Bullseye II on a Birdview

[DeputyDan]

I have a couple of questions for the good members of the BVOC (Birdview Owners Club).
I plan to mount a Bullseye II orthomode feedhorn on my Birdview solid. I have the scaler ring honed to accept a standard Chaparral feed. I had the 'rings' milled off of the front of a Co-rotor scaler and mounted it on the back of the BV scaler. I will use the set screw on the co-rotor scaler to secure the feedhorn. This setup works fine with a co-rotor feedhorn.

My problem is that the throat of the orthomode feedhorn is shorter than the co-rotor. When I insert the feedhorn as far as it will go, the mouth of the feedhorn doesn't project past the front of the scaler. See the attachments for more detail. Would one of you fellow Birdview w/orthomode owners take a few minutes to measure how far the mouth of your feedhorn projects from the scaler. If I only need a little more length I can cut a couple of notches into the co-rotor mounting ring. If I need more I may need to remove the co-rotor scaler and tap the Birdview scaler to accept a set screw.

My Birdview had the factory(?) sensor mod. When I got the dish the magnet wheel only had 4 magnets installed!!! I replaced them with 8 new ones. I just received my new GBox 3000 and I haven't had time to test the setup to see if 8 magnets will yield enough pulses to work accurately on ku. I have modded a couple of multiswitches to output 18v on all ports (Thanks for the instructions, pendragon) and have a Pansat 9000 on the way. Once I get the feedhorn installed I'll post an update. A new coat of paint won't hurt anything either... The other two dishes in the pictures are Unimesh 10's. I planted them in a straight line with enough distance between them so they will 'see' over the dish in front of them. That way every dish has an unobstructed view H to H.

 

[turbosat]

I was gonna say that sure looks like perfect land for a dish farm-flat like Kansas, but I see that's where you are. (close to Hays?) Nice array there.

 

[lonewolf85]

Paint that poor birdview dish now!

 

[DeputyDan]

I was gonna say that sure looks like perfect land for a dish farm-flat like Kansas, but I see that's where you are. (close to Hays?) Nice array there.

You have a good eye - I'm about 8 miles Southeast of Hays. I have 30 acres of land with a clear view to the South. Plenty of room for more dishes... Are you familiar with the Hays area?

 

[DeputyDan]

Paint that poor birdview dish now!

It is on my To Do list. I'm not sure why it turned that awful color. I'll give it a good power washing and get it painted before I get her hooked up.

 

[pendragon]

The throat of your orthomode should project essentially by the same amount as the corotor. Dual orthos are a real headache in this respect. I don't have a Birdview, but do have a Winegard 3m with a very low f/D (0.278). In my case I not only had to cut two notches out of my dual ortho mounting ring, but also two chunks out of the scalar rings themselves to clear the right angle LNB brackets. That made a VERY measurable improvement in performance.

C-band was essentially as good as an optimized single ortho, but Ku-band wasn't impressive. I could lock everything, but it seemed there was either a beamwidth or cross-polarization issue. Given that I couldn't expand the Ku beamwidth as easily as for C, I tried an experiment this weekend by readjusting the dual ortho feed and putting it on my 0.35 f/D dish. C-band was again great, but Ku was no better than on the low f/D dish.

As one dish is a perf and the other a mesh, I suppose there could be a surface irregularity issue. My next experiment will be to try the dual ortho on a solid, but I'm beginning to worry the problem may simply be cross-pol in the feed itself. That would be a bummer.

 

[DeputyDan]

The throat of your orthomode should project essentially by the same amount as the corotor... In my case I not only had to cut two notches out of my dual ortho mounting ring, but also two chunks out of the scalar rings themselves to clear the right angle LNB brackets. That made a VERY measurable improvement in performance.

I didn't install & fine tune the co-rotor before I decided to go with the ortho. I read a comment in another forum dealing with installing a co-rotor on a Birdview using the BV scaler. Their recommendation was to have the mouth of the co-rotor protrude about 3/4 inch. I assumed that would be a good starting point. It would be interesting to find a Birdview with the original feedhorn installed and measure the distance the factory set.

It might be time for some creative modding...

 

[turbosat]

Hays ? yes, a long story but I spent about a week there in 1994. Saw the sights, the buffalos at Ft Hays St, etc. Place has an interesting history. And I love the ways the roads run in a straight line, lol.

