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Old 01-13-2011, 07:33 PM   #21
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RE: TV antenna troubles

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Capn Chuck wrote:

Don't get overwhelmed with all of the tech speak. For a boat that moves around, get an omni-directional, a signal booster will help, mount it as high as you can, be sure it will receive UHF and VHF and don't worry about all of the rest. Be careful spending extra money for "long range" antennas, a lot of this is marketing ploys to get your money. Follow my suggestions and you will be fine. Chuck
I agree with everything except the necessity for an amplifier. The amplifier only overcomes the cable losses to the TV , it does not improve the efficiency and unless it is a really big boat or really tall mast, or you are splitting the signal it is not necessary and because the amplifier has a " noise factor " itself, it can actually make the signal worse if the noise contributed is greater than the cable loss that has to be compensated by the amplification in the TV tuner.

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Old 01-13-2011, 07:39 PM   #22
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RE: TV antenna troubles

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rwidman wrote:

*
rhprop wrote:These types of antennas were nothing but rabbit ears mounted in an enclosure with a small motor to rotate it.
There is no rotation.* It's an antenna that picks up signals from all directions.
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The pix of the antenna which is the topic of the original post is a bow tie *in an enclosure *rotated by a small rotor. That is what the pictured controller does. There are omnidirectional disc antennas, which are also junk, but this was not one of them.

I would sure like to be your Best Buy, Radio Shack, West Marine salesman. I would be fixed for life.

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Old 01-14-2011, 01:33 PM   #23
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RE: TV antenna troubles

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rhprop wrote:


rwidman wrote:

*
rhprop wrote:These types of antennas were nothing but rabbit ears mounted in an enclosure with a small motor to rotate it.
There is no rotation.* It's an antenna that picks up signals from all directions.
Quote:
Quote:

The pix of the antenna which is the topic of the original post is a bow tie *in an enclosure *rotated by a small rotor. That is what the pictured controller does. There are omnidirectional disc antennas, which are also junk, but this was not one of them.

I would sure like to be your Best Buy, Radio Shack, West Marine salesman. I would be fixed for life.

rhprop,

I didn't post on this thread so I could be insulted, I posted to try and help someone with a problem and clear up the misconceptions about "digital" antennas.

I assure you that my Glomex TV antenna has no motor turning a bow tie antenna and neither did the Dantronics UFO that it replaced.* I will also reassure you that either of these antennas perform better than a straight piece of wire (coat hanger).

While some folks may be proud of beating an antenna manufacturer out of a small profit by using a bent coat hanger for an antenna, I have a little more pride in my boat than that.* Perhaps a coat hanger antenna would look fine on a derelict boat tied to a run down dock somewhere.

Perhaps a coat hanger or rabbit ear antenna would work nearly as well as the marine antenna I installed, but as I pointed out above, I don't really want to get up and*climb to the flybridge in the middle of a show to reorient the antenna.* Besides, I wouldn't be able to see the TV while adjusting the antenna so it would be a two person job or I would need an additional TV receiver on the flybridge for monitoring purposes.

I believe your last few posts were not intended to add to the thread, only to start an argument.* You were partially successful, but I am done.* You may carry on if you wish.


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Old 01-15-2011, 09:57 AM   #24
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RE: TV antenna troubles

Not wanting to add to any debate or rancor on this subject, I would like to ask a related but different question.

Is there and relatively easy way to test the transmission capabilities of the coax that runs from my masthead antenna to the TV? I believe my old monster 2-foot diameter Shakespeare, which I've already disassembled once to clean out the spiderwebs and bee nests, is less than efficient anymore, and while replacing I would probably want to install a new coax, but pulling it off the mast and un-routing it is a pain. Can I measure gain/loss without any fancy RF - measuring equipment?
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Old 01-15-2011, 11:16 AM   #25
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RE: TV antenna troubles

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ARoss wrote:

Not wanting to add to any debate or rancor on this subject, I would like to ask a related but different question.

Is there and relatively easy way to test the transmission capabilities of the coax that runs from my masthead antenna to the TV? I believe my old monster 2-foot diameter Shakespeare, which I've already disassembled once to clean out the spiderwebs and bee nests, is less than efficient anymore, and while replacing I would probably want to install a new coax, but pulling it off the mast and un-routing it is a pain. Can I measure gain/loss without any fancy RF - measuring equipment?

*

The short answer is No, but if the ends have been sealed and the outer insulation does not crack when you bend it, you can be reasonably assured that it is OK. I have coax from the WW11 era that is just as good for the frequencies that it was designed for as it was when new.
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Old 01-15-2011, 11:29 AM   #26
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RE: TV antenna troubles

For those of you who can live with an omni-directional antenna, I ran across this on another board:

http://www.boatenna.com/index.htm

basic, no hype, well made, no enclosure to contribute to loss, low loss 300 ohm ribbon cable, actually made for a marine environment and the cost is pretty reasonable for anything with " marine " attached to the name. And no amplifier is necessary or recommended.
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Old 01-16-2011, 01:13 PM   #27
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RE: TV antenna troubles

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I agree with everything except the necessity for an amplifier. The amplifier only overcomes the cable losses to the TV , it does not improve the efficiency and unless it is a really big boat or really tall mast, or you are splitting the signal it is not necessary and because the amplifier has a " noise factor " itself, it can actually make the signal worse if the noise contributed is greater than the cable loss that has to be compensated by the amplification in the TV tuner.
Not to overwhelm anyone with tech speak, but there are some times when an amplifier can be usefull, so I wouldn't dismiss their use quite so quickly. I can easly see a cable run of over 20 feet on a lot of boats.......Arctic Traveller*
<h2>Signal Amplifiers, Preamplifiers</h2> Many people think that connecting an external amplifier to the antenna will improve the performance of the antenna.This is usually wrong.Receivers always have more gain than is necessary.(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.)

Normally the signal to noise ratio will be set by the receivers 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 receivers noise becomes insignificant.)Since there is no reason to think the external amps first transistor is quieter than the receivers first transistor, there is generally no benefit to the S/N ratio from an external amplifier.

But 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.

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).This author recommends 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 can to an extent ignore this advice.)

The preamplifier should have a gain equal to the loss in the cable (for your highest channel) plus another 10 dB (to keep the receivers 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.)

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
75W-to-300W balun0.2-2 dB(a balun is an adapter)

The antenna and the amplifier both have gains measured in dB, and some people add these two numbers (and then maybe subtract the losses) to find the strength of the signal at the receiver.But this sum is worthless.The net gain in front of the amplifier should always be kept separate from the net gain that follows.

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


<h2>Receiver noise</h2> Actually there is a reason to think the external amplifier is quieter than the receiver.Long ago designers made an effort to make the TVs 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 TVs noise is rarely a factor.Some TV makers no longer put any effort into making their sets quiet.

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.This author recommends that you try a Channel Master Titan or Spartan amplifier, but make sure you can return it if it is no help.
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Old 01-17-2011, 04:19 AM   #28
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RE: TV antenna troubles

Higher is better , so any antenna that can be "up top" is better than mounted down low.
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