I'll attach some
pix of the feedhorn and LNA for you.
The importance
of circular feeds is that as a linear wave passes through the ionosphere, it
undergoes Faraday rotation. So it may arrive at the station you are talking to
having been twisted 90 degrees. This is a slow progressing process and on all
bands except 23cM, may cause EU for example to be locked out for hours at a time
for linear stations.
With circular
polarity, Faraday is a non issue. The feedhorn almost all of us use is a VE4MA
that has separate TX and RX probes. The circular polarity is synthesized as the
linear wave propagates down the circular waveguide and encounters sets of
capacity stubs. The exact opposite occurs for waves entering the waveguide. The
result is we get CW and CCW without having to use any relays (loss) and phasing
lines (loss).
My LNA has a
noise figure of under 0.24dB and uniquely connects to a protection relay with no
cable or adapters (loss).
The position of
the feedhorn and its scalar ring is tediously adjusted by measuring
the difference between sun noise and cold sky. W4SC developed a very accurate
and repeatable process that uses an SDR RX for this.
I use a
modified C band satellite drive system known as a polar mount so I only need one motor drive to track the moon.
Anyway, hearing
my own echoes off the moon was and still is the highlight of my amateur
career.
The photos are
the feedhorn + LNA, My first water cooled 500W tube amp, my previous 400W solid
state amp (mounts right at the dish). My current design is 600W solid state and
will also mount at the dish.
BTW, that little
circle in the middle of the tube amp is a paddle wheel that turns as long as
water is flowing. A tachometer on the wheel sounds an alarm and shuts down plate
voltages should the wheel stop turning.
I'll keep you
up to date on my BB RX progress- thank you again Bill.
Dale W4OP
Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20
Posts
"noise figure of under 0.24dB".
ReplyDeleteI don't know if that reflects how much things can change, or how slowly it takes to improve things.
There was some review for a Nuvistor converter back when the Nuvsitor was new, and the writer compared it with a converter with a 417 in the front end, and the Nuvistor beat the 417 converter. The writer added "I guess it's time to tune up the 417 preamp.
I think it was a 417, which were regular tubes but uncommon and I think relatively expensive. I don't think it was a 416 was was an even fancier tube.
The point was that without any real effort, the Nuvistor gave a noise figure that was better than the 417, unless one put in extra effort to tune up the 417. The same could be said about the parametric amplifier, unheard of noise figure, especially at the higher frequencies where few active devices worked at all, but endless effort needed to tame it and adjust it.
I think even 20 years ago one could get GasFets that had about 2dB noise figure, though I can't remember if that was with lots of effort or not.
0.24dB noise figure at 1296 I think would be seen as impossible fifty years ago, certainly impossible without a lot of effort to choose a specific device and endless tuning, and maybe even supercooling things.
And then the transmitter is equally shocking. Back then you would have had to put a lot of effort into getting a high power transmitter at 1296, and likely it was horribly inefficient. Now it can be done with transistors?
Michael