep        | Sweep     | Pulse       | Duty Cycle  |
| Frequency    | Bandwidth(   | Time      | Period      |             |
| (MHz)        | kHz)         | (         | (T~pu       |             |
|              |              | T~sweep~) | lseperiod~) |             |
|              |              |           |             |             |
|              |              | Seconds   | (µs)        |             |
+--------------+--------------+-----------+-------------+-------------+
| 4.53         | 25.6         | 1         | 1946        | 50 %        |
+--------------+--------------+-----------+-------------+-------------+
| 13.46        | 49.4         | 0.5       | 669         | 50 %        |
+--------------+--------------+-----------+-------------+-------------+
| 24.65        | 101          | 0.5       | 486         | 50%         |
+--------------+--------------+-----------+-------------+-------------+

![](media/image1.png){width="6.05625in" height="1.7472222222222222in"}

Figure 1 -- Waveform structure of the HF radar.

The top picture represents the FMCW signal, and the bottom picture
represents the gated signal.

**4. Measurement Setup**

Table 2 contains the list of equipment that was used to perform the
measurements. The system was transportable and a van was used to rack
mount the equipment in the field.

Table 2.

Equipment List for HF Radar Test.

+------------------------+----------------------+---------------------+
| **Measurement**        | **Model/Unit**       | **Specification**   |
|                        |                      |                     |
| **Device**             | **Description.**     |                     |
+------------------------+----------------------+---------------------+
| Pre                    | HP-85685A            | 10 kHz -- 1 GHz     |
| -selector/preamplifier |                      |                     |
+------------------------+----------------------+---------------------+
| Spectrum analyzer      | generic              | High-speed laptop   |
| controller computer    |                      | running NTIA radar  |
|                        |                      | measurement         |
|                        |                      | software            |
+------------------------+----------------------+---------------------+
| Spectrum Analyzer      | HP-8563A             |                     |
+------------------------+----------------------+---------------------+
| Spiky discone antenna  | Tecom 201191         | 20 Hz-1 GHz         |
+------------------------+----------------------+---------------------+
| Spiky discone antenna  |                      | 1-50 MHz            |
+------------------------+----------------------+---------------------+
| Noise diode for        | Noisecom or          | Up to 26 GHz        |
| Y-factor calibrations  | equivalent           |                     |
+------------------------+----------------------+---------------------+
| Vector Signal Analyzer | Agilent              | Up to 26 GHz        |
+------------------------+----------------------+---------------------+

**4.1 Test Conditions**

The temperature was around 20 degrees Fahrenheit with some snow on the
ground. For the last two measurements the other radars were turned off,
once a harmonic was observed in the spectrum. The gated radars were
measured at Sandy Hook State Park in a dune area. Measurements of the
FMCW system were made near the COOL laboratory in a loading dock area.

**4.2 Measurement System Calibration**

Calibration of the system was performed with a noise diode over all
frequencies of interest. The calibration routine is contained in NTIA
Report TR-05-420, "[Measurement Procedures for the Radar Spectrum
Engineering Criteria
(RSEC)](http://www.its.bldrdoc.gov/pub/ntia-rpt/05-420/index.html)."

**5. Emission Characteristics Measurements**

CODAR Emission characteristics were made in the frequency domain
(spectrum) using the spectrum analyzer and in the time domain using the
Vector Signal Analyzer. A pre-selector/preamplifier was used at the
input to the spectrum analyzer. This improved the measurement system
noise figure to approximately 7-9 dB.

Linearity of the measurement system was checked periodically by
inserting and removing attenuation at the measurement system front end.
The linearity checks verified that feed-through into the spectrum
analyzer was not affecting the measurement, and verified that the
external low-noise preamplifier was not exhibiting gain compression. A
diagram of the test set-up is shown below in Figure 2.

**5.1 Spectrum Measurement Modes**

The radar spectrum for each waveform was measured in using 2 techniques.
One technique is the standard NTIA stepped-measurement technique
outlined in TR-05-420 and ITU-R M.1177. The second technique is similar,
however it dwells longer (5 to 8 times) than stated in either TR-05-420
or ITU-R Recommendation M.1177 and uses the VSA to time sample the radar
waveform long enough to capture many waveform sweeps.

