VE7TIL 2m EME Station
Last update: February 22, 2007
Summary
A couple of years ago Steve,
VE7SL planted the seed in my mind to get on 2m EME. With the seed planted I went about gathering the parts to build a
basic yet effective EME system. What
follows is a brief description of what I came up with.
Station Equipment
The station consists of the
following major subcomponents:
- FT-897D w/ TXCO installed. Note: despite Yaesu’s TXCO,
drift is still a problem with a stock unit so some beefing up of the TXCO
is needed with Styrofoam insulation due to the rapid temperature changes
in the rig during transmit. With
the insulation installed the rig has excellent thermal stability.
- AM-6154 linear power amplifier which provides
upwards of 400W when pushed to the limit.
- 4x7 DK7ZB Yagi antenna array
- 50’ of ½” hardline feeds the array on TX and RX.
- Phasing harnesses where constructed out of
flexible 3/8 “ hard-line cable. I
was able to find a surplus vendor that had suitable cable with connectors
at a reasonable price and only needed theirs lengths evened out…
- Mast mounted SSB Electronics SP-2000 Preamp, the
preamp includes the needed switching which is good to 750W.
·
Azimuth control was
done using an old AR-40 type rotor, which was rebuilt.
·
Elevation control was
done using an old satellite dish actuator arm also restored and a piece of 1 ½”
nominal AL pipe with a piece of 1 ½” O.D. tubing inserted thought the pipe to
provide a hinge that required no machining.
Grease nipples where installed to allow for periodic lubrication.
- For a power divider I used a used M^2 4 port
unit in the final array purchased from Joe, K9MRI. During my initial tests
I used a homebrew 2 port unit.
- AZ/EL sensors are presently resistive units and
calibration is done in a computer to ‘curve fit’ the resulting
measurements to the real angles. A heavy copper weight was used in the
shape of ‘T’ to ensure there was sufficient torque on the potentiometer to
ensure it moved during fine tuning…
- Sequencing the TX/RXing of the station is done
using a PIC16F84A micro-controller programmed with CCS C. The code is so trivial anyone with a
basic programming capability and any controller programmers could easily
do this for themselves. If you
need some help ask me!
Station Setup
The following are some
details of the construction of the station.
The images below depict the
element mounting method I used to construct my variant of the DK7ZB 3.3m boom
narrow bandwidth 2m yagi.
I used common marine
electrical enclosures to support the driven elements. Note: If you assemble
a stacked array, be careful of how the driven elements are phased! I spent days trying to understand why I
could not be heard by anyone and my SWR was normal! The elements should all be
the same.
Early successful attempts
where conducted with a single 7 element DK7ZB yagi. I was only successful with the largest EME stations such as W5UN,
KB8RQ and RU1AA. I worked W5UN with 50W and a single 7-element DK7ZB yagi.
However, I soon realized
that I needed much more gain but what can 3dB really do?
I experimented with the
above 2x7 array for a couple of months in what turned out to be the worst
winter in living memory here in Vancouver, BC.
Normally we get a lot of rain and not much else. This year we had high winds and heavy snow! Of course the snow caused chaos to the
array! The 2x7 system worked very well
and allowed me to work fairly small stations and 58 initials in 3 months… It
planted a seed in my mind that a very portable and effective EME system could
be made from a crossed pair of 7 element DK7ZBs as often what failed me during
QSO attempts was Faraday lock out… A
switchless combiner as described by SM5BSZ may be the answer for phasing…
Be careful not to place the
balun cable too close to the boom or the following will certainly occur at EME
power levels!!
The H-frame was first to be
placed on the small 20’ tower…
Station Testing
Prior to using anything on the air I wanted to understand the requirements of JT65 and ensure my equipment and systems where up to the task. Here are the test reports for the various components I tested. Early in my experiments I used a separate TX and RX but the principles in the following remain the same if you choose to do these tests on your station equipment…
Frequency Stability of the TX and RX
Once
I completed the station, I of course pointed my antenna at the moon and tried
to work as much DX as possible! But
there are many days in the month when the moon is in a very uncooperative
position in the sky… This led me to test the RX system by attempting to use the
Sun as a noise source.
The Yellow circles indicate
noise the correlated to snow fall during these tests. The Red circles indicate the Sun and Sagittarius A, AKA the
center of our galaxy! The time of year
I did this test happened to be when the Sun and Sagittarius where together in
the sky. The Blue circle is me up the
band on 2m FM.
About a month later I ran
the same test using the completed 4x7 array and was able to distinguish both
the Sun and Sagittarius A as distinct objects in the sky. Sagittarius being the
early bigger hump and the Sun the smaller later one…
The next task was to explore
the local noise field here in urban Vancouver, BC. I sweep 360 degrees at 0 degrees elevation and produced the
following plot of the noise all around me.
What came as a surprise was
the noise peaks corresponded to large objects on my horizon, namely mountains
and large buildings on the East side of the plot. The really big peaks are my house and a couple of neighbours to
my SW, W and NW.
I then asked Steve, VE7SL to
provide a weak signal from his QTH in CN88 and plotted the following:
Interestingly the plot of
Steve’s signal shows a correlation between the known locations of the large
objects on my horizon and peaks in his signal.
The reverse is true when the antenna passes through the nearby houses
blocking the path to Steve’s QTH which is on a bearing of about 200 degrees.
Links
DK7ZB's Excellent Antenna Page