This page describes the soft switching approach as implemented in the first PA8W conventional doppler.
The same basic approach was used to design the antenna drivers of all later models.
The antenna driver of the doppler RDF
is a very important part of the total system, and therefore it may
need some extra explaining.
Initially, trying to keep things as simple as possible, I started
off with the most simple version of an antenna driver, a hard
switching device that selected every antenna for 1/4 of the cycle
It turned out that it made the RDF pretty intolerant to strong
signals outside the passband because all switching signal harmonics
mix with the RF signals,
causing these undesired
You have to realize that the antenna switching diodes act like diode
mixers, so higher harmonics on the control signal would produce a lot
of undesired mixing products.
So it didn't take long before I took a closer look to see what could
be improved there.
Testing some intermediate driver with reduced switching speed I
noticed a big improvement but this driver wasn't flexible enough to get the best
so I designed a new one, which I currently use in the MK2 RDF.
This new driver was based on the knowledge that reduced switching
speed means reduced reciproke mixing problems.
Also, during my experiments I had found that some overlap between
the antennas seemed to be beneficial.
Although I found complex driver designs that used this technology, I
held on to the minimalistic approach I generally follow;
just keep it as simple as possible..
So I came up with a pretty straightforward but flexible design.
The new driver was tested and tweaked using a few potentiometers which
could set the most important parameters over a wide range, so I could
happily experiment with all kinds of driving waveforms.
After extensive testing, all unnecessary components were removed
from the driver circuitry, and the potentiometers were replaced by
Still the driver was maintaining 99% of its
The soft drive method produces much less noise and less additional
mixing products than the commonly used hard switching.
Additionally, I found that a substantial overlap of time in which two sequential
antenna elements are active, results in even better reduction of
noise and reciproke mixing.
(The actual switching is done with PIN-diodes or 1N4148 switching
diodes. See the bottom of this page for comments on that.)
The next screendumps illustrate the timing of the antennas:
We are looking directly at the voltage on the antenna driver output pins:
The red wave in the picture represents the control voltage of the first
The blue wave represents the control voltage of the second antenna.
The yellow wave represents the control voltage of the third antenna.
The second blue wave represents the control voltage of the fourth antenna.
The horizontal centerline is 0V, and one vertical division is 5 Volts.
So, the maximum ON voltage is +10 volts, which means that the
current through 3 PIN diodes in series will be around 8mA.
The maximum OFF voltage is about -6 volts, which is good enough to
ensure excellent isolation of the non-active antennas.
Also obvious is the serious overlap
between two antennas, at the 2V mark, where the PIN diodes will
start to conduct.
The next screendump will give you an even better impression of the
Note that there is some difference in rise and fall time of the control
This is due to the simple circuitry.
I did perform a test with additional hardware to match rise and fall
time but this revealed no further improvement.
So I decided to stick to the simple design.
Some argue about a real sinusoidal switching signal to be even
better as it contains no harmonics.
But this is only partly valid: The most important point is the on/off
transition point of the diodes.
If the control signal is passing this point relatively slowly, it
will virtually be as good as a sinusoidal control signal.
And using our circuitry, it is much simpler to realize.
Now the same control voltages but measured after the 1k series
resistors, so in fact directly on the first PIN diode:
The colours are the same for antenna 1,2,3, and 4.
The horizontal centerline is 0V, and one vertical division is 2
Just above 2V you recognize the threshold of the 3 PIN diodes in
series, so just above 2V the antenna ON status begins.
Also obvious is the 1,2 division (= 2,4millisecond = almost 1/8 array cycle) of overlap
between two antennas.
This means that with every transition from one antenna to the next,
there's a short period of time where two antennas are active,
simulating a pseudo 8-element array.
Improvement compared to hard
Below spectrograms show the frequency spectrum from zero to 100kHz
on the antenna control lines:
First: "Normal" hard switching, harmonics at 100kHz only
41.6dB down compared to the 501Hz fundamental frequency,
and the harmonics attenuation will slow down going up in frequency...
Second: The PA8W soft switcher, harmonics suppression at 6kHz
already exceeds the 100kHz value of the hard switcher!
Above 12kHz, all you see is the noise floor of my
simple FFT analyzer...
PIN diodes: are there alternatives?
Yes, there are.
In some designs found on the internet the 1N4004 is suggested, but I really can't support
That's a pity, because it would be a solid diode which would not be
zapped so easily if you happen to use a transceiver for the RDF and
squeeze the PTT by mistake...
However, for VHF the cheap 1N4148 is a good alternative, according to my
I took a normal switching circuit as we use in the Doppler RDF
antenna's, and put it in the coax between a vertical antenna and a IC746pro receiver.
Then I tuned to a solid carrier on 145MHz and made the following measurements:
ON: +5 or 10V
It is important to realize that the
dB indications are derived from the IC746pro's S-meter, for what it's
No absolute figures!
Another thing to remember is that in this test case, only one diode
switch was measured. (In a normal RDF array there are 2 or 3 diodes
per element, so overall isolation is much higher, probably more than
Also, no other antenna was connected and loading the coax with a 50
ohm impedance like in a real RDF array.
This would further improve off-state isolation.
However, the comparison shows that the cheap 1N4148 is a very good
alternative for the extremely small PIN diode.
Another remarkable thing is proven by the test; in contrast with
negative biasing of the 1N4148 does not really improve isolation in
the off state, but it does slightly improve the PIN diode
Whether you use PIN diodes or the 1N4148, the performance of the RDF
will hardly be degraded on VHF.
Isolation will be much higher than necessary, so other factors such
as mutual coupling of the elements will be amongst the more
On UHF however, I strongly recommend the use of PIN diodes.
More details in the pages describing the mobile and homebase