FAQ Technical

(Frequently Asked Questions - Technical)

Search this resource for more technical information.

Before installing an LNB or receiver.
Weak satellites reception tips & tricks
Focusing & Skewing the LNB

What is a C120 flange.
What is a C120 feedhorn.
What is a WF75 flange/ waveguide.
What is a waveguide.
What is a scala ring.
Coax Cable
Ground loops



ALWAYS switch off the receiver & any other satellite receiver connected to it as there is voltage on satellite cables to switch bands and power the LNB, diseqc switches & motors. 

Always make sure you have no electrical faults on any connected media device before installing

Always test any used equipment or older media device that has been allowed to cool right down separately before connecting it to your system

Plugging in an old TV/ VCR or satellite receiver .. anywhere on ‘any’ connected aerial or scart / phono system without testing it first … could easily damage a new HD flat screen TV or newer DVD/ BluRay or satellite receiver/ lnb/ motor or cause interference & disrupt band & motor control!

Regularly health check all your older media equipment for faults.




For weak/fringe signal satellites like Nilesat/7w, BADR 4 & 6 @ 26e, Nordic 1w beams etc. in the UK & satellites like 2d beam @ 28e (Freesat UK) around Europe/ Spain- Canaries etc...

Focusing & Skewing the LNB

After elevation & azimuth have been maximised to get the absolute best out of any lnb, careful, precise focusing & skewing of the lnb should be performed with a good meter that shows the transponder signal or an on screen meter from your receiver on a monitor screen at the dish ideally using your main desired weak channels and use fine movements of focus (foreword & back) and skew (turning clockwise/ anticlockwise) while carefully monitoring for the best signal quality.

If any LNB has it’s best focus at the most foreword or back of the LNB holder then you should consider modifying the LNB holder so that it is further back or foreword to check if you can get a better signal as many LNBs & dish/ LNB arm designs suffer this miss match. The long neck of the Inverto Black Ultra usually gives far more adjustment to focus on almost any dish.

Check elevation and alignment after fitting a new LNB as some slight differences can occur from LNB to LNB in this too with different internal feed horn/ waveguide/ antennae designs etc..

Dish Shape (bent dishes)

KU Band dishes should have no more than 0.6mm outage on their face.

Many dish manufacturers are producing weaker satellite dishes than can cope with moderate to strong winds without permanently bending. Once a metal dish is bent out of shape you not only lose some signal reflected into the LNB’s c. 18mm dia waveguide entrance in the feed horn- you also then will have noise/ interference from other satellites/ interference from space/ the sun, local noise & ground noise being directed into the LNB thus increasing noise over signal so it is vital that you check your dish for it’s accuracy.

To check your dish’s trueness… check the rim first by sighting from the edge with one open eye standing at the side of the dish 2 meters or more away and move you head from side to side slowly viewing the near and far edges - they should be straight and in line on most traditional dish designs. (this will not work on unusual dish designs- Sky dishes etc..). Go round the dish’s circumference to identify if there is any distortion. Next inspect the dish face for particularly damage from wind hitting the face causing lumps over the bolt area of the AZ/EL bracket in the middle of the dish.

Straightening a distorted dish.

With the dish on the ground … the easiest task is to true up the rim to rim alignment especially on thinner or weaker dishes by holding the dish vertical on the ground & turning it in your hands viewing the near rim to far rim once again with one eye closed ….carefully pushing & pulling distorted rim areas until you can not detect any more rim to rim outage. Aim for less than 0.6mm outage overall for best results (max = c. 2mm).

Lumps & dents on the dish face require basic dent straightening (panel beating) removal and if necessary, the use of car bodywork filler and reflective paint & 1 or 2 templates for the parabola/s  - Aim for less than 0.6mm for best results (max= 2mm).

We also supply satellite dish ‘bracing kits’ see BRACKETS

Good cables x Good cabling & joins.

Use good quality cable - (copper copper copper! … 1mm.. 100 type or better) ..

Webro wf100 has the lowest loss we know for 1mm core (xx100) copper satellite cable having 10% less loss over most standard good quality cable (but has a ¼ of the tensile strength more desired for commercial installations). Use good joins and use the minimum number of joins, avoid sharp bends or crushing the cable and discard cable that has been crushed or had water ingression and shows signs of copper sulphate(green/blue) or arcing/ carbon (black). Avoid running cables near other cables andany sources of induction or potential interference, especially near sources of L band.

We use & supply Webro WF100/ 125 & Triax TX100/ 125

Minimising interference & EM induction.

Avoid tracking mains cables with satellite cables, avoid aiming dishes through old unshielded overhead telephone lines, street lamps, substations, overhead power lines or any local electrical or rf /em source installation. Avoid DECT phones, low energy light bulbs (or keep them away from satellite equipment & cables).

