i gather that you lose 3db approx of signal using a dielectric plate...
You definitely gain signal on circular polarity signals by using a polo pipe compared to using a dielectric plate.
I have tested both and found the polo pipe consistently gives the stronger signal.
There is also a pin pipe which does the same thing, ie converts circular polarity to linear polarity
Lastly, there is the rib tickler. The rib tickler is made by Prodelin and is a very high quality piece of kit. It is quite a heavy unit and needs a square frame clamp.
A polo pipe has internal steps on opposite sides of the pipe.
A pin pipe has internal pins on opposite sides of the pipe.
A rib tickler is externally a square shaped waveguide with sequential ridges on opposite sides internally.
Polo pipes can be imported from Australia
Pin pipes can be imported from Brazil
Rib ticklers can be imported from the USA, sometimes listed on eBay.
Alternatively, they sometimes get sold second hand, or the polo and pin pipes can be fabricated rather than imported.
There are plenty topics on the forum discussing these pipes and ticklers, some with pictures of the internals.