Predicted coverage of the WB7FID ATV repeater

Remember:  "Receiving ATV signals isn't quite like watching normal TV."

Before you go too much further, make sure you read the following web pages:

A view of the 3 ATV transmit antennas
The three ATV transmit antennas.  From left to right:  The "Ogden" antenna, the "Salt Lake" antenna, and the "Provo" antenna.  The Salt Lake antenna is directly below the light, the camera view being slightly blocked by the Ogden antenna.
Click on the image for a larger version.

About the Repeater's Transmit Antennas:

From the outset, the design goal of the ATV repeater was to provide reasonable coverage along the major population center of the Wasatch Front, covering from Tremonton (to the north) to Payson (to the south.)  Owing to geography and population distribution, it turns out that this will allow line-of-sight coverage from the repeater site.  The complication of this is that this is that the azimuthal difference between these two cities is about 149 degrees!  The problem with this?  You just don't get an antenna that is horizontally polarized that has that wide of a beamwidth and have much gain and have it be relatively small.

Here are a few things we looked at:

Predicted pattern of the antenna system showing its orientation with respect to geography
This shaded relief map shows the shape and orientation of the antenna pattern with respect to the geography and the locations of the larger cities.  The "center" of this pattern is on a bearing of approximately 76 degrees relative to true north.  A cross marks the ATV repeater site.  Blue indicates lower altitude where yellow/reds indicate higher altitidues.
Click on the map for a legible version (165k jpeg image)

The antennas themselves:
 
What is the gain of the antenna array?

It is worth noting that the intrinsic loss of a 3-way splitter is 4.3 db.  This means that the effective gain of the antennas (after splitter losses) are 6.7 dbi and 9.3 dbi, for the 6 and 10 element antennas, respectively, not taking into account additional splitter and jumper cable losses. 

So, you can calculate the ERP one of two ways:  Take the pre-splitter power of 15-21 watts and assume 6.7 and 9.3 dbi gain antennas, or you could take the post-splitter power of 5-7 watts and use the 11.0 and 13.6 dbi gain figures:  The numbers come out the same!

If you consider the "transmit antenna array" to consist of the splitter and the three phased antennas, then you would be correct in saying that the gain in the center of the pattern is 6.7 dbi and is 9.3 dbi toward Provo and Ogden.

The transmit antenna system consists of three yagi antennas, all of them made by KLM.  The center antenna is a 6 element yagi (model 440-6x) which provides a rated 11.0 dbi gain while the other two antennas are identical 10 element yagis (model 440-10x) with a rated 13.6 dbi gain.  All antennas are fed in phase with equal amounts of power from a 3-way power splitter, resulting in a power of between 5 and 7 watts reach each antenna.

The patterns of these antennas actually overlap in order to prevent any "dead spots" or nulls along the coverage area.  In order to do this, the phase and orientation of the antennas need to be properly maintained:  Overlap could either result in the signals from the two antennas reinforcing each other, or in their cancellation.  If the latter were to occur, deep nulls may be present in the pattern.

The antenna sizes themselves are also no accident:  A lower gain (6 element) antenna was used to cover the Salt Lake valley - and that was a necessity as the valley being closer in, it represented a "wider" view from the top of the mountain - a fact accommodated by the wider pattern (60 degrees at the -3db points) of the lower-gain antenna.  This lack of gain is more than made up for the fact that everything in the Salt Lake valley is much "closer" than either Ogden or Provo.  In fact, downtown Salt Lake City is about 18 miles from the repeater site while Ogden and Provo just happen to each be about 40 miles away.

Being 40 miles away - and the fact that the coverage area is more-or-less a north-south ribbon, the subtended angle decreases as one moves either north or south from Salt Lake along the coverage area.  This works out nicely, as the higher-gain antennas (the 10 element yagis) help make up for the weaker signal due to the increased distance and their narrower beamwidths (48 degrees at the -3db points) are a good match for that narrowing angle.

What about coverage off the sides and backs of the antennas?  Unfortunately, this sort of thing is extremely difficult to predict.  We do know - from signal reports - that signals do radiate from the backs of the antenna.  We also know that these signals are quite weak.  In the Tooele Valley area, however, (directly west of the repeater) we know that reasonably good signals are obtainable using modest-sized (16 element) yagis.
 
Map showing the predicted signal strength of the ATV repeater along the Wasatch Front
Map colors and how they relate to signal quality and antenna requirements.

WHITE and RED - Only a modest (5-10 element) yagi is required to obtain P4 pictures. 
ORANGE - In the orange areas, a 5-10 element yagi will bring in P3 pictures while a 10-15 element yagi is sufficient to obtain P4 pictures. 
YELLOW - In the yellow areas, a 10-15 element yagi will bring in P3 pictures while a 16-22 element yagi will bring in P4 pictures. 
GREEN - A 16-22 element yagi will bring in P3 pictures, with the signals getting progressively weaker as you near the borders of the cyan and blue areas. 
CYAN, BLUE and VIOLET - You probably won't have much luck in these locations.

This map and table shows the predicted signal strengths along the Wasatch Front and what sort of signal quality may be expected.  The different colors indicate the signal strength. (Refer to the text for further explanations.)  The legend shows predicted signal strength at the terminals of a typical 11 element (14 dbi) yagi.
Click on the map for a larger (readable) version (350k jpeg image)

The coverage map:

Taking a look at the coverage map, you will get an idea of how well the repeater covers the Wasatch Front.

It is important to note that these are just predictions.  There may be some local phenomenon or geographical feature that will degrade your signal somewhat - or make it impossible to see the repeater at all!  As you can see from the map, there are few areas along the Wasatch Front that are badly shadowed by local terrain - and this is the same terrain that affects the broadcast TV stations as well.  A few of these areas include:


How good are these predictions?

As with any predictions of propagation, these are merely guidelines.  Your results may vary - and those results will be directly proportional to how good of a signal you have to fetch out of the air, and how good your receiving system is.  Although they are already mentioned on the Receiving the WB7FID ATV repeater at your QTH page, here are a few things that are worth repeating:




This page last updated on 20030102

Go to the Utah ATV Home Page...