----- Original Message ----- From: "Ronald Nutter" [email protected] To: [email protected] 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