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

"Convincing" your TV to receive on 426.25 MHz Why set the TV/VCR to cable mode for 70cm ATV?  You may (or may not) already know this, but there are big gaps in the frequencies and the numbering of the TV channels.  For example, all FM broadcast, aircraft, the 2 meter band, and VHF business band is between channels 6 and 7.  Another "break" occurs between channels 13 and 14:  All of the 220 MHz band, nearly 200 MHz of government frequencies, all of the 70 cm ham band, and a lot of UHF commercial frequencies are in this "gap."  Cable TV systems, on the other hand, since they don't actually radiate over the air (we hope) can "re-use" frequencies used by other services.  This means that there are actually cable channels that land within these other bands - including the ham bands.  Cable channels 57 through 61 lie within the frequency range of the U.S. 70cm amateur band and it so happens that 426.25 MHz corresponds with the frequency of cable channel 58.  Almost. As it turns out, cable channel 58 is actually on 427.25 MHz.  Strictly speaking, it isn't legal to use 427.25 MHz for repeater use (as the sound portion would be outside those frequencies allowed for repeater use as per FCC part 97.205[b].)  So far, every modern cable-ready TV and VCR that I have used will correctly receive 426.25 because they have some sort of AFC (Automatic Frequency Control.)  This AFC complicates things a bit, however:  If you tune a TV to a channel, and nothing is there, the AFC tends to "wander" around, trying to lock onto noise, or any signal that it finds.  If you were to tune your TV to cable channel 58 (hoping to see the ATV repeater) and it wasn't keyed up at the time, the TV's AFC might cause it wander off frequency.  When the ATV repeater keyed up, the TV might be "elsewhere" and you may see either no picture, or a noisy picture if the TV doesn't re-lock onto the signal.  The fix for this?  A few TVs or VCRs have an AFC that can be manually controlled, but another way to get it to "lock" on frequency is by setting the tuner to another TV channel (preferrably a commercial TV station that the tuner can lock to) and then back to the ATV frequency again.

To receive a good signal from the WB7FID ATV repeater, you will need to make sure that you are doing the following:

• You should use an outdoor antenna.  It must have a clear, unobstructed view of Farnsworth Peak in the Oquirrh mountains.  Preferably, it should be a "real" 70cm amateur-band yagi that is designed to work for ATV on 426.25 MHz.  (Farnsworth Peak is near the northern end of the Oquirrh range, due west of 4800 south.)  Remember that leafy trees can absorb a lot of signal if they are in the signal path, so keep this in mind when placing an antenna.
• The antenna must be horizontally polarized.  This is very important!  Almost all outside TV antennas are horizontally polarized, as are those used for CW/SSB operations.  If you use a beam on FM, it is probably vertically polarized.
• Your TV/VCR is in cable mode and tuned to cable channel 58.
• You have decent (low loss) coaxial cable between your antenna and your receiver.
• That you do not have any splitters in line.  If you do have a splitter and an amplifier (mounted at  the antenna,) you may have enough additional gain to overcome the losses of the splitters.
• You should note that most VHF/UHF TV antennas work very badly in the 70cm ham band.  To find out how badly, go to the Using VHF/UHF TV antennas on ATV page.
• The WB7FID ATV repeater must be TRANSMITTING!  This is a repeater and not a TV station.  Like a repeater, it is only transmitting if someone is using it.  The exception to this is during NASA Space shuttle missions, where the repeater will retransmit these missions in their entirety.

• The average UHF TV station runs between 20,000 and 60,000 watts out of the transmitter.  Hams are limited to 1500 watts maximum:  Very few can afford the expensive equipment required to operate at even the 1500 watt power level!
• The average TV station has antennas atop a huge tower or mountain.  Most hams don't even have a tower, let alone one that is at least 300 feet tall.  Here in the west, we have an advantage over those that live in the midwest in that we have these tall mountains and the WB7FID repeater, being atop one of them, is about 4700 feet above the surrounding valley floors!
• The average UHF TV station has an antenna that has a gain of between 13 and 17 dbi.  Antennas that can handle these power levels, have this much gain and are omnidirectional weigh tons (literally!) and are many tens of feet long!
• The average UHF TV station, because it has such a high power transmitter and high gain antenna has a typical ERP (Effective Radiated Power) well in excess of one million watts!  (No, really!)

