The following discussion about the minimum EME antenna gain requirements took place in the Moon-Net reflector. I found the subject so interesting that I decided to put together all the messages in this page.
(See also the discussion on lower limit for EME)
On 27-Jan-1998 KO6RD wrote:
Currently, I am slowly getting the station together here
but need some help in understanding what the MINIMUM ANTENNA GAIN REQUIREMENTS for EME on
the bands. I know 144 MHz is @ 20 dBd but i believe the path loss goes up 9 dB on 432 and
another 9 db for 1296 MHz. Does this mean that I need to make this up in the
antennas, 29 dBd for 432 and 38 dBd for 1296 ??
I know that sky noise does go down as the frequency goes up but I don't know
how much that will contribute to the antenna gain requirements. I also know there are many
scenarios for this example but am looking at just the relative differences between the
bands.
On 27-Jan-1998 JA9BOH wrote:
Jeff,
If you use legal limit power on 144MHz , 20dBd ANT gain is sufficient to do the job.
I understand that MINIMUM GAIN ANTENNA produce your own echo .
Pse refer to Darrell Emerson's article in QST June-July 1996 [The Radio Sky] on the sky
noise above 144MHz.
You must add your receiver noise and the earth noise pickup from your antenna .
On 27-Jan-1998 KK7KA wrote:
I, too, am just starting to set up a station, and much of the
information below may be inaccurate, but I am posting this to the
list in the hopes of sparking some discussion.
Indeed the path loss increases, as you say, with frequency squared. But if that were the
only effect, you would only have to "make up" half with your antenna, because
the other half would be made up by the antenna at the other station. Following your
example, if you go from a 20 dBd antenna on 144 to a 24.5 dBd antenna on 432, the other
station does the same, and you keep the same transmit power, then the power received at
the remote station should be roughly unchanged.
But things aren't so rosy. Although a 12 WPM CW signal in theory needs only about 10
Hz bandwidth, the practical reception bandwidth for 144 MHz EME is about 100 Hz. The
spread is caused by a number of factors, including libration and other effects in the
signal path, instability of transmitter and receiver oscillators, doppler uncertainties,
etc. These effects are all proportional to frequency, and result in an additional
noise penalty of about 3 dB per octave.
Although hefty amplifiers for 432 MHz are quite common, as you move into the microwave
bands, it becomes more difficult and expensive to generate high power. Looking at
typical setups from 432 MHz to 10 GHz, a rough rule of thumb is that for a given cost,
power output is inversely proportional to frequency, another 3 dB per octave penalty.
So the bottom line is that you are correct: for each octave increase in frequency, you
need about 6 dB more antenna gain!
Now it's not quite that bad: sky noise is less on the higher bands (a small effect if
compared with quiet sky conditions on two meters), and some folks here will say that
ionospheric attenuation is also much less :)
Finally, it would seem that when your antenna gain is so high that you are only
illuminating/seeing a portion of the moon, further increases in gain do not help as much.
This is dependent on the irregular nature of the moon's surface. Has anyone
measured or analyzed this effect?
On 27-Jan-1998 KK7KA wrote:
A friend pointed out that my previous post neglected polarization.
On 144 MHz, signals are received with essentially random polarity, which is on average a 3
dB penalty. If you correct for this with crossed yagis, you are roughly doubling the
elements/weight/cost, which could have gone into 3 dB more gain, so you are only buying
consistency, not more average signal. At much higher frequencies, polarity
distortion is not a problem; where the crossover occurs seems to be a matter of debate.
But there is also another economic factor favoring the low bands: Yagis are effective
there, and are much cheaper than a dish of
comparable gain. Hard to put a number on this.
On 27-Jan-1998 SM5BSZ wrote:
The cross yagi is more favourable than that. The probability of full
gain is always 100%. The station with the cross yagi can measure the polarisation of the
other guy and make sure to transmit correctly.
For a signal at the detect limit, an antenna with 3dB more gain will allow detection 50%
of the time (until misalignment is 45 degrees).
Sorting out the cases:
25% A can hear B , B can hear A
25% A can hear B , B can not hear A
25% A can not hear B , B can hear A
25% A can not hear B , B can not hear A
Despite the same total number of elements the probability of QSO is down at 25% compared
to the situation when one of the stations uses a cross yagi. In real life it is much worse
than that because Faraday rotation changes with time. The 25% case when communication is
OK will often not last long enough for a QSO to take place.
