|
COAXIAL
CABLE COMPARISON
Actual
cables sometimes exhibit lower losses than those specified and often vary
slightly from manufacturer to manufacturer.
WHAT
DOES IT ALL MEAN ?
From
the table of coax info, it indicates that with 33 metres of standard poythene
inner RG58 cable, the loss of signal at 400 MHz is going to be about 13dB.
With low loss Cellfoam RG58 it reduces to 6.3dB and with Cellfoil it should
be about 4.7dB. With RG213 coax (poly inner), it is only 5.0dB. When you
consider that this figure represents the signal level drop between the
bottom of the antenna and the receiver input, it is easy to see why you
must use a short length of good quality cable if it is going to work efficiently.
13dB
= 1/20th of the original signal power level is available from the output
end of the cable
9.5dB = approx 1/10th
6.7dB = approx 1/4
5.4dB = approx 1/3
It
becomes evident why we use low loss cable like CNT-400 or LMR-400 which
has a loss of about 2.6dB for the same 33 metre cable run at 400MHz when
we really want a good low loss system.
An
extra note about the likes of RG6 and RG11 coaxial cables as used for
TV signal distribution : You may see a reference to "tri-shield"
and "quad-shield" but this construction does not affect the
actual loss (/ attenuation) in the cable - only it's shielding factor..
i.e. the probability of something inducing signals into the coax inner
from other wiring (eg power & lighting) within the premises or from
other local RF sources (eg transmitters). The only truly "100% shielded"
cables are those with completely continuous jackets like heliax, not multi-layer
wound aluminium-tape jackets like quad-shield RG6 - which come very close.....
SELECTING THE RIGHT
COAX CABLES & UNDERSTANDING THE "LOSS" VALUES
EXAMPLE : A PROJECT
FOR WLAN USE
For those interested
in using extension coax cables for Wireless LAN (WLAN) applications, take
note of the following....
In the following examples we will ignore connector loss - which is minimal
with high quality connectors when correctly terminated.
As an example,
let's say you want a 20 metre extension cable to go from the wireless
lan card (SMA Reverse Gender) to an 18dBi external antenna with an N connector.
You contemplate
using RG58CU (cheap stuff, easy to find..) - the cable loss is 18dB at
2.4GHz negating all of your antenna gain. Connectors are available in
various qualities to go onto the ends of the cables but note that these
are generally crimp style and must be correctly terminated to minimise
loss.
That loss is far
too high so let's look at RG58 low loss (foam dielectric, easy enough
to find but just a little more expensive) - - the cable loss is 11.4dB
at 2.4GHz negating a good portion - 63% - of your antenna gain. This means
that your system gain is now around 6.6dBi. Again, connectors are available
in various qualities to go onto the ends of the cables but note that these
are generally crimp style and must be correctly terminated to minimise
loss. Use better quality to make sure that this loss is really minimised.
Yet another alternative
: RG213U (poly dielectric, easy enough to find but just a little harder
to handle) - - the cable loss is 7.8dB at 2.4GHz negating some 43% of
your antenna gain. Problem : SMA reverse gender connectors are not available
to fit this size cable so you must split it into 2 cables - the main cable
which is N to N style connectors plus a "tail" of N to SMA reverse
gender. Normally we recommend "tails" of up to 500mm as a maximum
value and generally draw this back to 300mm to minimise loss. Let's say
we elect to use a 19.7 metre RG213U cable plus a 300mm "tail".
We have 7.6dB main cable loss plus a 0.3dB tail loss so at 7.9dB (giving
44% effective antenna gain) very similar to having a cable for the entire
length. The effective antenna gain is now around 10dBi, not bad but can
we do better ?
