Jean, I have worked all the XW and CAS Birds regularly with both Linearly and RHC polarized antennas and using an SDR watching the entire passband, including both the PSK and CW beacons.
The results with linear vs. RHC are nowhere nearly as "clean" as the 3 dB you mention. Yes, it is certainly an advantage to have circular polarization on all these linear birds, but the signal losses due to polarization issues is often a very small part of the problem.
I have observed, routinely, linear birds dropping into the noise on RHC , that are 15 to 20 dB louder on linear vertical polarization. Then, within 20 or 30 seconds, the opposite is true. The only satellites where I see circular polarization "consistently" stronger than linear is with the weather sats, like NOAA-18, and I believe they are truly circular polarization on the Transmitter.
While the text books say Linear > RHC is max of 3 dB loss, there are so many other factors to consider that have dramatic effects on the rx signal strength as to make the theoretical 3 dB difference non-existent over short time intervals.
I have hours of passband recordings that show the following effects:
1. Uplink vs Downlink differential polarization 2. Shading of the satellite antennas by the body of the satellite. 3. Profound tumbling of the satellite (as in XW-2F). 4. Tropospheric Ducting at low incident angles preventing the signal arriving at ground level antennas
I am sure there are other factors. The one thing that is provably clear: the 3 dB loss of RHC to Linear is rarely evident on any of the LEOs that I use. It can much, much, much more or much much less, but the 3 dB theoretical is rarely evident on the LEOs.
I'm not saying RHC is undesirable on the rx end. It is VERY desirable based on my observations. It can show 10 to 15 dB increase over a linear antenna (at times). These times are NOT predictable and the duration is often less than 20 or 30 seconds, when the RHC antenna signal disappears when a vertically polarized antenna is quite strong.
Neither am I saying that this is all polarization...it clearly is not. What I am saying is don't expect to wipe out massive changes in rx or tx signal strength on the LEOs by using circular polarization. It can seem to help. It can seem to hurt. It can do absolutely nothing. All of which happen over very short (< 15 to 30 sec) intervals.
There are so many other confounding variables that any claims for practical on the ground signal improvement due only to Linear > RHC polarization changes just don't show up. They are masked by these other variables.
Why am I bothering to go through all this explanation of my own experience?
Because if someone goes out and spends $500 on RHC up and downlink antennas, with the expectation that their deep fades are going to limited to 3 dB against these linear birds....it is NOT going to happen....and they are going to be quite unhappy.
73, N0AN Hasan
On Tue, Sep 24, 2019 at 7:52 AM Jean Marc Momple via AMSAT-BB < [email protected]> wrote:
Rich,
Most of the birds (VHF/UHF) are using dipoles or monopoles which are linear polarisation. The reasons to use circular polarisation is to reduce deep fades and the convention for HAMs is to use RHCP, some also use antennas which can be switch between RHCP & LHCP but the difference is generally quite small except if the satellite has circular polarisation antennas.
To explain a bit more: if you use linear polarisation at your ground station, due to the spinning and/or tumbling of the bird the fades may be up to 30dB but if you use circular the fades will be about 3 dB max.
73
Jean Marc (3B8DU)
On Sep 24, 2019, at 6:01 AM, RG via AMSAT-BB [email protected] wrote:
Hi,I was thinking about getting the my eggbeater antenna,One thing I was
hesitant about it is that it does just RHP.Is it that most Leo's use RHP?I have no idea what satellites have RHP or LHP,If anyone can tell me where I can find that info that would be helpful.Please excuse my ignorance.ThanksRich
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