[amsat-bb] Re: HF Satellite Relay
Joe
nss at mwt.net
Wed Jun 30 11:57:48 PDT 2010
But how would you detect a doppler shift of something that lasts less
than a second?
Joe WB9SBD
The Original Rolling Ball Clock
Idle Tyme
Idle-Tyme.com
http://www.idle-tyme.com
On 6/30/2010 12:55 PM, Tony wrote:
> On 6/30/2010 10:35 AM, Joe wrote:
>
>> I agree,
>>
>> It's not the meteor or even the ionized portion near the head that you
>> bounce signals off of, it's the inonized trail that it leaves behind.
>> and that can be many many miles long.
>>
>>
> Joe - It would seem that the plasma cloud in front of the meteor would
> be moving at the same velocity as the meteor itself and would cause the
> fast Doppler shift we see prior to the trail formation.
>
> Tony -K2MO
>
>
>
>
>
>> Now about reflecting off of odd shaped things. A buddy an myself used
>> to in the late 70's have fun on 2 meters just playing around. we were
>> too far apart for direct communication, I was in the northwest suburbs
>> of Chicago, he was down in central indiana.
>>
>> We would point our 4 ele beams at eachother and run like a meteor
>> scatter transmission sequesnce. and would make a qso. it was flaky and
>> wide variations in signal strength. But what did we use?
>>
>> It was all the Jets coming in to land at Ohare! between us was the main
>> route for all the jets to com in on. The route killed TV all the time
>> by making signals in phase and then out then in then out, so we thought
>> if it's strong enough reflection to cancel out a direct megawayy
>> signal, hmmm. so we tried and it worked and worked faily well. each
>> "Opening" lasted between 10 and 15 seconds. and would go most times from
>> s zero to peg the meter.
>>
>> was fun
>> and thats an odd shaped reflector. that for me was ohhh 60 miles
>> away, and for him close to 100 miles.
>>
>> Joe WB9SBD
>>
>> The Original Rolling Ball Clock
>> Idle Tyme
>> Idle-Tyme.com
>> http://www.idle-tyme.com
>>
>> On 6/30/2010 3:31 AM, Tony wrote:
>>
>>
>>> Dominico,
>>>
>>> Your path loss calculations were very interesting. At first glance, I
>>> assumed it would be possible to hear 28MHz echoes off of the 10M sphere
>>> since smaller targets such as the ionized head of a meteor can reflect
>>> such signals with enough signal power to be heard.
>>>
>>> But, I think this is one of those apples to oranges comparisons since:
>>> a. the ionization around the front of the meteor head is responsible for
>>> signal reflection, not the rock itself. And b. the diameter of the
>>> ionized atmosphere in front of the meteor may be a lot larger than one
>>> would imagine.
>>>
>>> The 10M sphere brought another question to mind -- Mike Wantanabe,
>>> JH1KRC, managed to hear his own EME echoes on the 21MHz using a KW and a
>>> 6 element Yagi. Details and recordings are on his website (see below). I
>>> was wondering how the path loss calculations compare with his EME results.
>>>
>>> http://eme.dokidoki.ne.jp/sound/jh1krc/index.html
>>>
>>> Thanks Dominico...
>>>
>>> Tony -K2MO
>>>
>>>
>>> On 6/29/2010 9:52 PM, i8cvs wrote:
>>>
>>>
>>>
>>>> ----- Original Message -----
>>>> From: "Robert Bruninga"<bruninga at usna.edu>
>>>> To:<amsat-bb at amsat.org>
>>>> Sent: Monday, June 28, 2010 7:44 PM
>>>> Subject: [amsat-bb] HF Satellite Relay
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>> Heard today of a Passive HF relay satellite being proposed.
>>>>> Wondered if Hams could relay off of it.
>>>>>
>>>>> It's a 10m diameter sphere.
>>>>> I assumed a 10m signal and 1000 Watts
>>>>> And antenna gains at both ends of 10 dB.
>>>>> Unless I made a dumb error, it looks impossible?
>>>>> I get a received signal of -170 dBm
>>>>> Compared to a good HF receiver of -122 dBm
>>>>> So its 48 dB down in the noise.
>>>>> Going to narrow band, could improve things, but the Doppler of
>>>>> +/- 600 Hz would make that difficult.
>>>>>
>>>>> Anyway, if someone else wants to double check the link budget
>>>>> using the radar range equation, go for it.
