interesting c band results

[truckracer]

I have a dsr-905 and a pansat 9200HD running seperate coaxes out to a zinwell 4x8 directv multiswitch which is fed by a chaparall orthomode c/ku feedhorn on a 10' mesh dish.

My dish tracking is near perfect.

I can go to lets say G14 C:

I can tune the skew of the feedhorn to peak digicipher II tp's on that bird, but it drives down the signal a little bit on regular DVB FTA tp's.

I can tune for dvb fta, and it drives down the dcII signals a bit.

It seems that since both types of signals come off the same satellite on the same band that if one was peaked the other would be too.

I have noticed this on both my dishes. Its not a big deal, but an observation.

anybody have any ideas since i am curious now?

 

[Mr Tony]

truckracer
I've noticed the same thing on my 6 footer when I work with both DVB & DCII. G17 as example

Also with KU I've seen that happen too. I've been working with the C/K1 recently just seeing what I can get and when I go to 125W I can lock TBN at 90 quality but PBS is low. I skew the LNB a little different then PBS is at 85-87 but TBN drops to 65

 

[tvropro]

Could this be the way the feed handles c and ku frequency's? With a servo I would just adjust the skew for each band. I have noticed weird things with skew myself on C band. Why in theory it should not change on a polar mount across the arc. Although some satellites will benefit a + or - 5 off the norm. I attribute that to some kind of crosspolar from something.

I say the hell with theory... If it works go with it

 

[pendragon]

It also could be co-frequency interference from the satellites next door. Depending on which side is causing this and by how much, nudging your dish or skew one way or the other may help one signal but hurt another. There are also a lot of frequency dependent phenomena in your feed or dish that could masquerade the same way.

 

[truckracer]

I guess its safe to say that dvb and dc2 may handle cross pol interference differently.
I was always under the assumption however if the dish is "peaked", it is peaked regardless of what type of digital signal is coming down. On the chaparall bullseye ortho feedhorn, ku booms in almost regardless of focal distance and skew. C band is the most sensitive on my setup-which is certainly out of the norm. Usually you can hace c band tuned and still not have ku, this ortho feedhorn has the best ku reception on a big mesh dish i have ever seen.

 

[tdti1]

What size dishes do you have? Are they warped or in good shape?

 

[Hermitman]

Here's my 2 cents worth. I believe the variation is caused by the feedhorn opening not being exactly in the center. If the feedhorn opening is slightly off-center (we're talking fractions of an inch at these frequencies), the signals bouncing off the reflector will be slightly out of phase. Example: If the feedhorn opening is 1/2 inch off to the left, then the signals from the right reflector side will have to travel 1/2 inch farther to reach the focal cloud (1/2 inch may not sound like much but the c-band wavelength is less than 3 inches, even less for ku band). That's why its critical to get the opening as close to the center as possible. You then have to re-adjust the probe in the feedhorn throat for maximum wave amplitude. And since signal phase difference is frequency dependent, when you select a different transponder (different frequency) the maximum wave amplitude will occur at a different area of the feedhorn throat and you need to re-adust the probe position for the best signal. Its a theory anyway. Good luck.

 

[truckracer]

That sounds quite likely as it would create sort of a multipath condition. I am sure none of our dishes are exactly centered. Both my dishes are new and straight

 

[melgarga]

I have but one comment - All Hail Polarotors!.....huh tvropro
hehehehehe

 

[Mr Tony]

truckracer
I experienced the same thing + another quirk yesterday.
3 channels...3 different results

Had the Pansat 1500 & the 905 out yesterday just tinkering. Geosatpro c2 dual C-Band LNB on the 6 footer. Aimed at G1/G15 and locked Classic Arts. Dialed up the 905 on channel 150

Classic Arts...60 quality
California Chanel....45 quality
Starz 150...+3.0

but if I skewed the LNB a little more then Starz was up to +4.9
Classic Arts stayed close to 60
California dropped to 30

but if I moved the dish east then
California 75
Classic Arts 55
Starz dropped down to +2.8

Sounds like G1 and a 6 foot dish dont (bad pun) mesh well