Long cable runs (over 40m) / cable types x loss & amplifiers.

Long cable runs cause loss & increased noise pickup, the best solution is always to use better cabling if possible including using wf100 (c. 6% gains) or going up to 125 spec digital cable (eg: WF125 or Triax TX125 (c. 20%+ gains)) that give much lower loss and avoid amplifiers. If an amplifier is used choose a reliable manufacturer ideally with amp & Slope gain adjustment andavoid being tempted by cheap unbranded products. We recommend placing the amplifier in the cable run at an optimum point for the cable spec x length = optimum loss point & not at either of the ends.
*Connecting separate 'dwellings' with the same satellite &/or terrestrial signal is covered in the UK by special electrical safety regulations that must be followed.

Sensitive receivers, good motor control & Freesat & Linux receivers

Always use a sensitive receiver if you are after weak/ fringe satellite signals - these change year on year and if you are serious about weak/ fringe satellites you should search the net for the latest most sensitive.
(weak/ Fringe satellites usually carry free movies etc.. & in English).
Some receivers are good at weak signals or good motor control like the Technomates in the UK and can be paired with (looped through) with a less sensitive & poor motor control Linux or Freesat receiver to get the best of both.


High fences, large trees or buildings/ neighbour’s chimneys/ TV aerials, roofs etc…  in the right position can be used to block local, ground, solar & nearby unwanted satellite interference (while still just seeing the required satellite/s). Strong signal interference from an adjacent strong satellite can be blocked / knocked down by careful dish positioning so that a building/ chimney etc. knocks the strong signal down such as 28.2e &/or 23.5e in favour of the weaker 26e in the UK or visa versa in southern Europe/ Spain & North Africa & the Canaries. Or 8w &/or 5w in favour of the weaker Nilesat at 7w in the UK etc.. etc..

 Blocking Diagram 1a- Shows satellite noise blocking techniques

BLOCKING.. noise from local interference & adjacent strong satellites.

Use the roof of a property to block strong local interference from telephone lines, substations, overhead power lines, factory EM/RF noise, street lamps where a dish could be installed easily on a wall directly facing the interference or a position chosen on the opposite side of the building with an over gutter mounting that uses the roof to shield the dish from the local interference as the dish looks skyward well over the roof pitch.

Solid Installations & Wind Exposure.

Make sure your brackets & pole carrying your dish or motorised satellite are very strong- especially laterally (sideways) and adequate anchoring has been used. Use dedicated satellite brackets & avoid brackets designed for aerials.
Note should be taken regarding wind exposure &  the dish face size & type (wire mesh/[perforated/solid/steel/plastic aluminium/ offset or prime focus & it’s dead weight etc) & if a motor is fitted & as to what type of motor -diseqc / 36HH or polar mount x actuator.

A ground mounted dish surrounded on all sides by buildings, trees, fences and low in a valley surrounded by hills &/or in a built up area has a much lower wind loading exposure compared to an exposed hill location surrounded by open countryside or open sea views &/or if a wall mounted dish at 1st floor, 2nd floor, at gutter height, roof-chimney mounted (below the ridge) and the highest wind loading position over the roof ridge line/ over roof apex installations especially on a high exposed property. All these factors should be taken into account when choosing the equipment for their suitability/ functionality and long life. A poorly specified installation delivers poor results especially in windy weather, is difficult to align accurately and can have a short life.

Always try to choose a sheltered location as possible that still delivers the wanted satellite (or arc if motorised). Sheltering from strong wind usually also gives shelter from RF & EM interference.

We consider the best installation (especially motorised) to be 2 dishes- a small dish very high up & a large dish low down for weaker satellites.

We also supply satellite BRACKET ‘bracing kits’ see TEL: number at end>>>.

Keeping it cool…. Heat = Noise!

In southern Europe and in the hottest days in N Europe/ UK … keeping the LNB/ cable & receiver cool helps keep circuit based noise down for critical weak signals. LNBs & cables can be installed or painted white or silver to reflect heat &/or easily shaded from direct sun with good ventilation dropping mid summer exposed temperatures dramatically without special equipment.

Super cooling the LNB (dropping -20-30C etc..) is the next option - we favour non direct (not direct electrical) - heat exchanger techniques against using direct electrical methods such as Peltiers directly on the LNB to avoid EM/RF interference from the Peltier etc...
Keeping the receiver cool.

It is always a good idea to keep receivers well ventilated- avoid stacking receivers with other equipment and even adding a PC fan (that doesn’t cause interference) keeps the receiver well ventilated/ cooler and makes them last a lot longer.

Avoid Shorting cables & loose f connectors!

Shorting a cable, even if the receiver’s manufacturer claims the receiver has short circuit protection, can still damage the receiver’s critical low level sensitivity and could damage the LNB or motor. Always switch off satellite receivers before attempting to touch cable joins etc.. and always connect F connectors as though the power is on taking care still not to touch the inner to the outer etc.. even though it is all off…  because one day it might not be off accidentally or another receiver is supplying power that is connected and left on.

