It also has a null on an overhead pass.
But that is quite insignificant. Looking at the gain plot of a 3/4 wave vertical (the 19.5" whip on 70cm) it is only down say 6 to 10 dB above 85 degrees. BUT the satellie is 10 dB or more closer when it is above 50 degrees which more than makes up for any loss of gain straight up. see plots on www.aprs.org/rotator1.html
But yes, there can be a complete fade when it is perfectly directly overhead (extremely rare). But since the satellite is only above 50 degrees only 5% of the time, it is only above 85 degrees only 1/8th of that 5%, or much less than 1% of all access times. Again, losing less than 1% of access time due to a possible less than 1% chance of a fade is nothing to be concerned about. Just 2 cents worth... Bob, Wb4APR
For mobile work on AO27, SO50, AO51 a 2 meter quarter wave whip is all you need...
Absolutely, For a 19.5" whip in center of roof:
- Has 5 dBi gain above 20 deg on 2m
- Has 7+ dBi gain above 30 deg on 70cm
- Is an omni
- does not sacrifice 3 dB for circular
- Above 25 deg, satellite is 6 to 10 dB closer!
- works the birds solid for the center of high passes
- Simplicity at its best!
Read about it: www.aprs.org/rotator1.html
Disadvantage: The only disadvantage is TIME. On the above web page you can also see that satellites spend 70% of their daily pass times below 25 degrees. BUT! For those best passes in the morning and the evening (or whenever) you can make solid contacts while mobile for about 5 minutes.
Also note, that you do NOT need any tracking program to predict passes. AO51 schdule repeats evry 5 days for example. Just write down the CENTER pass of the morning and evening for each day for 5 days. Update those 10 times on a small 3/5" card on the dash about once a month or so will predict all passes whenever you are mobile. There will be a pass 100 minutes earlier and 100 minutes later each day too. So you can predict all 6 passes a day from those same 10 times.
See how: www.aprs.org/MobileLEOtracking.html
Bob, WB4APR