Two-band OH-beacons with millihertz precision and H/V-pol

Questions and answers on 4 m transceivers, transverters, antennas, etc.

Two-band OH-beacons with millihertz precision and H/V-pol

Postby OH1ZAA » Mon 10 Oct 2016, 09:18

There are a couple of new Finnish 70/50 MHz beacons coming up
with dual polarization. Some of these will be research beacons
with a scheme on the phase-shift between the polarizations in
order to extract propagation information of the path involved.

For the casual operator these will be identifiable like any CW
beacon. Antennas will be stacked turnstiles (X-dipoles) in two
(2 dBd omni) or four layers (5 dBd omni). Vertical polarization
will be an omni J-pole, or a 6 - 8 dBd yagi with the main lobe
into 220 degrees azimuth, depending on how crowded the top
of the mast is. Ground reflection gain is more efficient for
horizontal polarization. Main lobes tend to converge for the same
maximum field for both polarizations below the Brewster angle,
i.e. toward the horizon, below 5 degrees take-off. These will be
potent ionoscatter beacons (mainly on 50 MHz, as ionoscatter
weakens rapidly with the 5th power relative to the frequency
of operation).

Due to a fortunate coincidence related to the NEW 50 MHz beacon
band, milli-hertz precision carriers for these beacons can be derived
from a single LeoBodnar GPS-locked frequency reference. ... f5b059f0b8

When Output 1 of the Bodnar-unit is set for 50 MHz, there are not
too many options for Output 2. But fate wants it, that 50.400 MHz
projects to 84 MHz, which (divided by six and multiplied by five),
yields 70.000 MHz. In practice this can be achieved by an ACMOS
/6-divider, and by filtering out the 5th harmonic of the resulting
waveform (an XOR-gate function can be added for a part of the
sequence to add more spectral content to the 70 MHz component).

Due to the fixed relation of frequencies, 50.418 projects to 70.025
and 50.436 projects to 70.050, which is the limit of the Finnish 70 MHz
beacon band. This means that OH1SIX/b 50.412 projects to 70016.666
for OH1FOUR/b and correspondingly OH9SIX/b 50.417 to 70023.611
for OH9FOUR/b. I do not have info about the OH7SIX/b project and
whether there is a plan for a 70 MHz beacon there, but 50.433 would
yield 70045.8333 with the Bodnar-unit. Of course a separate unit
could be taken for each beacon if a dead-on round kHz frequency on
70 MHz is wanted (but it is the most expensive part of a beacon project).

ACMOS (5V or 3.3V) output levels (+13 dBm) suffice to steer out a 7 W
M67743L-module to full power. For OH9FOUR/b - the two polarizations -
2 x 7 watts is considered appropriate. For OH1FOUR/b two 50 watts
stages can be added (each having 2 x BLV80/28 running at a lower
voltage for long endurance in a harsh Arctic beacon environment).
Each PA-module will include notch-filters for the other band in order
to avoid reciprocal mixing of 50/70 MHz signals in the output units.

We hope to have some of this new technology running before the
commencement of the 2017 Es season. There is a perfect location
for the OH1FOUR/b + OH1SIX/b combo on a 200 m asl KP11TX hilltop,
with an isolated shack, grid power and 20 m of aluminum tower already
on the site, waiting to be immersed in beacon RF-signals. We have
some surplus 50 Ah telecom batteries to function as a primitive but
fool-proof parallel "UPS", during rare power grid breaks.


P.S. There will be no beacon for OJ0 Märket reef (JP90NH), but the
good news is that we will do a 70 MHz dx-pedition to OJ0/Märket
in May 2017. The 6-el LFA with a 4.9 m titanium boom and 10 mm
solid aluminium rod elements is tested and ready to ship (there is
already a small/light 70 MHz 4-el yagi on the island). A LDMOS-PA is
in preparation for 100 watts: will mount at antenna feed-point. Solar
buffer: 48 volts, 660 Ah nominal (12x 12V 220Ah, not just for radio).
OH9FOUR/b proto with 7 watts into dummy-load on old OH1FOUR/b-assigned frequency
OH9FOUR-proto-7watts-1610b.jpg (71.44 KiB) Viewed 625 times
Last edited by OH1ZAA on Mon 10 Oct 2016, 20:11, edited 1 time in total.
Posts: 155
Joined: Mon 30 May 2005, 13:37
Location and locator: KP01RO

Re: Two-band OH-beacons with millihertz precision and H/V-po

Postby GW8IZR » Mon 10 Oct 2016, 19:02

this is good news, please keep us updated with progress and I look forward to spotting them next year.

