[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: Primstar dishes at 10 Ghz? (again)



Jim White wrote:
> 
> I received no answers at all to my earlier inquiry about Primstar dishes on
> 10 Ghz.  So let me try again.
> 
> Has anyone had any luck at all using a Primstar dish and feed, and/or the
> electronics at the feed (I assume LNB) on 10 Ghz?  Some of these are
> showing up now and look like they might work with little or no
> modification.  Anyone know what the frequency is of the Primstar satellite?
>  How about the voltage the LNBs need to operate at that frequency?  What is
> the IF the LNBs put out, 70 Mhz like a big dish?  There are two LNBs, and
> two coax feeds from them, one for vertical polarization and one for
> horizontal.  The whole feed can be rotated manually to align the
> polarization.  How about using just the feed and dish with one of the more
> or less off the shelf 10 Ghz preamps/converters that are available?  Is the
> feed close enough to the P3d 10 Ghz freq to work?  How about to the
> terrestrial 10 Ghz ‘contest’ freqs?

Jim -- in addition to the good material that Kerry Banke posted, let me
add a couple of snippets.

(a) There are some of the K-band front ends that have a 10.000 GHz LO, 
done with a free-running DRO (Dielectric Resonant Oscillator). The DROs
have pretty good frequency stability -- typically better than 1 MHz: this
is adequate for the high-speed digital applications for which they were
intended, but not for SSB! One mod that can work is to drill/tap the lid
of the DRO cavity box for the 1/4"-32 threads of an SMA "bullet" connector.
A "nude" SMA connector is threaded into the hole and its lead serves
as a probe that can injection lock the DRO to the harmonic of a lower
frequency stable source (like 1 GHz).

(b) I know of one group at a University in Japan who modified 11.7 GHz
units down to the 10.7 GHz Radio Astronomy allocation to serve as the 
front-ends for an array (I seem to recall it was 12x12, i.e. 144 elements)
that was used for imaging the radio sun. The LNA packages they used had
a strip-line microwave bandpass filter between the LNA and the mixer.
After a lot of hacks at lengthening the filter elements to accommodate the 
~10% frequency change, they found that it was MUCH simpler to
dielectrically
load the filter by putting an appropriate sized slab of Teflon on top of
it. As I recall the numbers, with this mod more than 90% of the units
achieved < 100K system temperatures.

(c) I know Michael and the Helsinki group have played with similar 
packages as well, in part because of their love of microwaves, and in part
in anticipation of their X-band package flying on P3D. Perhaps they can
be enticed into announcing their results to the world!

(d) The (typically) 900-1600 MHz IF section in these units have a LOT of
discrete L/C components. Modifying them will be a challenge! It might
be worth considering using 23 cm (1269 or 1296) MHz as a convenient IF.
With P3D, you will only be using the TX side of your transverter since
23 cm is only an uplink (as stipulated in the International Radio Regs).
A good place to use the RX side of the transverter would be as a way to
save having to do a lot of hacking on the IF stages in the front-end
packages for the Primestar and/or DSS dishes.

It would be really GREAT is some AMSAT member would do a bit of reverse
engineering on the common small-dish K-band TVRO units. Things that we
need to know include 
  (1) What is the real LO frequency? Is it a DRO? How can it be locked 
      for better stability? 
  (2) What is the real IF used in the various common units? Is it feasible 
      to hack it to change the frequency?
  (3) How much filtering is there in the microwave signal path? Is there
      an easy way to change the frequency (like the Japanese use of 
      Teflon slabs)? 
  (4) What is the performance (i.e. Noise Temp & gain) of the unmodified
      (I presume) PHEMT LNA at 10.4 GHz? Is any input tweaking needed? 
      (I hope not!!). What is the input waveguide interface & is it
equivalent 
      to standard waveguide like WR75?
  (5) Most of the K-band LNA feeds I have seen use linear polarization, 
      and the two orthogonal polarizations are selected by sending a
different 
      DC supply voltage up the coax cable. How do they select the two 
      polarizations within the unit (electrical? Mechanical?)? Is there a 
      simple way to insert a quarter-wave plate in the waveguide path to 
      achieve circular polarization needed for P3D?
  (6) What is the bandwidth of the K-band feed? How well does it illuminate
      the dish at 10.4 GHz? How much does the feed add to Tsys at 10.4 GHz?
  (7) Given that there are several USA K-band systems (Primestar, DSS) and 
      a different set in use in Europe, and still a different set in Asia,
      is one of the COTS (Commercial, Off-The-Shelf) K-band systems
available
      somewhere in the world a real winner for use by amateurs at 10.4 GHz?

If there is a volunteer who is eager to tackle these problems, I'll find a
way to get enough funds to purchase the COTS samples. Please speak up!
There
are a couple of possible post-P3D AMSAT missions under discussion that are 
critically dependent on having the answers to questions like these.

Kerry -- since the SBMS folks constitute a likely candidate, I'm also 
cross-posting this to their address (as cloned from your message).

Michael -- can you speak up for the Finnish folks?

Tnx & 73 de Tom, W3IWI



AMSAT Top AMSAT Home