[Tech-Assist] STL problem now critical

[Jerry Mathis]

OK, here's an update, and an urgent request for some knowledge and suggestions.

One of my stations started experiencing STL dropouts this spring. At first I thought we had suddenly experienced a massive drop in our received signal strength. We had a tower climber go up the STL receive tower, re-aim the dish, and replace the coax jumper which appeared to be bad.

Problem not solved.

Today I had the tower climber go up the STL transmit tower (the studio tower) and re-aim the dish there. No improvement, even though a careful compass check had showed the dish was off the proper azimuth by about 8 degrees.

Here's the system description:

At the studio end, a Moseley 6010 transmitter, feeding a 7/8" foam coax going up to the STL dish at the top of a 100 foot tower. Dish appears to be a Mark-Anixter grid dish, full parabola, 8 feet in diameter. Transmitter RF output power is about 4 watts.

At the transmitter end, we have a Moseley 6020 receiver, fed by 7/8" foam coax from a 6 foot Mark-Anixter dish, full parabola, at the 370 foot level on the tower. Both ends with Vertical polarization. System is Composite. Path is 18.3 miles long.

In reviewing the meter readings I've taken over the years, the received signal strength, as best as can be interpolated with the meter on the STL receiver, has been around 300 microvolts. Running a path study and a signal budget link, I SHOULD be receiving about 934 microvolts. The LOS in the path is clear, and barely any intrusions into the .6 Fresnel Zone.

I took an STL transmitter to the receive site, after the coax jumper was replaced, and hooked it to the receive antenna with a directional wattmeter. Forward power was 6.3 watts, and reflected power was 0.15 watts.

I have hooked the same wattmeter into the STL transmitter path, and the forward power is 6 watts, and reflected power is ZERO.

While the tower climber was on the studio tower today, I asked him to measure as best he could, the length of the RF feed assembly on the STL dish. He did so, and said it was about 26.5 inches as best he could measure.

Now, after giving all the above details, is my first question: Can you calculate the focal point of an STL dish in the same way you figure it on a satellite dish? Logic tells me yes, you can, but never having needed to know until now, I've never asked.

Second question: Using data from the Gabriel website on their grid dish dimensions, it turns out that 26 inches is very close to the focal point of a dish antenna for their *6-foot* antenna. Running the numbers for their *8-foot* dish gives me a focal distance of about 34 inches. So, is this likely to be my problem?

I've heard recently about a station that discovered that their STL dish came with the wrong feed assembly. I'm wondering if I'm a victim of a similar problem, seeing as I can't find any *other* problem with the STL system.

Coincidentally, we happen to have in storage an RF feed for an STL dish. The label on it says it is for a Mark Antenna P-9A96GN dish. Its length is exactly 3 feet from the mounting flange on the feed to the radiating element at the end (not the reflector at the *very* end). The only odd thing about this RF feed assembly is that it has an additional flange added to it, exactly one foot down from the main mounting flange. This second flange has 3 holes in it. I can't figure its purpose on one of our STL dishes.

Depending on what I hear from others, tomorrow morning I'm planning to get the tower climbers to replace the existing feed on our studio STL antenna with the one I've found in storage, and see what happens.

BTW, yes, I've connected the dots and am also wondering, since there is a 6' dish on the other end, maybe the two got mixed up when the system was installed? I don't know yet, but I'm sure going to find out. I can't see the STL dish on the receive end very well, even with binoculars, because it's so high up.

I'll update everyone with what I find out. In the meantime, PLEASE comment if you have pertinent knowledge or information.

Sincerely,

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Jerry Mathis