How might this POTY antenna work for RX with no LNB?

[Basil]

I am slowly getting into amateur satellite work and run a 1.8 meter prime focus dish with a POTY antenna for 2.4GHz TX and a coaxial LNB for circa 10.5GHz reception. I know the LNB down converts to around 940 MHz using an intermediate frequency.

I heard a very nice SSB signal from a chap who appeared to know a lot about the hardware used, so looked him up on qrz.com He was using a POTY antenna the like of which I have not seen before. It has a feed horn and appears to feed the RX side without an LNB directly to a Kuhne down converter. How might that work, I thought down converters expected a lot of gain from the LNB and an already down converted signal, to some extent, done within the LNB?


I placed a photo of this nicely looking engineered device here:

http://www.chriswilson.tv/DJ8EI.JPG

It appears there's no LNB but the RX antenna is inside the POTY tube itself??

Chris 2E0ILY

[G8GKQ]

Chris

Remember that this setup is for SSB, not DATV.

There is a probe picking up the signal from the central circular waveguide and taking it to the input of the Kuhne LNB/downconverter. Kuhne LNBs have similar noise figures to domestic satellite LNBs, but less gain. However, for SSB, the receiver in use is a lot more sensitive than a TV satellite tuner, so less gain is required.

For DATV every dB matters. For SSB, you can afford to lose a few dB both on the transmit side and the receive side. So read-across from SSB "experts" to DATV should be treated with caution.

Dave

[Basil]

Ah, OK, thanks for the nice explanation Dave, I was mainly wondering if indeed the tube had a probe like inside a normal LNB, but didn't understand how they then increased the signal level from the probe to the Kuhne convreter. If the converter has extra amplification compared to a more generic one, that explains how it works very nicely, thank you!

So for DATV what is considered the optimum DATV RX antenna these days, a generic LNB perhaps modified for accepting an external reference, or are there speciality ones around? Thanks.

[radiogareth]

The 'Bullseye' LNB is good enough for SSB out of the box and has very good 'ears' from what I can tell, slightly up on an Octagon for instance...
Sadly seems it needs buying form the EU....so brace yourself for the shipping charges....https://www.passion-radio.com/lnb-pll/l ... t-978.html
Gareth

[g0mjw]

The Bullseye is also available from RTL SDR via Ali-Express. https://www.aliexpress.com/item/4001089 ... f0511oEvuR

They charge VAT at source so expect it to cost about £26.

Mike

[Basil]

Thanks, I have a Bulls Eye LNB and am also experimenting with a ready modded LNB from qro.cz that can be locked to a 25MHz reference up the one RX coax to the LNB. It seems to work well, but there seems to be a frequency offset. I have, in the below the dish cabinet, a Leo Bodnar GPSDO with 2 outputs. One at 40MHz is an external reference to my PlutoSDR, and one at 10MHz that goes to the qro.cz combined injection and BiasT. Inside this box from qro.cz I assume there's some form of multiplication up to 25 MHz. There's certainly a chip in there.

However, in use, whilst TX frequency is perfect, (measured with a GPS locked, in calibration frequency meter), the RX is off a tad, compared to the QO-100 beacons. I am not sure why and am currently using SDR Console's Geostationary Beacon locking ability. I had expected locking the LNB LO to a GPSDO would not require this, am I missing something here please?

Apologies for misusing the term POTY before, I am getting better as I frequently typed it as POTI for ages

Thanks for the help. 2E0ILY