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Re: Best type of coax to use between preamps and antennas



----- Original Message -----
From: "Ronald Nutter" <rnutter@networkref.com>
To: <amsat-bb@AMSAT.Org>
Sent: Tuesday, July 01, 2008 2:49 AM
Subject: [amsat-bb] Best type of coax to use between preamps and antennas


> After problems I had on Field Day in trying to operate the satellites, I
> am looking to upgrade the coax I use between the preamps and the
> antennas.  I am using 9913FX from the IC-910H and the preamps.  I had
> been using RG Mini 8/U for the connection between the preamps and
> antennas.  Should I also use 9913FX between the preamps and the
> antennas?  The run between the radio and the preamp is 50 ft.The run
> from the preamp to the antennas is about 10 - 15 ft depending on where I
> set the antennas up.  I am concerned about how flexible the 9913FX would
> be in terms of the antennas rotating back and forth.  I was planning on
> ordering the cables pre-made from CableXperts since working with N
> connectors isnt my strong point and dont want to worry about my
> soldering and having a good connection on both ends of the cable.
>
> Any suggestions appreciated.
>
> Thanks,
> Ron
> KA4KYI

Hi Ron, KA4KYI

Any loss introduced by the coax cable between the antenna and the
preamplifier add directly to the noise figure NF of your preamplier alone.

Example:

Preamplifier noise figure NF= 0.5 dB

Attenuation of coax cable + both connectors = 1 dB

In this condition the overall noise figure of your preamplifier becames
1.5 dB i.e. like to have a bad preamplifier with NF= 1.5 dB directly
connected to the antenna connector without any cable.

What is the attenuation in dB of  10 to 15 ft of  RG Mini 8/U including
connectors ? Make your calculations by your self and add it to the specified
noise figure of your preamplifier.

By the way the attenuation of the coax cable between the output of your
preamplifier and the input of your IC-910H do not degrades significantly
the overall noise figure of your  receiving system because in general the
gain of a good preamplifier is in the order of G= 20 dB wich is enought to
compensate for the losses intoduced by 50 ft of 9913FX

More important the attenuation introduced by 50 ft of 9913FX reduces
directly the power generated by your TX and reaching  the antenna.

Example:
In general if the attenuation of the coax cable between TX and antenna is
3 dB and your TX is 100 watt then the power at the antenna connector
will be only 50 watt and this is no good.
By the way if the gain of your preamplifier is 20 dB and the attenuation
betwen preamplifier output and RX  input is 3 dB then instead to have a
preamplifier with G=20 dB it is like to have a preamplifier with G= 17 dB
but only from the receiving point of view this is not too bad because it
do not make a significant degradation of  the overall noise figure of your
receiving system.

For more information about the above matter read the following article:

"Receiver Noise Figure Sensitivity and Dynamic Range. What the numbar
mean" by James R.Fisk, W1DTY  Ham Radio Magazine october 1975

If you cannot find the above super article I have it scanned and I can send
a file to everybody is interested.

73" de

i8CVS Domenico









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