# [VHFcontesting] PA Question - corrections

George Sintchak wa2vnv at optonline.net
Wed Oct 24 01:31:43 EDT 2012

```I was just re doing the numbers when ur email came in....

I made a (common) mistake in the case #2 calculation. I should have known
better - but it's late.
The Gain number(s) should be Power Gain NOT dB gain.
You have to convert the dB to Power Gain to use in the equation and then
back into dB's.
G = [10^(dB/10)]
10 dB becomes [10^(10/10)] = 10^1 = 10
20 dB = 10^(20/10) = 10^2 = 100
7 dB = 10^0.7 = 5.01

And you did the rest of the calculations.
Tnx for seeing my error

----- Original Message -----
From: "Tom Holmes" <tholmes at woh.rr.com>
To: "'George Sintchak'" <wa2vnv at optonline.net>
Sent: Wednesday, October 24, 2012 1:15 AM
Subject: RE: [VHFcontesting] PA Question

> One minor problem with this example is that the equation works in linear
> power, not dB. Convert the dB values back to their ratios to get the right
> result.
>
> A 10 dB pad is .1 in linear terms of its gain, but 10 in terms of its
> noise
> factor. The output S/N is 1/10th of the input S/N in linear terms. A 7 dB
> Noise Figure is a Noise Factor (the linear form) of 5. If the amp has a
> gain
> of 20 dB, that is 100 in linear terms.
>
> So if the pad is first in line, then Friis predicts that Ft= 10 + (5-1)/.1
> =
> 50 or just a hair over a 17 dB NF.
>
> If the amp is first (and having described it as a power amp, this might
> raise some eyebrows), then:
>
>    Ft = 5 + (10-1)/100 = 5 + .09, or just over 7 db.
>
> Working the Friis equation in dB might come close to the right answer in
> some cases, but the error could lead to some bad conclusions.
>
> Regarding the description of this as a power amplifier, one might wonder
> why
> noise figure is important. Remember that the definition of noise figure is
> the reduction in signal to noise ratio from input to output of a device.
> An
> amplifier takes the incoming signal and accompanying noise and amplifies
> it
> all by the same amount. A perfect amplifier adds no noise, thus the output
> has the same S/N. But a real amp adds some noise of its own to the input
> noise, and so deteriorates the S/N. Noise figure is the dB change
> (decrease)
> in S/N at the output.
>
> If a power amp (think high power linear foot warmer) is adding noise to
> its
> output, that is energy that will also be transmitted and potentially cause
> interference. It also wastes some of the amps power on useless noise and
> reduces efficiency. And then even if the added noise from the atmosphere
> is
> insignificant, the S/N that the signal carries with it is now the best you
> can see at the receiver.
>
>
> Tom Holmes, N8ZM
> Tipp City, OH
> EM79
>
>
>> -----Original Message-----
>> From: VHFcontesting [mailto:vhfcontesting-bounces at contesting.com] On
> Behalf
>> Of George Sintchak
>> Sent: Wednesday, October 24, 2012 12:15 AM
>> To: 'VHF Contesting Reflector'
>> Subject: Re: [VHFcontesting] PA Question
>>
>> You don't say what the gain of the amplifier is.
>>
>> Case 1.  If you put the 10dB pad on the input of an amplifier, the Noise
> Figure
>> becomes 17 dB,  (10+7) and the overall gain is reduced by 10 dB.
>> If you put the pad on the output of the preamplifier, it also reduces the
> gain of the
>> preamp by 10 dB, and the Noise Figure will also be reduced
>> (somewhat) depending on the gain of the amplifier.
>>
>> Case 2.  With the pad on the output, let's assume the amplifier gain is
>> 20
> dB.
>> The Ft = 7 + (10-1)/20 or 7+ 0.45 = 7.45 dB NF The Friss equation is
> applied for
>> (usually) amplifiers in series, showing that the overall NF is dominated
> by the 1st
>> stage NF (and gain) as long as it has resonable (high) gain. You need to
> be more
>>
>> 73's, George, WA2VNV
>> HF+50-1296, SOLP, FN30kv
>>
>>
>>
>> ----- Original Message -----
>> From: "John Santillo" <u1004467 at warwick.net>
>> To: "'VHF Contesting Reflector'" <vhfcontesting at contesting.com>
>> Sent: Tuesday, October 23, 2012 9:39 PM
>> Subject: [VHFcontesting] PA Question
>>
>>
>> > Hello,
>> >
>> >
>> >
>> > I have a PA Noise Figure Question.
>> >
>> >
>> >
>> > If a power amplifier has a Noise Figure of say 7dB and a series pad of
>> > 10dB
>> > is added does the Friss Equation hold true for calculating the
>> > increased
>> > Noise Figure?  For example, using the equation below I calculated a
> Noise
>> > Figure of just over 19dB with the pad inline.  If this is the case, can
>> > the
>> > carrier coming out of the PA be overwhelmed by the noise?
>> >
>> >
>> >
>> > Is the 7dB Noise Figure really the noise out of an amplifier with the
>> > input
>> > terminated above kTB?
>> >
>> >
>> >
>> >
>> >
>> > Ft = F1 + ( F2-1) / G1
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> > 73,
>> >
>> >
>> >
>> > John
>> >
>> > N2HMM
>> >
>> >
>> >
>> > _______________________________________________
>> > VHFcontesting mailing list
>> > VHFcontesting at contesting.com
>> > http://lists.contesting.com/mailman/listinfo/vhfcontesting
>>
>> _______________________________________________
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>
>

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