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Old 23-Aug-2013, 12:24 PM   #1
serengety
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What does any of this mean?

Hi all.

I have a bit of a problem. I found an antenna that MIGHT be interesting but I don't really know.

What they say is "This VHS/UHF/FM/TV antenna with built-in high gain amplifier (up to 15 to 18 dB) delivers amazing picture quality and has a remote to help you position it! You'll enjoy clear images from even distant or weak stations. Also has 45-860 MHz operation frequency and 75 ohms output impedance. It's crafted from weather-resistant ABS and aluminum and mounts in just minutes. Plus it comes with a high powered receiving booster, low noise microwave amplifier tube for enhanced resolution, 360 degree rotation and more. Receives channels 2-69 and operates up to 2 tv's."

Now, I do understand a little bit of that, but most of it goes over my head. Is this combo good, bad or indifferent? It is supposed to have a 100 mile range.
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Old 23-Aug-2013, 4:33 PM   #2
Stereocraig
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Quote:
Originally Posted by serengety View Post
Hi all.

I have a bit of a problem. I found an antenna that MIGHT be interesting but I don't really know.

What they say is "This VHS/UHF/FM/TV antenna with built-in high gain amplifier (up to 15 to 18 dB) delivers amazing picture quality and has a remote to help you position it! You'll enjoy clear images from even distant or weak stations. Also has 45-860 MHz operation frequency and 75 ohms output impedance. It's crafted from weather-resistant ABS and aluminum and mounts in just minutes. Plus it comes with a high powered receiving booster, low noise microwave amplifier tube for enhanced resolution, 360 degree rotation and more. Receives channels 2-69 and operates up to 2 tv's."

Now, I do understand a little bit of that, but most of it goes over my head. Is this combo good, bad or indifferent? It is supposed to have a 100 mile range.
You will only have a problem, if you buy it.
Those are flimsy garbage and the claims are ridiculous.

Post your TVFOOL report and somebody will be able to make a sensible recommendation.
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Old 23-Aug-2013, 5:31 PM   #3
GroundUrMast
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Agreed...

You're describing an antenna product that's been promoted under several brands. In every case, they seem to depend on the consumers' lack of knowledge and a lot of extravagant claims that sound too good to be true.

Antennacraft, Antennas Direct, Channel Master and Winegard publish credible specifications for their antennas. There are a few other reputable antenna manufactures, but they choose not the provide much in the way of technically useful data.

I see from your previous posts, you have a desire to understand the technology. Please, feel free to ask more questions.
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Old 24-Aug-2013, 2:14 AM   #4
serengety
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I thank you both very much, you just saved me some money. Which believe me, I cannot afford to waste. Especially right now. (Like who can?) And you are right GroundUrMast, I would like to understand the technolgy but I'm afraid that my brain sometimes has trouble processing new information. Like "output impedance", for example. What the heck does that mean? Even the concept of impedance is hard to wrap the mind around. And is there any such thing as "input impedance"?
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Old 24-Aug-2013, 5:51 AM   #5
GroundUrMast
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Impedance Lesson 1 ???

http://en.wikipedia.org/wiki/Electrical_impedance
http://en.wikipedia.org/wiki/Output_impedance
http://en.wikipedia.org/wiki/Input_impedance

http://www.williamson-labs.com/

As you can see in the above links, there's lot's of theory to get overwhelmed by, but there are also many tutorials available... A search will give you dozens to look through.

To keep it simple, I think of output impedance as just a resistor in line with the output lead of a generator that by itself could deliver infinite current without a change in voltage.



Let's say the generator produces 75 volts, no matter how much current flows in the circuit. And lets also say that the 'Output Resistance (Impedance)' is 75 ohms.

Q1. How much current would flow if nothing was connected to the output terminals? A. zero, the load resistance or impedance is virtually infinite so no current can flow.

Q2. How much current would flow if a short circuit was connected to the output terminals? A. 1 amp, the load resistance or impedance is virtually zero ohms in the case of a short circuit so the current is only limited by the resistance inside the generating devise. Ohms Law says, Voltage divided by Resistance equals Current so in this case 75 volts / 75 ohms = 1 amp.

Q3. How much power will be dissipated outside the generating devise in the case of Q1 & Q2? A. Zero watts. Because If either voltage or current is zero across or through a resistor, Ohms/Watts Law shows that no power is dissipated.

Q4. How much power will be dissipated in a load resistance of 75 ohms when connected to the terminals of the generating device?

If you like, I'll let you answer that...
Attached Images
File Type: jpg Generator with resistive output impedance.jpg (26.6 KB, 751 views)
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Old 24-Aug-2013, 10:14 AM   #6
Stereocraig
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Fortunately, a simple installation does not require us to be concerned w/ impedance, since using a BALUN to convert from the 2 wire antenna output, to a coax input, changes the impedance for us, automatically.

An analogy that helped me understand the concept, was to picture a clothesline, with the ground as the operating threshold and the line as the signal.

Too many items, or a few heavy ones, will pull the signal down too much.
Some may still work fine, while others may then be below threshold.

If a distribution amp is used for multiple sets, it's like having individual clotheslines.
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Old 25-Aug-2013, 1:32 AM   #7
GroundUrMast
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Quote:
Fortunately, a simple installation does not require us to be concerned w/ impedance, since using a BALUN to convert from the 2 wire antenna output, to a coax input, changes the impedance for us, automatically.
I agree. I know of no consumer grade hardware that provides for adjustment of output impedance. You'll find that most OTA hardware is configured to operate at 75 ohms... Done, move along, nothing more to look at there... And the few examples of 300 ohm hardware interfaces neatly with the aid of the matching transformer you mentioned already. Again, no provision for fine tuning the impedance... Just a fixed transformation ratio by design.

Still, as an academic exercise, I'm certainly happy to see people take interest in the technology.

I'd like to offer a different analogy, that I think addresses the concept of output impedance more directly...

In a plumbing or hydraulic system, pressure is analogous to voltage in an electrical system. Resistance to flow of fluid and resistance to flow of electron current are related concepts. Volume of fluid per second is comparable to electrical current.

Consider a pump that supplies water or hydraulic fluid to a system... With the pump running and the flow stopped by a closed valve you would expect the outlet pressure of the pump to be at maximum... True? Then, in a real world application, as you open the valve, and as fluid begins to flow... you expect the output pressure to drop somewhat. That's because the internal parts of the pump are limited in size and have some resistance to the flow of fluid. There is something inside the pump the impedes flow, and so as the flow rate increases, so does the pressure drop.

That 'internal' resistance in a pump is analogous to the resistance and reactance inside any real electrical devise that can supply voltage and current. We refer to that combination of resistance and reactance as 'impedance' and when we are referring to an impedance that affects the output of an electrical power source, it's referred to as 'output impedance'.

Batteries, generators, amplifiers, antennas, splitters, etc. all can deliver voltage and current to a load connected to their output port... they also have integral resistance and reactance. So each will have an output impedance which in some cases will be constant and in cases such as a battery, will increase as they discharge.

The goal of the impedance lesson 1 was to set the stage for showing that when the impedance of a source such as a generator, antenna or amplifier is matched by the impedance of a load, maximum power transfer can occur. Conversely, when impedance is not matched, inefficient power transfer takes place.
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Last edited by GroundUrMast; 25-Aug-2013 at 1:45 AM. Reason: sp.
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