[amsat-bb] Antenna separation on crossboom

Jean Marc Momple jean.marc.momple at gmail.com
Sat Dec 15 15:44:30 UTC 2018


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)
>>>> _______________________________________________
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>> _______________________________________________
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> _______________________________________________
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