[amsat-bb] Re: Bob's Message
kl7uw at acsalaska.net
Wed Dec 19 07:44:43 PST 2007
At 11:12 PM 12/18/2007, Nate Duehr wrote:
>On Dec 17, 2007, at 10:12 PM, Edward Cole wrote:
> > Realities of digital EME:
>Great detail Ed, this will help me plan my setup here for trying this.
>Nate Duehr, WY0X
>nate at natetech.com
I hope it helps understand that eme no longer means huge antenna
arrays and lots of power, though it does require some good
construction with low noise receiving in mind.
Something I forgot to mention: EME signals are subject to
polarization shift when they pass thru the ionosphere. EME is linear
polarization so if your 2m satellite antenna is circular pol you
should consider rewiring so that either the horizontal or vertical
elements can be used. This a little more complication if you want to
retain circular pol for satellite use. My eme antennas have crossed
elements just like many satellite antennas but the vertical elements
are all connected thru a 4-way divider and brought to a T/R coax
relay at the tower-top where the preamp is located. The horizontal
elements are similarly connected in another 4-way divider. Thus I
have two feedlines coming to separate coax T/R relays. The Tx side
of each relays goes to a third coax relay that selects whether I use
H or V polarization. The preamp has a fourth smaller coax relay
(since it does not handle high RF power) so that the receiver can
independently chose H or V polarity. One could simplfy this with
less coax relays but then the TX and RX would always be the same polarity.
EME signals are not necessarily the same pol at the other station you
are working so often I may transmit in one polarity and receive in
the opposite polarity. Having them separately controlled makes this easier.
The point of telling you all this is that if you are running a QRP
eme station (less than four antennas) you want to be able to shift
polarization or your signal may not make it due polarization mismatch
losses as high as 20-dB. EME signals are almost always in the
noise. If you run a small one or two antenna station that is your
condition all the time on eme. JT65 makes this all possible by
enabling signal decoding of signals 10-dB weaker than what one can
copy on CW. The signals can often be not audible under the noise,
yet fully copyable with JT65 due to its superior narrow band digital
processing. JY65 occupies 4.3 Hz bandwidth.
Two cross-pol satellite antennas (KLM-22C or the M2 2mCP22) will make
a pretty good eme stations, today. If you can add a 400w PA to that
you will work hundreds of eme stations using JT65. My M2 2m-xpol-20
are quite similar to the 2mCP22 except not wired for circular pol.
You do not need the antennas mounted high on a tower for eme (same as
for satellites) since the moon is often high in the sky. AZ-EL
antenna rotation is needed to do much eme, but that is standard on
most satellite antennas setups. NOVA and many other satellite
trackig programs have the Moon listed so that is all you need to
point your antenna. Keeping on the moon requires moving the antennas
about every 20-30 minutes so no need for auto-tracking.
There is a lot of help for those that are interested in trying eme.
Ed - KL7UW
BP40IQ 50-MHz - 10-GHz www.kl7uw.com
144-EME: FT-847, mgf-1801, 4x-xpol-20, 185w
DUBUS Magazine USA Rep dubususa at hotmail.com
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