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--- 1420mhzfeed [2026/05/17 21:36] – [RF System and Performance] dsheen
+++ 1420mhzfeed [2026/05/17 23:43] (current) – [1420 MHz] dsheen
@@ Line 3: / Line 3: @@
 The 1420 MHz frontend is a receive-only system designed by Daniel Sheen (KC1EPN) for radio astronomy. It is tuned to 1420 MHz for the hydrogen emission line and incorporates custom bandpass filters to reject high power OOB signals which otherwise overload the LNAs. The LNAs used are the first generation Indium Phosphide devices designed for the deep synoptic array, and incorporate noise calibration diodes which are used for amplitude and phase calibration of the full RX chain.
-Tsys for this frontend is around 45 K within the 1420 MHz protected band (1400 - 1427 MHz), and it has a gain of about 36.3 dBi. Performance degrades rapidly outside of the band.
+Tsys for this frontend is around 45 K within the 1420 MHz protected band (1400 - 1427 MHz), and it has a gain of about 36.3 dBi (~2.9 degree beamwidth). Performance degrades rapidly outside of the band.
 {{::1420mhz_frontend_simplified.jpg?direct&800|}}
@@ Line 18: / Line 18: @@
 Additionally, we performed our own measurements at Haystack during early testing which are documented here:
- {{ :2023.003_cmt_ulna_measurements_at_haystack.pdf |}}
+{{ :2023.003_cmt_ulna_measurements_at_haystack.pdf |}}
 These devices receive calibrator control signals and power over their RF output connector. A dedicated control board located at the feed generates the required voltages and control tones and is in turn commanded by the SDR GPIO.
@@ Line 24: / Line 25: @@
 ===== RF System and Performance =====
-The 1420 MHz feed is a dual-polarized septum feed. One port receives left-hand-circular polarization and the other receives right-hand-circular polarization. More details on the original design and measurement of the feed may be found here {{ :2023.001_a_1420mhz_septum_feed_for_wr66.pdf |}}
+The 1420 MHz feed is a dual-polarized septum feed. One port receives left-hand-circular polarization and the other receives right-hand-circular polarization. More details on the original design and measurement of the feed may be found here:
-A subsequent, more detailed analysis of the system performance (prior to upgrades which substantially improved the system temperature) may be found here {{ :wmmaf_2024_001_full_system_temperature_models_for_the_westford_uhf_multimode_array_feed_and_the_wr66_l_band_feed.pdf |}}. Note that the performance of the system in its current configuration was dramatically improved by the installation of lower loss filters ahead of the LNAs. The current performance curves are as shown below.
+{{ :2023.001_a_1420mhz_septum_feed_for_wr66.pdf |}}
-{{ ::wr66_g_over_t_new_filter-2.png?direct&800 |}}
+A subsequent, more detailed analysis of the system performance (prior to upgrades which substantially improved the system temperature) may be found here:
-An accurate model for the antenna gain pattern can be found here in ticra format: [[https://github.com/dsheen2019/BigDishTools/tree/main/antenna_data|https://github.com/dsheen2019/BigDishTools/tree/main/antenna_data]]
+{{ :wmmaf_2024_001_full_system_temperature_models_for_the_westford_uhf_multimode_array_feed_and_the_wr66_l_band_feed.pdf |}}.
+Note that the performance of the system in its current configuration was dramatically improved by the installation of lower loss filters ahead of the LNAs. The current G/T and system temperature curves are shown below.
+{{ ::wr66_g_over_t_new_filter-2.png?direct&600 |}}
+An accurate model for the antenna gain pattern can be found on github in ticra format, along with scripts to read and manipulate it as needed for modeling observations:
+[[https://github.com/dsheen2019/BigDishTools/tree/main/antenna_data|https://github.com/dsheen2019/BigDishTools/tree/main/antenna_data]]
+The actual beamwidth is approximately 2.9 degrees with 36.3 dBi gain and between 25 and 26 dB suppression of the first sidelobe. The pattern is depicted below, and is discussed in further detail in
+{{ ::6_meter_dish_2024_001_antenna_pattern_modelling_and_measurement_overview.pdf |}}
+{{ ::big_dish_beam_pattern.jpg?direct&600 |}}
+The filters used for the frontend are described in detail in the two memos below. They add about 6 K of additional noise ahead of the LNAs, which is about a 20 K improvement over devices from minicircuits which were originally used for the first year of operation. Note that these limit operation to strictly within the 1420 MHz band and fall off rapidly outside of it.
+{{ :6_meter_dish_memo_2025_002_ultra_low_insertion_loss_1420mhz_filter.pdf |}}
+{{ :6_meter_dish_memo_2025_003_assembly_and_initial_measurement_of_ultra_low_insertion_loss_1420mhz_filter.pdf |}}
 TODO: Link simulations, analysis, electromagnetic simulations, calculated gain pattern, etc.
@@ Line 36: / Line 57: @@
 ===== Control Interface =====
+Calibrator control is via a dedicated control board located at the feed. This board accepts 12 V power and two opto-isolated GPIO pairs, and provides a 5 V supply for the LNA summed with the calibrator control tones. These are then combined with the RF lines from the LNAs by minicircuits bias tees.
+[[https://github.mit.edu/w1xm/ULNA_Bias_and_CTL_v2.git|Control Board Kicad Files]]
+The calibrator signals for the two channels are enabled by a 3.3V active high signal on outputs 0 and 1 of the Ettus X300 radio GPIO. Setting this requires running the following commands to configure the radio correctly:
+usrp0.set_gpio_attr(''FP0A'', ''CTRL'', 0x000, 0xFFF ^ 0b11)  #set first two bits to manual control
+usrp0.set_gpio_attr(''FP0A'', ''DDR'', 0xFFF, 0b11) #set first two bits as output
+After which the desired output stat may be set by
+usrp0.set_gpio_attr(''FP0A'', ''OUT'', <desired state>, 0b11)
+This can be implemented as a python snippet within gnuradio companion flowgraphs (note the correct alias for the radio must be used). Alternately, if using thor.py for recording, message passing can be used using the code here:
+[[https://github.com/dsheen2019/BigDishTools/tree/main/radio_client|https://github.com/dsheen2019/BigDishTools/tree/main/radio_client]]
+===== Other Documents =====
+See Also:
+MRT Phase and Amplitude Calibration implementation details. Relevant to the calibration of standalone observations, and to the implementation internal to srt-py ([[https://github.com/w1xm/srt-py/tree/dual-polarization|srt-py:dual-polarization]]):
+{{ :polarimetric_fits_import.pdf |}}
+{{ ::polarimeter_phase_calibration.pdf |}}
+{{ :new_phasecal_coefficients.pdf |}}
+{{ :6_meter_dish_memo_2025_001_computing_stokes_parameters_from_observations.pdf |}}
+Filter measurements:
+{{ ::measurements.zip |}}
+LNA data:
+{{ :copy_of_03oct2022_cmt_ulna_data_ganesh_to_daniel.xlsx |}}