Is there any benefit to using any other type of feed other than cantenna or loop feed for a small amateur radio astronomy dish for hydrogen line work & is there any benefit to adding a sleeve balun on Loop Feed?
andrew....@googlemail.com
Is there any benefit to using any other type of feed other than cantenna or loop feed for a small amateur radio astronomy dish for hydrogen line work & is there any benefit to adding a sleeve balun on Loop Feed?
Following is a discussion I have had recently with Alex Pettit on these two subjects – I thought the outcome was so useful that I would post it here (hope you don’t mind Alex!).
Andy
Does this mean in practice that there is no practical reason to need any other type of feed other than cantenna or loop feed for a small amateur radio astronomy dish for hydrogen line work? And if so where does dipole in Discovery Dish feed fit in?
Andy
When the two are equally well fabricated and tuned, they perform similarly.
This is getting into a complex discussion .
For Small dishes, loop feeds or cantennas win slightly over dipoles. There is a large benefit from the beam shaping of choke-ringed cantennas.
Various features are important: Design / reliability / production costs / weight / mounting requirements / weatherproofing = There is never one simple answer to these questions.
The “ Dipole ” is not as trivial as it seems : it also incorporates a ” 1/4wl Common Mode Isolation Balun ” in its implementation (At least the Nooelec ‘attempts’ to do so).
Is the balun the short parallel bit at end of coaxial cable before either splits into two arms?
Andy
YES… ( it may not be req’d for a directly connected LNA )
I made one for the Loop Feed (a real pain), but did not see any performance change.
(It is more complex to fab that it may appear due to the need for exact lengths within the sleeve.)
Alex
James Abshier
Ideally, the loop and dipole feeds should be balanced feed lines.
The balund provides the transition from unbalanced coax to a
balanced feeder. Although you might not see a big difference in
return loss or VSWR, between balanced vs unbalanced feed, the
radiation pattern will be distorted with an unbalanced feed.
From my experience with loop feeds, the spacing between the loop
and the reflector is very critical. The antenna impedance changes
very rapidly with this spacing. Making this spacing variable
facilitates optimization of the feed.
Jim Abshier
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