Differences

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--- 1296mhzfeed [2026/04/23 03:16] – olt
+++ 1296mhzfeed [2026/08/22 14:42] (current) – [Control Interface] dsheen
@@ Line 2: / Line 2: @@
 The 1296 MHz feed is an amateur radio feed intended for Earth-Moon-Earth (transmit and receive, half-duplex). It is a scaling of the design for the [[1420mhzfeed|1420 MHz Feed]].
+The [[https://www.rfhamdesign.com/downloads/septum-dish-feed.pdf|accepted convention within the amateur radio community]] for Earth-Moon-Earth (EME) communication is to transmit using right-hand-circular polarization (RHCP) and receive using left-hand-circular polarization (LHCP), since reflection off the Moon flips the polarization.
+Note that reflection off the dish flips the polarization, so we have configured this feed with the receive path on the RHCP port of the feed (which produces LHCP after reflecting off the dish) and the transmit path on the LHCP port of the feed (which produces RHCP after reflecting off the dish). The ports are labeled with Sharpie in case of confusion.
+There is around 23 dB of RF isolation between the RHCP and LHCP ports. This isolation is not sufficient to protect the LNA from high power transmit, so an additional TR relay is used to disconnect the LNA from the feed during TX.
 ===== Control Interface =====
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 The 1296 MHz feed borrows version 2 of the [[https://github.mit.edu/w1xm/Antenna_Base_Switching_Board|W1XM Control Board Design]] by Noah Haefner (K9EA). This board provides regulated 12 V power for the LNA, as well as opto-coupled control of the LNA power and TX/RX relay coil.
-Currently, we have replaced R6 and R11 with 510 Ohm resistors to make the board controllable by the 3.3 V I/O of the Ettus USRP X300 SDR. Measurements indicate that this modification causes the RX_EN pin to draw around 2 mA from the Ettus's GPIO, which is within the safe 5 mA limit specified by Ettus. The board itself provides opto-isolation and has FETs which switch the higher currents for powering the LNA and relay coil.
+R6 and R11 were initially replaced with 510 Ohm resistors to make the board controllable by the 3.3 V GPIO pins of the Ettus USRP X300 SDR. These have since been put in series with a 10k resistor (currently jumping the feed connector) since it is running on 26V again.
+Like the rest of W1XM, the default state of the LNA protection relay at the feed is the transmit state (in which the LNA input is isolated from the feed). To be able to receive, the bigdish sequencer MUST be in the receive state. this can be achieved by logging in through the UI program provided with the client library [[https://github.mit.edu/w1xm/BigDish_Sequencing/tree/main/client|status_window.py]]
-Similar to how the board is used for the rest of W1XM, the relay is configured such that it is in the transmit path by default (when unpowered). Putting 3.3 V across the RX_EN_P and RX_EN_N pair switches the relay into the RX path and turns on the LNA. This configuration decreases the probability of accidentally having the LNA in the RF path when transmitting and destroying the LNA.