OTHER Channel Master 1.2m Focal Point

[Captain Midnight]

I am using a CM 1.2m fiberglass dish for ku with a C120 feed and an Eagle Aspen LNB.

I finally got around to peaking the thing after realizing performance was just a bit subpar.

The Feedhorn throat has a slot that fits the LNB holder perfectly. That's why I thought my focal point was set. But after playing with it, I noticed signals boomed in when I shoved the feed as far forward as I could. I even had to remove one of the little C shaped LNB adapters that fit in the holder to secure the feedhorns with skinny throats. It works but not as expected.

Anyone else notice this or have this issue?

 

[RimaNTSS]

Captain Midnight if you have time look in THIS thread
But, if in 2 words, this is not only issue of the antenna's focal point but also issue of the phase center of the feedhorn. And main target of this exercise is to get those 2 points co-located. Secondly, the feedhorn has to be pointed to the proper point on the surface of the dish.

 

[Captain Midnight]

Captain Midnight if you have time look in THIS thread
But, if in 2 words, this is not only issue of the antenna's focal point but also issue of the phase center of the feedhorn. And main target of this exercise is to get those 2 points co-located. Secondly, the feedhorn has to be pointed to the proper point on the surface of the dish.

That what has me thrown for a loop. The feedhorn and LNB are still perpendicular to the holder assembly.

 

[Captain Midnight]

Does anyone know if this measurement should be taken from the translucent cover on the feedhorn or from where the feedhorn meets the "throat" of the waveguide?

 

[RimaNTSS]

if this measurement should be taken from

The measurement should be done to phase center of the feedhorn. Phase center is located inside the feedhorn, just a couple of centimeters in front of the end of the waveguide. On the attached picture, I have with the red circle (very approximate). The green circle shows the approximate location of the focal point of the dish. In ideal situation those 2 circles should be co-located.

 

[Captain Midnight]

The measurement should be done to phase center of the feedhorn. Phase center is located inside the feedhorn, just a couple of centimeters in front of the end of the waveguide. On the attached picture, I have with the red circle (very approximate). The green circle shows the approximate location of the focal point of the dish. In ideal situation those 2 circles should be co-located.

Thanks for the clarification. I did have my feed back in that position but my C/N readings were not as good as a friend of mine with a GeoSatPro 1.2m. That lead me to try experimenting with the feed assembly. For whatever reason, it is working in the current position really well. I need to take some time to experiment longer with it and check for any moisture inside the feed while at it.

I have an Inverto Pro Black Dual LNB that I can install on the feed to compare. This Eagle Aspen is a Standard LNB, the Inverto a Universal. Since I run my dish through 22khz switches, I had thought that I needed standard LNBs. But I recently was told the Universal will work but only be able to scan the standard 'upper Ku' band.

I wish I had a dual output Maverick LNBF. I tested it on there after first buying the dish and was impressed by the performance. I only got the C120 feedhorn after reading it is perfectly matched to the CM1.2 reflector.

 

[Captain Midnight]

I went back and installed it the conventional way and had around 14.3dB on 11880H at 103w. Then I set it up close where I had the better results and hit 17dB on 11880H and 11760H! Not sure why. I am using my Inverto Black Twin Pro now too. Same results

 

[Captain Midnight]

I went back and installed it the conventional way and had around 14.3dB on 11880H at 103w. Then I set it up close where I had the better results and hit 17dB on 11880H and 11760H! Not sure why. I am using my Inverto Black Twin Pro now too. Same results

Anyone else have an idea on what is happening? I went with this C120 feedhorn because I read it was designed to minimize interference for the 1.2m.
While testing this, I went ahead and pulled out a new Maverick LNBF and the meter registered 16.4db on 11880H with it in the proper conventional position in the LNB holder.

 

[a33]

I went back and installed it the conventional way and had around 14.3dB on 11880H at 103w. Then I set it up close where I had the better results and hit 17dB on 11880H and 11760H!

Just checking: You did in fact also finetune elevation again?

Greetz,
A33

 

[Captain Midnight]

Just checking: You did in fact also finetune elevation again?

Greetz,
A33

Yes.

 

[a33]

Yes.

Then I have no ideas.....

Alas,
A33

 

[Captain Midnight]

Then I have no ideas.....

Alas,
A33

I too have no idea why it would be better. I would think I would have trouble with the feed not collecting the gain of the entire reflector.

 

[RimaNTSS]

Captain Midnight Unfortunately, original CM feedarms do not give you enough freedom to freely adjust the position of the feed. That is why I have changed all the feedarms on all my CMs.
I personally think that it is nearly impossible to manually find the proper position of the feed. I think the position must be carefully calculated then the feed must be installed and only then the whole antenna system should be pointed towards the satellite to get maximum signal. Afterwards, the feed can be moved closer/further from the dish and also skew can be adjusted.
Do not forget that the offset angle of the CM dish is 22.6* and it is possible to use this constant to check whether the feed is in right position. It might sound difficult but, in fact, very easy procedure - take a digital protractor and measure the leaning (elevation) of the dish when you have maximum signal from the satellite. Then extract 22.6 from the elevation of particular satellite and compare the result to the measured leaning angle.

 

[Captain Midnight]

Thanks for the input. I carefully played with the placement for an hour, trying to find the "sweet spot" and never could beat my current position. I did the elevation adjustment with the feed in the "conventional" placement, so I believe I have kept my current feed position facing the dish with the same offset angle. But maybe I can experiment with it more once I have cooler weather! Over 35°C here today! Heat index near 40!

Captain Midnight Unfortunately, original CM feedarms do not give you enough freedom to freely adjust the position of the feed. That is why I have changed all the feedarms on all my CMs.
I personally think that it is nearly impossible to manually find the proper position of the feed. I think the position must be carefully calculated then the feed must be installed and only then the whole antenna system should be pointed towards the satellite to get maximum signal. Afterwards, the feed can be moved closer/further from the dish and also skew can be adjusted.
Do not forget that the offset angle of the CM dish is 22.6* and it is possible to use this constant to check whether the feed is in right position. It might sound difficult but, in fact, very easy procedure - take a digital protractor and measure the leaning (elevation) of the dish when you have maximum signal from the satellite. Then extract 22.6 from the elevation of particular satellite and compare the result to the measured leaning angle.