For better HD ....

Like broadcast tv & commercial AV production in the past, it is now increasingly more vital to maintain the highest HD & DVBs2' compliant LNB, equipment & cabling and satellite 'noise-clean' installation techniques throughout any system to maintain the highest HD -- dvbs2' (& SD) stream delivery to satellite receivers & flat screen TV’s and minimise 'lossy' MPEG digital compensation processing by satellite receivers or flat screen TVs that can, ‘at any processing stage‘, introduce subtle as well as abrupt (obvious blocks or blocky) ..lower HD & SD detail in order to deal with time consuming action such as any noise and error processing in the HD digital stream from your satellite dish & LNB that it's processing limitations & components can't handle on top of high action scenes.


Most LNBs sold today in the mainstream market are LNBs plus a feedhorn inside and more correctly calledLNBf's. The feedhorn not only guides signals into the LNB from the dish it also (and it's main job) acts to reject unwanted signals from outside the dish area. These all in one LNBs are a compromise and often don't match the dish they are fitted to that well, especially Prime Focus dishes where the LNB is in the middle.

C120 LNBs are bolted to separate feedhorns (with the same C120 fitting) - that way they can be better matched to the dish and helping produce better signals & less unwanted interference- especially on Prime Focus dishes.
Once a C120 LNB is bolted to a feedhorn (of the right type for your dish) it performs much like a pro' version of an LNBf. 

To use a C120 LNB then you need to have a feedhorn, if you do not have one, you can do a search for feedhorns - for offset dishes (most dishes are nowadays) the popular choice is from ASC-Signal (Channel Master)- now owned by Raven. For Prime Focus dishes the popular choice is an adjustable one by Invacom. C120 LNBs can also be bolted to waveguideswith another LNB or transmitter or filter or polarotor etc..... search for microwave waveguide or KU band waveguide if you want to learn more-


The quattro version of LNBshas 4 outputs - Vertical low band..... Horizontal low band......... Vertical high band......... Horizontal  high band..... and is usually used today with multiswitches that take the 4 bands from a QUATTRO LNB and distribute them to multiple outups/ rooms or properties so that every output has all bands correctly as though each was connected to a dish and the multiswitch handles the switching instead of LNB/s- this can be to 4 8 16 or more rooms or properties using just one slightly larger dish. Most also include an aerial input for TTV & FM & DAB and 2 outputs per socket for plus (twin tuner). Also some multiswitches can relay multiple satellites. (distributing TV & radio signals to seperate properties requires special EP Bonding wiring)

The quattro LNBs  can also be used for sensitive/ weak satellite reception such as Nilesat (pub football) in the UK & 2d beam (Freesat/ 28e) in Spain & around Europe.

Strongest Satellite Channels  


diseq 1.2/ 36v MOTOR CONTROL 

These are older systems & far less used now** compared to the automatic USALS satellite motor system **unless the dish size is large &/or very wind exposed.
Diseqc 1.2 & 36v moves the motor by steps & as each satellite is found you store each satellite as a position from say1-60 until you have manually found all the satellites you can receive . USALS is a subset of instructions of Diseqc 1.2 with motors that are Diseqc 1.2/ USALS compatibly & uses mathematics to calculate all the satellite positions correctly without the need to find them.
Some motors are Diseqc 1.2 only but very few now as most use a version of software that recognised USALS & there are 36v horizon to horizon motors or polar-mounts fitted with 36v Actuators (jacks). 
36v generally requires 36v DC @ 3A- 5A or above with either a satellite receiver with 36v controller on board or an interface box - most popularly named a V Box that takes the diseqc 1.2 controls from any modern receiver with diseqc 1.2 & converts the diseqc instructions to 36v pulse counts usually on a read or optical switch that counts the cycles of the 36v motor in the 36v HH or actuator on a geared wheel set to so many counts for a full movement (eg 1000 pulses/ turns). 
You can control any size satellite dish from a receiver wirh diseqc 1.2 like the TM5402HD - we have installed large motorised dishes (1.2m 1.8m 2.4m 3m etc..)  controlled by the TM5402HD which is actually very good at big dish motor control.
36v normally uses 4 extra wires (so 5 including the feed) - 2 thicker copper multi-strand wires or the 36v DC @3A-5A & 2 for the simple on/off pulse counting generated by the read or optical switches but you can have 5 wire using instead a 3 wire variable resister to show the motor's true position as a relative resistance which is not as exacting but does show the true position, other systems are still used.


Contact purplesat.com for any queries regarding satellite or aerial & HD/3D installations.
TEL: 01625 615379
TEL: 07931 846757
International TEL: 00 44 1625 615379