[OZAPRS] Antenna Patterns & HAB [was: VK2HAB]
Matthew Cook
vk5zm at bistre.net
Mon May 30 12:43:00 AEST 2016
Hi Norm,
New thread. Correct big can of worms which was partially deliberate...
I too can remember in the way back machine working out which direction that
AO13 spun using two linear antennas, provided that the spin direction and
cir-polarisation of the antenna was in alignment you were good.. however
try the other cir-polarisation and things were as you say not good,
horizontal or vertical induced amplitude modulation on the FM envelope,
which made life tuff depending on orbit and orientation of the antenna. A
pair of 2m and 70cm K1FO's at 45 degrees to each other with appropriate
switching allowed all sorts of interesting "experiments".
The biggest problem with HAB work is the payload will spin due to the wind,
it will usually wind up in one direction and then unwind the other, you've
already expressed the answer why cir-pol is not a good choice at the HAB
end. We have put tails on payloads to hold them into the wind for
photography, but these are cumbersome and have broken off due to violent
changes in direction. So it's best not to rely on these as LOS means we
have trouble getting back our gear. We fly payloads that are a little
larger than most.
Satellites can control their spin direction and do so for altitude and
directional control. They have it easy.
As Mark and I have both expressed we use compromised antennas that have a
very broad pattern (sacrificing gain over pattern) to reduce the effect of
the nulls. Both at the payload and receive site location. We are lucky in
that over such a short FSPL we have plenty of signal at both ends that we
can throw away 6dB or more and still receive the payload.
On the payloads we typically run an inverted 1/4 wave which should have a
donut shape in free space. However the ground plane is not ideal (just 4
radials) and changes the pattern sufficiently that the null towards the
ground is not as severe. We also make the antennas out of soft copper so
that when the payload comes down to earth, we do not hurt anything ie
people, livestock or payloads.
There is plenty of information and pictures on the Project Horus website of
our first 30 or so flights. We learnt amazing things for the first few
years, we continue to learn new things every flight. We hope to putting a
few more balloons into the air this year.
Tis a good topic to sink your brain into first thing on a Monday morning.
73
Matthew
VK5ZM
On 30 May 2016 at 11:16, Norm McMillan <vk3xci at gmail.com> wrote:
> Mathew VK5ZM said
> >" No circular polarised antennas are not the solution"
> Boy, there's a can of worms right there, just waiting to be opened.
> Sherman, crank up the Way Back Machine!
>
> In the days of AO-10, 13 etc a LOT of work went into circular, v oblique v
> linear poz. Seems for satellite work, when the squint angle was constantly
> shifting, matched sense circular poz at each end provided the best
> compromise, about 3db loss max from true circular to oblique to linear..
> Trouble is, the sense reversed as the bird went overhead, so switching poz
> sense was required at the ground station.
>
> Next best, circular to linear, 3db again and no need to switch sense.
> Unfortunately, the incoming sig was not always true circ. poz., often
> oblique, so 6db is a closer figure.
>
> Worst... linear to linear cross poz, around 20 to 30 db loss, and circular
> to circular crossed sense, 30 to 60 db loss.These are pretty much real
> world empirical figures... like I said... a LOT of work.
>
> Seems to me the best compromise would be linear at one end (balloon) and
> circular at the other. On the other hand if it aint broke, don't fix it :-)
>
> Radiation pattern is a different story.
>
> Maybe we need a new thread?
>
>
> norm
> vk3xci
>
> On Sun, May 29, 2016 at 11:27 PM, Matthew Cook <vk5zm at bistre.net> wrote:
>
>> I'd suggest you research polarisation of antennae a little more and then
>> think about that some more. No circular polarised antennas are not the
>> solution.
>>
>> 73
>>
>> Matthew
>> VK5ZM
>>
>> On 29 May 2016 at 05:59, Andrew Rich <vk4tec at tech-software.net> wrote:
>>
>>> Hang a yagi looking back at earth
>>>
>>> -----------------------------
>>> Sent from my iPhone
>>> Andrew Rich
>>> vk4tec at tech-software.net
>>> www.tech-software.net
>>> 0419 738 223
>>>
>>> On 26 May 2016, at 11:08, Matthew Cook <vk5zm at bistre.net> wrote:
>>>
>>> Hi Norm,
>>>
>>> That sounds about right with respect to range, height is might !
>>>
>>> Also don't forget that if you've got local traffic on a digi that this
>>> will tromp on a balloon payload even at altitude. It all comes down to
>>> sneaking through the gaps. Even with altitude you still have free space
>>> path loss and the capture effect to contend with a the digi receiver. It
>>> is always sensible to run open squelch with software DCD if your packet
>>> engine supports it. That means weak signals don't have to crack the mute
>>> of the radio first or get truncated if it drops too low.
>>>
>>> Another quirk with balloons is the antenna pattern at altitiude can have
>>> severe nulls in it, so being directly under a balloon can make it very
>>> difficult to decode (from bitter experience) when people 50km or more can
>>> copy it fine. YMMV.
>>>
>>> 73
>>>
>>> Matthew
>>> VK5ZM
>>>
>>>
>>>
>>>
>>> On 25 May 2016 at 17:44, Norm McMillan <vk3xci at gmail.com> wrote:
>>>
>>>> Thank you Mathew,
>>>>
>>>> 100,000 feet is 30480meters, range 625km.
>>>> VK3RBT to VK2HAB 510 km.
>>>>
>>>> Seems odd that VK3RBT picked it up but not VK3RBX, which is closer.
>>>> Better check the squelch/sensitivity on RBX I guess
>>>>
>>>> norm
>>>> vk3xci
>>>>
>>>> On Wed, May 25, 2016 at 4:55 PM, Matthew Cook <vk5zm at bistre.net> wrote:
>>>>
>>>>> Norm,
>>>>>
>>>>> Yes 100,000ft for a HAB is not uncommon. See if you can find an
>>>>> online radio horizon calculator and punch in 100,000ft. Take the position
>>>>> of the balloon when you received the posit and draw a circle around it from
>>>>> the horizon calculator. I'm fairly certain you'll find Bambil tanks is
>>>>> within this circle.
>>>>>
>>>>> We regularly have VK3 stations receive our HAB telemetry with the
>>>>> balloon 25kms or more in to the stratosphere. Height is might !
>>>>>
>>>>> 73
>>>>>
>>>>> Matthew
>>>>> VK5ZM
>>>>> AREG/Project Horus
>>>>>
>>>>> On 25 May 2016 at 16:12, Norm McMillan <vk3xci at gmail.com> wrote:
>>>>>
>>>>>> Copied a few (20 or so) packets from this event. the path is
>>>>>> interesting to say the least. Is there a digi not correctly identifying or
>>>>>> did VK3RBT at Bambil Tank REALLY receive the balloon direct.
>>>>>>
>>>>>>
>>>>>> 0:TNC->
>>>>>> VK2HAB-11>APT314,VK3RBT-1,VK2RBX-1,WIDE2*:!3349.O1S/14701.26E0161/003/A=100594/UpLift
>>>>>> Balloon Flights|$"!!!!|
>>>>>>
>>>>>> And did it really make it to 100594 feet?
>>>>>> And what does the |$"!!!!| at the end mean. I noticed the first
>>>>>> two characters change as the flight progressed
>>>>>>
>>>>>> Due to my inexperience with xastir I didn't log it, but I have a
>>>>>> screen shot!
>>>>>>
>>>>>>
>>>>>> norm
>>>>>> vk3xci
>>>>>>
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