BATC Forum

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

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

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.

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

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

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