Antenna basics
This presentation is the property of its rightful owner.
Sponsored Links
1 / 30

Antenna Basics PowerPoint PPT Presentation


  • 188 Views
  • Uploaded on
  • Presentation posted in: General

The TV channels Hertz (Hz) means cycles per second . (Heinrich Hertz was the first to build a radio transmitter and receiver while understanding what he was doing.) KHz means 1000 Hertz, MHz means 1,000,000 Hertz, and GHz means 1,000,000,000 Hertz. Antenna Basics.

Download Presentation

Antenna Basics

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Antenna basics

The TV channels

Hertz (Hz) means cycles per second. (Heinrich Hertz was the first to build a radio transmitter and receiver while understanding what he was doing.) KHz means 1000 Hertz, MHz means 1,000,000 Hertz, and GHz means 1,000,000,000 Hertz

Antenna Basics


The radio frequency spectrum is divided into major bands

Frequency: Wavelength (meters):

VLFvery low frequency3 KHz – 30 KHz100 Km – 10 Km

LFlow frequency30 KHz – 300 KHz10 Km – 1 Km

AM Radio

MFmedium frequency300 KHz – 3 MHz1 Km – 100 m

Short wave

HFhigh frequency 3 MHz – 30 MHz100 m – 10 m

TV and FM

VHFvery high frequency30 MHz – 300 MHz10 m – 1 m

UHFultra high frequency300 MHz – 3 GHz1 m – 100 mm

Satellite

SHFsuper high frequency3 GHz – 30 GHz100 mm – 10mm

EHFextremely high freq.30 GHz – 300 GHz10 mm – 1 mm

The radio frequency spectrum is divided into major bands:


Frequency allocations for tv channels in the u s

VHF Low Band:|VHF High Band:

ChannelFreq. (mhz)| (mhz)

254-60|7174-180

360-66|8180-186

466-72|9186-192

|10192-198

576-82|11198-204

682-88|12204-210

|13210-216

Frequency allocations for TV channels in the U.S.


Frequency allocations for uhf

UHF Band:

ChanFreq (MHz)| ChanFreq (MHz)| ChanFreq (MHz)| ChanFreq (MHz)

14470-476|32578-584|50686-692|68794-800

15476-482|33584-590|51692-698|69800-806

16482-488|34590-596|52698-704|70806-812

17488-494|35596-602|53704-710|71812-818

18494-500|36602-608|54710-716|72818-824

19500-506|37608-614|55716-722|73824-830

20506-512|38614-620|56722-728|74830-836

21512-518|39620-626|57728-734|75836-842

22518-524|40626-632|58734-740|76842-848

23524-530|41632-638|59740-746|77848-854

24530-536|42638-644|60746-752|78854-860

25536-542|43644-650|61752-758|79860-866

26542-548|44650-656|62758-764|80866-872

27548-554|45656-662|63764-770|81872-878

28554-560|46662-668|64770-776|82878-884

29560-566|47668-674|65776-782|83884-890

30566-572|48674-680|66782-788|

31572-578|49680-686|67788-794|

Frequency allocations for UHF


Note 2 below is part of the switch to dtv

Channels 70-83 were de-allocated in 1982. They are now cell phone frequencies.

Channels 52-69 have been de-allocated on February 17, 2009.

Channel 37 is allocated to radio astronomy

.

NOte -2 below is part of the switch to Dtv


6 mhz frequency spectrum

6 Mhz frequency spectrum


Basics

Any of these channels could contain either an analog channel or a digital channel. Note that “channel 25-1” is a virtual channel name and does not indicate what channel the station physically occupies.

Channels 2-13 are the VHF channels. They are split into two groups so that antennas will work better: In general, an antenna designed for frequency N will also work well at 3N, but very poorly at 2N.

Basics


Wavelength

The wavelength of a radio wave is:

λ = 300/F where F is the frequency in mega-Hertz and λ is the wavelength in meters. Antenna elements are typically about a half-wavelength long.