The reason commercial TV stations run as much power as they do is to make up for the (lack of) performance of our (meaning TV viewers) lousy antennas!  For example:  Let's say that our "average" ATV repeater runs 50 watts and has a 6 db gain antenna.  This means that the ERP of this repeater is 200 watts.  Comparing this to a "typical" commercial UHF TV station at 1,000,000 watts ERP, there is a 37 db (5000-fold) difference in these two power levels!  This difference can make up for a lot of antenna deficiency on the part of the viewer.

Compare this to a rural area that is served by a TV translator (most of the state of Utah is served by the hundreds of low power translators scattered around the state.)  It is not uncommon for a rural TV translator to run a mere 10 watts into a 10 db antenna, resulting in an ERP of 100 watts:   The signals, while definitely much weaker (about 10,000 times weaker than in the "big city!") are still very watchable.  How?

The key is in the receive system.

In rural areas, most people do not use rabbit-ear antennas to pick up the signal.  They usually have some rather large rooftop antennas (or if they don't, the cable system that they are connected to does!)  In most cases it really doesn't take too much effort to get a good picture out of a fairly weak signal.

"It's all in the antennas..."
 

"What's this 'Noise Figure' stuff about?" System "noise figure" correlates with overall receive system sensitivity.  The sensitivity of any receive system depends on many factors.  The most important of these are:  At the antenna, how much stronger is the desired signal than the "background" noise. How much noise does the receiver itself contribute? How much signal does one lose in the feedline. There are many sources of noise that can be intercepted by the antenna such as powerline noise, noise from appliances, or weather.  Assuming absence of these noise sources, there is also thermal noise.  Since the Earth itself is about 300 degrees (Celsius) above absolute zero, and since any object warmer than absolute zero actually radiates noise, any antenna with the Earth in its beam pattern (i.e. not pointed skyward) will intercept a lot of this noise.  The wider your receive bandwidth, the more noise energy you pick up!  This thermal noise effectively limits the ultimate sensitivity of any "earth based" receiver.  The receiver itself also contributes its own noise:  An "infinitely" sensitive receiver would have no noise of its own to mask even the weakest signals.  Any noise that is added by the receiver covers the weaker signals, effectively reducing receiver sensitivity.  Losing signal along the feedline has the same effect of reducing receiver sensitivity since the feedline loss puts the signal that much farther below the receiver's noise.

As it turns out, the WB7FID ATV repeater manages to deliver about 5 watts average power to each of its three transmit antennas:  There are a 10 element yagi trained on Ogden, another 10 element yagi on Provo, and a 5 element yagi pointed at the Salt Lake metro area.  These antennas are phased so that the beam pattern is contiguous over more than 180 degrees of arc.  The larger (10 element) antennas deliver more power toward Ogden and Provo because those cities are twice the distance of Salt Lake city (resulting in more path loss) and because those cities (and their surrounding communities) describe fewer degrees of arc than the closer cities.  The result of this "composite" antenna system is that we are "putting the power where it needs to go."  True, other types of antennas could have been used to deliver an appropriate amount of horizonally-polarized energy, but we are rather limited in our tower space.

The 5 watts into each of the antennas translates to about 63 watts of power being radiated in the direction of Salt Lake and about 125 watts (each) being radiated toward Ogden and Provo on beam centers.  Given this information, we have calculated how much signal should arrive in various communities within range of the WB7FID ATV repeater and what kind of antenna system you might need to get a certain quality of signal.

