Topband: Hi Z amplifiers for 160m

[John Kaufmann]

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