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Some hints for Newcomers to Earth-Moon-Earth communication on 144 MHz
Introduction:
We've been QRV now on 2m EME since almost one year (started end of May 2009).
The following summarizes some basic findings from that one year of EME activity
on the 2m Ham Radio band.
This all is related to a minimum 2m EME set-up which we consider to be required
in order to perform 2m EME QSOs on a regular base.
Findings:
Mode:
The by far most used and most reliable mode of operation is JT65B, due to the readability
of signals with respect to noise. The decoding is done with the "help" of a software program
running on a PC ("WSJT" developed by K1JT). "WSJT" is Freeware and can be downloaded
from the Internet.
I'd dare to say that 95% of all EME QSO's on 2m are performed in JT65B mode (not taking
into account EME contest situations etc.).
From our experience, JT65B still decodes signals which are some 25 to 28 db below
the noise. This is about 10 db better compared to what the human ear may be able to copy
as a signal (CW signal, i.e.), when filtered appropriately (narrow bandwidth).
In order to participate in 2m EME communication, you absolutely need to enable your
equipment to run JT65B flawlessly (no audio "hum", appropraiate sequencing on
RX/TX change-over, etc.).
Transceiver hints:
Saying that, you need to double-check that your transceiver (i.e. TX (and RX) part of it)
sends-out the various JT65B tones at about equal output levels. We've seen one transceiver
where the sent-out tones varied in RF output power a lot, most likely due to the ripple
of the incorporated SSB filter, causing problems at the receiver side ("bad reads" or
"no decodes" even if our signal was "good enough" off the moon).
So, watch out for this. The output power should not change for much more than 10 percent
related to the various JT65B tones sent out.
Antenna(s), etc.:
In order to perform 2m EME QSOs on a regular base and given the fact that you are
most likely *not* a "rare bird (call sign)", who is being called anyway by strong stations
any time and any frequency, you need to provide a minimum of 15 dbd of antenna gain
on 2m.
This amount of gain can be achieved e.g. by two horizontally stacked yagis with
about 4 to 5 meters of boom length. A single Yagi would require a boom length of about
10m in order to achieve about 15 dbd of gain.
VE7BQH provides a table in the Internet where performance data of various 2m antennas
can be looked up as a reference. Based on this data we selected the I0JXX 8-element
Yagi as it has a pretty good price-performance ratio for a 4.1m long antenna (12.1 dbd of
gain for a single Yagi). There are many other choices, though. YMMV.
If you mount 2 horizontal antennas side-by-side at the required stacking distance, make
sure that you have an insulated (non-metallic) mounting structure for the horizontal
mounting arm(s).
Also make sure, that none of the feeding lines gets into the radiation field of the antenna(s).
Before considering of upgrading to a horizontal 4x antenna system ("4x H" config),
you should consider upgrading to a 2x H/V (horizontally/vertically) switched array.
We've seen many times that during an EME session there are effects like "Faraday
Rotation" which basically turns the polarisation of an EME signal around by up to 90 degrees,
which results in a degradation of the received signal - if the polarization cannot be switched -
by 20 db or even more. Needless to say that under such circumstances the received EME
signal is completely lost. The bad news with Faraday is that this situation can last for hours!
So, being able to switch the polarisation of the used antennas by 90 degrees is a very
efficient way of getting around this polarization change issue vs. adding just another 2 or
so antennas of the same polarity and not being able to address the Faraday effect.
The "bad news" with Cross-Yagis (H/V-switchable):
H/V polarized yagis require substantial more hardware efforts, both for the combiner part
(double effort vs. H-only yagis), for the mounting structure (weight, insulated mounting
frame), as well as for the feeding line part and antenna relay switching.
In any case:
If you are out for "long distance" 2m EME communication, my recommendation would
be:
go for Cross-Yagis from the beginning!
Elevation:
If you are bound to "no elevation" (no vertical rotator available or no manual means of
adjusting the antenna elevation):
You'll have a very limited choice time-wise for EME operation as you'll be only able to run
EME QSOs up to a moon elevation of some 20 degrees. This may include a moon window
of operation of an hour+ or so at moon rise or set.
That requires that you need to have a free sight at the moon at moon rise or moon set in
order to direct your signals to the moon. You may additionally gain from a so-called "ground
gain" effect under these circumstances, which means a ca. 6db added gain at low moon height.
In any case: at low moon elevation angles there is an absolute need for "free sight" at
the moon at those times!
Feedline:
Don't fool-around with RG.58 etc. at 144 MHz as your antenna feed line!
Get the *best* (i.e. lowest attenuation) feedline you can get for feeding you EME
antenna and make your antenna feedline as short as possible!
This is especially true if you don't have an RX preamp close to the antenna!
Here are some data on 50 Ohm Coax feedlines as provided by "wikipedia".
The values indicate the cable loss for a 100m line length at 145 MHz:
RG58C/U: 17.8 db
RG213/U: 8.5 db
Aircell 7: 7.9 db
Aircom Plus: 4.5 db
Ecoflex 10: 4.8 db
Ecoflex 15: 3.4 db
Dont't be shy of investing into a "good" feeding line. It will definitely pay-off!
TX Power level:
For JT65B EME communication a TX power level of 300 Watts minimum needs to be
considered. As the TX periods are only 1 minute separated in JT65B, this amount
of output power needs to be delivered at a continuous duty level (like in FM operation).
Your antenna relay, TRX, Final Amp plus PSU's need to cope with this amount of power!
Sequencing receive/transmit turn-around:
Sequencing the change-over from receive to transmit and vice versa is a *very* important
subject. Not only for EME-operation but also for any kind of switch-over of receiving and
transmitting devices.
The purpose of "sequencing" is to avoid damage to the equipment/components caused by
RF energy hitting a device at a non-appropriate point in time.
In an EME kind of station environment the "sequencer" takes care of the proper switching
in the time domain of components like
Pre-amplifier for RX
Antenna relay
Power amplifier for TX
Transceiver
etc.
Please search the Internet for more details on this subject.
-...-
Enjoy 2m EME and hope to see you off the moon soon!
--... ...--
To be continued...
... to be continued
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