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Re: Topband: Hi Z amplifiers for 160m

To: topband@contesting.com
Subject: Re: Topband: Hi Z amplifiers for 160m
From: Michael Tope <W4EF@dellroy.com>
Date: Thu, 12 Mar 2020 13:37:02 -0700
List-post: <mailto:topband@contesting.com>
Hi Lee,

Yes, if you are combining coherent signals that are not in phase, then the each of the voltage vectors is weighted by cos(phi-i) where phi-i is the angle between the i-th voltage vector and the 1st vector. If phi=0, then you have the case I described previously. I can see how this can get tricky, however, with an electrically short baseline where you are striving for cancellation in the rearward looking direction. It's like you cancel in the rearward direction and almost cancel in the preferred direction :-). This degrades the SNR not because the noise is adding up, but because the signals are subtracting down.

73, Mike W4EF.............

On 3/11/2020 10:23 PM, Lee STRAHAN wrote:
    Mike and all,
       Well stated Mike. It's been a long time since we have conversed. The 
modifier to this is when the signals coming into the combiner are no longer in 
phase or coherent. This as a result of delay lines and time of signal arrival 
at the many elements. Most often in our small portion of a wavelength low 
frequency arrays the combination of signals is subtractive to form a given 
pattern per array dimension. This then lowers the signal to noise ratio. It 
gets pretty complicated to arrive at a noise figure. The only way we have been 
able to do this with amplified arrays is to simulate the array in NEC being 
excited with a known signal many wavelengths away. We can extract the actual 
amplitude and phase of these multi element array signals and then combine them 
mathematically as you have done by example to arrive at a signal gain number 
from signal combination. The noise gain is easy. I say we because I have a 
retired very smart Ham friend in Finland that has helped me through this. I
t h
  as caused me to rethink gain distribution in some of my arrays.
Lee   K7TJR  OR

What matters is the signal-to-noise ratio (SNR). Take the canonical example of 
an ideal 2-port Wilkinson power combiner with in-phase coherent signals of 10 
Vrms applied to each input along with 1 Vrms random thermal noise from the 
respective element amplifiers applied to each input (i.e. each input signal has 
a 20*log(10 Vrms/1 Vrms) = 20dB SNR).

The power loss of the combiner is 3.01 dB [i.e. 10*log(2)], so voltage of each 
signal is attenuated by 1/sqrt(2) = 0.707. Thus, the components of each input 
signal appearing at the output are 7.07 Vrms each and
0.707 for each of the noise inputs.

The signal components add coherently at the combiner output yielding a total 
signal voltage of 14.14 Volts rms. The noise voltages are incoherent, so they 
add as root-sum-square at the output of the combiner. This yields a total noise 
voltage of sqrt(0.707^2 + 0.707^2) =
sqrt(1) = 1.0 Vrms. Thus, the combined noise voltage is unchanged, but the 
signal voltage goes up by sqrt(2).

The SNR of the combined output = 20*log(14.14Vrms/1Vrms) = 23dB, a 3dB 
improvement.

The same things holds for an ideal N-way combiner with equals noise components 
at each input. The noise power at the combined output equals the noise power of 
any of the equal input components (i.e. 0dB gain).

73, Mike W4EF..................



On 3/11/2020 7:22 PM, Lee STRAHAN wrote:
     Hello John and all,
     Concerning the adding the noise in a typical array. If the noise was 
coherent or exactly the same signal from each element/amp the summed noise 
would indeed be 8 times. However circuit noise is always random and incoherent 
which causes the summation to be a single noise power times the square root of 
the number of elements assuming equal levels from each amp. In the case of 8 
elements 4.5 dB increase which is no small matter as well. In the case of the 
three elements the noise summation would be about 2.4 dB higher than a single 
element.
Lee  K7TJR  OR

As the designer of the YCCC high impedance feedpoint amplifier, let me address 
some issues related to the design of the YCCC amplifier as well as feedpoint 
amplifiers in general.  If you don't want to read a lot of technical 
gobbledygook, please disregard this message.

The YCCC uses an AD8055 RF amp as the gain element.  As Lee, K7TJF, points out, there are most 
certainly better op amps out there.  However, the AD8055 was the "best" part I could find 
in a DIP-8 package.  The "better" op amps are all SMT parts but given that the YCCC 
preamp was a kit, I intentionally limited the selection to DIP-8 parts that kit builders could work 
with relatively easily on a PCB.  Not everyone is able to do a competent job soldering tiny SMT 
parts.

