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RE: Beginner birds



Hi Wayne,

>Wayne replies:
>
>Three things make Mode A easier for a beginner to use than Mode B (AO-7) 
>or Mode J (FO-29).
>
>First, Mode A has much less Doppler shift.  +/-600 Hz on the 10m downlink 
>and +/-3 kHz on the 2m uplink.  I agree that LEO Doppler is very 
>challenging on Mode B (AO-7) and Mode J (FO-29).  I was never able to make 
>FO-29 contacts until I got computer-controlled Doppler tuning.
>
>Second, Mode A transponders are typically non-inverting, which makes 
>tuning much more intuitive for a beginner.  Up is up.  Down is 
>down.  Always USB.  AO-7 (Mode B) and FO-29 both have inverting 
>transponders which add another layer of confusion for a beginner.  The 
>inverting transponder keeps the Doppler from being quite so ornery, but it 
>makes tuning much less intuitive.

Both points are well taken.  I hadn't considered the amount of Doppler 
shift as a factor. In my experience inverting didn't mess me up once I 
understood the math (excellent example in the ARRL Sat Handbook Figures 
4.1-4.3) but I could see how it might some people.

I haven't had the need for computer controlled tuning on FO-29 - but it 
did, as I mentioned, take practice.  And it was frustrating - I felt - well 
- dumb!  It still can be frustrating on FO-29 trying to keep up with the 
very fast doppler shift when it is directly overhead.  But even that has 
become second nature now.

>Third, Mode A is viable with omnidirectional antennas because the path 
>loss is low.  You can use a 10m inverted Vee and a 2m 1/4 wave ground 
>plane antenna.  No need to mess with any kind of antenna pointing (manual 
>or motorized).  Both antennas are extremely easy and cheap to homebrew.

I have to ask - why is the path loss lower in Mode A than in Mode 
B/J?  Most of it is in a vacuum anyway. I'm probably wrong, but wouldn't 
the opposite be true since you can get partial scatter in and refraction 
coming through the E-Layer on 10M that doesn't happen at the 2m/70cm?

I don't see how a 2m 1/4 wave ground plane uplink would work better than a 
beam,  given that the power is measured in EIRP and the power arriving at 
the satellite has to be the same.  If you are using a ground plane or 
colinear for transmitting you are throwing most of the energy away from the 
satellite, which means you need more power.  That's OK out in the country, 
but in high density living situations like a condo complex you easily run 
the risk of exceeding MPE levels. You are also subject to splatter and 
multipath.  Think entry level housing - studio apartments, dorm rooms, 2 
bedroom condos and townhouses.  (OK - I left out mobile homes - but they 
seem to be a dying breed.)

I think a small beam antenna pointed up into the sky that you can control, 
and that allows you to radiate less power is a better solution.

0.02 & 73,

Emily  

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