The Official AVS Antenna and Related Hardware Topic! - Page 82 - AVS Forum
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post #2431 of 16235 Old 07-26-2005, 04:32 PM
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Originally Posted by Neil Laffoon View Post

Yes, thanks Bob. Finally an educated explanation for the observations I have made the last three years, from about 70 miles from the Memphis towers. Over the years some of the self proclaimed "experts" on this forum have called me crazy for believing that I would get better reception during the summer than in winter, even citing water content in the atmosphere as a reason summer reception should be worse. But your clear explanation of the effects of dew point on reception coincides exactly with what I have observed.


The problem with summer are that things are so much more unpredictable (at least for me). This is likely due to intermittent tropospheric ducting and interference from other markets (analogs mostly). I'm hoping this will clear up after analog goes away.
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post #2432 of 16235 Old 07-26-2005, 08:23 PM
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Originally Posted by cpcat View Post

If it blocks line of sight the result could be multipath. If you're analogs are from the same towers and are ghost free you're likely o.k.

Thanks for the reply. The analogs are from the same towers and they are not ghost free. The antenna that I am playing with is a Zenith Silver Sensor. I am assuming a directional antenna may be better for me.

Alan
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post #2433 of 16235 Old 07-27-2005, 04:45 AM
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Your Silver Sensor is highly directional - but it's basically an indoor antenna, although some have used it with success outdoors.

What you may need is a directional outdoor antenna - and more than likely, since you have ghosting on your analog channels, orient the antenna for strong reflected signals.

Gary
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post #2434 of 16235 Old 07-27-2005, 05:06 AM
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Quote:
Originally Posted by Alan Curry View Post

Thanks for the reply. The analogs are from the same towers and they are not ghost free. The antenna that I am playing with is a Zenith Silver Sensor. I am assuming a directional antenna may be better for me.

Alan

I'd agree. I'm assuming you've tried moving the silver sensor around. You could also try it in the attic. Another thing to try would be the variable attenuator from RS. This can decrease ghosting/mulitipath and you could use the analogs as a guide. The CM 4221 in your attic/outside might be the next option. Even safer would be the CM 4228 outside/ in the attic. You shouldn't need a preamp and this would typically make multipath worse, anyway.
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post #2435 of 16235 Old 07-27-2005, 06:09 AM
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Originally Posted by bobchase View Post

While there are many factors that cause the bending, moisture content is the biggest factor, swamping out all the other factors combined.

Really? I'd put tropospheric ducting at the top of the list.

Here in Minnesota, we get extremes of moisture content in the air. Dewpoints in the winter have actually been below zero, and in the summer as high as 86. While generally speaking, reception in the winter is better than in the summer, nothing beats a good tropospheric event to stretch the radio horizon. I've gotten UHF signals from over 250 miles away, clear as a bell, during tropospheric events which usually seem to happen on calm, hot and humid summer evenings more than any other time of year. But kick up a little breeze and the signals, even from relatively close stations over the radio horizon, break up like ice in 100 degree heat.
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post #2436 of 16235 Old 07-27-2005, 10:05 AM
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Originally Posted by cpcat View Post

If it blocks line of sight the result could be multipath. If you're analogs are from the same towers and are ghost free you're likely o.k.

The ghosts in this case will be too close together to see.

Alan,
The fairly large distance from the water tower will be helpful. Even better, if you were a mile from the tower. Think of it like an island in the ocean. The farther you get from the island, the less if effect the ocean waves. As you probably know you will need a bigger antenna than you would if you did not have the water tower.
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post #2437 of 16235 Old 07-27-2005, 10:34 AM
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I've been moving the Silver Sensor around including the attic while playing. I've known I have needed a larger one for sometime now. My biggest question is which one to start with; bay style like the CM4228 or a yagi style like the 42XG?

Thanks for all the replys,

Alan
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post #2438 of 16235 Old 07-27-2005, 11:03 AM
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Quote:
Originally Posted by Alan Curry View Post

I've been moving the Silver Sensor around including the attic while playing. I've known I have needed a larger one for sometime now. My biggest question is which one to start with; bay style like the CM4228 or a yagi style like the 42XG?

