The visual detail
below shows how a Ku-band LNB was modified to improve
the front-end performance
for use at 10.368 GHz to monitor the ARRL's EME
contest in 1997.
After the mod this LNB's front end was vastly improved and the receiving system sensitive enough to manually track the moon on its moon-noise signature which was up to 2.5 dB. Only the input matching and interstage matching is relevant to this discussion, but the positioning will indicate where we are going.
front-end of a
Improving the LNB's noise figure, here the system noise figure NF or system temperature T, increases the noise power ratio (Pn=10 log(kTB)) and thus the downlink signal-to-noise ratio in the case of a satellite such as Es'Hail 2.
The noise figure is defined as NFsys dB = 10log10 (Tsys /297 + 1) where the ambient temperature is typically about 297k, and system T is composed of the antenna noise as collected plus the temperature contribution of the 1st and 2nd RF stages.
The S+N/N on a satellite's downlink boils down to: downlink EIRP - (-noise power ratio of the receiver). My standard unit under test had a noise power ratio of 10log(1.38 x10-23 x 2500 x 180) = -172 dB.
In this case the variables are fixed, only the system noise temperature T can be improved.
The front end can be improved to provide a system noise temperature possibly around 42k by the look of it. The noise power ratio would then be 10log(1.38 x10-23 x 2500Hz x 42k) = -178.3 dB. This is a 6 dB improvement in S+N/N which would otherwise have to be found by enlarging the Rx dish antenna's diameter 4x, i.e. a typical 0.8m to 3.2m.
Front end of an Octagon OTLSO LNB
Possible zones to install patch material
This popular LNB shows a steep drop-off in performance between the bottom of the satellite TV Ku-band at 10.950 GHz and the amateur X-band allocation around 10.4 GHz. These frequencies are quite close.
Accordingly, minimum patching of the RF inputs will produce a substantial reduction in noise figure.
A device capable of
operating on the
IF frequency 740 MHz, in wideband AM mode, the wider the
better. Do not use
non-linear detection such FM. A
s-meter on a
suitable Rx is ideal, or the input of a dongle such as a
· At least 20 dB of IF attenuation between the IF output of the LNB and input to the Rx. Ideally the s-meter should indicate about s-2 to s-4 when connected, the s-meter should not 'bottom' at s-0 nor be hard driven in excess of about s-5 or so. It should show some 'life' and move around on the random noise peaks.
· A handful of 1mm squares of thin copper or brass shim. These squares are temporarily fixed in the green zones illustrated by wiping them in "stick' such as Pritt so that they do not fall off during the optimisation process. They are then slid around using a pin or needle between the readings taken.
The LNB may be mounted on the dish antenna or waved around by hand. Apply 12v to the LNB's input and note which RF input stage has a small negative voltage on the gate circuit attached to the waveguide input. This is the front end handling vertical polarisation, suitable for reception of the Es'Hail 2 satellite's narrowband transponder. The other first RF stage will be off and not have a bias voltage on the gate. Point the LNB's input to a solid object nearby such as the ground or a wall, this is the 'hot' body. Take the s-meter reading. Then point to a portion of clear sky, the 'cold' body, take the s-meter reading. Note the reduction in received signal strength between the 'hot' and 'cold' body readings. Take several readings and average the result. My standard LNB yielded a RATIO about 2.8 dB assuming 6dB per s-unit (I actually used a 1 dB precision stepped attenuator), from this the noise figure was calculated to be 2.1 dB and the system noise temperature as 180k (see my test on this site).
Starting with the input (gate) of the active 1st stage, place a 1mm square shim piece on the possible zone illustrated above. Do not short to the nearby ground plane of the PCB. Repeat the hot/cold procedure. Take the average of several results. Note the RATIO, this is important, more than the actual peak readings. If worse move the patch to a nearby position and retest hot/cold. Add patches freely, noting the results and positions. Eventually the best result will be obtained using patches on the input and possibly the output of the 1st RF stage. There might be some benefit doing the same on the second stage to improve the conjugate match between the 1st and 2nd RF stages.
Up to about 1 s-unit of improvement (5-6 dB) should be quite easy, indicating the LNB's noise figure has improved from approximately 2.1 dB to around 0.6 dB. I reckon with a lot of care even more is available as the LNB in native form might be good for a NF around 0.45 to 0.5 dB at 12 GHz and the same at 10.4 GHz.
When complete it is not necessary to solder the patches, simply secure them in position with a tiny drop of superglue on the point of a pin.
Apply 18v to the IF line and do the same on the horizontally polarised input. In practise improving the horizontal polarisation of this LNB is more important because the wideband DATV and RB-TV signals from Es'Hail 2 are marginal on small dishes of around 80cm.