The significant quantity is not the average signal, it is more a quantity like the average
of the product of the signal levels at A and B.
Clever operators can select times for EME skeds to make sure polarisation
alignment/disalignment happens simultaneously at both ends. Then the probability is 50%
that both stations hear each other and 50% that no one hears the other. Just by making
very long skeds the good situation will come sooner or later - and the peak level is 3dB
better than in the cross yagi case.
BUT like always, my arguments are biased with random contacts in mind....
On 27-Jan-1998 W2RS wrote:
In addition to the comments you'll be receiving from everyone else,
the amount of antenna gain that you need depends upon what you want to accomplish.
If your criterion for "minimum" is consistently hearing your own echoes, without
ground gain, most of the time, then approximately 20 dBd at 144 MHz is in the right
ballpark.
Many people, however, have done very well at this frequency with a lot less. Most
recently, W5UN and F/G8MBI worked each other with a single Yagi each. PA0JMV has had
hundreds of initials with two Yagis, approx. 17 dBd. Several years ago, CO2KK worked KB8RQ
running just 22 watts or so to two 5-el Yagis. Before local noise levels knocked me
off the air, I had 37 initials with 13 dBd and 150 W.
Results such as these at 144 MHz are made possible by using propagation mechanisms which
are not as useful at higher frequencies, such as ground gain (for the record, Dave and
Graham did not use ground gain), libration enhancement and ionospheric scintillation. For
further info about these and how to use them, see my articles in Feb '89 and Oct '90 QST,
as well as my paper in the 1992 CSVHFS Proceedings (published by ARRL).
On 27-Jan-1998 PA0JMV wrote:
Many operators haven proven that there is almost no minimum antenna
gain required to make EME contacts at all on 144 MHz. IMHO it really depends on "how
easy" you want to make your EME QSO's.
If you don't like to "work or struggle" for each QSO, your estimated 20dBd won't
even do that job easily; then you might
even need 23dBd or more...BUT: If you are smart and take advantage of many phenomena, like
low sky noise, ground gain, perigee, relative good G/T (by creating a
"clean" antenna pattern), polarity rotation, digital receiving techniques,
and above all : persistance and patience, you will be able to run dozens of EME
contacts with some 14-15 dBd. (@144 MHz)
I have made 1020 EME QSO's (502 initials) with my "old" 2 yagi's, only 17 dBd,
and since I relocated last summer to a new QTH, I have operated a temporary yagi
with12 dBd (2.5WL boom) and worked again some 30 stations already.......(10x 4 yagi-stns)
Even if you have a lot of gain, but Murphy decides for bad condx, you won't be able to
work much. (Murphy will be able to destroy a "more gain" antenna very easily
though.....) I would say: just give it a try by starting with one decent long yagi,
and NEVER give up reaching your target !!! (my 17dBd antenna gave own echo most of the
time, unless Faraday rotation was very bad)
On 28-Jan-1998 AL7EB wrote:
During the November 97 EME contest I thought I'd try to copy off the
moon with my satellite antenna. I was able to copy K5GW consistently over about 20
minutes with an antenna elevation from 15 to 20 degrees (thru the tree tops no less).
This was using the vertical half of a twelve year old cushcraft 20T (9 elements on
10 foot boom...even missing tenth director!) and feeding an ARR gasfet preamp into a DEMI
144/28 transvertor and FRG-100 receiver with 2.4 Khz bw. I'd guess signals were -6
to -8 db below noise; I usually can't detect a cw carrier below about -10 db with my
hearing (64 db loss in left ear and 35 db loss in right).
This discusion has interested me since I have been thinking about an new 144 array of two
crossed yagis for satellite that might also serve as a minimal eme system. Been
looking at using two C3I FO12 crossed yagis. Any opinions on these antennas; C3I
states they have 12.6 dbd (14.75 dbi) so two would be roughly 15.1 dbd (17.25 dbi)taking
in phase line loss. Running df9cy's pathloss program for eme predicts a -6 db snr with 100
hz bw working a station with 20 dbi and 1000 watts. Opinions?
(Note: I'm not showing any E-Mail
address here in order to avoid them from being collected by SpamBots. You can
possibly find the E-Mail addresses of the above OM at QRZ.COM.)
This discussion is still not closed. If you have any opinions or additional
related information you would like to be published here, just send
me an E-Mail
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