Bowtie-styles seem to work better in attics. Yagis work better outdoors.
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post #2439 of 16235 Old 07-27-2005, 01:08 PM
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Originally Posted by the_bear89451 View Post

The ghosts in this case will be too close together to see.

Hmmm. I guess that'd make them true "ghosts" alright.

I understand what you're getting at. If the reflections are arriving very close together the tuner would see them simultaneously. But that'd be a good signal wouldn't it?

Anyhow, Alan did reply that his analogs "were not ghost free".
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post #2440 of 16235 Old 07-27-2005, 01:20 PM
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Quote:
Originally Posted by Alan Curry View Post

I've been moving the Silver Sensor around including the attic while playing. I've known I have needed a larger one for sometime now. My biggest question is which one to start with; bay style like the CM4228 or a yagi style like the 42XG?

Thanks for all the replys,

Alan

The more "fair" comparison would be between the 4228 and the 91xg, and they should both be more directional and perform better for multipath than the 42xg. If you're definitely going to stay in the attic, I'd say toss a coin (or I guess you could put that coin in your pocket since the 4228 is half the price). If you think you might go outside on a rotator at some point, the 91xg is lighter and less wind load.
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post #2441 of 16235 Old 07-27-2005, 02:05 PM
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Quote:
Originally Posted by cpcat View Post

Hmmm. I guess that'd make them true "ghosts" alright.

I understand what you're getting at. If the reflections are arriving very close together the tuner would see them simultaneously. But that'd be a good signal wouldn't it?

Anyhow, Alan did reply that his analogs "were not ghost free".


If the wave bending around the right side of the water tower hits the antenna at its peak at the same time the wave bending around the left side hits the antenna at its trough, the two will cancel each other out. This is the potential problem with short distance multi-path.

My guess is the ghosts are caused by something other than the water tower. You could calculate out what the time it takes light to travel 20-30 feet translates into in inches on the TV screen. I'm feeling too lazy to do this myself.
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post #2442 of 16235 Old 07-27-2005, 03:04 PM
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Quote:
Originally Posted by the_bear89451 View Post

If the wave bending around the right side of the water tower hits the antenna at its peak at the same time the wave bending around the left side hits the antenna at its trough, the two will cancel each other out. This is the potential problem with short distance multi-path.

My guess is the ghosts are caused by something other than the water tower. You could calculate out what the time it takes light to travel 20-30 feet translates into in inches on the TV screen. I'm feeling too lazy to do this myself.

The ghosts are likely caused by reflections through the back or to the sides of the antenna. This is common when line of sight is lost whether due to buildings, mountains, etc. This is why narrower beamwidth and higher front to back ratio is so important in mulitpath prone areas and why omnidirectional antennas don't work. In most cases it's still possible to receive the direct signal at an acceptable level if you can narrow the beamwidth/increase FB ratio enough. Sometimes not, however, and it's necessary to try non-intuitive aiming and try to catch a reflection which is strong enough to tune.

If the water tower is the only obstruction to LOS, then either that's the cause or the receiving antenna is just too omnidirectional to begin with. Either way, the remedy at the antenna is the same=narrower beamwidth, higher FB ratio.

Two signals in perfectly opposite phase nulls the signal. This would not produce multipath but rather attenuation of the direct signal. It's hard to imagine one signal refracting so much differently around a water tower as to alter the relative phase by 180 degrees since the signal is emanating from the same point, but I'll not argue it's possibility. More importantly, the direct signal is simply blocked (reflected elsewhere or absorbed) by the obstruction and so the tuner sees proportionately more reflected signal.

Nulling can be used to advantage if stacking antennas to cancel unwanted interference.
See http://www.kyes.com/antenna/sca/scaint1.html
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post #2443 of 16235 Old 07-27-2005, 04:34 PM
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Originally Posted by cpcat View Post

The ghosts are likely caused by reflections...

I think I see what you are saying the water tower is not the cause of the ghosts, but rather allows ghosts caused by something else (ie. a hill) to be seen. Because the water tower makes the more direct signal much weaker, the difference between the multiple-paths (around water tower vs bounce off hill) is not as great. This makes sense.

