As
you can see, some coax has high loss. Loss is RF energy that the
coax turns into heat or "leaks" instead of passing on to the antenna
(or to the receiver from the antenna). The lower the db of attenuation
the better the cable is. Think of cable loss as negative
gain! The higher the attenuation, the less efficient our antenna
system is.
Loss
is primary dependent on the coax's shield and dielectric. The
shield is the outer wire braid that surrounds the inside of the
cable. A thick, tight braid results in less loss. Also, the dielectric
(usually white), the plastic type material that separates the
inside wire from the outside braid has an effect on cable loss.
Cables that use foam dielectric, that is where the insulation
is mixed with an inert gas, have very low loss. It is important
to use quality low loss cable! As you can see from the chart,
the losses can be quite high. You must make perfect connections
at the coax ends or, even higher losses will occur. It is also
important to note that old coax has high loss also. The cables
properties break down over time, resulting in very inefficient
cable. If you are still using that coax from the 1970s, its time
to replace it! New coax is manufactured better than the coax was
in the 1970s also, so this newer cable should last a lot longer.
Two
special cables are listed. One is Belden 9913. Belden is the name
of the company that makes the cable and they call it "9913". It
is a special coax that has two outside shields! The first is a
foil material that is on the outside of the dielectric, then over
that is the regular copper braid. As a result, the cable is very
efficient (low loss) and also STIFF (though they now make a 9913F
that is supposed to be flexible)! With low loss comes cost, this
cable is expensive. The other special cable listed is hardline.
This cable has a solid aluminum cover on the outside for the shield.
It is thick, and very efficient---stiff (can't really bend it)
and costly. It is used by cable TV companies. Since they run miles
and miles of cable, they need low loss cable. Cable loss is still
so bad, they still need to have amplifiers along the cables every
few miles or so. You can see hardline on telephone poles if your
area has cable. It is usually a silver cable about 3/4 inches
in diameter.
I
said before that loss becomes even worse as SWR increases. These
attenuation numbers in the chart are assuming a perfect 1:1 match.
If your SWR is over 3:1, cable loss is horrendous no matter what
kind of coax it is!
Coax
Impedance
Again,
the term impedance in "Coax Impedance" has different meaning...you
can not measure it with your trusty Ohm meter. It is determined
by the spacing (ratio) of the inner wire and outer braid. In CB
service, the two impedance's mainly used are 50 Ohm and 75 Ohms.
Velocity
Factor
Wow,
doesn't that sound like a serious high tech term! You can be king
nerd of your CB group if you know things like "velocity factor".
Ok, ok we said before that waves travel different speed through
different materials, if you missed it, its under the "1/2 Wavelength
Dipole" Section of "Antenna Basics". Velocity factor is simply
a number we use to determine how fast or slow a wave travels through
coax. Different coax models have different velocity factors.
Lets look at some numbers. Say we want to make a coax that is
exactly 1/2 wavelength long (this means when the wave travels
through the coax, it makes exactly 1/2 of a cycle while it travels
from one end of the coax to the other). If this sounds confusing,
better check out the "Antenna Basics" section. We will take our
formula for figuring out 1 wavelength and modify it.
One
Wavelength in coax, in feet = 984 * (Velocity Factor) / Frequency
in Megahertz (MHz)
Ok,
say we want a 1/2 wavelength RG-8/U Foam on channel 40 (27.405)
984
is for a 1 wavelength, so we want a 1/2 wavelength or half of
984, 984 / 2 = 492. Get the Velocity Factor from the table above
for RG-8/U Foam, which is .80. Put these numbers into the formula:
1/2
Wavelength, RG-8UFoam, Ch.40 = (492 * .80) / 27.405
1/2
Wavelength, RG-8UFoam, Ch.40 = (393.6) / 27.405
1/2
Wavelength, RG-8UFoam, Ch.40 = 14.362343 feet
The
length of coax is 14 feet 4 inches! Practice and see if you can
get lengths for other coax types with different velocity factors.
This will become important if you ever "stack" or co-phase antennas.
You must cut certain length coax lines for co-phased antennas
to work!
Assemble
Your Coax Correctly
Bad
connection cause loss. If you are going to solder connectors on
the ends of your coax, be sure to do it right. You must have the
right tools. Most Cbers and Ham radio operators think that they
can solder on connectors to coax with their 25 Watt pencil tip
soldering iron. You can't. You should use a high wattage iron,
preferably over 100 watts. You must heat the connector up quick,
so you do not damage the coax and connector, and the only
way to do this is with a high wattage soldering iron. I am not
going to go into detail of soldering on PL-259 connectors to coax
but let you look at figure 2. Trim the coax carefully, do not
nick the inside when cutting. And I have one big tip you do not
want to forget, before soldering the PL-259 plug to the coax,
do not forget to slide the PL-259 collar over the coax!