73 de Paul GW8IZR
73 de Paul GW8IZR
IO73TI Anglesey
Posts: 351
Joined: Sat 12 Jun 2004, 05:37
Location and locator: IO73TI

Re: Two-band OH-beacons with millihertz precision and H/V-po

Postby LA3EQ » Tue 11 Oct 2016, 15:00

Great news.....millihetz precision and ionoscatter takeoff is like music to my ears...
73's LA3EQ -Jan :D
Posts: 1
Joined: Wed 26 Oct 2011, 19:28
Location and locator: JO28XJ

Re: Two-band OH-beacons with millihertz precision and H/V-po

Postby OH1ZAA » Tue 28 Mar 2017, 15:16

2 x BLV80/28 standing tall, and 2 x M67743L in the diecast box (with name plate)
OH9FOUR-8W+50W-growing.JPG (154.85 KiB) Viewed 347 times

There is slight progress with the Finnish beacons.
OH9FOUR/b mock-ups have been made in several
variants. The picture shows one with dual PA's.

The M67743L 8W modules can be driven directly
with ACMOS TTL level signals. For horizontal
polarization the M67743L is driving a pair of
BLV80's (capable of 130 watts) for a modest
50 watts output.

An alternative is to use a M67742 30 watts module.
I have used a BFG135 to drive these from TTL-level
ACMOS. The BFG135 is biased Class A with 47 ohms
at the collector (in series with a small RF-choke) to
V+ and a 15 k feedback resistor from collector to
base. Depending on the DC-amplification of the
BFG135 this resistor can be fine-tuned for 8V
DC on the collector with about 100 mA of bias.
At this level the Class A amplifier is well matched
with the module input impedance to provide enough
drive for about 35 watts output on 70 MHz @12.5V.

Actually with TTL drive the BFG135 works more like
a switch, which is perfectly OK for a beacon carrier.
Also M67742 modules are not allowing Class AB
bias for any linear modes, but the same BFG135
circuit works fine for the M57735 (AB) on 50 MHz.
A DC-blocking capacitor (1 nF) should be used at
the BFG135 base input. The modules are floating for
DC, so those can be directly bridged to the collector.

The tin box contains two SWR-samplers for the chains.
Those were made with BNC/RCA adapters soldered
directly to the wall of the box. Those turned out 500 MHz
wide for 1 : 1 SWR when terminated with 50 ohms, and
the FWD/REF ratio in detected voltage was 70 : 1 under
that condition. The parallel sampling line (actually the
resistor leads) was terminated with 150 ohms resistors
on both ends. (The grounded leads were cut to 5 mm).
Level detection with a two-diode voltage doubler.
High SWR (e.g. due to ice forming on the antennas)
will lower the beacon supply voltage to 8 volts. A note
about this can be embedded in the CW-message.

During construction my curiosity was quenched for the
input impedances of some of the modules and circuits.
The M67742 (70 MHz) showed 75 ohms on the operational
frequency. For the RF-designers: the BFG135 had almost
infinite input SWR without load on the collector. This is
due to the feedback resistor compensating any RF-current
variations coupled into the base. But with the M57735
(50 MHz) module as a load, the transistor input hovered
around 17 ohms real (SWR 3 : 1). One can make it 1 : 1
with a series 33 ohms resistor (with a -10 dB gain loss),
or one can make a simple asymmetric pi-filter to match
lossless to 50 ohms. The RigExpert AA-1000 instrument
directly on the base of the BFG135 was able to steer the
module near to maximum power during SWR-measurement.
In beacon mode, while driving with nearby ACMOS-logic,
impedance matching is not required. ACMOS-logic is used
to vary the phase difference between H/V-polarizations.

Cheers/73, "Zaba" OH1ZAA
Posts: 155
Joined: Mon 30 May 2005, 13:37
Location and locator: KP01RO

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