Lets' get serious
now : LMR400 / LM400 / CNT400 - all basically the same cable but made
by different manufacturers to the same general specification (foam dielectric,
harder to find, harder to handle and more expensive and mainly only crimp
connectors to suit) - the cable loss is 4.6dB at 2.4GHz negating just
25% of your antenna gain. Same problem : SMA reverse gender connectors
are not available to fit this size cable so you must split it into 2 cables
- the main cable which is N to N style connectors plus a "tail"
of N to SMA reverse gender. Normally we recommend"tails" of
up to 500mm as a maximum value and generally draw this back to 300mm to
minimise loss. Lets say we have a 19.7 metre LMR400 cable plus a 300mm
"tail". We have 4.53dB main cable loss plus a 0.3dB tail loss
giving an overall 27% loss - not quite as good as having a CNT400 cable
for the entire length but unavoidable becasue we can't get SMA connectors
to suit this cable style. The total cable loss plus antenna gain gives
an overall system gain of a little over 12dBi - a far cry from using cheap
RG58 !
Now you may say,
is the 2dB worth the extra cost over the RG213 ? Since 3dB represents
a 2:1 power ratio or a 4:1 voltage ratio, the signal voltage at the far
end is basically tripled by using the CNT400 cable. What about the foam
RG58 ?- it ended up with a system gain of just 6.6dBi so only about 6dB
worse than the CNT400 installation. Now just remember that the extra 6dB
happens in both directions - transmit and receive. It means that the incoming
signals from the WLAN link at the far end are also attenuated by that
extra 6dB as well and if you have a questionable path in the first place
then your data rate may fall considerably as soon as it starts raining
! If your need is for stable signal path, spend the money.
If you want to
spend big money, look at the full heliax style cables but remember that
you still need a "tail" on the end and you may find that the
"tail" has more loss then the entire heliax cable run. Connectors
for heliax often run upwards of $50/$60 each (2 required) and new heliax
cable is usually at least $15/metre, usually more. Not a move for those
"faint of wallet" but if performance is the target, spend the
money.....
Cable
Type
|
Insulation
Type |
Attenuation
dB/metre
2.4GHz |
Attenuation
dB/metre
5.8GHz |
|
| RG174 |
PE |
1.20 |
1.85 |
|
| RG316 |
PTFE |
1.10 |
1.70 |
|
| RG58CU |
PE |
0.90 |
1.40 |
|
| RG58U |
Foam |
0.57 |
0.89 |
|
| LMR195 |
Foam |
0.56 |
0.88 |
|
| RG213U |
PE |
0.39 |
0.51 |
|
| C2FP |
Foam |
0.23 |
0.36 |
|
| LMR400
, CNT400 |
Foam |
0.23 |
0.35 |
|
| LDF4/50A |
Foam
(Heliax) |
0.13 |
0.21 |
|
| LDF5/50A |
Foam
(Heliax) |
0.08 |
0.13 |
|
|
|
|
|
|
| PE
: Polyethylene (ie normal low cost coax) |
