Ku Frequency Issue With Prodelin 1.8m Dish

[stogie5150]

I have on my hands an engima that I hope someone can solve. Awhile back I scored a Prodelin 1.8m dish with Dual Ortho feed, Norsat .8 LNB's. At the time folks said they had never seen a 1.8 with the feed setup I had. I took some time, adapted the dish to a polar mount, and got it tracking. Installed the LNB/Feedhorn combo, and off it went. Great signal, great quality. This is the pic of it as installed:

Shortly after, I noticed that the ortho LNB/feedhorn was scanning the Ku frequencies 18mHz HIGH from my other Ku dish, a BV with a quad ortho with Norsat .6 LNB's. I know LNB's shift with age, but 18mHz? No matter what the receiver, no matter what the switch.

So I said well, I have a regular Eagle Aspen linear LNB, that may just bolt up to the horn, I'll try THAT. Somehow the horn was drilled correctly ( couple guys I talked to said theirs wasn't so who knows).... like so:

Same issue. 18mHz high. I changed multiswitches, changed diseqc switches. Ran new coax. Changed the ends. Bypassed the ground block at the dish with barrels. Ran two new coaxes bypassing everything to the multiswitch. No change.

So today, I decided on a test. I had an SatAv Bullet single LNB, went out to the shop, and cobbled up a quick and dirty LNB holder, ran a cable from the LNB to the diseqc switch, scanned a satellite...BAM right on. Scan a satellite with the BV, the bullet was on the same frequency. Scan with the bullet, watch the same freq with the BV.

So the question is....whats causing it? the only thing I can figure is the feedhorn itself, although I don't see how that could be, feedhorn is just aluminum, no electrical connection with anything to affect the frequency. But that is the ONLY constant with the whole situation, its the only thing I didn't change.

My course of action at present is to order a regular dual LNB and fab up a better mount, and just use that. The bullet and the Norsats were dead even quality wise, so I am not worried about signal loss.

What am I missing?

 

[Anole]

I know you've been wrestling with this for a while.
And you've tried everything several of us could come up with.
Regardless if it -should- affect the problem or not.

I know I've hinted at this one last idea before, and I think you tested the theory, but...
Could you duplicate the odd results on a different satellite, please.?.?
If so, they I'm totally out of ideas.

 

[Pixl]

I have nearly the same setup on one of my Prodlins.

The dish originally had one lnb and a Vsat transmitter mounted on it.
I removed the transmitter at the tail end of the orthomode and bolted on a second lnb for the opposite polarity just as yours is set up. Only difference, and it should not make any difference is that I removed the right angle elbow that contains transmit reject filters. My lnb's run at 10750 L.O. and do not have any 18 MHz displacement. since the dish and the feed horn and the waveguides are all mechanical devices, it is not likely they are the cause of this. I haven't read what you have already tried, but I got to believe it is something to do with your lnb settings in the receiver, not the dish.

By the way, the feed on these are drilled for 4, 6, or 8 bolt patterns and is why you can bolt almost anything up to it.

 

[FaT Air]

I got to believe it is something to do with your lnb settings in the receiver, not the dish.

Because, I don't see, read, that you tried a different receiver. You could have used the same receiver on this and the BV.(???) May not be the settings, may be an oscillator in the receiver is 18mhz off. But on 2nd thought, that wouldn't explain why the SL2 Bullet got the correct freqs(???) Unless it's got something to do with levels. Maybe the troublesome setups generate too much signal and overloads the receiver, causing the 18mhz discrepancy somehow (??) Nice, more questions than answers.

 

[stogie5150]

Because, I don't see, read, that you tried a different receiver. You could have used the same receiver on this and the BV.(???) May not be the settings, may be an oscillator in the receiver is 18mhz off. But on 2nd thought, that wouldn't explain why the SL2 Bullet got the correct freqs(???) Unless it's got something to do with levels. Maybe the troublesome setups generate too much signal and overloads the receiver, causing the 18mhz discrepancy somehow (??) Nice, more questions than answers.

