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Old 04-06-2011, 21:30   #93
olegsher
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Join Date: Jul 2009
Location: Mountain View, CA
Posts: 2
Again, good points!!

Thank you for keeping good and interesting discussion.
I have all documents that you mentioned, except SPAWAR one, in my library, but SPAWAR is a real good read - thanks again.
My argument will be - if you look at the distribution of the Rx frequencies on majority of the 25KHz channels, all of them start at 252MHz and up. The 244-252 MHz band is used for narrowband 5KHz channels, and usually they are allocated for ANDVT traffic. In all our requests for SATCOM we were always given 25KHz channels, both for DAMA and dedicated. That's why I decided to assign a lesser weight to that 244-252 Mhz band.
Second, the SWR is much more critical to Tx side - in HF radios (PRC-138, PRC-150 ALE for example), the SWR minimization tuning is only done on Tx, but not Rx side. And there is a valid reson for that - if you have bad SWR on transmit, then in all radios ALC (Automatic Level Control) kicks in when the SWR is usually more than 1:1.5, and as the result, the output power is immediately reduced, hitting you with the double wammy. That also prevents the output cascades of the power amp from frying when your antenna is not connected or is short circuited (SWR is really bad then). On the other hand, the receive side SWR is not that important. Basically it boils down to "You can receive with any piece of wire, but you cannot transmit into any piece of wire".
And that brings us to your good point about directivity (beamforming) of the antenna. This is what important on receiving, when it is necessary to properly filter useful signal from the environmental noise (as the article you suggested points). And for this antenna by its design, without additional reflectors (like in AV 2055-3) there is no way to get any serious suppression of the sidelobes.

Again, the point of computer simulation and optimization was to create an antenna that will behave nicely in a wide frequency range, without the need to recalculate and retune.
To convert the dimensions that I provided to the L = 1005/F formula, the equivalent sizes are: FTX = 318MHz, FRX = 251MHz, so this is not a big change from the original design. The most important thing that I would like to stress is the addition of the polarizer - the 8" wire that gives you extra 3dB of gain for RHCP relative to a isotropic antenna.

Last edited by olegsher; 04-07-2011 at 00:46.
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