How Well Does Your Orthomode Feed Perform?

[Pixl]

Pendragon,

I have access to a spectrum analyzer, if I want to evaluate a few lnb's can I make some meaningful measurements just by connecting without pointing at anything in particular, such as looking at the noise floor? Or does each have to be mounted up and peaked.

 

[pendragon]

Pixl - I wish it were that easy. Because LNBs all have different gains, you need to have a reference. Unfortunately you and I lack an anechoic chamber with signal generators, transducers and waveguides. So at best we have to mount the LNB and feeds, and tweak before measuring. But signals change, satellites move in their boxes, the sun heats the dish, pole and ground, and clouds come and go. And LNBs do not perform consistently across the band. This makes careful comparisons rather difficult.

I spent a weekend doing this for all my C-band dishes and feeds. I used one dish as a reference and changed variables on another, comparing the same signal. But I had to keep tweaking even the reference to make sure it stayed absolutely peaked (0.1 dB is a big number for me). From this I gathered a substantial amount of data, and at least some opinions of what worked best. I've quoted the gross results here and at other forums, because it would be difficult to place error bars on the measurements. I'm certain I can accurately distinguish a 0.5 dB difference in those tests, and maybe much less.

I haven't mentioned the issues with measuring a CNR. That can be a headache on its own.

 

[truckracer]

my 10' sami out performs my other smaller c band dishes by a good bit as far as signal quality percentages. it will lock high FEC's when the others either macroblock or see nothing. all my dishes are well tuned and aligned.

BIGGER is better on satellite dishes.

one of the members pointed out in an old thread that basically any multiband setup is a compromise at best. just like buying a tv antenna-a combo uhf/vhf/fm antenna will never stack up against dedicated antennas for each band or if you want to go all out channel specific antennas that are high gain.

i would say seperate c and ku dishes are the ideal way to do it. a 6' or larger even for ku band that is solid metal.

 

[Lone Cloud]

I wish I knew what some of you guys know. I do have two dishes - my 8.5 foot Birdview solid and a 1 meter Winegard - hardly a dish farm but workable.

But with my Geosatpro C2 I basically get no S-2 and never have.

All this lingo about "orthomode" feedhorns and dual and single and quad and octuple. I tried looking it up but got nowhere.

I have decided you are speaking French

 

[Anole]

All this lingo about "orthomode" feedhorns and dual and single and quad and octuple. I tried looking it up but got nowhere.

I have decided you are speaking French

Yea, I brought this up before, and nobody paid attention.
An ortho mode feed needs two LNBs for each band.
So, a C-only needs two C-band LNBs.
C & Ku would need four LNBs.

When someone says "quad orthomode", you know he means dual band.
When someone says "single orthomode", you know he means single band.
But all this talk of "dual orthomode" is meaningless because some are saying "dual LNBs", and some mean "dual bands".

I try to make it a point to be clear when I talk about 'em , so everyone knows what I'm saying.

edit:
And, it's already too late to "be correct".
The damage is done.
From here forward, we all need to be clear what we are saying.

Also -
If you'll look in the Classifieds, Stogie5150 has pictures of two ortho feeds.
The short one is the Chaparral (it's labeled)
And the long-neck one is a different brand.
Note each has two C-band LNBs on it.

 

[Lone Cloud]

Yea, I brought this up before, and nobody paid attention.
An ortho mode feed needs two LNBs for each band.
So, a C-only needs two C-band LNBs.
C & Ku would need four LNBs.

When someone says "quad orthomode", you know he means dual band.
When someone says "single orthomode", you know he means single band.
But all this talk of "dual orthomode" is meaningless because some are saying "dual LNBs", and some mean "dual bands".

I try to make it a point to be clear when I talk about 'em , so everyone knows what I'm saying.

...

Also -
If you'll look in the Classifieds, Stogie5150 has pictures of two ortho feeds.
The short one is the Chaparral (it's labeled)
And the long-neck one is a different brand.
Note each has two C-band LNBs on it.

You mean that wasn't French?

 

[Anole]

You mean that wasn't French?

Well, I take your meaning.
And a search for a good wiki to explain them wasn't productive.

Basically, it's a feedhorn with two ports, which get opposite polarities of signal.
So, one port gets only Vertical signals, and the other gets only Horizontal
You mount a stock C-band LNB on each, and you can receive both polarities all the time.

By combining both LNB outputs in a 2x4 (or bigger) multiswitch, you can exercise voltage control to select between V (13v) & H (18v) signals.

Here are several pictures of the Chaparral C-band only feed, from various angles.

First, a view that the dish sees of the feed, minus LNBs.
Second, a view of the backside, showing where the LNBs bolt on.
Third, a side view of the whole assembly, with LNBs attached.

Needless to say, a dual band ortho feed is more complicated, sporting two C-band LNBs, (one H & one for V)...
... and two Ku band LNBs, (again, one for H & another for V).

Hope that was close enough to English.

 

[pendragon]

When someone says "quad orthomode", you know he means dual band.
When someone says "single orthomode", you know he means single band.
But all this talk of "dual orthomode" is meaningless because some are saying "dual LNBs", and some mean "dual bands".

Whoa, I think you may want to re-read what you wrote. Orthomode means that the device will operate on orthogonal polarizations (H & V or L & R) and not just one.

A single polarization would be handled by a single mode device. One LNB.

A single orthomode feed will receive two polarizations in a single band. Two LNBs.

A dual orthomode feed generally will receive two polarizations in two bands. Technically it might also encompass receiving four polarizations (H, V, L, R) in a single band, but I have never seen anyone use the term in this manner, and it doesn't make a lot of sense. Four LNBs.

A quad orthomode feed is a holy grail for some people in FTA. Presumably in our little world, this would receive H, V, L, R for both C-band and Ku-band. Eight LNBs. However it is rather impractical unless you are willing to live will some significant losses. A siamese feed, like I documented in a thread some time back, is probably a much better solution.

Let's not make the nomenclature worse than it already is