Does not matter receiver wise. Vantage, Sathawk, hell I even tried the Coolsat 5000 I have in the shed...all the same, 18mhz high. It has me flummoxed too. I have been trying different stuff for two months now or longer.

 

[turbosat]

STogie I will consult the bible on that one, that really is curious. I think you figured it out already though. (feedhorn) but I can't yet explain how or why.

edit: Ok the bible was no help in this one. Of course, it was written in 1985, but feedhorn design probably hasn't changed that much, waveguides are waveguides, physics is still physics, etc. How it could introduce a shift in frequency (if it is what does it) is a mystery!
Let's hope pendragon reads this one, I bet he will have a theory.

 

[pendragon]

The feedhorn is not going to cause a frequency shift. Could the polarizations be swapped? Perhaps if you could post a few examples from a particular bird on both H & V, that might help us. Is there any chance the feedhorn was intended for circular polarization?

 

[stogie5150]

The feedhorn is not going to cause a frequency shift. Could the polarizations be swapped? Perhaps if you could post a few examples from a particular bird on both H & V, that might help us. Is there any chance the feedhorn was intended for circular polarization?

I can't answer the circular question. I just don't know. The dishes were on an old department store ( I got a 1.2 and a 1.8 with the same feed setup on them), and had Tx and Rx units on the feedhorns. I removed the Tx and installed a Rx.

Polarization, well, the LNB's are installed straight up, the eagle aspen the line molded into the LNB indicating 90 degrees is straight up. Anyway....this is scanned with the Quad Ortho on the BV, confirmed with my temporary bullet LNB too....

And this one was scanned with the Eagle Aspen with the stock Prodelin Feedhorn:

Thanks everyone for weighing in!

 

[Cham]

Stogie,
What is the local oscillator frequency on the Norsat and Eagle Aspen lnbs? Maybe moving the LO up or down 18MHz in your receivers for these feedhorns would correct your frequency issue... but if they are standard 10750MHz it would be a really strange phenomenon...
Are the LNBs seeing the correct voltage? The Norsats might want 18V full time and maybe go squirrely like some Motorola PAs when the voltage is a bit low.
Maybe interference though a switch or other component in the circuit somewhere. Combining the LO from a universal or DSS LNB with the 10750 might cause mixing in the circuit, but to me that's too far away frequency wise...
Interesting problem for sure!
-C.

 

[stogie5150]

Stogie,
What is the local oscillator frequency on the Norsat and Eagle Aspen lnbs? Maybe moving the LO up or down 18MHz in your receivers for these feedhorns would correct your frequency issue... but if they are standard 10750MHz it would be a really strange phenomenon...
Are the LNBs seeing the correct voltage? The Norsats might want 18V full time and maybe go squirrely like some Motorola PAs when the voltage is a bit low.
Maybe interference though a switch or other component in the circuit somewhere. Combining the LO from a universal or DSS LNB with the 10750 might cause mixing in the circuit, but to me that's too far away frequency wise...
Interesting problem for sure!
-C.

Results were the same no matter the LNB, the separate Norsats or the Eagle Aspen. The Eagle Aspen is 10750, the Norsats, well, can't tell ya because there is no model number on them I dont think, faded out from sun exposure. I thought about adjusting the LO, but I'd rather fix the problem.

 

[Pixl]

The feed horn is not going to cause a frequency shift. Could the polarizations be swapped? . Is there any chance the feed horn was intended for circular polarization?

I can't answer the circular question. I just don't know.

Pendragon,

Judging from the pictures stogie has the same feed I have (see photo above). The feed horn is round, funneling down to a short length of 18mm round waveguide. The orthomode has a round waveguide input and transitions to rectangular waveguide with a vertical port out the top and the horizontal out the back as currently mounted.
I had noticed but didn't measure, that the back port looked slightly smaller where the transmit module used to be mounted. I thought maybe the higher TX frequency might call for a smaller dimension waveguide and may compromise Rx performance. But rotating the assembly 90 deg, effectively swapping polarities produced identical receive levels. I still don't know why stogie has 18mhz displacment. Mine works ok.