 

[pendragon]

It would be interesting to find a Birdview with the original feedhorn installed and measure the distance the factory set.

I don't know the f/D of your Birdview; with that one can estimate a 'good' starting point for the feedhorn projection. There's nothing magic about this because some situations will work better with a little more or a little less. I'm sure someone here can provide the f/D for a dish like yours. Otherwise measure the diameter and depth, and you/we can calculate it.

 

[DeputyDan]

I'll get some measurements tomorrow. I found the formulas for focal length and f/D ratio. I'll post my calculations.

 

[DeputyDan]

Dish measurements

I measured the dish and came up with these results:

Measuring the portion of the dish face that can see the feedhorn I came up with:
Diameter = 100.5 inches and Depth = 15.5 inches

focal length (f) = Diameter squared (DxD) divided by 16 times the depth (16 x d)

f = (100.5 x 100.5) / (16 x 15.5) or 10,100 / 248 or f = 40.7268
f/D ratio = 40.73 / 100.5 = 0.405

The edge rolls off a little at the very edge so I also measured the Diameter that seemed to be parabolic and came up with: Diameter= 99.5 inches

f = (99.5 x 99.5) / (16 x 15.5) or 9900.25 / 248 or f = 39.92
f/D ratio = 39.92 / 99.5 = 0.4012

The actual measurement from center of dish to the face of the scaler is 39.25 which is less than the shortest focal length (f) that I calculated. For those who are not familiar with the Birdview the scaler is mounted on rigid rods that are threaded at the tips. No adjustments to the scaler are possible (without modding)

If the orthomode feedhorn is like the co-rotor it likes the focal point to be inside the mouth about 0.25 inch. In that case the feedhorn wouldn't project past the face of the scaler.

Pendragon, thanks for reminding me of the focal length calculations. I must have had a brain fart -

 

[pendragon]

My experience with ortho feeds is they like the focal point more inside the feedhorn than one would expect. In terms of the feedhorn projection beyond the scalar rings, I'm calculating about 5mm (about 0.2") for a f/D of 0.40 with typical feedhorn and scalar ring dimensions. If the scalar rings are flush, that would correspond to roughly a f/D of 0.42.

You will lose about 0.4 dB of SNR if the scalar rings are set to 0.42 and the actual f/D is 0.40. That may not sound like much, but it will make a measurable difference. You would be losing about half a foot of your dish diameter. I have found cheating a f/D setting is usually better on the other side, but I do a lot of low elevation work in Colorado with the Atlantic birds.

I'm not familiar with the mounting options and mods for a Birdview, but I've become very liberal with hacksaws, sawzalls, files, spacers and the like when it comes to ensuring my focal point is in the right place and the scalar rings are in the correct f/D position. I use both a bench spectrum analyzer and a USB receiver on Linux that allow me to measure very fine differences in SNR/SQ. I've been surprised how often factory-set or recommended positions for mounting arms, feeds and scalar rings are in the wrong places and result in significant degradation. Feed adjustments and tweaks have been the easiest way for me to improve reception and the ability to lock tough signals.

Calculations are a good starting point, but measurements are often approximate/ambiguous and mathematically modeling feeds/rings/dishes can be almost as much art as science. I always assume I will explore the neighborhood of a calculated position and allow enough mechanical adjustment range to do so.

 

[DeputyDan]

I modified the back of the scaler to allow the feedhorn to project about .3" past the scaler rings. I start at .2" and adjust both ways to get the best signal.

I don't have a spectrum analyzer so I'll have to rely on a Channel Master signal level meter and the Pansat's meter to peak the signal. Would Ku band be more sensitive to focal length adjustments due to the shorter wavelengths?

Thanks for all your help and patience.

 

[pendragon]

I don't have a spectrum analyzer so I'll have to rely on a Channel Master signal level meter and the Pansat's meter to peak the signal. Would Ku band be more sensitive to focal length adjustments due to the shorter wavelengths?

Go by the SQ readings on the Pansat for these types of measurements rather than signal level. Signal level can send you on wild goose chases because there are many ways it can deceive you. More level does not mean more SNR and SNR is the crux of getting f/D and the focal point optimized.

I would recommend peaking f/D and focal length only on C-band and then check Ku to make sure it is ok. The scalar rings are only tuned for C-band and will have little positive effect on Ku. Ku is far more useful when tuning the mount to track the arc accurately. It sounds like you are on right path with your mods. Good luck and happy hunting!