![](media/image2.png){width="5.303472222222222in"
height="4.7027777777777775in"}

Figure 2

**Diagram of Test set-up**

**5.1.1 Frequency Domain Measurements with Peak Detection**

The spectrum analyzer measurement resolution bandwidth, defined in
TR-05-420, is equal to 1 kHz for the FMCW mode. For the gated mode,
based on the given pulse period, the measurement bandwidths should be
3.6 kHz or less for the 4.53 MHz center frequency waveform, 8.6 kHz or
less for the 13.46 MHz center frequency waveform, and 14.4 kHz for the
24.65 MHz center frequency waveform. These were found by setting B~c~
equal to the chirp bandwidth, and$\tau$equal to the total pulse period.

The measurement resolution bandwidth was then defined as being equal to
or less than$\sqrt{B_{c}\tau}$. All the measurement bandwidths are
summarized in Table 4. Figures 3 through 6 show the measurements of the
radar waveforms made using the M1177 techniques in the frequency domain.
Note that high power external interference has been removed from the
peak-detected measurements from the 13.46 MHz gated FMCW waveform.

Table 4

Measurement Bandwidths for the HF Radar Modes

  ----------------------- ----------------------- -----------------------
  Center Frequency        FMCW Measurement        Gated Mode Measurement
                          Bandwidth               Bandwidth

  4.53 MHz                1 kHz                   3.6 kHz

  13.46 MHz               1 kHz                   8.6 kHz

  24.65 MHz               1 kHz                   14.4 kHz
  ----------------------- ----------------------- -----------------------

![](media/image3.wmf){width="5.654861111111111in"
height="4.163888888888889in"}

Figure 3

**Emission Plot of Gated Waveform at 4.5 MHz**

**Distance between measurement antenna and radar antenna was 60 feet**

![](media/image4.wmf){width="5.104166666666667in"
height="3.736111111111111in"}

Figure 4

**Emission Plot of Gated Waveform at 13.46 MHz**

**Distance between measurement antenna and radar antenna was 300 feet**

![](media/image5.wmf){width="5.148611111111111in"
height="3.763888888888889in"}

Figure 5

**Emission Plot of Gated Waveform at 24.65 MHz**

**Distance between measurement antenna and radar antenna was 60 feet**

![](media/image6.wmf){width="5.65625in" height="4.145138888888889in"}

Figure 6

**Emission Plot of FMCW non-continuous Phase Waveform at 13 MHz**

**Distance between measurement antenna and radar antenna was 60 feet**

**5.1.2 Corrections for Bandwidth in Peak Power Measurements**

For measurements of peak power in which the measurement bandwidth is
equal to or wider than the inverse of the radar pulse width, there will
be no correction for radar peak power\--the measured power value
accurately indicates the maximum power incident.

For cases in which the inverse of the radar pulse width exceeds the
measurement bandwidth, the incident power level will be computed as
20log of the ratio of the radar emission bandwidth (essentially the
inverse of the pulse width) and the measurement bandwidth.

As an example, if the radar pulse width is 50 ns (which equates to an
emission bandwidth of 20 MHz), and the measurement bandwidth is 8 MHz,
then the correction will be 20log(20/8).

As a second example, if the radar pulse width is 20 ns (which equates to
an emission bandwidth of 50 MHz), and the measurement bandwidth is 8
MHz, then the power correction value will be 20log(50/8).

**5.2 Time Waveform Measurements**

For the time domain measurements, data is needed that has not been
envelope detected (i.e. both the amplitude and phase information is
needed). Taking the FFT of time sampled data allows it to be transformed
to the frequency domain to see how this technique of spectrum
measurement compares with the technique in ITU-R Recommendation M.1177.
The time sampling of the radar waveforms was accomplished with an
Agilent Vector Signal Analyzer. Each radar waveform was sampled long
enough (8-10 seconds) to ensure that hundreds of sweeps were recorded.
The time and sampling rate for each signal using the VSA is shown below
in Table 5. The VSA recorded its data in a file format that was easily
readable by Matlab. A pre-amplifier/selector was *not used* in these
measurements so it does not have the same dynamic range as the frequency
domain measurements. The VSA was directly connected to the cable coming
from the measurement antenna. Software is till being developed for using
the VSA with the pre-selector that was used in the analyzer for the
frequency domain measurements.