|
| PTFE
: Teflon |
|
| Note
: The dB/metre value is based on power loss |
|
TYPICAL
COAXIAL CABLE DATA
|
Type
|
Nom. Imp
Zo
|
Outer Dia.
(mm)
|
Nom loss
(dB/100m) @ 50 MHz
|
Attenuation
(dB/100m)
|
Vel. Factor
%
|
Cap. pF/m
|
Voltage
Rating {Vrms}
|
|
100 MHz
|
200 MHz
|
400 MHz
|
1GHz
|
|
RG - 6A/U
|
75
|
7.0
|
4.73
|
6.37
|
8.89
|
13.1
|
~26
|
82
|
53.1
|
2700
|
|
RG - 8/U
|
52
|
10.3
|
5.2
|
7.2
|
10.5
|
15.4
|
29.2
|
66
|
96.8
|
5000
|
|
RG - 9/U
|
51
|
10.67
|
5.2
|
7.2
|
10.5
|
15.4
|
29.2
|
66
|
98.4
|
5000
|
|
RG - 11/U
|
75
|
10.29
|
4.3
|
6.6
|
9.5
|
13.8
|
23.3
|
66
|
67.3
|
5000
|
|
RG - 58/U
|
53.5
|
4.95
|
10.2
|
14.8
|
22.3
|
32.8
|
55.8
|
66
|
93.5
|
1900
|
|
RG - 58A/U
|
50
|
4.95
|
10.8
|
16.1
|
23.9
|
37.7
|
70.5
|
66
|
101
|
1900
|
|
RG - 58C/U
|
50
|
4.95
|
10.8
|
16.1
|
23.9
|
37.7
|
70.5
|
66
|
101
|
1900
|
| RG-58
Cellfoam (9001) |
50 |
4.9 |
9.6 |
12.7 |
18.9 |
27.4 |
51.1 |
76 |
- |
- |
| RG-58
Cellfoil (9006) |
50 |
5.1 |
6.6 |
9.5 |
14.1 |
19.3 |
34.9 |
80 |
- |
- |
|
RG - 59/U
|
73
|
6.15
|
7.9
|
11.2
|
16.1
|
23.3
|
39.4
|
66
|
68.9
|
2300
|
|
RG - 59B/U
|
75
|
6.15
|
7.9
|
11.2
|
16.1
|
23
|
39.4
|
66
|
67.3
|
2300
|
|
RG - 62/U
|
93
|
6.04
|
6.2
|
8.9
|
12.5
|
17.7
|
28.5
|
84
|
44.3
|
700
|
|
RG - 62B/U
|
93
|
6.15
|
6.6
|
9.5
|
13.8
|
20
|
36.1
|
84
|
44.3
|
700
|
|
RG - 122/U
|
50
|
4.06
|
14.8
|
23
|
32.8
|
49.9
|
87
|
66
|
101
|
1900
|
|
RG - 141A/U
|
50
|
4.83
|
6.9
|
10.5
|
15.4
|
22.6
|
42.7
|
69.5
|
95.1
|
1900
|
|
RG - 142B/U
|
50
|
4.95
|
8.9
|
12.8
|
18.4
|
26.9
|
44.3
|
69.5
|
95.2
|
1900
|
|
RG - 174/U
|
50
|
2.56
|
21.7
|
29.2
|
39.4
|
57.4
|
98.4
|
66
|
101
|
1500
|
|
RG - 178B/U
|
50
|
1.83
|
34.4
|
45.9
|
62.3
|
91.9
|
150.9
|
69.5
|
95.1
|
1000
|
|
RG - 179B/U
|
75
|
2.54
|
27.9
|
32.8
|
41.0
|
52.5
|
78.7
|
69.5
|
64
|
1200
|
|
RG - 180B/U
|
95
|
3.56
|
15.1
|
18.7
|
24.9
|
35.1
|
55.8
|
69.51
|
49.2
|
1500
|
|
RG - 187A/U
|
75
|
2.66
|
27.9
|
32.8
|
41
|
52.5
|
78.7
|
69.5
|
64
|
1200
|
|
RG - 188A/U
|
50
|
2.59
|
31.5
|
37.4
|
46.6
|
54.8
|
101.7
|
69.5
|
95.2
|
1200
|
|
RG - 196A/U
|
50
|
1.93
|
34.4
|
45.9
|
62.3
|
91.9
|
150.9
|
69.5
|
95.2
|
1200
|
|
RG - 213/U
|
50
|
10.29
|
5.2
|
7.2
|
10.5
|
15.4
|
29.2
|
66
|
101
|
5000
|
|
RG - 214/U
|
50
|
10.8
|
5.2
|
7.2
|
10.5
|
15.4
|
29.2
|
66
|
101
|
5000
|
|
RG - 223/U
|
50
|
5.38
|
10.1
|
14.8
|
21
|
30.2
|
53.5
|
66
|
101
|
1900
|
|
RG - 303/U
|
50
|
4.31
|
6.9
|
10.5
|
15.4
|
22.6
|
42.7
|
69.5
|
95.2
|
1900
|
|
RG - 316/U
|
50
|
2.49
|
30.8
|
34.1
|
43.3
|
54.1
|
101.7
|
69.5
|
95.2
|
1200
|
|
BEL 9913
|
50
|
10.29
|
3.0
|
4.6
|
5.9
|
8.5
|
14.8
|
84
|
78.7
|
-
|
| CNT-400 |
50 |
10.29 |
2.9 |
4.3 |
5.5 |
8 |
12.7 |
85 |
- |
- |
| LM-400 |
50 |
10.29 |
2.8 |
3.9 |
|
9.1 |
13.8 |
85 |
77.1 |
2500 |
|
