Re: [amsat-bb] AO40 Downlink, RG213 verses 1/2 inch

["hasan schiers" <schiers@xxxxxxxxxx>]

I have attached an AO40 spreadsheet in Excel format. If you enter the
information on the respective feedlines you will see EXACTLY what effect it
will have. (attachment was sent direct to you, this is a copy of the text
only)

In general:

The base sensitivity of your system is set at the first RF amplifier and any
coax in front of it. (no change to antenna)

You describe a DEM downconverter, so it looks like this:

Ant > Very short Coax run > DEM D/C > Long Coax Run RG-213 > Radio

Let's assume the DEM D/C has a NF of .9 dB and a gain of at least 25 dB.
Let's also assume there is 0.2 dB of coax loss between the antenna and the
input to the DEM.
(If the DEM is directly mounted to the feed, you can leave this out)
Then you have your RG-213 run.

Replacing the 213 with any other coax isn't going to change your system
sensitivity hardly at all. It's a waste of money. A complete waste. (If the
layout is as I have described).

I use LMR-400 between the output of my UEK-3000 (NF 0.6 dB, Gain 30 dB) and
the shack. About 80'. Changing it to even RG8/x will not significantly
affect system sensitivity. The D/C output freq is assumed to be 2m. If so,
the loss after the NF has been "established" by the front end of the D/C, is
inconsequential.

Play with the spreadsheet and you'll see what I mean.

Any loss in the feedline BEFORE the input to the DEM is added directly to
the NF of the DEM, thus making it more and more "deaf". Any coax added after
it, has very little effect....it serves as nothing but an "S-meter"
attenuator...it has no real effect on overall system sensitivity.

In fact, if you had a separate preamp at the antenna and put the DEM in your
shack, you could still tolerate quite a bit of loss (even at 2.4 gig), if
the preamp had a very low NF and high gain.

The most typical arrangements however are something like this:

Ant > .2 dB line loss coax  @ 2.4 gig > .7 dB NF/25 dB gain DownConveter> 2
dB of long run coax loss @ 2 meters > Radio NF 8 to 12 dB at a 2500 hz
bandwidth.

Assuming your antenna has at least 21 dB gain (after subtracting 3 dB if
linear instead of circular), the system described above will produce
downlink S/N ratios on the beacon of at least 20 dB. In other words, it is a
quite decent system.

Hope this helps. 73

p.s. The apparent jump in signals your S-meter will see by replacing a 2 dB
loss feedline with a .5 dB loss feedline is an illusion as far as system
sensitivity goes. Don't trust your S-meter...it means next to nothing as far
as system sensitivity. It's great for pointing the antenna, but NOT as
measure of sensitivity. You can put all the gain in the world in front of
that radio (after the preamp/downconverter at the antenna) and it won't help
at all. The S-meter will go way up, but your ability to copy weak signals
will NOT IMPROVE, and most likely will get WORSE.

...hasan, N0AN


>
> > Hello List,
> > I am running RG213 from my DEM downconverter to the shack,it is
> > 25 Meters long..I have the DEM 2401/144 at the dish feed point.
> > Is it worth the effort in laying down 1/2 inch Andrew hard line to my
> > 144Mhz rx. Will I notice a big jump in signals?
> >
> > Hope one of the experts out there can help..
> > Thanks in advance.
> > Etienne
> > ZS6Y
> >


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