**Table 5**

VSA Parameters

  --------------------- -------------------------- -----------------------
  Waveform              Sampling Time              Sampling rate

  4.53 MHz Gated        390.6 ns                   2.56 MHz

  13.46 MHz Gated       390.6 ns                   2.56 MHz

  24.65 MHz Gated       260.4 ns                   3.84 MHz

  13.46 MHz FMCW        1.95 us                    512 kHz
  --------------------- -------------------------- -----------------------

The FFT process was accomplished in Matlab, and plots of the results are
shown in Figures 7 through 10. Figure 7 shows the HF Gated FMCW waveform
at 4.53 MHz center frequency. To get a correct estimate of the
frequency, 520 pulses were needed in the FFT. This is because the
frequency sweep (T~sweep~ from Figure 1) takes approximately 514 pulses
(T~pulseperiod~ from Figure 1) to complete 1 cycle. Therefore, by taking
slightly more pulses, the entire frequency domain of the radar could be
represented. The same holds true for the other Gated FMCW waveforms,
where they needed 750 pulses (13.46 MHz center frequency waveform,
Figure 8) and 1030 pulses (24.65 MHz center frequency waveform, Figure
9) to show the entire frequency content of the waveform. Figure 10 shows
the HF FMCW waveform at 13.46 MHz center frequency.

> ![](media/image7.png){width="5.271527777777778in"
> height="3.9590277777777776in"}
>
> **Figure 7**
>
> **FFT of time sampled, HF Gated FMCW waveform at 4.53 MHz.**
>
> ![](media/image8.png){width="5.190972222222222in"
> height="3.8993055555555554in"}
>
> **Figure 8**
>
> **FFT of time sampled, HF Gated FMCW waveform at 13.46 MHz.**
>
> ![](media/image9.png){width="5.151388888888889in"
> height="3.8631944444444444in"}
>
> **Figure 9**
>
> **FFT of time sampled, HF Gated FMCW waveform at 24.65 MHz.**
>
> ![](media/image10.png){width="4.959027777777778in"
> height="3.7194444444444446in"}
>
> **Figure 10**
>
> **FFT of time sampled, HF FMCW waveform at 13.46 MHz.**

**6. Discussion of Test Results**

A discussion of the frequency domain and time domain measurements is
contained in the following paragraphs. A *comparison* of the two methods
is discussed in JRG paper "*Comparison of Radar Emissions using M1177
and Time Sampled Techniques*".

**6.1 Frequency Domain Measurements with M1177**

The plots of the radars emissions using the technique in M 1177 show
that it is a valid for measuring HF radars, if the measurement system is
properly designed and operated. In cases where the measurement system
antenna was placed close enough to the radar transmitter, the dynamic
range of the measurements was on the order of 70 to 80 dB. This is more
than enough dynamic range for regulatory purposes to measure the radar's
fundamental, out-of band, and spurious emissions components. The results
show that stepped frequency technique used in M1177 does not need to be
modified to accommodate HF radars. The methodology and techniques within
it are fine for measuring HF radars.

Some delegates questioned whether or not radar measurements at HF
frequencies would be corrupted by noise form external natural and man
made sources. Although some extraneous signals were present during the
measurements, they were easily identifiable from the radar emissions and
could easily be removed from the data post processing. In some cases, by
simply re-measuring a part of the spectrum that had an anomaly during
the measurement, the extraneous signals could be removed real-time.

2.  **Time Domain Measurements with VSA**

Plots of the radar emissions obtained by using a FFT to transform the
time sampled data to the frequency domain show that the spectrum can be
obtained in that manner. However, unlike with the frequency domain
measurements, it is not possible to review the time sampled data
real-time for a specific frequency component to verify that the
measurement system is still operating in a linear mode nor is it
possible to re-measure any part of the spectrum that appears to be
corrupted by extraneous signals. The time-sampled data is obtained "all
at once" due to its sampling nature. Furthermore, the resolution
bandwidth, sampling time and sampling rate of the VSA must be carefully
chosen to insure that enough signal components can be obtained to
transform the signal to the frequency domain for regulatory
applications. Clearly, these types of measurements need more
investigation before they can be applied to such purposes.