The only think I would like to add is if Alan was closer to the water tower, a less directional antenna might be helpful to pickup signal curving around each side simultaneously.
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post #2444 of 16235 Old 07-27-2005, 05:16 PM
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I agree completely with "bear". But even then, you would have to be really unlucky to have that tower directly on the line of your transmission path. If it is, it would seem to me that if you bought a 500 foot spool of RG-56 and held a test antenna 500 feet to the left or right of where your antenna is located now, you'd get much improved reception because the RF shadow of the tower is no wider than the tower, and, with the penumbra, it is even narrower than that.

Or it might be easier to do something like that with an A/C inverter and a portable test rig, if you are in an urban situation where you can't string a long coax to a distant reception antenna.

Scrounge up a copy of Satellite, Off-Air SMATV by Dr. Frank Baylin, probably published in the late 1980s (my cover and first few pages are missing, so I can't find the copyright date). On page 93, it has the formulas for determining phase cancellation distances between horizontally stacked antennas. One way that cable companies cancel the effects of interfering signals is by having two horizontally stacked antennas pointed at the main signal path, but spaced so that the reflected signal gets to one antenna exactly 180 degrees out of phase with when it reaches the other and thus, those signal components, which are visuallly equal in magnitude, cancel each other out. Unfortunately, spacing is unique for each channel, so this technique can only reliably clean up one channel with each antenna pair.

On page 195 of my edition is a useful table that lets you use the distance between the main analog image and the ghost to determine how much further the reflected signal is traveling.. Figure 9-1 there has "Ghost Displacement versus Reflection Distance" plots for different size TV screens. For example, if a ghost image is one inch to the right of the main image on a 15" diagonal TV screen, then the multipath signal has traveled about 5000 feet further than the direct one, whereas that one inch of image displacement looks like about 2,700 feet of extra travel distance on a 25" TV.

On the same page is Figure 9-4, which demonstrates that an ellipse is the locus of all points of the same distance differential between the direct path and multipath signals. You can stick pins on a map that has both the transmitting and receiving sites on it and connect them with a thread that is equal in length to the multipath distance and use that thread to guide a pencil in developing your own plot of that ellipse on the map, and then you can find something big on that ellipse and that is what your signals are bouncing off. For all the good it does you.

It did me a lot of good several years ago. I was trying to mitigate really harsh analog ghost images at a campground in College Park, Maryland, and I knew, roughly, the direction that the multipath signals were coming from, because I identified secondary antenna peaking in that direction that exceeded what a sidelobe would develop at that azimuth, but I couldn't tell how far away the reflecting object was because of the treeline. So I knew I had to move my antenna, but I needed a more informed basis to speculate where else on the property to put it. Otherwise. I'd have to walk around the property with light, 25 foot pole with a tiny UHF antenna on it and then try to make sense of my haphazard collection of prospective relocation analyses.

I drew the ellipse, using the thread and pencil, and bingo, it went right through Byrd Stadium at the University of Maryland. Armed with that knowledge, it was much easier to certify the efficacy of the new antenna location than it would have been if I hadn't known how far away the reflection was coming from. I set up two new antennas, about 500 feet closer to the transmitters, and they almost totally eliminated the ghosts. Obviously, I had a little bigger budget to work with than most of you do. The campground serves 400 sites and I did this before DirecTV made local TV available in that market.

Some of you can surely figure out how to generate such an ellipse with a computer. I can't. About forty years ago, I arrived at a fateful, erroneous conclusion that I now regret. I attended a science exposition at which a behemoth called the IBM 360 was the center of attraction. Computer technology was to be the wave of the future, and someday, we'd all be using these things. That's what they told us.

Well, I could do anything that I saw them try to programing it to do in less time than it took them to program it, using the punch cards that they did, and the only thing it could tie me at was tic-tac-toe. It couldn't even play chess.

