ScannerDude244
Member
Would their be a big difference between 50ft r6 with F to PL259 adapters to 50ft rg8x with no F to PL259 adapters for low band?
RG8x and low band
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w2xq
Mentor
If you mean low like 30-50 MHz, at <=50 feet I wouldn't expect you to notice any difference in reception between the coaxial cable versions. HTH.
W2NJS
Member
Not enough of a difference to worry about, and probably hard to even measure if you tried to do so.
The RG8X with no adapters would theoretically have the edge but, again, the differences between the
two examples you give would be minimal.
The RG8X with no adapters would theoretically have the edge but, again, the differences between the
two examples you give would be minimal.
RG-6 is a 75 ohm TV feedline where the RG-8X is a 50 ohm radio feedline. You would be better served using the RG-8X since the impedence is 50 ohms matching your receiver. (Unless you use a balun)
ScannerDude244
Member
I did do the coax loss calculator didn't see much difference between the two, but I would like to do away with the adapters anyway!
Thanks guys!
Thanks guys!
N1BHH
Member
Impedance differences are hardly noticed by the ear. It's all a myth that you have to supply 50 ohm coax for a 50 ohm antenna. The majority of all antennas in use today by police and fire agencies and listeners alike are nominally 50 ohms, the actual feed point impedance could actually be anywhere between 40 and 90 ohms depending upon design. RG-8X does very well on low band as well as high band. And adapters have very minimum insertion loss, so don't sweat it.
mpddigital
Member
50 ohm vs 75 ohm
The choice of transmission line should be based on the equipment you are using. It involves price and performance trade offs. For you a Times Microwave LMR-400 or LMR-240 cable run might be complete overkill and RG-6 and RG-8x may work fine. Cable transmission loss is based on many factors, one of which is the case that all things being equal transmission loss is cut in half as cable size doubles. Since RG-6 is much thicker than RG-8x this will sometimes offset any impedance mismatch. There is also less of an issue in RX only systems. So in the case of your particular use there may be less loss or no difference in loss between 6 and 8x at the specific frequencies you are operating on.
YMMV
Um, nope. There are a lot of myths in our world but I'm afraid this isn't one. Not going to debate it here but for every system there are designed operating parameters. This does not mean you can't USE mismatched feed line or antennas it just means there is a cost in doing so. Depending on your set up the difference between RG-6 and RG-8x may be negligible in your specific case but it is there.
The choice of transmission line should be based on the equipment you are using. It involves price and performance trade offs. For you a Times Microwave LMR-400 or LMR-240 cable run might be complete overkill and RG-6 and RG-8x may work fine. Cable transmission loss is based on many factors, one of which is the case that all things being equal transmission loss is cut in half as cable size doubles. Since RG-6 is much thicker than RG-8x this will sometimes offset any impedance mismatch. There is also less of an issue in RX only systems. So in the case of your particular use there may be less loss or no difference in loss between 6 and 8x at the specific frequencies you are operating on.
YMMV
Don't sweat the difference on low band for your setup.
Re 50/75 ohm - one thing NOBODY seems to test when you want to get picky about it, is if the manufacturer is just ball-parking their 50 ohm specification for the scanner. Is it REALLY a 50 ohm impedance across the entire range of 30-900 mhz or so? Or does it swing or have dips and peaks from 40 - 90 ohms, and 50 ohms is just spec'ed to be the nominal impedance? For transmitters, the 50 ohm spec is likely to be more accurate. But for a receiver/scanner manufacturer, is it going to be that important for the average consumer without lab equipment?
Thus, without knowing if the actual impedance of the scanner's antenna input jack is stable across the entire spectrum to be 50 ohms, we may be making some false conclusions.
Re 50/75 ohm - one thing NOBODY seems to test when you want to get picky about it, is if the manufacturer is just ball-parking their 50 ohm specification for the scanner. Is it REALLY a 50 ohm impedance across the entire range of 30-900 mhz or so? Or does it swing or have dips and peaks from 40 - 90 ohms, and 50 ohms is just spec'ed to be the nominal impedance? For transmitters, the 50 ohm spec is likely to be more accurate. But for a receiver/scanner manufacturer, is it going to be that important for the average consumer without lab equipment?
Thus, without knowing if the actual impedance of the scanner's antenna input jack is stable across the entire spectrum to be 50 ohms, we may be making some false conclusions.
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Gotta agree with hertzian. If the OP said he was connecting a TRANSMITTER... well, that's a whole 'nother story. But a scanner receiver? 50 or 75 ohm.... use what ya want. Mox nix.
