Regarding some comments on the active elements, there are a
few corrections that need to be made. If this doesn't pass
through on Towertalk I hope someone forwards it.
First, someone posted that a simple Minicircuits transformer
could replace the voltage follower. This appeared on
Towertalk a year or so ago, and again on Topband just now.
I'm very surprised that comment was made. That idea probably
comes from confusing voltage gain and power gain.
In a source or emitter follower, voltage gain is
theoretically unity. Power gain is a function of impedance
ratios of the input and output circuits. The power gain is
NOT unity and unlike a transformer it can actually be quite
high.
Let's consider a perfect voltage follower. If we apply 1
volt on the input, the output is 1 volt. If the input
impedance is 3000 ohms the input power would be 333
microwatts. The load power, if 50 ohms, would be 13.3
milliwatts.
This means if a whip or small antenna was sourcing 1 volt
into 3000 ohms and the receiver load impedance was 50 ohms,
the system would have a power gain of .133/.000333 log10 or
26dB.
26dB gain is certainly NOT unity gain!!!
A second factor enters the equation. The transformer, if
16:1 ratio and perfect (which it isn't even remotely close
to being) would have an input impedance of only 800 ohms.
This means the voltage delivered from a voltage probe would
be considerably less into the transformer, giving negative
gain even with a perfect transformer. The real gain is more
like 30dB when this is considered.
The statement a voltage follower can be replaced by a
conventional transformer with no change in performance is
very clearly not even remotely close to being factual,
although there are certain cases where a transformer can be
made to "work".
Second, let's look at the claim a thick antenna would drive
an amplifier better than a thin whip.
What is being missed there is a large part of the antenna
capacitance is shunting the input just like the input
capacitance of the amplifier does!! While not all of the
capacitance is shunting the input, a large amount is.
The actual measured difference in signal level between a 10
foot long 1 inch diameter tube driving 3000 ohms shunted
with 30pF when excited by a kilowatt into a 200 foot
vertical 1 wavelength away is 3.6 volts with the one inch
tube and 3.2 volts with a .12 inch diameter conductor the
same height.
This represents a signal loss of about 1dB, which is
insignificant.
Finally a comment about phase. There are two ideal
conditions of controlled stable phase shift. One is where
the antenna is so reactive phase shift is almost perfectly
90 degrees. The other condition is where the load is purely
resistive. Either one can make a very phase stable array.
The WORSE condition for a source is where the source is
resonant with fairly high Q, which means a large ratio of
reactance to resistance. In a case like this the sources are
the most sensitive to tuning or frequency changes. The phase
changes dramatically with small frequency changes. This is
why small self-resonant probes are to be avoided at all
costs if you want a stable array. You'll never find them at
my QTH!
My own personal choice in *monoband elements* at my station
is a 20 foot tall pole with a large capacitance hat at the
top. This allows a minimal amount of base loading to be used
along with a series resistance to raise impedance to a
reasonably high value. This makes a fairly broad bandwidth
low-Q resonant system. The performance change from 1.8-2 MHz
would be minimal, and the array would be fairly stable with
temperature and moisture changes if proper components were
selected. Not as easy as buying a Hamstick, but a heck of a
lot better in performance!
By far the best **broadband solution** is a voltage follower
and an untuned vertical. A matching transformer won't even
come close to having the same sensitivity.
73 Tom
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