Naked Science Forum
Non Life Sciences => Geek Speak => Topic started by: CZARCAR on 21/08/2011 13:34:56
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New aerial tv signal [digital,vhf,uhf] in the US. My antenna is high abve roof,properly aimed. Got a 18db amplifier on the tv end which i tried on antenna end already with no diff. Q= Can I connect another amp in series with the first so that 2 amplifiers will boost the signal? & How effective might this be? 18dB boost + 18DB boost = 36 DB total boost? Any concerns over "noise" interference?...........thanx
As is, weather affects some channels sometimes so they dont come in at all.
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You could try it, but I doubt it would work. The problem is not just that the signal isn't strong, it's that the noise (from things like the CMBR among others) is "louder" than the signal you want. If you amplify it you will amplify the noise just as much as the signal.
The best bet would be to try a bigger/ better antenna.
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I was just about to post this. I agree with BC:
The "easiest" way to find out is to haul a TV up to your roof and plug the antenna directly into the TV. That will show you the best you can hope for, and it probably won't be any better than what you already get. Amplifiers can compensate for signal attenuation on long cable runs, but if the signal from the antenna is weak, amplifying it will also amplify the noise, and it's really the signal to noise ratio that is most important.
You might improve the signal strength slightly with a larger antenna, but it sounds as if the signal strength at your location is very weak, so I don't think you are likely to see a great improvement.
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thanx, i'll try with a real short cable be4 i buy a 36db amplifier. current cable is ~ 110' long + 18 db amp.
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Please do not fall off the roof!
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Bigger antenna, plus a channelised amplifier ( only provides gain for selected channels, and nothing for the others, so reduces noise) at the head. All connected with good quality coax ( you have a long run, so need a cable with low loss and good shielding. Try another position on the roof, you could get a big improvement just moving the antenna a few feet either way. At the Tv end a low gain amplifier ( 6dB is enough) is all that is needed to get the signal to the TV set. Replace the cables that came with the TV with cable made up of the coax you bought and using good quality connectors. The cable that comes with the TV is so bad that in strong signal areas it makes a fair antenna on its own. For safety connect the antenna and cable to a ground rod or three driven into the earth directly below the antenna, and connected with an appropriate cable and good connections. Take care on the roof.
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It is an interesting point that when the signal very weak and coming from an ill defined direction an antenna with fewer elements and nominally lower gain will often give a better picture.
Amplifiers placed close to the receiver can only compensate for substandard receiver performance and seldom do any good, the best return for your outlay on antennas is more height although a sideways move in bad locations sometimes helps.
PS have you considered satellite reception.
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Quick pedantic point: (Sound Pressure Level) decibels (dB) are a logarithmic scale (thus 18dB + 18dB = 21dB) - I presume this would be the same for gain through an amplifier?
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Quick pedantic point: (Sound Pressure Level) decibels (dB) are a logarithmic scale (thus 18dB + 18dB = 21dB) - I presume this would be the same for gain through an amplifier?
(Scratching head and trying to remember.)
That would be the case for power gain, but I think we are only interested in voltage here. Assuming the input and output impedances are the same, the voltage gain is 20log(Vout/Vin).
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Quick pedantic point: (Sound Pressure Level) decibels (dB) are a logarithmic scale (thus 18dB + 18dB = 21dB) - I presume this would be the same for gain through an amplifier?
Pedantically true, but unhelpful. What we are actually doing is calculating 18dB times 18dB which is 36 dB.
The first amplifier multiplies something (power or voltage- it's not specified) by about 63 (i.e. by 10^1.8)
So does the second. The overall effect is to multiply by 63*63 i.e. about 4000
You convert that to Bel by taking the log
log(4000) = 3.6 and convert to dB by multiplying by 10
to give 36.
None of which will help as much as a bigger antenna or a better sited one.
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Thanx. After climbing the roof & seeing what a great job I did[= hassle] I bought a 36db amp from ebay- after reading feedback [AOK, & WORKS GREAT] so a week from now I'll have the amp installed near the tv & will post results. $20, no pay till 2012 so i might can charge it on my Mayan account, - possible $10 refund= $10 total. Funny is I got 2 stations coming from same direction, 1 is hi vhf & the other is medium vhf & I CANT GET BOTH STATIONS!
Got a dish once & kil-a-watted the standby loss from the receiver OFF [NOT UNPLUGGED]cost me over $5/mo.
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It is a waste of time and money installing a 36db amplifier close to your receiver, install a 18db amplifier close to the antenna before the cable and it might do some good.
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Remember that the amplifier amplifies both noise and signal ( and cannot differentiate between them either) and contributes noise of it's own. The cable has a loss as well ( and this can be quite high, and vary with frequency as well, and not be constant, though generally it increases with increasing frequency) and adds noise as well. Thus you get a weak signal at the antenna becoming a very weak signal plus some noise at the end of the cable. The amplifier does it's job, giving you a stronger signal plus a lot of noise ( the noise from the cable and the antenna is amplified by the amplifier, plus it's noise is added as well. It may work for a digital signal, and may be enough to get a level where the decoder can recover the errors, but for analogue TV you will have a noisy picture and poor sound.
I did a similar thing for my car, where the radio is well known ( being designed for Germany) for being a poor low signal performer, but quite good in a strong signal area. I added a simple 2 stage amplifier, giving about 12dB of gain, right at the base of the short antenna that comes with the car. Just this did a massive improvement, giving a signal that does not fade under bridges, and will still be a noiseless stereo there. The amplifier does 2 things for me, matching the high impedance of the antenna to the cable, and providing modest gain. The actual unit was a Tv distribution amplifier that was replaced after a lightning strike, which popped the input stages and power supply. I cut out the output stages, which worked perfectly, and used that.
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Quick pedantic point: (Sound Pressure Level) decibels (dB) are a logarithmic scale (thus 18dB + 18dB = 21dB) - I presume this would be the same for gain through an amplifier?
Pedantically true, but unhelpful. What we are actually doing is calculating 18dB times 18dB which is 36 dB.
The first amplifier multiplies something (power or voltage- it's not specified) by about 63 (i.e. by 10^1.8)
So does the second. The overall effect is to multiply by 63*63 i.e. about 4000
You convert that to Bel by taking the log
log(4000) = 3.6 and convert to dB by multiplying by 10
to give 36.
None of which will help as much as a bigger antenna or a better sited one.
Thanks I now understand why it is a multiplication rather than an addition as expressed in the question. 10 out of 10 for showing your workings as I would have relied on the shortcut of simply adding logs(in the same base) when multiplying them...