[P12] What's up?

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For some reason we suddenly have solid reception of all the weak channels. Instead of the low 50s. Quality is now in the high 60s. Even as high as 70% on one channel! Signal strength S remains at about 78.

The weather is almost totally overcast but bright. With only occasional sunny periods. After a completely still start and an overnight frost, the wind is picking up to a breeze.

I have no explanation for this sudden change in signal quality. The wind cannot affect the dish very much even when it is blowing hard. Otherwise the stronger channels would disappear along with the weak ones. A large dish demands a much stricter pointing accuracy with its acceptance beam width of less than 1 degree.

I have found a second hand galvanised pipe to get rid of the ugly concrete fence posts which I use to load down the present iron and steel base. A simple pole will be much neater and allow the grass to be mown tighter to the dish.

By the time the new pole is cast in concrete it should provide a much more stable base for the new dish. A flange on the bottom will ensure the pole (and mounted dish) cannot rotate under wind loads. Normally one would have to add rods or fins to stop pole rotation in the concrete base.

A pole set in bare earth, even with added rocks or boulders in the excavation is not nearly stable enough. The concrete greatly increases the surface area of the pole. Thus avoiding the localised pressure a simple pole is able to exert on the soil. An "elephants foot" excavation will avoid the pole lifting due to frost heave or wind storm effects on the large dish. Channel Master recommend a cubic meter of concrete for their 1.8m dishes!

Yesterday's improved reception excitement was short lived. Q had dropped to 53% by 14.30 on most Spot Beam channels without a safe lock possible. Nothing pm, overnight or early morning.

Today the higher signal strength had returned by about 10.30am. S was back up to 80 with Q up to 70%. By 2.30 many channels were becoming only borderline watchable with low 50s% Q. Though some channels were still fine at 17.30pm! A very strange variation in Q across the range of transponders on the UK Spot Beam. 

I found this YT video on checking satellite dish focus using the sun and some small mirrors:

http://youtu.be/TTXvpES1_ZA

Provided the reflections are all equal and all overlap on the face of the LNB (or feedhorn) they must be centred and in focus. I believe the spoken language is an Indian dialect.

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[P11] Bigger AND better?

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As I hinted in my last post; there is the option to go on increasing dish diameter if stable reception is impossible with a smaller one. However there are many obvious and less obvious disadvantages to up-sizing. Not least is having a huge round object cluttering up the garden. Then there are the inevitable problems of wind drag literally tearing the dish from a weak support. "Hoist the spinnaker!"

There is the increasing narrowness of the field of view or signal acceptance angle. This problem is severely exacerbated by any wind movement in the mounting of the dish. Massive concrete foundations and thick walled pipe of requisite diameter to suit the dish size and its mounting are a must.

Dish flexure, or that of the feed support arms, is very possible in very high winds.

Sturdy stands (usually of tripod or four-legged construction) are available to allow the adjustable mounting itself to be bolted down. Usually onto buried concrete footings. Though these stands may also be bolted own onto flat, concrete roofs if necessary. The emphasis here must obviously be on absolute strength and stability in extreme wind speeds. Nothing else offers the level of public safety required.

Or the dish can be attached to a massive aerial mast as seen here in abundance. (These are all high quality, Channel Master/Andrews (now Raven) offset dishes except for the very top one which is prime focus)

The bottom one is about 6' or 1.8m in diameter at a guess. It is very difficult to judge from below and impossible to reach with a tape measure. These communal lattice masts are seen in many towns and villages throughout Denmark. Formerly providing a local TV service "pack" by cable for a regular fee. While avoiding the need for aerials or dishes to be installed on every home. Most of these TV services are now provided by a small dish and payment card to go in the receiver. Or TV packs can often be provided on the Internet these days against a subscription.

Signal gain from increasing dish diameter is hard won and still highly dependent on dish quality and accuracy. The cost rises extremely quickly if a brand new, larger dish is purchased. There may even be heavy charges for delivering the over-sized beast. Anything to special order is likely to enjoy a heavy premium. Shopping around is probably wise to minimize the expense.

Trying to save money on cheap, kit-built, segmented dishes is arguably a waste of money. That is, unless great care is taken in assembling and supporting the dish to a very high standard. Any misalignment in the construction of the "petal" segments will result in far inferior performance over a high quality, one-piece dish. The segmented dishes can't be simply assembled and expected to offer upmarket performance for a small fraction of the cost of the best quality dishes. Some dealers will not even stock them for this very reason. Buying, assembling and installing one badly is likely to be a total waste of time and resources. 

Then comes the choice of an offset or prime focus dish. Certain manufacturers, such as Andrews (Previously Channel Master but now called Raven) have produced very high quality offset dishes. These are made in a whole range of sizes from moulded plastic with a reflective mesh hidden under the curved surface. These dishes are easily recognized by the unique rear ribbing. Designed to maintain the accuracy and rigidity of the parabolic dish surface. These dishes are certainly not cheap unless they can be obtained second hand. Their quality is legendary.