>>>>>
>>>>> The beauty of this system is that it is perfectly spherical, so
>>>>> the reflection coefficient would be constant within 1 dB. That
>>>>> is the advantage over trying to use the ISS or other large
>>>>> rocket body... They vary by 20 dB making communication by
>>>>> reflection impossible.
>>>>>
>>>>> Oh, and it would be in space for 30 years or more. So with
>>>>> something that reliable, it would be worth developing an amateur
>>>>> capability to use it.
>>>>> It is not designed for comms, but as a calibration sphere for
>>>>> over the horizon radars that have LOTS more power and LOTS more
>>>>> gain than we do.
>>>>>
>>>>> Bob, Wb4APR
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>> Hi Bob, WB4APR
>>>>
>>>> I have assumed that the altitude of the Passive HF relay satellite over the
>>>> earth is 1500 km and as we know the diameter of the sphere is 10 meters.
>>>> Also I assumed that the reflectivity coefficient of the sphere is 50%
>>>>
>>>> The 28 MHz Round Trip Isotropic Attenuation using the concept of Radar
>>>> Equation is as follows:
>>>>
>>>> Pt x Gt x Ar x Sigma
>>>> Pr = ------------------------------
>>>> (4 x 3.14 x R^2)^2
>>>>
>>>> where :
>>>>
>>>> Pr = received power
>>>>
>>>> Pt = transmitted power = 1watt
>>>>
>>>> Gt = gain of a 28 MHz isotropic antenna = 1 in power ratio
>>>>
>>>> Ar = Aperture of the isotropic antenna at 28 MHz in square meters.
>>>>
>>>> R = Radius of a sphere wich distance from the earth is 1500^3 i.e
>>>> the distance from the Passive HF relay satellite and the earth
>>>> expressed in meters.
>>>>
>>>> Sigma = Surface of the target in square meters i.e. of the Passive
>>>> HF relay satellite as seen as a radar target disc multiplied
>>>> by the reflectivity coefficient of 50%
>>>>
>>>> Computing:
>>>>
>>>> / 2 2
>>>> /\ 10.71
>>>> Ar = ---------- = ----------- = 9.13 square meters
>>>> 4 x 3,14 4 x 3,14
>>>>
>>>>
>>>> Sigma = 5^2 x 3.14 x 0.5 = 39.2 square meters
>>>>
>>>>
>>>>
>>>> 1 x 1 x 9.13 x 39.2
>>>> Pr = --------------------------------------- = 4.47 ^ -25 watt
>>>> (4 x 3.14 x 1500000^2) ^2
>>>>
>>>>
>>>> 1
>>>> Round trip attenuation = 10 log --------------- = 243.5 dB
>>>> 4.47^ -25
>>>>
>>>> Link budged calculation:
>>>>
>>>> Assuming that we are using a good HF receiver with a NF= 8 dB
>>>> equivalent to 1539 kelvin we must consider in addition that the receiver
>>>> sensitivity is limited by the external available noise power.For quiet,rural
>>>> locations the galactic noise is the limiting factor and at 28 MHz the noise
>>>> temperature is around 29.000 kelvin so that reducing the Noise Figure
>>>> belove 8 dB at 28 MHz do not improve too much the S/N ratio.
>>>>
>>>> With the above data the noise floor of this receiver for SSB into a
>>>> bandwidth of 2500 Hz can be calculated as follows:
>>>>
>>>> Noise Floor = KTB = 1.38 x 10^-23 ( 1539 + 29.000 ) x 2500 = - 151dBW
>>>> or - 121 dBm
>>>>
>>>> TX power 1000 watt.............................+30 dBW
>>>> TX Antenna gain....................................+10 dBi
>>>> -----------
>>>> Transmitted EIRP .................................+40 dBW
>>>> Round trip attenuation 1500 km..........- 243.5 dB
>>>> -----------
>>>> Received power Pr on isotropic
>>>> antenna on the earth ..............................-203.5 dBW
>>>> RX antenna gain....................................+ 10 dB
>>>> -----------
>>>> Available power at RX input............... - 193.5 dBW
>>>> RX noise floor...................................... - 151 dBW
>>>> -----------
>>>> Signal received with a S/N ratio.......... - 42.5 dB
>>>>
>>>>
>>>> So according with Bob calculations the signal is 42.5 dB under the noise and
>>>> so it is not detectable.
>>>>
>>>> Best 73" de
>>>>
>>>> i8CVS Domenico
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
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>>>>
>>>>
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