Notch Filters for AO-40 Mode L/S
Two notch filters have been developed to prevent de-sense of the S-Mode converter while transmitting on a nearby Mode L antenna. They were developed for use with the DNA helix antenna described elsewhere on this website, but could be used with any system where de-sense problems are being encountered.
Filter 1
This provides a deep notch (about 47dB) at 1269MHz using a quarter wave open stub connected to the through 50 ohm line. It is intended to be placed between the antenna and the 2401MHz preamp or converter. At 2401MHz the stub is nearly half-wave long and hardly affects signals at 2401MHz, but the small shunt inductance of the stub is cancelled out at 2401MHz using a small open circuit stub. The through loss including 2 sma connectors is only 0.14dB, so the filter should not add significantly to the system noise figure. It is hoped that this 47dB added to the 15dB coupling loss in the antenna should be sufficient not to overload the S-Band LNA or converter with the 1269 transmit signal, to allow proper duplex operation.
Filter 2
This provides a deep notch (about 47dB) at 2401MHz using a quarter wave open stub connected to the through 50 ohm line. It is intended to be placed between the 1269MHz transmitter and the antenna to prevent any noise at 2401MHz coming out of the 1269 tx reducing the sensitivity of the S-Band receiver . At 1269MHz the stub adds a significant amount of shunt capacitance which would affect the tx signal at 1269MHz, and this is cancelled out by a shorted stub. The through loss including 2 sma connectors is only 0.2dB, so the filter should not reduce the transmit power significantly. .
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Filter 3
A double-stub version of Filter 3 has been prototyped, where two stubs are printed on one board. This filter has over 100dB measured rejection at 1269MHz, with only 0.5dB loss at 2401MHz, as shown below:
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Filter 4
The latest filter to be developed is a single-stub filter designed to provide a notch at 435MHz, for stations using Mode U uplinks. The filter has about 0.3dB loss at 2401MHz, and provides more than 30dB attenuation at 435MHz. The filter also has reasonably low loss at 145-146MHz, making it also suitable for use with 70cm uplink / 2m downlink satellites.
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On-air tests with dual-helix feed
On-air tests are ongoing. One station has reported that Filter 1 was sufficient to prevent de-sensing, while in my own station I needed two of them in cascade, plus a Filter 2 in the transmit line. With this combination I could not detect that I was transmitting, with 15-20W of transmit power. The same result has been with one Filter 3 replacing the two Filter 1 combination. This is now in use with the dual helix feed in the 10ft dish in our AO-40 system.
These filters are now available through the MCCS, currently as ready-made tested units. Kits may follow in future, and details will be posted here.