Topband: RX 4 square

[Tom Rauch]

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