λ = 186000 miles /F where F is the frequency in Hertz and λ is the wavelength in mile (you would need to convert to feet).

wavelength


Decibels

Decibels (dB) are commonly used to describe gain or loss in circuits. The number of decibels is found from:

 Gain in dB = 10log(gain factor) or:

Decibels


Decibels1

In some situations this is more complicated than using gain or loss factors. But in many situations, decibels are simpler. For example, suppose 10 feet of cable loses 1 dB of signal. To figure the loss in a longer cable, just add 1 dB for every 10 feet. In general, decibels let you add or subtract instead of multiply or divide.

Decibels


There are some special numbers you might want to memorize

20 dB = gain factor of 100

10 dB = gain factor of 10

3 dB = gain factor of 2 (actually 1.995)

0 dB = no gain or loss

-1 dB = a 20% loss of signal

-3 dB = a 50% loss of signal

-10 dB = a 90% loss of signal

There are some special numbers you might want to memorize:


Noise

  • Whether a signal is receivable is determined by the signal to noise ratio (S/N).

  • For TVs there are two main sources (classes) of noise:

    • Atmosphere noise. There are many types of sources for this noise. A light switch creates a radio wave every time it opens or closes. Motors in some appliances produce nasty RF (radio frequency) noise.

    • Receiver noise. Most of this noise comes from the first transistor the antenna is attached to. Some receivers are quieter than others.

  • Receiver noise dominates on the VHF and UHF bands, and atmospheric noise is usually insignificant. On an analog channel, noise looks like snow. If there were only a barely perceptible amount of snow, this would correspond to how noise-free a DTV signal must be for a DTV receiver to lock-on to it.

Noise


Signal amplifiers preamplifiers

Many people think that connecting an external amplifier to the antenna will improve the performance of the antenna.

This is usually wrong. Receivers always enough gain built in to work with an appropriate signal.

The problem is that you need a strong enough signal from the antenna to begin with. Then if you have a long run or multiple taps you can add an amplifier to boost the signal.

The receiver has an Automatic Gain Control circuit, AGC, which will reduce strong signals.

The AGC makes all stations the same strength at the demodulator. When you add a preamplifier, the TV receiver lowers its own gain, usually by an equivalent amount.

So when do you use an amplifier?

Signal Amplifiers, Preamplifiers


Signal to noise s n ratio

Normally the signal to noise ratio will be set by the receiver’s first transistor. But if an external amplifier is added, the first transistor in that amplifier determines the S/N ratio. (Since the external amp will greatly magnify its own noise as well as the signal, the receiver’s noise becomes insignificant.)

Since there is no reason to think the external amp’s first transistor is quieter than the receiver’s first transistor, there is generally no benefit to the S/N ratio from an external amplifier.

Signal to noise - S/N ratio


External amplifier

an external amplifier will compensate for signal loss in the cable if the amplifier is mounted at the antenna.

Without this amplifier, a weak signal, just above the noise level at the antenna, could sink below the noise level due to loss in the cable, and be useless at the receiver.

external amplifier


Mast mounted amplifier

RG-6 will lose 1 dB of the signal every 18 feet at channel 52.

For a DTV channel, 1 dB can be the difference between dropouts every 15 minutes (probably acceptable) and every 30 seconds (unwatchable).

Recommendation- use a mast-mounted amplifier whenever the cable length exceeds 20 feet.

If you are in a good-signal area or you have no high-numbered UHF channels, you may be able to get by without one.

mast-mounted amplifier


Rule for amplifier power

The Mast mounted amplifier should have a gain equal to the loss in the cable (for your highest channel) plus another 10 dB (to keep the receiver’s first transistor out of the picture

The amplifier can usually exceed this target by another 10 dB without causing trouble.

If you follow the above rule, the cable length becomes irrelevant, and reducing the cable length yields no benefit under normal home construction sites (antenna cables limited to ~ 100’ or less).

rule for amplifier power


Antenna basics

When figuring the cable loss, be sure to include the loss in any splitters and baluns. If a 2-to-1 splitter were 100% efficient then you would figure a 3 dB loss since each TV gets half of the power. But splitters are usually 80% to 90% efficient.