First, here are a few of the general parameters:

• The frequency is 426.25 MHz, the output of the WB7FID ATV repeater.
• Horizontally-polarized antennas are used throughout.
• It is assumed that there are no sources of interference to degrade the received signal.  (There are no coordinated repeaters or links in the passband of the repeater's output.)
• Your receive antenna is pointed at the transmitter (i.e. peaked on the signal.)
• Your receive antenna is in the clear and the path to the transmitter is well clear (line of sight) of any obstacles such as hills, buildings, and trees. (Trees with leaves can cause an amazing amount of signal loss.)  Since the repeater is atop a mountain, it is about 4700 feet above the surrounding valley locations.
• The WB7FID transmits 5 watts average (including the aural carrier) into each of its three antennas:  A 5 element yagi (about 11 dbi gain) pointed at Salt Lake City, Utah and two 10 element yagis (about 14 dbi gain each) with one pointed at  Provo, Utah and the other pointed at Ogden, Utah.

• One column assumes that your ATV receiver (a converter especially made for ATV, or a cable-ready TV or VCR) has a noise figure of 12 db.  The "noise figure" is a measurement of how sensitive your receiver is.  This factor can be influenced many things such as the intrinsic sensitivity of of your receiver and the amount of signal loss your antenna's feedline exhibits.  As it turns out, most modern TV receivers have better performance than this - a fact that can be exploited if you keep the feedline losses low.  The theoretical maximum sensitivity for such a receiver system (terrestrially-based) is approximately -93 dbm (about 5 microvolts) at the antenna for a 0 (that's zero) db signal/noise ratio.
• The far right column in each table shows what the signal would look like if you had a system noise figure of 2db.  This could be easily achieved by using a GaAsFET preamplifier mounted at the antenna.  The theoretical maximum sensitivity for such a receiver system (terrestrially-based) is approximately -103 dbm (about 1.6 microvolts) at the antenna for a 0 (that's zero) db signal/noise ratio.

• An isotropic (0dbi) gain antenna.  This is an antenna with no directivity and with 2.1 db lower gain than even a dipole. (Note:  2 db loss is a reduction of approximately 40%.)  Note:  The performance of this antenna is roughly comparable to that which may be expected from an "average" VHF/UHF TV rooftop antenna, assuming that you had no splitters inline and it had a clear, unobstructed view of the transmit site.  An "isotropic" would be roughly comparable to how well a pair of "rabbit ear" antenna would work if mounted in the clear, with a line-of-sight path (i.e. on your roof!)
• A "typical" 5 element 70cm yagi tuned for the ATV frequency.  This sort of yagi exhibits a typical gain of about 11 dbi (almost 13x improvement.)
• A "typical" 16 element 70cm yagi tuned for the ATV frequency.  This sort of yagi exhibits a typical gain of about 16dbi (almost 40x improvement.)

For a complete description of the "P-Signal" system, go to the "P's and Q's of Video Signals" page.  In a nutshell, the rating system is as follows:

• P5 - Completely perfect, absolutely noise-free signal (video S/N > 45 db)
• P4 - Very slightly noisy (video S/N from 35 to 45 db)
• P3 - "Somewhat" noisy - as noisy as it could be without the noise being overly distracting or obscuring finer details (video S/N of 20-35 db)
• P2 - "Definitely" noisy - the noise is obscuring picture detail, making watching somewhat tedious (video S/N of 8-20 db)
• P1 - "Extremely" noisy - the picture is barely visible, with only the largest details being visible (video S/N 3-8 db)
• P0 - Barely detectable signal.  Only the presense of the signal is detectable:  No picture details are visible.  (video S/N <3 db)

Here are tables with the calculated signal qualities:
 

Location:  East Benches of Salt Lake City (in the main lobe of the 5 element Salt Lake transmit antenna) 
Distance:  23 miles

