[Namaste-dev] Antenna Feed Choices

Roger Thompson rogert at traceroad.net
Fri Jun 20 03:34:32 PDT 2008


Tom Clark, K3IO mentioned the fiberglass designs WA5VJB has used for the LP
feeds, which were designed, I think, mostly for convenient rover use.  I
looked back earlier at some of the information about the design in the NTMS
Feed Point newsletter of June/July 2000, where he discussed the choice of
fiberglass based on costs to design, where repeated trial designs would just
be too expensive for Teflon. An updated version of this discussion is at
http://www.wa5vjb.com/references/DesigningPCB-LPs.pdf, where a cost to
design a Teflon version is given as $2000.  If a Teflon LP would be
sufficiently better than the fiberglass versions, and actually would be
useful in the Namaste ground station, I don't think this level of cost would
be a significant barrier.

In the same discussion, the problem of non-optimal phase center alignment is
raised, and the Teflon boards would be even less desirable than fiberglass.
If the higher band part of the LP is placed at a small dish phase center, as
seems to be recommended, then the lower frequency would be impaired, which
is unfortunate in our case as the DSS dish is pretty small at 3.4 GHz
already.

If the design is constrained to a single small dish, then some sort of dual
feed, maybe an improved WA5VJB LP would be useful?  What are the other
single small dish feed alternatives?

Roger
AD5T

> > Tom Clark said:  
> One caution on Kent's PCB antennas -- he has designs that work over a 
> fairly wide frequency ranges, but since they are implemented on 
> fiberglass dielectric stock, they tend to be more lossy than 
> you might 
> desire. I have seen one of his dual-band feeds (see 
> http://www.wa5vjb.com/images/logperio400.jpg) burned on TX 
> when it was 
> used as a dual 23/13 cm feed. And remember that every dB of 
> loss in the 
> antenna/feed is a dB added onto an LNA's noise figure (since 
> it not only 
> increases the noise but also decreases the signal, the S/N 
> ratio takes 
> an even bigger hit!).
> 
_




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