Gotta agree with hertzian. If the OP said he was connecting a TRANSMITTER... well, that's a whole 'nother story. But a scanner receiver? 50 or 75 ohm.... use what ya want. Mox nix.
jhooten
Member
mpddigital,
What is the radiation resistance or a 1/2 wave dipole?
A 1/4 wave ground plane?
What is the radiation resistance or a 1/2 wave dipole?
A 1/4 wave ground plane?
Yikes - I have to jump in.. got to be careful we don't mix terms between mere impedance and radiation resistance:
Small antennas and radiation resistance
The classic folded monopole is the grand-daddy of misused / miscalculated terms here.
Small antennas and radiation resistance
The classic folded monopole is the grand-daddy of misused / miscalculated terms here.
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There's a big difference between radiation resistance and input impedance, they are not the same things at all. Careful with them.
The typical input impedance of any antenna can be manipulated to some degree by how the antenna is constructed and mounted. In general, a 1/4 wave has an input of something between 20 and 40 ohms, a 1/2 wave between 60 and 80 ohms. But that varies to a large degree according to the relationship between that antenna's 'elements'/'halves'. Either can be adjusted in shape to get you very close to a 50 ohm input, that's quite common.
- 'Doc
(And just because most of the antennas I deal with are connected to transmitters, I tend to favor 50 ohm cable. Most of those antennas are also resonant on the frequency(s) of use, which is nice, but not absolutely necessary for just receiving.)
The typical input impedance of any antenna can be manipulated to some degree by how the antenna is constructed and mounted. In general, a 1/4 wave has an input of something between 20 and 40 ohms, a 1/2 wave between 60 and 80 ohms. But that varies to a large degree according to the relationship between that antenna's 'elements'/'halves'. Either can be adjusted in shape to get you very close to a 50 ohm input, that's quite common.
- 'Doc
(And just because most of the antennas I deal with are connected to transmitters, I tend to favor 50 ohm cable. Most of those antennas are also resonant on the frequency(s) of use, which is nice, but not absolutely necessary for just receiving.)
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jhooten
Member
Theoretical input impedance of a halfway dipole according to several references on my book shelf and the internet is 73 ohms. So I can run a 50 ohm cable from a 50 ohm radio to a 73 ohm antenna and have the mismatch at the antenna end of the cable or run a 75 ohm cable from a 50 ohm radio to a 73 ohm antenna with the mismatch at the transmitter end. Since the "normal" station has the antenna "tuner" at the radio end of the system, which choice would give the lowest losses and better system performance?
The point being that to make a blanket statement that just because your radio has a 50 ohm output the best choice for coax is always 50 ohm is not correct.
The point being that to make a blanket statement that just because your radio has a 50 ohm output the best choice for coax is always 50 ohm is not correct.
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mpddigital
Member
Man we've taken this one down the Rabbit Hole For every rule there are exceptions, for setups using ladder line, co-phasing, single wire transmission antennas and others there are a number of different set-ups. For most folks out there in most situations the 50 Ohm vs 75 Ohm debate is simpler.
TX/RX systems are built around a 50 ohm standard. In reality the coax could be of a lower impedance for many systems but 50 Ohms was selected as a <I>Compromise</I> for these systems. Just as 75 Ohm coax is a compromise for wideband and satellite systems. For most folks sticking to these standards works best.
There are lots of set-ups that will work, YMMV
For every rule there are exceptions, for setups using ladder line, co-phasing, single wire transmission antennas and others there are a number of different set-ups. For most folks out there in most situations the 50 Ohm vs 75 Ohm debate is simpler.TX/RX systems are built around a 50 ohm standard. In reality the coax could be of a lower impedance for many systems but 50 Ohms was selected as a <I>Compromise</I> for these systems. Just as 75 Ohm coax is a compromise for wideband and satellite systems. For most folks sticking to these standards works best.
There are lots of set-ups that will work, YMMV
As others have suggested go with some 50 ohm cable you wont regret it.
If you MUST use RG6 (not recommended) make sure you add a balun to the line. RG6 is VERY lossy .. RG8 is much better and better still is RG213. If you can afford 1.08 a foot there is some RG-8x FOAM cable that is pretty much the same as LMR400.
If you MUST use RG6 (not recommended) make sure you add a balun to the line. RG6 is VERY lossy .. RG8 is much better and better still is RG213. If you can afford 1.08 a foot there is some RG-8x FOAM cable that is pretty much the same as LMR400.
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