Advice on 1.9m prime focus dish & 4.8'e

[Channel Hopper]

Hi All,

I just wanted to give a quick update on how I have been progressing. I have straightened the dish using the structure shown in photo 1 & 2. I had split the dish into 12 segments using fishing line and then used the threaded bar to ease the dish into shape so that all the lines just touched in the centre. I have centred the Invacom feedhorn on the axis of the dish and set it to the calculated focal point and I have maxed out the signal as far as possible using alignment and adjusting the lnb (including skew). I have been maximising it using the SD transponder in image 4. I haven't as yet hooked up the HD machine- but I wouldn't be that confident as I had been hoping to push a higher reading on the above TP.

One questing I have relates to the Invacom adjustable Feedhorn. I did the calculations for the projection required and accurately set it using Vernier callipers. On Saturday, as an experiment, I tried adjusting the corrugated feedhorn as I though it would need 'tuned'. I tried it at half turn intervals. And you know what?- It made zip difference!! I tried the ring right along the threaded section and the signal strength and quality stayed exactly the same. I even tried the ring projecting a few threads beyond the waveguide and again it made no difference. Next I tried the corrugated ring completely off (just leaving the waveguide). In this instance the signal strength stayed the same but the quality dropped by 4%. Does this seem right? I admittedly don’t fully understand the purpose of the corrugated ring but I would have thought that it should have had more of an impact than this- especially as the projection is supposed to be important. Am I missing something?


Thanks Trev

You are aware that the feed centre point has to be on the central line of the reflector, boresight and for full efficiency you have to ensure the feed is perfectly aligned with the perpendicular axis of the reflector surface, no margin for error.

Additionally the screw in/out part of those feeds has to mimic the focal point of the dish, so alignment is done with extending the feed arms to the optimum distance and then adjusting the scalar by turning the thread. It does take a number of small adjustments to get right, especiially with a feed arm system like yours that has three reflector bolt holes that require synchrony in where they need to be tightened.

In cases like this, I would conclude that if you have done the above as best you can and without firm measurements with an analyser, the reflector is sending received signals everywhere, both to the feedhorn but also elsewhere and no matter how many adjustments you might make, the dish parabola will not perform any better than you currently get without distorting it.

It could be you have a defective or 'second' manufacture dish or feedhorn but only a proper check with the right equipment is going to give you an answer.

Where did you get it and how was it advertised ?

 

[trev1978]

The dish was on the roof of a former Stan James bookmakers!! I Paid the building owner £50 for it and the stand 7 years ago- although I wouldn’t say that represented valve for money given the time spent on it! I would estimate it is a 1980’s reflector. I would accept that the reflector is probably not top end by today’s standards but there has been an improvement with correcting the slight warp over the rim. The Invacom feedhorn was purchased from a retailer on eBay around the same time.

I did spend a lot of time centring the feedhorn on the dish. You cannot perhaps see it in the photo but I set up a fishing line running from the centre of the dish and struck it through the crossing point of the lines that I had set up running between opposite points on the rim of the dish. I adjusted the feedhorn so that it was centred exactly on this axis.

What is the exact point of the scaler rings? I remember reading (I think this forum) that the reflected signal from the dish should form a ‘cone’ running into the waveguide of the feedhorn with the focal point just inside the waveguide. If this is the case what function does the scaler ring serve? Certainly in my case is doesn’t seem to be doing a lot.

 

[Channel Hopper]

It looks like a Levick reflector to me which would be an 80's design, though I don't have the specifications to hand. Did the feed come with the dish or is it aftermarket, and more importantly, is it at the correct focal distance for the reflector ?

Additionally older style feeds (and many earlier reflectors), were manufactured and tuned at 10 to 11 GHz frequencies, and the upper Ku band might not be received with the same efficiency owing to a mismatch of transfer port causing reflections and subsequent losses.

Centering is all about boresight, which can be accurately achieved with a laser down the feed throat, though it is possible to do with some line down each of the feedarms to the edge of the reflector, though after adjustment you will have to stick the LNB back on and realign the dish to the satellite.

 

[trev1978]

There was an original feedhorn on the dish but it had a rectangular throat at the end of the waveguide so that is why I purchased the adjustable invacom feedhorn. My dish diameter is 1900mm with a depth of 296mm which calculates a focal length of 762mm. I set the waveguide to this then hooked up the lnb & meter and moved the feedhorn in and out to max the signal.

As I said I cannot understand why the position of the scalar ring on the waveguide has no impact. Perhaps on a high f/d figure (mine is 0.4) the scalar ring doesn’t have that big of an influence?