I concluded that computers were a fad and that, rather than get in on the ground floor, I'd let them pass by, like the hula hop did, and catch the next fad. Fool on me!
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post #2445 of 16235 Old 07-27-2005, 05:30 PM
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Those among you willing to go to heroic lengths to maximixe the performance of your reception antenna installation may choose to visit HERE.
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post #2446 of 16235 Old 07-27-2005, 06:30 PM
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Well, I didn't expect this much discussion from my original question but I've enjoyed it. To add a little more information to the equation I'm on the backside of a hill. If the cheap mapping software I have for my GPS receiver is close then the elevation of the tower is at 1036 ft. and I'm at 1029 ft. To further muddy the water, if antennaweb.org's distance and bearing are fairly accurrate then the antenna farms elevation would be around 500 ft.

Right now I'm going to get a CM 4228 and try it out.

Thanks again,

Alan
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post #2447 of 16235 Old 07-27-2005, 06:34 PM
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Quote:
Originally Posted by AntAltMike View Post

About forty years ago, I arrived at a fateful, erroneous conclusion that I now regret. I attended a science exposition at which a behemoth called the IBM 360 was the center of attraction. Computer technology was to be the wave of the future, and someday, we'd all be using these things. That's what they told us.

Well, I could do anything that I saw them try to programing it to do in less time than it took them to program it, using the punch cards that they did, and the only thing it could tie me at was tic-tac-toe. It couldn't even play chess.

I concluded that computers were a fad and that, rather than get in on the ground floor, I'd let them pass by, like the hula hop did, and catch the next fad. Fool on me!

15 years ago I took a job working with the descendant of the IBM 360. Since then, I'm glad I stayed on the mainframe. If not, I'd probably be of a job right now.

Alan
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post #2448 of 16235 Old 07-27-2005, 08:06 PM
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Originally Posted by AntAltMike View Post

Scrounge up a copy of Satellite, Off-Air SMATV by Dr. Frank Baylin, probably published in the late 1980s (my cover and first few pages are missing, so I can't find the copyright date). On page 93, it has the formulas for determining phase cancellation distances between horizontally stacked antennas.

This info is also here: http://www.kyes.com/antenna/sca/scaint1.html
http://www.kyes.com/antenna/sca/scaint2.html

I've tried it with some success (not as much as I had hoped) to try to null an analog 34 which interferes with a digital 34 here. The problem as I remember was that the 1/2 wavelength spacing which is supposed to work best decreased the overall performance of the stack.
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post #2449 of 16235 Old 07-27-2005, 08:21 PM
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Quote:
Originally Posted by cpcat View Post

This info is also here: http://www.kyes.com/antenna/sca/scaint1.html
http://www.kyes.com/antenna/sca/scaint2.html

I tr(ied) to null an analog 34 which interferes with a digital 34 here. The problem as I remember was that the 1/2 wavelength spacing which is supposed to work best decreased the overall performance of the stack.

What is the angle between the towers. How far away are each?

You might try pointing an antenna at the source of the offending signal, and coupling it 180 degrees out of phase with the main antenna. It is tough to do but doable. it is often done to get rid of unwanted harmonics of FM signals that develop in the VHF high band.

Microwave Filter makes a phase shifter that you can use to continuously shift the phase up to 180 degrees, so that, coupled with a variable attenuator, would allow you to tweak until the problem is minimized. The retail price of it is about $700. I have a couple, but unfortunately, mine are VHF lowband and highband
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post #2450 of 16235 Old 07-27-2005, 08:47 PM
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Quote:
Originally Posted by AntAltMike View Post

What is the angle between the towers. How far away are each?

You might try pointing an antenna at the source of the offending signal, and coupling it 180 degrees out of phase with the main antenna. It is tough to do but doable. it is often done to get rid of unwanted harmonics of FM signals that develop in the VHF high band.

Microwave Filter makes a phase shifter that you can use to continuously shift the phase up to 180 degrees, so that, coupled with a variable attenuator, would allow you to tweak until the problem is minimized. The retail price of it is about $700. I have a couple, but unfortunately, mine are VHF lowband and highband

WBKI analog 34 Campbellsville KY bearing 303.5 at 84 miles
WTNZ DT 34 Knoxville bearing 176.1 at 65 miles
Angle between 127.4
My spacing calculation for nulling assuming ch. 34 centered at 593 mhz:
12.53 inches for .5 wavelength
37.6 inches for 1.5
62.7 for 2.5

I do have a couple of Band A antennas down right now to play with, but all of my uhf antennas have PCB baluns. Would turning one upside down flip the phase? I'd also expect this to have the potential of creating unwanted multipath for other channels unless I used a 34 jointenna or something similar on the out of phase antenna.

