||Although the same
arrangement as the current front-end (converter at the
antenna with an 18 MHz IF output) could have been
used, or a PLL LNB with a 618 MHz IF output, the
long IF interconnecting lead (up to 100 foot) would be
prone to IF pick-up at either frequency. It is quite
frustrating when eagerly monitoring a weak waterfall
signal to find that the signal is not on 3cm at all
but a spurious finding its way into the IF. For this
reason, it seemed better to pipe the raw 3cm from the
antenna down to a receiver located in the equipment
Cable loss at 3cm:
Standard quarter-inch RG223 has an attenuation of about 2 dB/m at 3cm, and cheap satellite IF cable measured an impressive 1.2 dB/m the last time I checked, so for a 100 foot run, the loss is going to be at least 40 dB. Modifying a satellite LNB to come out after the final rf amplifier will provide an antenna with integral 20 dB gain pre-amplifier - not enough to give a simple solution with these cables on this occasion. However, a reel of half-inch CLF-400 became available and the attenuation in this was measured as 0.4 dB/m, so this looked a good bet at 15 dB for the 100 foot length, once the connectors were taken into account.
What LNB to use:
A few current LNB types were modified and tried, but the noise increase at the end of 100 foot of cable, though noticeable when the LNB supply connected, was low enough to impair the overall system noise figure. No matter - some of the older Cambridge LNBs had three rf stages (and were easier to modify). Two were found and modified. Now there was a hefty 15 dB of noise increase when the supply was connected, and we appeared to have a working solution.
One advantage of these older LNBs is that it is easy to take the output from the 3rd rf amplifier (from where it had fed the band pass image filter) out from the back of the pcb and take it straight through a larger hole drilled in the LNB casting. The pcb picture above is not that clear, but a dc blocking capacitor has also been added into the output feed. To provide a good mechanical anchorage, a bracket was attached to the casting so that flying lead semi-rigid output could be terminated at that point.
To interface with the WR-90 antenna feed, it was noticed that if the LNB horm was cut away at the right place, the remaining horn section would fit nicely into the choke ring (or gasket ring) on a standard flange, so the overall assembly ends up being self centering. A couple of matching screws are required in the WR-90 waveguide to tune-out the mismatch caused by the simple way in which the LNB is being offered up to the rectangular waveguide.
Initially, the noise figure looked poor at about 1.5 to 2 dB. It was then remembered that these LNBs were old enough that they were not designed for use below 10.9 GHz. As soon as an extra mm or two was added to the probe antenna, the noise figure improved to a sub 1 dB noise figure, as can be seen below:
The HP noise source used has a calibrated ENR at 10 GHz of 5.67 dB, indicating that the LNB noise figure is about 0.6 dB.
Supply chokes/resistors were removed from parts of the LNB that were not required (such as the LO anf IF amplifiers), resulting in a supply current of only 25 mA, which is fed in via the LNB 'F' connector.
Ground level receiver:
By bringing the 3cm signal all the way to the equipment room, we have the advantage of flexibility - any receiver can be added there quite easily. This could be a duplicate LNB with probe, fed from the existing receiver rack unit, as shown below, or a completely new shelf that could contain a PLL LNB, for example. Or any any other configuration that someone might want to try. In that sense, having two completely individual equipments might not be a bad idea, and as a starter there is a spare PLL LNB based rx here that could be used.
The following LNB box, connected to the mast head feed allows easy swapping between it and the current set-up - just requires two plugs to be changed over. It uses exactly the same LNB type as in the existing equipment room roof assembly.