Should I use 50 or 75 ohm coax?
- Thread starter Bill2k
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The scanner's (any receiver's) input impedance should be the same at any frequency. That's a matter of design. If that input impedance changes with frequency then the scanner/receiver is poorly designed.
The characteristic impedance of coaxial cable has nothing to do with the losses at some particular frequency. That characteristic impedance also has nothing to do with the power handling ability of coaxial cable. Some 75 ohm cable can handle much more power than any 50 ohm cable. There's just not much market for that impedance cable with high power capability so you very seldom ever see any.
Part of any coaxial cable's losses is resistive in nature. Smaller center conductors mean higher resistive losses no matter what the cable's impedance.
Lot's of misconceptions in all this, most of which are fairly easy to 'think' through.
- 'Doc
The characteristic impedance of coaxial cable has nothing to do with the losses at some particular frequency. That characteristic impedance also has nothing to do with the power handling ability of coaxial cable. Some 75 ohm cable can handle much more power than any 50 ohm cable. There's just not much market for that impedance cable with high power capability so you very seldom ever see any.
Part of any coaxial cable's losses is resistive in nature. Smaller center conductors mean higher resistive losses no matter what the cable's impedance.
Lot's of misconceptions in all this, most of which are fairly easy to 'think' through.
- 'Doc
About all scanners specify 50 Ohms which should be what they do, plus minus a certain tolerance, across the entire spectrum they cover. For consumer grade items you can expect a tolerance around 10%.
Since you asked about *exactly* 50 Ohms l assumed you wanted something like "at which frequency the impedance is very close to 50 Ohms like in 1% or 0.1% tolerance". For that you would need to know the exact model and measure it or have the service manual & schematic diagram handy and try to compute it.
It's supposed to vary around 50 not 75. You can play here to see what different degrees of mismatch can give:
Return loss and mismatch loss Calculator
The practical view is that, if you bother with a big, external antenna, then you probably need every bit of gain, and it doesn't look very productive to (partially) annihilate the antenna's gain with an impedance mismatch. Especially since the cable is not such an expensive component as to justify cutting some corners.
With a severe mismatch you may find that the whole thing is not any better than using the original rubber duckie antenna. So why bother at all.
for the record
you will not see that much of a differnce in 75 vs 50 ohm cable for receive. yes there is a missmatch but in 35+ years of working with radios i have never seen enough differnce to matter.
if you are talking about transmit then yes , use 50 ohm cable.
When i was installing alert monitors in public schools i used RG-6 becase i had it by the spool.
and if you look at the spcs of com scope coax RG-6 and RG-11 has LESS signal loss at 50-800 MHZ then RR-8 or even RG-213.
short story, for an scanner it really doesnot matter.
you will not see that much of a differnce in 75 vs 50 ohm cable for receive. yes there is a missmatch but in 35+ years of working with radios i have never seen enough differnce to matter.
if you are talking about transmit then yes , use 50 ohm cable.
When i was installing alert monitors in public schools i used RG-6 becase i had it by the spool.
and if you look at the spcs of com scope coax RG-6 and RG-11 has LESS signal loss at 50-800 MHZ then RR-8 or even RG-213.
short story, for an scanner it really doesnot matter.
Let me offer you a more relevant link:
Antenna Fundamentals
Quote:
"Reciprocity Theorems
Many antenna properties are the same for both transmitting and receiving. It is often easier to calculate the gain of a transmitting antenna than the collecting area of a receiving antenna, and it is often easier to measure the receiving power pattern than to measure the transmitting power pattern of a large radio telescope. Thus this receiving/transmitting "reciprocity" greatly simplifies antenna calculations and measurements. Reciprocity can be understood via Maxwell's equations or by thermodynamic arguments."
So could you please elaborate on how impedance mismatch would affect transmission only?
We are discussing transmission line not antennas so that link isn't really relevant.There is an effect on receive but it is not significant.RG-6 has less loss than RG8X and RG11 has the same loss as RG8 @100MHz. Look at the loss chart at the bottom of this link:
Strong Signals - Co-ax Cable
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From that link I would select LMR-400 as:
- it has matched impedance
- it has the lowest loss when compared to the mentioned cables
- it has the proper size to be fitted with a standard N connector
It's very seldom just a matter of resistive losses versus losses from impedance mismatching, it's not that simple. 'Resistive' losses are dependent on the length of the feed line, and are more or less 'fixed'. But the impedance mismatches occur in more than just one place and can be both detrimental or beneficial. If a system is designed for one impedance, why upset that 'balance' by possibly saving a penny or two by using a different impedance.
The impedance mismatch that's the most common is between the feed line and antenna. If that antenna is used for more than one frequency, then there's a guarantied mismatch condition in almost every case. The only exception to that is if that antenna is 'tunable', can be adjusted for frequency shifts. If that antenna is remotely/conveniently adjustable, then the cost of the feed line isn't going to 'touch' the cost of that antenna. That impedance mismatch is certainly more noticeable for transmitters than for receivers, but it's still there.
I find that it's mechanically easier to deal with the typical 50 ohm coaxial cables rather than the 75 ohm cables. That deals with making connections, soldering to aluminum with RG-6 cable.
- 'Doc
The impedance mismatch that's the most common is between the feed line and antenna. If that antenna is used for more than one frequency, then there's a guarantied mismatch condition in almost every case. The only exception to that is if that antenna is 'tunable', can be adjusted for frequency shifts. If that antenna is remotely/conveniently adjustable, then the cost of the feed line isn't going to 'touch' the cost of that antenna. That impedance mismatch is certainly more noticeable for transmitters than for receivers, but it's still there.
I find that it's mechanically easier to deal with the typical 50 ohm coaxial cables rather than the 75 ohm cables. That deals with making connections, soldering to aluminum with RG-6 cable.
- 'Doc
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