I'd say the 700 dollar option is pretty much out.
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post #2451 of 16235 Old 07-27-2005, 11:21 PM
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Originally Posted by the_bear89451 View Post

In many Yagi antennas, there is no easy way to ground the director elements because the are connected to the boom with an isolator.

A gas filled lightning discharge unit instead of a grounding block handles the job. If there is a static buildup to the degree specified on the unit, the gas ionizes and the charge goes to ground.

Pat

While I may link to and mention products as examples, I don't recommend specific products.
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post #2452 of 16235 Old 07-28-2005, 01:43 PM
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A gas filled lightning discharge unit instead of a grounding block handles the job. If there is a static buildup to the degree specified on the unit, the gas ionizes and the charge goes to ground.

I was more expecting someone to respond with,
The TOTAL charge is not very high, because of the small volume.
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post #2453 of 16235 Old 07-28-2005, 03:00 PM
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Thanks to a number of members for the links here mentioned the past few weeks, I'll add & sort these in:

Antenna basics - excellent source for technical antenna & accessories info 7/28/05
On-air DTV test with various length Coax Baluns 7/28/05
Entry point to purchase custom made Coax Baluns ($40) 7/28/05
Winegard transformers 7/28/05
Half-Wave Balun compared to wider bandwidth, low loss, Log Periodic Balun 7/28/05
Some basic Ferrite Balun design information 7/28/05
Maximixe performance of reception antenna installation 7/28/05


If I were that concerned for higher UHF frequencies in my area, Europeans seem to like the Televes DAT-75. I guess to each his/her own area, situation & requirements. I've seen vicious but informative arguments w/the Channel Master 8-bay vs. Winegard & other yagis between what seems to be experienced pros/installers... in other threads/forums. When does one say they've looked @ say Comparing the common antenna types long enough?

AntAltMike's link is interesting on improving reception drawing a big ellipse & finding optimum location. I've designed sealed concrete cast ellipsoid speaker enclosures using Fibonacci cancellation with a lot of help from Cantrell's ellipse formulas in a spreadsheet, mitigating need for 1/2 wave resonance damping material which can also muffle sound. VERY precise & no Calc III needed.
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post #2454 of 16235 Old 07-28-2005, 03:12 PM
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Hey gang,

Just saw this now, but...

Stark Electronics, which has a good reputation among antenna buyers, put the
ChannelMaster 4228 UHF antenna on sale for $39 this week. Just caught it.
If you were thinking of getting one, grab it before the end of the week;


http://www.starkelectronic.com/allant.htm

The funny thing is, I hate the website (WAY too long of a list), but love their
service. A good compromise. ;-) Full disclaimer: I don't know anyone from
them; they came recommended to me last year and I bought some of my
stuff for my antenna install through them, and they got me the stuff I needed
quickly. (Read; satisfied customer, nothing more!)

Edit: Well, well...price war!!!

http://www.warrenelectronics.com/antennas/4228.htm

I am also a satisfied customer with them. I would be even more satisfied
if they dropped the price even lower! :-)

Gilbert
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post #2455 of 16235 Old 07-28-2005, 03:27 PM
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Quote:
Originally Posted by AntAltMike View Post

So I knew I had to move my antenna, but I needed a more informed basis to speculate where else on the property to put it. Otherwise. I'd have to walk around the property with light, 25 foot pole with a tiny UHF antenna on it and then try to make sense of my haphazard collection of prospective relocation analyses.

Imagine walking around with a 30' pole trying to find the best location for a wind turbine. Knowing Rayleigh wind distribution helps here, but they usually have to measure (over time) chosen spots @ least about 30 feet.
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post #2456 of 16235 Old 07-28-2005, 07:38 PM
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...I hate the website...


