linear C-Band frequencies with AVcomm? it is possible ?

[stephan94]

Is it possible to receive linear C-Band frequencies with an LNB fitted with AVcomm?

AVcomm is normally used in C Band to receive circular frequencies without a depolarizer plate

But can we receive linear frequencies? And how do we do this?

thanks in advance

 

[John]

Is it possible to receive linear C-Band frequencies with an LNB fitted with AVcomm?

AVcomm is normally used in C Band to receive circular frequencies without a depolarizer plate

But can we receive linear frequencies? And how do we do this?

thanks in advance

Yes - it's possible stephan , can be done with either a lnb or lnbf.
Short answer - It just requires the AV-Comm barrel to be in a static fixed position and the lnbf inserted into the rear of the barrel.
Use an additional C band feed with the lnb attached and insert that into the rear of the barrel.
There are quite a few detailed posts and threads on the subject within the forum , i'll try and winckle out the good ones later.

 

[stephan94]

thank you john for your quick reply.
for the moment i use my Avcomm with my LNB C ( as you indicate installed behind AVcomm) but currently
it is only for circular, not for linear ..

 

[John]

Here's a quick sketch of how i used my newly obtained Titanium C138 5G lnbf used with an AV-Comm pol barrel during the experimental and evaluation stages to give me H/V-L/R polarities.
In my case the rotation of the lnbf was motorised, controlled from the shack.

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[halliwell]

Is it possible to receive linear C-Band frequencies with an LNB fitted with AVcomm?

AVcomm is normally used in C Band to receive circular frequencies without a depolarizer plate

But can we receive linear frequencies? And how do we do this?

thanks in advance

Hi,

you turn your LNB by 90 degrees, that's all!

best
Hallo

 

[Channel Hopper]

Hi,

you turn your LNB by 90 degrees, that's all!

best
Hallo

Or maybe 45 degrees (and then only with a switchable polarity vesrion otherwise it would be 45 and 135 degrees)

 

[Lexzie]

I thought you'd need to place a dialectrum plate to make circulair signals lineair? (And remove it to get lineair signals).
Can anyone explain how this 45° rotation works?
What kind of LNB's are you referring to?

 

[s-band]

Can anyone explain how this 45° rotation works?

This may be simplified too much but the AVCom is similar to a squeezed pipe differential phase shifter. First, 2 orthogonal linear waves are created then differentially delayed to give 90 degree phase shift between them. Combining them again gives circular polarisation. This is the case in John's 2nd diagram.

To get linear polarisation, see the 1st diagram. Where the V & H are in line with the squeeze or no-squeeze sections, the differential phase shift does not take place on the single polarisation hence keeping linear polarisation.

Reference: W2IMU, A differential phase shifter for X-band in dominant mode circular waveguide, 1989, Crawford Hill VHF club