Receive antenna	Signal Strength in dbm [microvolts] at antenna (50 ohms)	Signal/Noise ratio assuming 12 db receive noise figure	Signal/Noise ratio assuming 2 db receive noise figure
Unity gain (0 dbi - this is 2.1 db worse than a dipole)	-69 dbm [79 uV]	25 db ("noisy" P3, "good" P2)	35 db ("solid" P3, "noisy" P4)
5 element yagi (11 dbi)	-58 dbm [281 uV]	36 db ("solid" P3, "noisy" P4)	46 db (P5)
16 element yagi (16 dbi)	-53 dbm [501 uV]	41 db ("good" P4)	51 db (P5)

Note -  The above numbers for Tremonton also happen to apply for a Santaquin, Utah station under the following conditions:

• Distance:  52 miles
• TX ERP degradation of 6 db due to being 21 degrees off the main lobe of the Provo transmit antenna
• Line-of-sight to Farnsworth Peak with no fresnel effects

How good are these predictions?

The tables above are predicted signal strengths.  In order to determine if these calculations are even close to real-life calculations (the gap between practice and theory can, at times, be very wide) the predicted signal strength readings from several locations have been compared with some actual readings.

These calculations were based on the following:

• Average visual transmit power level:  +36 dbm at each transmit antenna (approximately 4 watts.)
• Transmit antenna gain:  11 dbi (the Salt Lake TX antenna) and 14 dbi (Provo and Ogden TX antennas)
• Receive antenna gain, accounting for feedline loss:  10 dbi
• Received signal measurements were made with a Sadelco MiniMax 800 signal strength meter.  This meter is rated for an accuracy of +- 0.25 db over a 5-862 MHz operating frequency range and lab measurements tend to corroborate this.

The above table shows pretty good correlation between the predicted signal strength and the actual signal strengths.    It is interesting to note that in the "overlap" zone between the Salt Lake and the Provo transmit antennas (i.e. the Draper measurement) the readings would indicate that the composite gain of the antenna system is equal to that of one of the 10 element transmit antennas.

Conclusions:
 

"What antenna and coax should I use, then?" In the Salt Lake area, even a relatively small (5 element) yagi will work fairly well - but an 11 element antenna (which is still quite compact and not a whole lot more expensive than a 5 element antenna) would provide a better degree of margin. If you are in Ogden or Provo (or points in-between) then an 11 element antenna is the minimum recommended for good, solid pictures.  If you live outside these areas (refer to the coverage map - see the Predicted coverage of the WB7FID ATV repeater page) then you'll want to get a larger antenna. If you are on a budget, consider building an 8 element quagi:  These should give "good" results from Provo to Ogden and "excellent" results in the Salt Lake valley.  (Go here for some links to some construction articles.) What about coax?  If you are going to transmit, use RG-8 type coax (the larger stuff...) at a minimum and do not use RG-58 or RG-59.  For receive-only use, RG-6 (which is used for satellite TV) is almost as low-loss as RG-8.  Do not use any splitters!

In the "Provo to Ogden" area, it would appear that even a small 5 element yagi (with no preamp) and a reasonable run of low-loss coax will give "good" results (i.e. a good "P3" picture.)  This does not allow for things like splitters, etc. which (which typically eat about 4db for a 2-way splitter.)  Also, it assumes a line-of-sight path and NO QRM or QRN. Generally speaking, a mast-mounted preamp can certainly be counted on to turn a P2 signal into a P3 (assuming no other QRM or QRN, of course.)

If you have a choice, do not use an indoor antenna.  Buildings generally cause tremendous signal (10 to 20 db are typical, translating to 90% to 99%) loss due to the intrinsic lossiness of building materials (concrete with or without rebar is very lossy, and foil vapor barriers or aluminum siding don't help either!.)  Worse yet, other signal sources (appliances, computers, etc.) cause even more degradation to the already weak signal.  If you must use an indoor antenna, try to point it through a window that has a clear view of the repeater.  If you don't have the option of doing any (or all) of these things, then you will have to be creative.

It should be pointed out that typical rooftop VHF/UHF TV antennas perform poorly at 426.25 MHZ - the ATV repeater's output frequency:  To find out how poorly they may work, go to the Using VHF/UHF TV antennas on ATV page.

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