[amsat-bb] Antenna separation on crossboom

Sat Dec 15 16:09:40 UTC 2018

I would add to this that there is one workhorse satellite on orbit that 
is circular and changes with location.  In the northern hemisphere FO-29 
is RHCP when to the south of the station and LHCP when to the north.  It 
makes a very large difference.

In addition, while cubesats of the 1U to 3U size typically use a whip 
(linear) because there isn't real estate for anything circular, for 6U 
and larger a canted turnstile is a better choice and is circular.  In 
that case they are typically wired for RHCP when the antenna ends faces 
earth but if it tumbles or flips the sense will change.  When facing 
away it will be LHCP to the ground station.

This question has been rattling around for at least since the 1990 
AMSAT-NA microsats (which were canted turnstiles on the UHF downlink).  
For those satellites the downlink was RCHP in the northern hemisphere 
and LCHP in the southern, except for LO-19 which was reversed.  I wrote 
a paper on this for the 1991 AMSAT Space Sympoisum (or it might have 
been 1992).  So I tend to suggest that folks just getting started with a 
hand held antenna a linear antenna is fine because they are a bit 
lighter and less cumbersome to handle, and less expensive.  But if you 
are building a fixed "base" station I still suggest including sense 
switching. It will make a large difference with FO29 and you will be 
ready for the next one that uses circular.  Given there are probably 100 
in various stages of design and construction and rides are plentiful it 
seems worth the investment.

Jim

Jim White, WD0E
jim at coloradosatellite.com
On 12/15/2018 8:44 AM, Jean Marc Momple wrote:
> Bob,
>
> So that all may fully understand:
>
> Most LEO’s use monopoles or dipoles (on VHF/UHF) which are actually linear and not circular polarization, but the birds are spinning and tumbling therefore the polarization can be H, V or any angle. If you use H or V pole at the ground station the fading may be as high at 30dB therefore it is recommended to use circular polarization at the ground station as maximum fade would be around 3 dB whatever the satellite polarization may be.
>
> I suggest that you use RHCP without polarisation switch, except if you wish to spend some money to experiment with same. I have the possibility to switch polarization but it is not used at all.
>
> Hope this help.
>
> 73
>
>
> Jean Marc (3B8DU)
>
>> On Dec 15, 2018, at 1:40 AM, Mark D. Johns <mjohns+K0JM at luther.edu> wrote:
>>
>> On Fri, Dec 14, 2018 at 10:58 AM Bob Hammond <propgrinder at gmail.com>
>> wrote:
>>
>>>> Do you use polarity switches on your yagis?  If you do, is it worth
>> the effort?
>>
>> No and no.
>>
>> I had polarity switching on long yagis back in AO-10 & AO-13 days and it
>> was helpful. But for the LEO satellites we have now, it's rare to have a
>> fade so deep that one can't copy.
>> --
>> Mark D. Johns
>> KØJM / MØGZO / ex-9H3DJ / ex-KØMDJ
>> Brooklyn Park, MN USA   EN35hd
>> -----------------------------------------------
>> "Heaven goes by favor; if it went by merit,
>>    you would stay out and your dog would go in."
>>     ---Mark Twain
>>
>>
>>
>> Greg,
>>>
>>> 73s,
>>>
>>> Bob
>>> W7OTJ
>>>
>>> On Mon, Nov 26, 2018 at 10:08 AM Greg D <ko6th.greg at gmail.com> wrote:
>>>
>>>> That's pretty much what I did, though with different antennas.  Also put
>>>> the AO-40 dish on the other side of the rotor from the 23cm helix, so it
>>>> went 70cm, 2.4gig, rotor, 23cm, 2m.   Put things as far away from each
>>>> other as you can, without running out of crossboom.
>>>>
>>>> I was very concerned about having the 70cm beam (uplink) so close -
>>>> inches - to the 2.4 ghz dish (downlink), but it was fine.  Good input
>>>> filtering in the 2.4 ghz Khune preamp, I guess.  More recently, Mode L/v
>>>> on AO-92 seems ok too, with the 23cm antenna near the 2m, but I have a
>>>> lot less power on 23cm than I did with AO-40 on 70cm.
>>>>
>>>> Greg  KO6TH
>>>>
>>>>
>>>> Bob Hammond wrote:
>>>>> I'm putting the following antennas on an 8-foot fiberglass crossboom:
>>>>>
>>>>> M2 2MCP8A, 143-148 MHz C{
>>>>> Gulf Alpha 70cm 8x8 CP
>>>>> WiMo Helix 23
>>>>>
>>>>> I'm not sure how to calculate the proper spacing on the crossboom.  I
>>>>> assume the 2m antenna should go on one end and the 70cm antenna on the
>>>>> opposite end.  Can I mount the 23cm helical antenna very near the
>>> rotator
>>>>> without harm since it has the solid reflector behind the helix?
>>>>>
>>>>> Bob
>>>>> W7OTJ
>>>>> (was WL7CQX)
>>>>> _______________________________________________
>>>>> Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available
>>>>> to all interested persons worldwide without requiring membership.
>>>> Opinions expressed
>>>>> are solely those of the author, and do not reflect the official views
>>> of
>>>> AMSAT-NA.
>>>>> Not an AMSAT-NA member? Join now to support the amateur satellite
>>>> program!
>>>>> Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
>>>>
>>> _______________________________________________
>>> Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available
>>> to all interested persons worldwide without requiring membership. Opinions
>>> expressed
>>> are solely those of the author, and do not reflect the official views of
>>> AMSAT-NA.
>>> Not an AMSAT-NA member? Join now to support the amateur satellite program!
>>> Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
>>>
>> _______________________________________________
>> Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available
>> to all interested persons worldwide without requiring membership. Opinions expressed
>> are solely those of the author, and do not reflect the official views of AMSAT-NA.
>> Not an AMSAT-NA member? Join now to support the amateur satellite program!
>> Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
> _______________________________________________
> Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available
> to all interested persons worldwide without requiring membership. Opinions expressed
> are solely those of the author, and do not reflect the official views of AMSAT-NA.
> Not an AMSAT-NA member? Join now to support the amateur satellite program!
> Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb