Quote:
Originally Posted by olegsher
Really good point!!
But this was done on purpose - the SATCOM system is asymmetrical - the Satellite Vehicle Platforms (Satellites) are more powerful (150W) than ground stations (5-20-50W). Additionally, SWR is much more important on the transmit side - we can pull out the signal lost on the receive side with som LNA (Low Noise Amplifier), but you cannot do anything with the transmit power lost due to the impedance mismatch.
Ground Stations Rx frequencies are 240-270Mhz, Tx Frequencies 270-320Mhz.
The optimization on SWR was favoring Tx portion of the band with the ratio of 4:1 over the SWR on Rx side.
Quite opposite was done for the Gain - the geometry was optimized to get the maximum gain on Rx portion of the SATCOM band.
The final optimization was to find the best balance between those two contradicting requirements.
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I read your response with some interest and I will offer you a few things to consider.
When I said you had a “significant variance” well, that was an understatement. Because of the sensitivity of TACSAT receivers TACSAT antennas designs try to stay below 2:1. The entire premise that you optimize the uplink band is incorrect, as you are only comparing power transmitted without considering the receiver portion of the link. Unlike the ground terminals, the satellite has an array of high efficiency antennas, sensitive receivers and a clean electromagnetic environment. The ground terminals, on the other hand, have a lower sensitivity and are very susceptible to harmful interference from other emitters. They are further handicapped by scintillation effects as well as ground effects (diffraction, super and sub refraction, etc…) in certain environments. LNA’s will often exacerbate the noise issue, as they themselves may only generate a small amount of noise, but they will happily amplify any environmental noise or other interference in the receive path. All of these effects degrade the receive sensitivity and require more signal in order to achieve link—something the satellite cannot compensate for. They also have to accommodate the loss of power from the satellite transponders as they age.
You stated that you optimized the gain of the RX side (and I still would like to see your plots), but your model significantly degrades the efficiency of the downlink band from barely acceptable at 259 MHz to atrocious at 240 MHz via the SWR indicated. I was also troubled by your plots, as they indicate gain at zenith (dBic) to be 6.3 and your 3dB beamwidth to be ~110 degrees. Assuming an Omni-directional radiation pattern your gain at peak of beam should be closer to 0dBic. Either you have an error in your chart, or the model does not exhibit a 360 degree radiation pattern.
The antenna in this thread has been around for a while and is generally accepted to work. I recommend you review your model more before you make further suggestions to change it. A couple of links that provide background:
Combating Low UHF-SATOM Downlink Margin—a White paper prepared by Raytheon E-Systems
http://www.argreenhouse.com/society/...pers/38-03.pdf
MIL-STD-188-181A Interoperability Standard for Single Access 5 KHz and 25 KHz UHF SATCOM Channels (also available on AKO)
http://rodolfo.ips.es/HTML/06radio/S...S/m188181a.pdf
Interference on UHF SATCOM Channels, Barrie Strachan, SPAWAR, San Diego
http://www.argreenhouse.com/society/...ers99/36_6.pdf
Multi-Service Tactics, Techniques and Procedures for UHF TACSAT and DAMA Operations, FM 6-02.9 (available on AKO)
http://www.uhf-satcom.com/uhf/r3403g.pdf