Offset dishes have the LNB set low on the nearly upright dish. The satellite signal is reflected downwards at a considerable angle above the dish's apparent axis before it reaches the LNB at the focus. Raven offers a special, offset, C120 feedhorn to match its own offset dishes.

Most other commercial offset dishes use a 40mm LNB specifically designed for offset us. Offset dishes are reputed to have theoretically better performance than prime focus dishes. Though this difference may be the result of the quality differences between various manufacturers. One must compare like with like. Raven provide prime focus dishes as well as offset. It might be instructive to study this manufacturer's technical literature for each type for a more accurate comparison. One must also remember that the Ku-band requires far greater accuracy in dish manufacture than the lower frequency C-band. 

Channel Master (Andrews/Raven) offset dish. Probably a 1.8m [6']in diameter. Note the tiny feedhorn and professional quality LNB.

Prime focus dishes are easily recognized by having equal length, LNB support arms. With the LNB sitting absolutely central to the dish and the dish usually pointing up in the sky directly at the satellite. Prime focus dishes tend to look better in that they are tilted back and thus their more oval appearance looks smaller. They also look more like traditional radio astronomy dishes. The obvious sense of purpose may be more acceptable to some eyes.

After years of using a 120cm Gilbertini offset dish my foray into larger dishes began with a prime focus one. Though there was absolutely no conscious decision involved. Beggars can't be choosers and I found my neglected 6' fibreglass dish quite nearby. A search of the local small ads may save a  lot of money if one is extremely lucky.


Large dishes (over 1.5 meters diameter) have only a very small amateur market in most places. Being of interest only to those seeking very weak TV signals. Which usually means trying to receive TV channels out of their intended geographical range. Perhaps of greatest interest only to immigrants seeking the pleasures of their own domestic TV stations. Which are broadcast in their own, native tongue. Though there are a few hobbyists around who seek weak TV channels as a pastime, of course. These few must represent only a very small proportion of the large dish market. Though there seems to be more interest in the hobby in the USA judging from the number of YouTube videos on the subject.

Depending on your country of abode there may be planning restrictions on satellite dishes. Particularly in built-up areas, on (historical) listed buildings or in their grounds. A neighbour may consider his vast, white,  mobile home (caravan) as something to be immensely proud of. Yet, ironically, the same caravan owner may greatly resent the intrusion of quite a modest sized dish into his view from the terrace.

It doesn't help your cause if your dish is as toothpaste white as their own caravan! Though dishes may be painted with non-metallic paints it must reduce efficiency slightly. A well upholstered, rural garden probably offers a far greater chance of hiding your "carbuncle on the landscape" from view  While a city garden is usually overlooked from a number of angles and is often much smaller. Greatly increasing the sense of scale of the intruder. It's similarity to an upturned UFO may not have been missed by the mischievous neighbour who sees parallels between your dish and your alien status.

An old, well-weathered 2.2m [7'2"] spun aluminium (?) prime focus dish by Kathrein on a Danish communal TV mast.

The dish must obviously have an absolutely clear view of the sky. Though only in the desired direction of the host satellite and it must be clear over the entire surface area of the dish. No building, trees or bushes must block the delicate signal path from the satellite. Imagine a broad beam of light coming down from the satellite of the same diameter as the dish. No shadow may be thrown anywhere on the dish. Or think of the dish as a huge searchlight. The parallel beam must not shine on anything except the sky.

Larger dishes do not look well attached to a house wall. Nor from a balcony on a block of flats! The scale is all wrong and the supporting wall would need to be very strong! Wind loads on satellite dishes rise extremely rapidly as their area increases!

The bottom line is always: Are you feeling lucky? Enough to invest lots of effort and possibly thousands of pounds (or currency equivalent) in a very large, high quality dish, its mounting and stand in the (perhaps) vain hope of successful reception? A secondhand dish of suitable diameter might be well worth a try. The technology of TV reception via TV packs has changed dramatically. Small dishes and the internet now supply this public demand.

So a large dish may be sitting on an obsolete mast somewhere nearby. Simply because it is far too expensive for the limited funds of the local "TV club" to take it down. Though this is only likely in Denmark it may also be true elsewhere.

I missed a large (1.8m) CM/Andrews dish on a telephone exchange simply because I could not sum up the confidence to ask about it. It disappeared between my weekly visits to the town. As far as I know it was just scrapped. Getting one of the mast-mounted dishes down will probably require a strong cherry picker and formal permission from the owners. Plus a modest sum towards the mast owner's funds. These lattice masts are usually protected at the base from would-be climbers, drunks and over-ambitious children.

Click on any image for an enlargement.

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