Detailed prime focus operation

[ssjenkins]

Hi All ,

I've got a 2.4m prime focus in the south of Spain . This allows me to get most of Astra 2D (no BBC2) 24 hours a day .

Last week I put up a 7m vertical ham antenna which has 6 2m horizontal radials at the base . I've attached a photo . 2 of these horizontal radials pass over part of the face of the dish which is located about 2m away . As soon as the vertical antenna was mounted I noticed that I started to lose a lot of the weaker 2D channels at around 11pm at night . Yesterday , the ham antenna installers came back over and started to measure the prime focus signal strength (during the day) removing the radials at the same time . There was no difference in the signal strength . Eventually yesterday we decided to remove the ham antenna temporarily and last night I made some tests and I had no signal loss on Astra . Basically that means that the ham antenna is the guilt party . I'm now trying to convince the (quite tecnically competent) ham guys that the vertical is causing the problem .

To cut a long story short I'd like to learn more about the tecnical properties of the prime focus dish . I know that the LNB always blocks part of the signal but I'd like to see some tecnical details showing the face of the dish and how the signal is transfered to the LNB .

I've done a bit of Googling but I haven't been able to find anything sufficiently technical enough .

Any help would be appreciated .

Sammy

[freebe]

@ssjenkins
Any Structure that is in front of an Antenne will reduce the Signal.
But i am not shure that that is whats causing your Problem.
I would be more inclined to turn your L.N.B. and see if you get an improvement. The world down there is a bit on the side.
Turn you L.N.B. about 10deg anti clockwise from looking into the dish.
The 2D signal is Very tight over England but with a 2.4 Prime i would say you might have Problems.
Take a look here.
> _http://www.lyngsat-maps.com/maps/astra2d.html

Sorry i cant give you a live link

[ssjenkins]

Thanks freebe . I'm doing some testing this weekend removing bits of the antenna to see what happens . So hopefully I'll find a solution . I don't see any difference on my meter with the radials removed buy my meter is not exteremly sensitive so I wouldn't notice the drop anyway . Changing the skew may help but I'd rather mot touch the dish at the moment as it works fine by itself . I may try rotating the vertical bit by bit . Just so you know there's a better footprint map of 2D here .

_http://www.astra2d.com/iberia2d.htm

Cheers

[freebe]

@ssjenkins

Yes the first map is a bit confusing :-)) but if you go deeper then it seems some of the people have picked it up OK.
Well all i can say is good luck, and try that with the L.N.B. skew it will make a big difference.
Think of it like a Post Box and if you turn it the Postman will also have to turn the letter to get it in.
The world is not flat but it follows the Ark.
:respect-048:

[oldfart]

Been told we need a 2.8m channelmaster (min) to get 2D here :cuss:

[ssjenkins]

2 of the radials are causing the problem . They're about 2m away from the dish but obviously close enough to cause some signal loss during the evening . Going to move the vertical on Friday which will fix the problem .

@oldfart . Depends where you are . I'm in Alicante and a 2.4 will get you most channels 24/7 . No BBC2 , but BBC1 (Not Scotland) , ITV, CH4 (+1) , CH5 etc. I've only got a FTV card but with a subscription you'll get most movie and sports channels .

Sammy