RadiationX
Member
Any one know?
What is better for a scanner 50ohm or 75 cable?
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Since scanner antennas are rated at 50 ohms, it would be best to use feedline (coax) that has the same exact impedance value. Using 75 ohm feedline, would cause a mismatch with the receiver; thus, reducing it's overall sensitivity, and there would be a great deal of signal loss. You can however, purchase specialized transformers that will make 75 ohm feedline balance out to 50 ohms, but they can be rather expensive. It would be more economical to purchase 50 ohm, low-loss feedline from manufacturers like Belden and LMR that are more efficient.
73's
Ron
73's
Ron
scansomd
Member
RadiationX said:
50 Ohm is better, although you probably won't notice the difference in 75 OHM.
If you have a choice, go with the 50 OHM cable.
Also keep in mind that all brands / types of coax are not created equal.
n5ims
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Additional information would yield a better answer, but I'll try.
Scanners generally are designed for 50 ohm coax. That said the quality of cable is much more important than the impedance. For example, high quality RG11 (75 ohm about 1/2" thick) will work vastly better than RG174 (50 ohm, about 1/8" thick).
My example may give you the impression that thickness of the cable is a good guide. Well, not really. RG8X and RG59 are nearly the same size, but RG8X generally works better than RG59, especially at lower frequencies. Speaking of frequencies, the desired frequency can greatly affect the choice of the correct cable. The loss at lower frequencies is much less than using the same cable at a higher frequency.
There are sights on the web that will provide you with the estimated loss for various types of coax at a given frequency. These can be quite helpful in choosing the correct cable for a specific application. However, don’t worry too much about small differences. Differences of 3db or less are generally not noticeable. I’d still choose the coax with the lower loss, assuming that the cost, size, and ease of installation are the same.
Now to answer your question more specifically, based on common antenna installations I’ve encountered. LMR-400 is good for medium length runs (100 foot or so). It is moderately expensive and fairly stiff (50 ohm, large solid center conductor and about 1/2" thick). For shorter runs (about 50 foot or so), I’d use a good grade of RG6 (quad shield would be preferred). It is pretty cheap and easy to maneuver (75 ohm, small solid conductor and about 1/4" thick).
Hope that this helps point you in the right direction.
Scanners generally are designed for 50 ohm coax. That said the quality of cable is much more important than the impedance. For example, high quality RG11 (75 ohm about 1/2" thick) will work vastly better than RG174 (50 ohm, about 1/8" thick).
My example may give you the impression that thickness of the cable is a good guide. Well, not really. RG8X and RG59 are nearly the same size, but RG8X generally works better than RG59, especially at lower frequencies. Speaking of frequencies, the desired frequency can greatly affect the choice of the correct cable. The loss at lower frequencies is much less than using the same cable at a higher frequency.
There are sights on the web that will provide you with the estimated loss for various types of coax at a given frequency. These can be quite helpful in choosing the correct cable for a specific application. However, don’t worry too much about small differences. Differences of 3db or less are generally not noticeable. I’d still choose the coax with the lower loss, assuming that the cost, size, and ease of installation are the same.
Now to answer your question more specifically, based on common antenna installations I’ve encountered. LMR-400 is good for medium length runs (100 foot or so). It is moderately expensive and fairly stiff (50 ohm, large solid center conductor and about 1/2" thick). For shorter runs (about 50 foot or so), I’d use a good grade of RG6 (quad shield would be preferred). It is pretty cheap and easy to maneuver (75 ohm, small solid conductor and about 1/4" thick).
Hope that this helps point you in the right direction.
K8PBX
Member
For what it's worth...RadiationX said:
I use high quality 75 Ohm RG-6 Quad-Shield coax for all three of my receive antennas and I've yet to encounter perceptible loss as a result.
Plenty of signal - even after feeding an 8-Output Electroline splitter (Model EDA-UG2802). Works like a champ for me.
w0fg
Member
For receiving purposes the mismatch between 50 and 75 ohms will not matter nearly as much as the dialectric loss. Satellite-grade RG6QS should work fine, though if you want to be really picky, you could go with Andrews Heliax or Davis Bury-Flex, both of which have less than 6db loss per 100 ft. length. If you want to transmit, on the other hand, the impedance matching becomes much more critical as the match affects the efficiency of the transmitter.
3db down is a half power point. Personally, if I have a choice, I would not run a cable that is eating half of my power ...n5ims said:
Illinistripes
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Rg6-rg8
n5ims....where do I find RG6 (quad shield would be preferred). It is pretty cheap and easy to maneuver (75 ohm, small solid conductor and about 1/4" thick)? I'm trying to find RG8 at Radio Shack and can't find in the stores or online. Thanks
n5ims....where do I find RG6 (quad shield would be preferred). It is pretty cheap and easy to maneuver (75 ohm, small solid conductor and about 1/4" thick)? I'm trying to find RG8 at Radio Shack and can't find in the stores or online. Thanks
N
N_Jay
Guest
crayon said:
I have no idea why people get so hung up with the fact that a 3dB loss is (OhMyGod) 1/2 your power gone.
3dB is 3dB, and 3dB is imperceivable to most receiver applications.
if 3dB was so friggin big, you would think the first step on your attenuator button would be a whole lot less than 20 db (OhMYGod) 99% or your power!