2-to-1 splitter3.5-4 dB

3-to-1 splitter5-6 dB

4-to-1 splitter7-8 dB

75-to-300balun0.2-2 dB

(a balun is an adapter)


Antenna basics

You might not need an amplifier if the antenna is too big. But an amplifier can never make up for an antenna sinal that is too small.


Receiver noise

Actually there is a reason to think the external amplifier is quieter than the receiver.

Long ago designers made an effort to make the TV’s first amplifier stage very quiet.

But now 90% of homes use cable or satellite boxes (strong sources) and most of the rest are rural homes using antennas that have mast-mounted amplifiers.

So the TV’s noise is rarely a factor. Some TV makers no longer put any effort into making their sets quiet.

Receiver noise


Antenna basics

Suppose you live in an apartment 15 miles from the transmitter.

Your indoor antenna mostly works, but you are troubled by dropouts and some snow appears on analog channels.

Will adding an amplifier right at the TV improve things?

Yes, if it is quieter than the TV. Unfortunately TV makers see no reason to publish the noise figures for their receivers.

So buying an amplifier for an indoor antenna is a total crapshoot.

recommended to try an amplifier, but make sure you can return it if it is no help.


Transmission cable

Twinlead (ribbon cable) used to be common for TV antennas. It has its advantages.

But due to its unpredictability when positioned near metal or dielectric objects, it has fallen out of favor.

Such objects, even if not touching the cable, cause a portion of the signal to bounce, return to the antenna, and get retransmitted.

Transmission cable


Coaxial cable

Coaxial cable is recommended. It is fully shielded and not affected by nearby objects.

Transmission cable has a feature called its characteristic impedance, which for TV coax should always be 75 ohms. (50-ohm coaxial cable is also common. Avoid that cable.)

Although rated in ohms, this has nothing to do with resistance.

A resistor converts electric energy into heat. The “75 ohms” of a coaxial cable does not cause heat.

Coaxial cable


Resistance

But coax also has ordinary resistance (mostly in the center conductor) and thus loses some of the signal, converting it into heat. The amount of this dissipation (loss) depends on the frequency as well as the cable length.

Type: Center conductor:Cable diameter:

RG-59 20-23 gauge 0.242 inches

RG-6 18 gauge 0.265 inches

RG-11 14 gauge 0.405 inches

resistance


Cable losses

Cable losses


Mast mounted amplifier gains

  • If the mast-mounted amplifier gain exceeds the cable loss then it shouldn’t matter what cable you use. But there are two problems with this:

  • 1.    Some cable has incomplete shielding. This is most common for RG-59, another reason to avoid it.

  • 2.    When the cable run is longer than 200 feet, the low-numbered channels can become too strong relative to the high-numbered channels. In this case, RG-11 or an ultra-low-loss RG-6 is recommended. (These alternatives are expensive.) Alternatively, frequency compensated amplifiers will work.

mast-mounted amplifier gains


Sealing connections

Recommendation - use RG-6 for all TV antennas.

It can be stapled in place using a staple gun with common 9/16” T25 staples.

How long the cable lasts depends solely on how long you can keep water out of it.

3M Vinyl Electrical Tape is a good water proofer.

Even better is an asphalt putty called “Coax Seal”, but it is so tenacious it should not be used for temporary connections. Cover the connectors completely.

Sealing connections


Baluns

A balun is an adapter that adapts a balanced line to unbalanced line.

If a balanced transmission line (such as twinlead) is connected directly to an unbalanced line (such as coaxial cable) the two lines become a long-wire antenna, which is undesirable for VHF and UHF.

All baluns are passive bi-directional devices. They are usually above 90% efficient.

Baluns


Baluns continued

4-to-1 balun - This will connect 300-ohm twinlead to 75-ohm coaxial cable. This balun is usually a ferrite transformer. This is the type you find mounted at the antenna

1-to-1 balun - This will connect a 75-ohm balanced load to 75-ohm coaxial cable. This balun is often just some ferrite beads slipped over the coax.

Baluns continued


Comparing some common 4 to 1 baluns

Comparing some common 4-to-1 baluns

The 15-1253 is not suitable for outdoor use.


  • Login