No subject

The idea of links over water has been discussed previously on MW to 
link areas surrounding Port Phillip Bay.  I believe there is one such 
link functioning in the Williamstown area.

The following discussion covers technical aspects of these links, 
problems encountered, and how to get the best results, from people 
who have actually done this.

Clae.

/some snipping for brevity/

>Date: Sun, 25 May 2003 16:11:22 -0700
>From: Tim Pozar <pozar at lns.com>
>
>On Sun, May 25, 2003 at 07:06:18PM -0400, S Woodside wrote:
>>  On Saturday, May 24, 2003, at 11:45  PM, Ben Johns wrote:
>>  > Has anyone on the list done something similar? I'm interested to know
>>  > how
>>  > ocean/sea could affect a signal. The water is pretty calm most of the
>>  > time, sheltered by the islands/reefs, but could reflection/refraction
>>  > still have an effect?
>>  >
>>  I remember reading about a link over water, at a nature preserve I
>>  think over a lake. IIRC the polarization made a difference, don't
>>  remember which way was better though.
>
>You want to be vertically polarized to minimize the horizontal
>polarized signals from the water.
>
>Tim
>------------------------------
>
>Message: 3
>Date: Sun, 25 May 2003 13:47:10 +0700
>From: Stefan Probst <stefan.probst at opticom.v-nam.net>
>
>At 13:45 25.05.2003 +1000, you wrote:
>-------------------------
>  >Has anyone on the list done something similar?
>
>I was line-up engineer for a 4.7 GHz radio link over wet (wet rice fields)
>and water terrain.
>
>The problem is, that you are very likely to get reflection from the water
>and from air layers of different humidity above the water, caused by
>temperature differences. The result is, that you get at the receiving side
>not only the direct signal, but also the reflected one. The reflected one
>has to travel a longer distance and is therefore delayed, i.e. phase
>shifted, with the original at the receiving antenna. Depending on the
>distance, the phases will be in sync, i.e. the signal gets stronger. If it
>is 180ƒ out of sync, the both signals will be subtracted, i.e. get much
>weaker, or can even disappear. Since this is not a constant, and not a
>single reflection, the result is, that the signal strength fluctuates. The
>result is known as "fading".
>
>Solutions:
>- Very, very highly directional antennas that shield the reflected signal.
>    Usually only feasible over short links and high towers, i.e. if there is
>    a sufficient large angle between the direct and the reflected signal.
>- If you are lucky, you can chose a signal path that has a natural obstacle,
>    like a mountain, in such a way that it shields the reflected 
>signal from the
>    receiving antenna.
>- Frequency Diversity: Since the phase difference depends on the frequency,
>    there are good chances, that in case the resulting receive level on one
>frequency
>    is different from another frequency.
>    Means: The signal is transmitted simultaneously on two different
>frequencies.
>    When the bit error rate exceeds a certain level on the receiving side, the
>    receiver would quickly switch over and take the signal from the other
>    frequency.
>- Space Diversity: The signal is received by two antennas, that are mounted
>    with a certain distance above each other.
>    The reflected signal changes at the two antennas, it will sometimes add
>    "in phase", sometimes completely "out of phase", i.e. cancelling the direct
>    signal. By choosing an appropriate distance (there are some ways to
>determine
>    it), it is possible to minimize the likelihood, that there is a 
>cancellation
>    at both antennas at the same time. The receiver will choose the antenna
>    with the best signal. (Well, in fact there are two receivers and 
>a circuitry
>    determines which signal to choose).
>    Space diversity if often also used with wireless microphones, high quality
>    cordless phones, WiFi base stations.
>    As a modification (or in addition), you can save the second 
>receiver and the
>    switch-over circuitry by combining the signal of both antennas in such a
>way,
>    that the direct signal is "in phase".
>    This means usually to add some extra RF cable length to the lower antenna,
>    so that both cables are same long. By this, the direct signal adds always
>    correctly. The reflected signal will be different, and sometimes 
>"in phase",
>    sometimes "out of phase". Only if it is "out of phase" at both antennas
>    would the signal be completely lost. You should be able to increase the
>    effect by creating an array of several antennas above each other.
>In reality, Space and Frequency Diversion is often used at the same time.
>
>Good Luck!
>
>Stefan
>
>
>------------------------------
>
>Message: 4
>Date: Mon, 26 May 2003 10:39:34 +1000 (EST)
>From: "Ben Johns" <benj at marinanet.com.au>
>
>As Stefan Probst has recommended, I will be using two antennae on each end
>of a link for space diversity. Although I would like to know if having the
>two antennae set to different polarizations is of any benefit. Ie, have
>one set to horizontal and the other to vertical. Or should I just have
>them both set to vertical?
>
>--
>Regards,
>
>Ben Johns
>
>------------------------------
>
>Message: 5
>Date: Sun, 25 May 2003 18:22:05 -0700
>From: constantinos at shaw.ca
>
>We have a 23 mile link going over a strait of ocean.  As long as you 
>have enough power, your high enough so the fresnel zone is clear, 
>good LOS, you should be good to go.
>
>Cheers,
>
>Constantinos Tsakonas
>www.constantinos.ca
>
>------------------------------
>
>Message: 6
>Date: Mon, 26 May 2003 08:44:51 +0700
>From: Stefan Probst <stefan.probst at opticom.v-nam.net>
>
>At 10:39 26.05.2003 +1000, you wrote:
>-------------------------
>>As Stefan Probst has recommended, I will be using two antennae on each end
>>of a link for space diversity. Although I would like to know if having the
>>two antennae set to different polarizations is of any benefit. Ie, have
>>one set to horizontal and the other to vertical. Or should I just have
>>them both set to vertical?
>
>My tip:
>Try, try, try.
>Usually, it should both be vertical. But there are sometimes crazy effects.
>
>I would even start with one antenna. Maybe it is enough. I guess that in
>colder climate fading over water is not such a problem like in tropical and
>subtropical areas like here.
>If one antenna is not enough, try the second one and observe the difference.
>Maybe you find somewhere on the Web a way on how to determine the best
>height difference.
>
>And be prepared that things change over time, with seasonal changes of the
>weather.
>
>Also, the thing with the two "electrically same long" antenna cables sounds
>a bit easier than it is. Depending on the type of antenna mast etc., the
>lower antenna (where the mast is often thicker) might be some cm "closer"
>to the sending antenna. The result was, that I usually had to find the
>right additional cable length empirically, i.e. try different additions and
>observe the level of the received signal. This can be done of course only
>during daytimes with little fading....
>
>Good Luck!
>
>Stefan
>
>------------------------------
>
>Message: 8
>Date: Sun, 25 May 2003 18:17:45 -0700 (PDT)
>From: Michael Przybylski <mikep at core.ucsd.edu>
>
>Another standard trick I have heard of to minimize reflections with
>over-water links is positioning one antenna high, the other one low.
>
>Regards,
>Mike Przybylski
>

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