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Coax question

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#21
75 ohm coax are actually better to use than 50 ohm. The voltage will be higher in the coax and have higher level above the noise floor.
That's only true if the antenna matches the impedance of the 75-ohm coax. Connecting 75-ohm coax to an antenna to a 50-ohm antenna isn't going to raise the voltage above the noise floor at all, unless you use an impedance matching transformer, or a preamp with a 50-ohm input and a 75-ohm output. Connecting directly, you lower the signal voltage somewhat because less of the signal from the antenna feeds into the coax, and more signal reflects back into the antenna.
 
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#22
That's only true if the antenna matches the impedance of the 75-ohm coax.
But you forget that the antenna are far from a perfect 50 ohm impedance match when we use it for broadband scanner use and a higher impedance coax are prefered.
But if you google it you'll find some results with charts and diagrams with impedance vs signal levels from senior engineers
and I believe professors that state that even a 50 ohm antenna performes better with a 75 ohm feedline when it comes to receiving small signals.

/Ubbe
 
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#25
Bob Grove, the former publisher of Monitoring Times magazine, has information regarding 50 ohm and 75 ohm coax in his 5-part All About Antennas series (http://www.monitoringtimes.com/antennabook.pdf). The 13 pages make for a very good read if you want to learn more about antennas, coax, impedance, preamps, splitters, coax connectors, etc.

Here is one excerpt from page 5 of the pdf file:

"Why have we chosen impedance standards
like 50 and 75 ohms for coax? For transmitters,
the best power-handling capability is at 77 ohms,
while the best voltage tolerance occurs below
30 ohms. 50 ohms is a good compromise and it
matches several standard antenna designs.

For receiving purposes, 75 ohms is optimum
for low coax losses, so it was adopted
by the cable TV industry. Conveniently, it also
matches several antenna designs."


Here is another excerpt from page 7 of the pdf file:

"Don’t let 70 ohm (instead of 50 ohm)
impedance throw you; you won’t hear the difference
for receiving, and the impedance mismatch
for a 50 ohm transmitter is only 1.4:1 which is
inconsequential, resulting in a loss of less than
0.2 dB, which is imperceptible."
 

JamesO

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#26
I use RG6 quad sheild for almost all my RX systems. It is cheap, easy to find, easy to run, small diameter and easy to terminal with connectors that cost pennies.

The length of my RX path runs are so long I would require 1 1/2" hard line and this would be like adding a bathroom in my attic to plumb all of this to my locations inside the house. I woudl also go broke and would have to worry about the large bend radius the deal with expansive and hard to install connectors.

I have decided to put my money at the base of the antenna in the way of filters and a good LNA and I have been very happy with the end result.

Currently have my feed split 8 ways and have no regrets.

Even if I had a shorter run and did not run a LAN, I probably would still use RG6.

Plenty of people will not agree and will do things very differently, that's fine. I know what I have that works very well and I have no current plans on making any major changes to the set up.
 
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