Does a feedhorn behave as a waveguide ?

Hello.

Planning to replace the Ku section of a LNBF (inside the top section in the attached picture); would like to know if its depth in the feedhorn throat is critical or not.
The LNBF has a ~7 cm diameter aperture. (Bottom end in attached picture)
If the waves entering the opening can travel with no major attenuation a few cm. more -or less- to reach the new Ku antenna electrodes position, or if am missing something...

Externet said:

Hello.

Planning to replace the Ku section of a LNBF (inside the top section in the attached picture); would like to know if its depth in the feedhorn throat is critical or not.
The LNBF has a ~7 cm diameter aperture. (Bottom end in attached picture)
If the waves entering the opening can travel with no major attenuation a few cm. more -or less- to reach the new Ku antenna electrodes position, or if am missing something...

Click to expand...

I cannot see any picture. The LNBF is designed with a horn to be used as the primary radiator in a parabolic dish.
What do you mean by "Ku antenna electrodes"?
From the LNBF horn to LNA input(s) there is a circular waveguide section, some 5 cm long. You can also tap the signals along this "tube' but the LNA inputs are backed by a short for matching which such "tap" cannot use.

What is your intention?

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The ridged horn itself is not a waveguide but a radiator. The cylindrical pie is the waveguide, in your picture it looks like there is a filter inside. The rear section with the F-connector is waveguide to coaxial transition.
Best LNBs use no coaxial-cable jumper as it is lossy. The LNA input is directly coupled to the waveguide end and is mechanically integrated with it. You can improve LNB noise figure by attaching it using a F(M)-F(M) straight link instead of cable jumper.

I don't think "Ku" term written in red on the picture came from the name of Ku Band which is 12GHz-18GHz, because any of the standard coax cables wouldn't work at those frequencies.
Most of the old C-band LNA+downconverter have input at 3.4GHz-4.2GHz and IF output of 950MHz-1750 MHz (using an internal 5.15GHz LO).

By the way, why you want to "Abolish the deciBel" ? I think is still useful...

Thanks.
By antenna electrodes, i refer to what here is called 'pins' in the hole of the picture
----> https://en.wikipedia.org/wiki/Low-noise_block_downconverter#mediaviewer/File:LNB_dissassembled.JPG
That protude into the feedhorn or waveguide.

Yes, Ku is for the 12GHz RF input LNB that has a ~1GHz downconverted output to feed my 2m dish receiver via a RG59 feedline.
Am not confident of its health and plan to replace only that Ku section, which resides at the deep end of the feedhorn.

The unit has a built-in 22KHz switch to select C or Ku bands LNBs, that is why the short coaxial for the independent Ku section comes to play.

The near 'pins' are C band; beyond and deeper in the throat, the Ku 'pins' :
----> https://s589.photobucket.com/user/misfits1977000/media/bsc621-2dinside.jpg.html

The intention is replacing the original Ku section with another brand :
----> https://www.satpro.tv/images/orbital-single-lnb-1.jpg

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The logarith is useful; I believe the deciBel is not necessary, as the electrical unit is the Volt / Watt. It complicates concepts to new learners.

For the orbital Ku component, it need some protocols between the itself and the control unit, so that it can adjust its LO freq to produce the IF freq.
It seems the new and the old component have different SW protocols.
It should be easy to design a new component.
1. Does the cable carry Ku signal or IF signal?
2. If IF signal, then you need to design a LNA and a mixer and LO in it? It can be very compact and seems has enough space.

Thanks.
The Ku LNB has its local oscillator built-in. RG59 cable carries IF out. Same cable carries DC in to power it. The receiver superimposes 22KHz to DC for selecting which LNB.