Feedhorn installation tips
Proper installation and adjustment of the feedhorn is critical to system performance. It is particularly important if you are installing a feedhorn that receives Ku-Band signals. In order to find the correct focal distance for tracking, the feedhorn must be at the correct distance from the center of the dish, properly oriented, centered and perpendicular to the plane of the antenna. Follow the alignment procedure outlined below:
- Set the scalar ring adjustment for the f/D ratio that is called for in the antenna specifications. If you do not know the f/D ratio, you can calculate the focal distance and f/D ratio yourself using a formula.
- Rotate the feed to it’s proper orientation using the “polar axis template”. The polar axis is a line that runs through the center of the dish pivot points. It is the axis around which the dish will rotate. Another way to look at it is ……If your dish is positioned so that it is pointing at it’s highest point of travel (the zenith of the arc)…….when you stand directly in front of the dish, the “polar axis” runs from 12 o’clock to 6 o’clock. Proper orientation in these terms means that you point the arrow of the polar axis template at 12 o’clock (directly in line with the axis). If you do not have a template, you can get close by siting down the long side of the servo motor; pointing it at about 11 o’clock.
- Centering the feed in the dish is also critical to proper reception. This can be done by measuring from the feedhorn to at least 3 different points around the rim of the dish (i.e. measure from the feed to the left side, right side and bottom). The 3 measurements should be equal. Use the adjustments in the feed support legs (or guy wires if you have a buttonhook support) to make any necessary adjustments.
- The opening in the feedhorn (face) should be parallel to the face of the antenna (dish). The easiest way to check this is to use an inclinometer or universal protractor. Check the angle at the center of the dish and across the throat of the feedhorn; the measurements should be the same.
The f/D ratio and scalar rings – why it is important to set properly
Proper setting of f/D on the feedhorn allows the feedhorn to take advantage of all of the signal being reflected off of the dish, without receiving interfering ground noise or terrestrial interference.
The f/D ratio is the focal distance of the dish (f), divided by the diameter (D). When dealing with most prime focus antennas, the number should come out between .28 and .42. If you notice, most of those numbers are also on scale on the side of the feedhorn. You simply set the top edge of the scalar ring even with the line that corresponds to your correct f/D setting.
What this adjustment actually does is determines how wide of an angle the feedhorn can “see”. If the dish is very deep(example: 10ft diameter dish that is 24 in. deep), having an f/D of .28 for example, then the focal distance is relatively short. When that is the case, the focal distance is often only a few inches greater than the depth of the dish. Therefore, the feed needs to be able to “see” nearly straight to the side of the opening in the throat.
Conversely, if the dish is very shallow (example: 10ft diameter dish that is 11 in. deep), the f/D ratio would be closer to .42 and the focal distance would be much longer. In that case, the feed would need to have an narrower field of view so it would “see” the whole dish, yet not see past the edge of the dish.
Formulas for calculating focal distance and f/D ratios
To calculate the focal distance, you have to measure the diameter (D) and the depth (d) of the dish. Measurements should be in like units (you can’t use feet for the diameter and inches for depth). For the example, we will say we have a dish that is 120 inches in diameter (D) and 18 inches deep (d).
focal distance (f) equals the diameter squared (D x D) divided by 16 times the depth (16 x d) or :
D x D = 120 x 120 = 14400
16 x d = 16 x 18 = 288
D x D/16 x d = 14400/288 = 50
focal distance f = 50 inches
After you have calculated the focal distance (f), you can use that figure to calculate the f/D ratio of your dish. In this case, using the same diameter (D) = 120; and the calculated focal distance (f) = 50
f / D = 50 / 120 = .416
f /D = .416 which you would round up to give you a setting of .42
The list below shows how far the throat is out from the scalar rings for different f/D settings.
EXAMPLE: A dish with a .42 f/D will have the throat about flush with the rings.
Inches — f/D
.12 —— .42
.32 —— .40
.52 —— .38
.72 —— .36
.92 —— .34
1.12 —- .32
Feedhorn troubleshooting tips
How to recognize a polarity problem
Polarity problems are usually very easy to recognize. They are usually indicated by the fact that every other channel is bad. You will notice that on some satellites, only the even numbered channels will come in, while on other satellites only the odd numbered channels will come in. This happens because the probe inside the feedhorn will not turn the 90 degrees that is required to change from a horizontally polarized channel to a vertically polarized channel. If your satellite system is several years old, the problem is most likely that the servo motor that drives the probe has failed. Here are some steps to take to find the problem:
- Use a volt meter to check the voltage at the back of your receiver to make sure that the voltage is coming out of your receiver. The connector to check is usually labeled “Polarizer +5v” or Polarity +5v”. Disconnect the wires that go to the dish and measure the +5 connector to GND. You should have approximately +5 to +6.5 volts dc. Chaparral receivers put out a constant +5 supply, so the voltage should be there as long as the receiver is turned ON. Other brands of receiver may only put out the +5 when the channel is being changed or when the polarity/skew is being adjusted.
- Check for dc voltage at the pulse connector. The pulse output is what tells the servo motor how far to turn the probe. You will read from .2 to .9 (+)volts dc here. In most receivers, this voltage will only be present when the channel is being changed or when the polarity/skew is being adjusted.
- If the receiver is putting out the proper voltages on the pulse and +5v connectors, re-connect the wires that go to the dish. Then, go out to the dish and remove the feedhorn cover. Disconnect the 3 wires that are connected to the servo motor. Measure to verify that you are getting the pulse and +5 voltage on each respective wire. If you are NOT getting the same voltage as you had at the receiver, then you have a wiring problem. If you are getting the same voltage, reconnect the 3 wires, proceed to step 4.
- Have someone inside change channels on the satellite receiver. If you hear the servo motor turning, but there is no apparent change in the position of the probe (remove the throat cover and look inside the throat to see the probe), remove the servo motor and pull up gently on the amber colored drive shaft that couples to the servo motor. If the shaft pulls out, you will need to send the entire feed to Chaparral for repair.
- If the servo motor does not turn, and you have the correct voltages getting to the motor, that normally indicates that the motor is bad and needs to be replaced. You can usually purchase a servo motor at any satellite dealer.
If you find that the servo motor seems to be buzzing all of the time or if you are watching a program that seems to fade out intermittently and will come back by itself or if you change the channel up or down and back, the problem is also likely to be a bad servo motor. But try these steps to determine if the problem is more serious:
- Take the servo motor off of the feedhorn and hook it up directly to the back of your receiver. You must disconnect the wires going to the dish for this test to be valid.
- Watch the servo while you change channels, then let it sit for a couple of minutes. If it turns when you change channels and does not drift or buzz when you are not changing channels, that tells you that the receiver and servo motor are working properly and the problem is likely to be noise being pick up by your unshielded pulse line. The only way to correct this problem is to make sure that the pulse line is shielded and the shield is grounded at one end.
- If the servo motor behaves the same way when it is hooked up directly behind the receiver as it did out at the dish, then it is most likely bad. You need to replace it.
We hope this information was helpful. If you can’t solve your polarity problem after following the instructions and tips above, we recommend calling out your local satellite dealer to troubleshoot the system further.