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Re: AO40 downlink signal strength



I have no practical experience with this (yet!) but I can't help but
wonder...

The modified commercial S-band downconverters seem to generally be
around 2dB NF and 20dB gain.

Assuming the following (probably close to typical) station parameters:
no cable loss between downconverter and antenna (probably the area of
maximum variability.  Definitely a simplifying assumption)
100' RG-8 foam (2.3dB loss at 150MHz)
15dB RX NF (which seems generous to me)

The system NF of a modified commercial system will be
NF=F1+(F2-1)/G1+(F3-1)/G1G2

F1	2dB  	G1 20dB
F2	2.3dB	G2 -2.3dB
F3	15dB

NF=3.25dB
Noise Floor=-139.5dBm (approx.)

Now use a customized unit like the UEK3000 (no commercial endorsement
intended)

F1	0.7dB	G1 30dB
 
NF=0.9dB
Noise Floor=-147dBm (approx.)

So a 7.5dB improvement in receive ability.  This is a LOT!  And if 15dB
RX NF is indeed generous, the difference in Noise Floor between the
modified commercial system and the customized system will be even larger
(with these specific numbers, the relative contribution to the Noise
Floor of the RX NF is 1.25dB for the modified commericial system, and
0.2dB for the customized system).

Obviously I haven't considered external/galactic/sky noise.

Is this an accurate synopsis of the situation?  Is it possible that
"easy to get on mode-S downlink" is that suboptimum?

I'm not trying to ruffle feathers here, but these thoughts are what so
far have paralyzed my efforts to get mode S going.  

Estes Wayne-W10191 wrote:
> 
> When transmitting with 20W to a 15+15 CP yagi on 435 MHz, I got an easy-to-copy downlink signal.  On the Sunday pass my downlink was stronger than most of the other stations I heard.  I'm amazed how WEAK most other downlink signals were.  Most of the stations had much weaker downlink signals than mine, yet they were making contacts.  They must have much better receiving setups than mine, because I had a very difficult time copying most stations.
> 
> My receiving setup is a 60cm (2 foot) dish with an AIDC 3733 downconverter.  I didn't have to "see through trees" for the Saturday and Sunday passes.  The downlink is not near as strong as the old AO-13 V-band downlink, even though my S-band antenna has much more gain than my V-band antenna.
> 
> So I'm really curious how much more stronger downlink signals will be with the "final" transponder configuration.  Right now the downlink signal is marginal for the commonly available receiving equipment.  It's very hard NOT to be an alligator with the current configuration.
> 
> Other comments:
> 
> 1. The "transponder frequency" was 36 kHz lower than the expected 2401.225+435.800 MHz.  I had to turn my sub tune knob for a LONG time before I finally found my downlink.  Then I noted how far I tuned and put a new value into ITUNE.CFG.  Now my downlink is right on frequency every time I start up InstantTune.
> 
> 2. This weekend was my first time to copy the beacon with my AIDC 3733.  With a 60C PTC thermistor on the crystal, I found the L.O. frequency to be about 2277.98 MHz (20 kHz low).  After putting this rxconverter frequency into ITUNE.CFG, the beacon is audible every time I start up InstantTune.  Hopefully I can tweak the L.O. at work to be exactly 2278 MHz.  That would make it much easier to determine the "actual" frequency.
> 
> 3. Signals seemed a lot stronger on the Saturday pass than on the Sunday pass.  This is probably because the satellite set on the Sunday pass before getting to the strongest part of the orbit.  On Saturday I made SSB contacts, but on Sunday I could only make CW contacts.  Sunday night I worked two LZ stations on CW. (don't bother QSLing to LZ1JH, he won't respond).
> 
> 4. I sure would like to know the ALON/ALAT prediction for the next several days.  Are the high-gain antennas used now?
> 
> 5. I tried spotting my first AO40 contact to the local PacketCluster.   I entered the actual RX frequency of 2401.385 MHz and PacketCluster returned an amusing error message: "invalid frequency".
> 
> Wayne Estes W9AE
> Mundelein, IL, USA
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