NRAO Home  >  Green Bank  |  Wiki Topic:    GB > Electronics > MicrowaveGroup > DualBandPFfeed

Changes | Index | Search | Go

GBT Dual-Band Prime Focus Feed Discussion Area

Please keep comments brief, linking to other pages for lengthy expositions, and please sign your comments. Thanks, RDN.

Since very early in the GBT concept development, scientific interest was expressed in dual-band observations, primarily by pulsar observers. That is, observations at two widely spaced frequencies (~2:1 frequency spacing) simultaneously, or at least closely in time. Accordingly, the PF1 receiver was designed to support that capability. However, to optimize G/T and due to the technology available in the early 1990's, the 300-920 MHz frequency range was divided into 4 bands, each with about 1.3:1 bandwidths. Feeds were designed for each of the four bands. The receiver can simultaneously produce 2 IF outputs from either the 340 or 450 MHz band, and 2 IF outputs from either the 600 or 800 MHz band. What is missing is a dual-band feed that can produce simultaneous beams.

Recently, renewed interest in a 340 and 800 MHz dual-band feed has arisen, both from scientific needs, and to ease scheduling pressure. (Changing from one feed to the other takes 3-4 hours in fairly decent weather, 3-4 people, and is normally done only during regular working hours.)

• Power Point File ; PDF

• Summary of results and discussion
  • The concept considered is to stack vertically two of the Short-Backfire type feeds. Doing so requires some compromise in the front reflector of the lower frequency feed, and in the back reflector of the higher frequency feed.
  • Substituing a short-backfire type for the corrugated horn type at 800 MHz decreases the G/T, mostly due to increased Tspillover.
  • The reflector size compromises degrade the stacked Short-Backfire performance from that of the optimized individual feeds.
  • The net performance loss varies with frequency, and is summarized in the table below (to be added).
  • Using a short-backfire feed, because of the coaxial output ports, will result in some increase in Trx compared to the corrugated horn/ cooled OMT combination currently in use. This effect has not yet been quantified, but we suspect will be 5-10K.
  • Stacking the feeds means the focus position will differ between high and low band. It was thought that would not be a serious impediment to simultaneous band observations if a compromise focus position was used.
  • The initial concensus of the scientists present was that the hit in G/T relative to the existing feeds is probably too large for spectroscopy, and marginal for pulsar observers, even given a new simultaneous band capability. It was noted that (G/Tnew)/(G/Told) = 0.7 => 2X observing time.
  • Additional tweaking of the design might yield some improvement, so the scientists were asked to consider what decrease in G/T would be allowable.
  • Fairly small changes in the ratio of high to low frequency bands could help, so the scientists were asked to consider what could be done there.
  • The engineers will continue to consider other designs or refinements of this one.