N
N_Jay
Guest
Illinistripes said:
RG-6 is better at high frequencies. It is designed as a low loss receiver or distribution cable for frequencies up to 1GHz. (over in some cases)
RG-8 was originally designed as a moderate power cable for frequencies below 500 MHz.
One other quick note. QS is irrelevant for receive systems unless you have very specific local noise issues.
w0fg
Member
RG6QS from RadioShack would be a whole bunch better than RG8 from RS. If you feel compelled for some reason to go with larger cable then at the very least go with something like Belden 9913 for VHF/UHF use from somewhere like Texas Towers. RS's RG8 is designed for CB use and, in my view, rather low quality. Their RG6QS satellite cable, on the other hand, is very good.
Because, omg, not everyone lives two blocks from a transmitter.N_Jay said:
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N_Jay
Guest
crayon said:Because, omg, not everyone lives two blocks from a transmitter.
Once again, it shows more about what you don't understand than what you do!:roll:
:lol: :twisted: :lol: :twisted: :lol:
I switched from 50 to 75 ohm good quality Belden or Commscope cable about 25 years ago and it has worked great. I use mostly RG11 now for lower loss at 700-800 Mhz.
Dave
Dave
OH TOSH! This is surely the place to be confused over cable!
The slight VSWR loss with 75 ohm cable means nothing compared to the comparative inherent losses in the cable designs themselves. In other words, compare the loss in RG-58U against RG-8U and RG-6U against RG-11U. You'll see it's about the same, then take the jump to hard line. There's a BIG difference there, the mismatch losses pale by comparison. That's why many hams scrounge 75 ohm CATV hard line spool ends ignoring the 1.5:1 SWR, the exceptionally low loss more than makes up for it.
Just for the fun of mentioning it, scrap CATV hard line can usually be had for free while enough Andrew Heliax (tm) needed to reach the antenna costs more than your scanner.
"Microsoft is a cross between The Borg and the Ferengi. Unfortunately they use the Borg to do their marketing and the Ferengi to do their programming. -- Simon Slavin"
Obviously Mr. Slavin isn't a Trekkie, it's the other way around.
The slight VSWR loss with 75 ohm cable means nothing compared to the comparative inherent losses in the cable designs themselves. In other words, compare the loss in RG-58U against RG-8U and RG-6U against RG-11U. You'll see it's about the same, then take the jump to hard line. There's a BIG difference there, the mismatch losses pale by comparison. That's why many hams scrounge 75 ohm CATV hard line spool ends ignoring the 1.5:1 SWR, the exceptionally low loss more than makes up for it.
Just for the fun of mentioning it, scrap CATV hard line can usually be had for free while enough Andrew Heliax (tm) needed to reach the antenna costs more than your scanner.
"Microsoft is a cross between The Borg and the Ferengi. Unfortunately they use the Borg to do their marketing and the Ferengi to do their programming. -- Simon Slavin"
Obviously Mr. Slavin isn't a Trekkie, it's the other way around.
I suppose that it would make a difference if you are into DX or not.N_Jay said:
Todays word, kids, is: Sensitivity.
That being said, if you have a cable run that is clobbering your signal, the radio, no matter how sensitivity it has, is not going to hear it.monitoringtimes.com said:
Basic math, N_Jay. If 3db drops the signal below a radio sensitivity threshold for any particular band, then yeah, it going to make difference.
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N_Jay
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crayon said:I suppose that it would make a difference if you are into DX or not.
Todays word, kids, is: Sensitivity.That being said, if you have a cable run that is clobbering your signal, the radio, no matter how sensitivity it has, is not going to hear it.
Basic math, N_Jay.
First, I doubt that anyone asking such a general question and referencing a scanner as the receiver is into DX.
Second, I did provide the math, 3 dB is 1/2 the POWER and is basically irrelevant to an FM receiver.
Third, as I said, I doubt that many scanner listeners can tell the difference between two signals 3dB apart.
You seem to be of the opinion that the sensitivity is a cliff. Not so, you will find a 3 to 6 dB difference between radios on any system and even between channels in a radio.
If it sounds like crap, 3 dB more signal will NOT make it clear.
If it sounds good, 3 dB less signal will not make it sound like crap.
When was the last time you played with a step attenuator?
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No, I provided that. You restated it.N_Jay said:
I guess where I am confused at it is Volt's relationship to Power:
V squared / R = P
V x I = P
It does not matter if you are talking power or microvolts, mess with anyone of them and the signal degrades. Degrade it enough and you have a deaf radio, no?
N
N_Jay
Guest
crayon said:
Very true, but equally true (the point I am making), that if you degrade just a little you can not tell the difference.
3dB is "just a little" and is not "enough" to cause noticeable degradation.
People get hung up on the 1/2 thing because it sounds like a lot.
In terms of receive signal levels it is almost NOTHING.
Cummon guys, when are you going to stop arguing with meat head and do something constructive? (;->)
Steve, that's a pretty neat avatar! The MAD kid Alfred E. with a Weller 8200 puts a whole new slant on the wonderful world of geekdom. That gives me an idea, who said they don't have cowboys in Shropshire UK?
Steve, that's a pretty neat avatar! The MAD kid Alfred E. with a Weller 8200 puts a whole new slant on the wonderful world of geekdom. That gives me an idea, who said they don't have cowboys in Shropshire UK?
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