I like it. I don't have time to develop a price schedule, so when I am drawing up a materials list for a big job, I just go to their site and factor some multiple of their prices.
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post #2457 of 16235 Old 07-28-2005, 07:39 PM
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Originally Posted by sregener View Post

Really? I'd put tropospheric ducting at the top of the list.

Here in Minnesota, we get extremes of moisture content in the air. Dewpoints in the winter have actually been below zero, and in the summer as high as 86. While generally speaking, reception in the winter is better than in the summer, nothing beats a good tropospheric event to stretch the radio horizon. I've gotten UHF signals from over 250 miles away, clear as a bell, during tropospheric events which usually seem to happen on calm, hot and humid summer evenings more than any other time of year. But kick up a little breeze and the signals, even from relatively close stations over the radio horizon, break up like ice in 100 degree heat.

srenger,
I was not ignoring tropoducting. The person who I responded to is 60 mi from the xmitter, probably very close to end (or just beyond) the stations Grade B coverage area. From 60ish miles out to 100 miles is an area that I call the 'deep fringe' area. (Mainly because you see that listed on antenna charts and consumers can relate to it.) In the deep fringe, the signal is typically from the station or a reflection of the station from a tall object, like a mountain. While there is no hard rule to tropoducting, it tends to be further out and more sporadic. Your 250 mile example is a classic case. It's very far and sporadic. It would be rare, but not impossible, for a signal to get all the way up to the troposphere and get back down to a receive antenna within a 60 mile radius of the transmitter.

I was explaining a completely different scenario where the signal gets better or worse with the seasons or with weather fronts within the stations normal viewing area. This is true at 30 miles out, especially with an indoor antenna.

Most people don't realize how thin a UHF television beam is. Folks see the receive antenna charts and think that the transmit antenna is similar. The transmit beam is closer in shape to a frisbee than to those charts. If it's a directional antenna, then it's a frisbee the dog chewed on. At 60 miles, a 1/4 of a degree change in beam tilt can make a huge difference in signal strength.

Bob Chase
KHWB-TV

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post #2458 of 16235 Old 07-28-2005, 07:54 PM
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Originally Posted by bobchase View Post

I was explaining a completely different scenario where the signal gets better or worse with the seasons or with weather fronts within the stations normal viewing area. This is true at 30 miles out, especially with an indoor antenna.

Ah, understood. I'm kind of in this situation, with stations just over 75 miles distant and no line-of-sight. Winter is better than summer, but evenings in summer can still kick the pants off the best days in winter. I've been attributing it to tropospheric ducting, but I'm not an engineer and I don't play one on TV.

For instance, WCCO-DT (32) comes in nearly 24x7, 365 days a year for me. I'm shocked if I don't get it. KSTP-DT (50), supposedly on the same tower, at about the same height and power, is an elusive catch year round. The atmosphere has to be "helping" me a lot to get it, day or night, summer or winter. In the winter, on a "good" night, the signal meter will list KSTP-DT in the middle of the "Normal" range. In the summer, on a "good" night with no wind and high humidity, I've seen the signal peg high on the "Good" range on my meter. Now, KSTP's engineers insist that the problem is that KSTP drew a "bad card" with channel 50, and lower channels aren't as fussy about multipath, and they may be right. I find it ironic, though, that KSTC-DT, on channel 44 and about 1/5th the power, comes in far more regularly and reliably not just for me, but for just about everyone in their viewing area. In any event, I'm a good 15 miles from all these station's coverage patterns, so I have no business expecting them to come in, especially with my "below average grade" location.
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post #2459 of 16235 Old 07-28-2005, 07:59 PM
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Originally Posted by bobchase View Post

At 60 miles, a 1/4 of a degree change in beam tilt can make a huge difference in signal strength.

Bob Chase
KHWB-TV

I've played around with tilt +/- 7 degrees or so and I've never been able to convince myself that it makes much of a difference. The problem is that conditions with my reception seem to vary enough constantly any time of the year that something that makes it a little worse or a little better is just really hard to confirm (or maybe there's just no significant difference at my location).
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post #2460 of 16235 Old 07-29-2005, 04:56 AM
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I think that a 1/4 degree difference in beam tilt might make a little difference in transmision at that distance, but I can't imagine it ever having any measurable effect on reception.
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