Within the universe of available RF op amps, tradeoffs must be made in terms of 
noise, linearity, and bandwidth.  The AD8055 is not the lowest noise part but 
it has excellent linearity and plenty of bandwidth for HF use.  At my QTH there 
is an AM BCB station 3 miles away, which makes it a somewhat challenging EMI 
environment.  The decision to run the op amp in a unity gain configuration 
comes down to linear dynamic range.  It is easy to design for more gain, but it 
is also easily demonstrated that you will begin to suffer in terms of unwanted 
intermods.  With the YCCC preamp, I get absolutely zero BCB intermods or 
distortion products in the 160m band at my QTH.

In general I do not like to use an outboard preamplifier between the
output of the phased array combiner circuit and my receiver because it
degrades the linear dynamic range of the system.  The YCCC system
user's manual (Section
12.1) does specify several outboard preamps that could be used.  In a low EMI 
environment, I think they all work fine.  However, at my QTH, with the nearby 
AM BCB station, all of them, without exception, generate increased distortion 
and intermod, which I find unacceptable.

It is always desirable to apply RF gain with a roofing filter in front, which 
is becoming common practice in high performance receivers.  With my K3S 
receiver, the use of a unity gain antenna feedpoint preamplifier is perfectly 
fine if you also turn on the preamp in the K3S.  This gives the best overall 
linear dynamic range with a preamplified short vertical system.
There is no loss in noise performance because the noise on 160 and 80 is 
totally dominated by atmospheric noise.  In measurements I made at my QTH, the 
internal noise of the YCCC preamp is about 10 dB lower than my daytime 
atmospheric noise on 160m when using a vertical about 20 feet high.

You must also consider the number of active elements in an amplified
antenna array when evaluating overall system noise performance.  This
is because the amplifier circuit noise power of all the feedpoint
amplifiers is added together when the elements are phased up in a
combiner.  If you have N elements in your array, the effective circuit
noise contribution gets multiplied by N.  The YCCC array has 3 active
elements at a time.  However, the YCCC design is somewhat unusual in
that maximum RDF is achieved when the signals from the elements are
combined in unequal ratios.  As a result the effective amplifier
circuit noise contribution is less than 3 times (or 4.8
dB) the noise of a single amplifier.  In fact because of the unequal combining 
ratios, the actual effective noise goes up by a bit less than 2 dB compared to 
a single amplifier.  An array like the Hi-Z array with 8 active elements 
combines the elements in equal proportion so the effective amplifier circuit 
noise of the system is 8 times (or 9 dB) higher than the noise of a single 
amplifier.  For this reason, the YCCC array can tolerate noisier amplifiers 
without degrading system noise performance.  The objective is to keep circuit 
noise well under atmospheric noise.

On the subject of op amp noise specs, you must consider *both* input voltage 
noise and input current noise because, in general, both contribute to the total 
output amplifier noise.  It is not good enough to pick an op amp with low input 
voltage noise without also considering the input current noise.
For a good noise analysis, download a copy of the datasheet for the CLC425 op 
amp:  http://www.elektronikjk.pl/elementy_czynne/IC/CLC425.pdf.  Refer to pages 
8-10.  (The CLC425 is a very good RF op amp but has been obsoleted by newer 
parts).  I put the noise equations into an Excel spreadsheet, which allowed me 
to compare many different op amps in terms of total noise performance, using 
their input current noise and voltage noise specs.

Not all op amps publish specs on linearity.  It is safe to assume that if no 
specs are given, the linearity is not particularly outstanding.  Look for 
harmonic distortion (HD2 and HD3) as well as TOI (third-order intercept) data.  
You do have to be careful in interpreting the data because the linearity is 
directly tied to the amplifier gain configuration.

If I were to recommend a particularly outstanding RF op amp, it would be the LMH6622, an 
inexpensive but very high performance SMT part.  It comes as a dual op amp package but I 
only use one of the op amps.  There is no single op amp equivalent part.  The noise is 
very low and the linearity specs are outstanding.  It is intended for use in RF systems 
with very stringent linearity requirements.  I have built a "beta" version of 
an antenna feedpoint amplifier using this op amp in a very unique configuration (not a 
high impedance design).  The effective circuit noise floor is about 8 dB lower than the 
AD8055 preamp with similar linear dynamic range performance and about 8 dB higher RF 
gain.  I am still working some tradeoffs in this design, so I'm not ready to go public 
with it just yet.

73, John W1FV



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