I have done this so many times! Soon as you start working just
slide that collar on, push it back far down the coax out of the
way so it does not slide back off..you will thank me for this!
If you solder the connector on without sliding the collar on,
you will have to start over (the collar will not fit over the
connector once it is soldered on)! After you are done (or to check
coax you suspect is shorted or bad) take an Ohm meter and check
you coax as shown in figure 3.
Figure
2 - How to solder on a PL-259 to coax.
Figure
3 - How to check to make sure your coax is ok. This does not
check for cable loss, it just makes sure the cable is not shorted
or an open circuit.
When
you are done, be sure to waterproof the ends of the cable. Wrap
it in quality electrical tape (I like 3M electrical tape) or
use a special wrap you can get at radio shack. Water will easily
find its way into coax ends. Remember I said old coax has high
loss? This is probably the number one way coax is ruined.
Why
does my coax length affect the SWR of my antenna?
How
many of you change the length of your coax to tune your antenna?
One of my good friends said to me, "I think changing the length
of the coax is the same as moving the gamma rod adjustment on
my Moonraker 4". Sorry to say, this is not true. As most people
will find, varying the length of coax to the antenna will vary
the SWR that the SWR meter is reporting. Actually, SWR should
remain relatively constant no matter how long the coax is or
where it is placed on the line (if its 5 feet down the coax
from the radio or 50 feet down the coax from the radio). In
most cases, the cause of inconsistant SWR meter readings is
from poor SWR meter design or component aging / failure. For
the SWR meter to read consistant SWR readings on the coax, the
meter has to have an impedance itself of exactly 50 Ohms.
Any deviation of the SWR meter's self impedance (from 50 Ohms)
from poor design or component aging / error / failure will cause
slightly inconsistant SWR readings when the SWR meters
position on the coax or length of the coax is varied. In practice,
generally you will find varying the coax length seemingly effects
the SWR reading. Most SWR meters (built into radio and external
type meters) and impedance "humps" in coax lines and connectors
will cause minor variations in SWR as jumpers and coax length
are varied. In reality, the mismatch at the antenna's feedpoint
/ coax junction is unchanged. Therefore - the actual SWR is
unchanged.
Another
reason SWR could vary is from the situation where the coax is
acting as part of the antenna. Not a favorable or normal situation.
The signal is traveling back down the outside of the of the
coax braid (note power should only be traveling on the inside
on the coax braid). Therefore, the coax is part of antenna system
and changing the coax length will change the SWR. This situation
is more likely to occur in mobile installations. You can try
to eliminate this situation (called "Common mode currents")
by winding an "RF Choke". Wind about 6ft of RG-213 or RG-8 into
a coil (6 to 8 turns). For RG-58 use 4ft with 6 to 8 turns.
Wind the coax up, placing each turn right next to one another.
Use electrical tape to secure turns together. You should place
these as close to the antenna as possible. Right at the antenna
coax connection point being optimum. Most times, you can verify
that you have common mode currents flowing back down the coax
by grabbing hold of the coax while transmitting and moving the
coax around. You can watch the SWR waver by moving the coax
while transmitting (don't speak into mic!). You have to do this
with all the doors closed from inside the vehicle. SWR should
waver, if you notice that SWR jumps rapidily between two values,
you might have a intermitant (bad) connection in the connectors
(PL-259s) on the coax. In most cases of "common mode currents",
just grabbing the coax will cause the SWR to change.
The
"RF choke" described above stops the signal from traveling back
down the outside of the coax. The signal inside the coax is
* u n a f f e c t e d * by the choke (contrary to what
you may have heard about coiling up excess coax). Common mode
current kills antenna efficieny. You could have a decent SWR
and not realize half your signal is being broadcast into you
car (result very poor antenna performance). If your linear amplifier
causes serious problems with your car's computer, lights, etc....you
may have common mode currents. If moving the coax around the
vehicle results in SWR change, this is a good indicator you
have common mode currents flowing back down the coax line.
This
doesn't happen often with base station antennas. Most base antennas
have some type of device that will decouple the antenna from
the feedline (gamma match, balun, etc.). Make sure you run your
feedling (coax) straight down from the antenna, taking care
not to run close to antenna to prevent "common mode" currents
which could still occur if coax is oriented in a way to pick
up strong antenna signal.
Coax
Length Issues Simplified
Question:
What is the "correct" length of coax?
Answer: The shortest length that makes it from the radio to
the antenna.
Question:
Are there any exceptions to the above rule?
Answer: 75 Ohm harnesses for Co-phasing is the only exception.
Question:
Why do most mobile antenna makers recommend 18 feet of coax?
Answer: You got me, they claim you should use 1/2 wavelength
multiples of coax. 18 feet isn't even close to being
a 1/2 wavelength in any 50 Ohm coax you will find. Check some
commonly used coax using the above formulas. RG-58, the most
commonly used mobile antenna coax length would have to be 12
feet to be a 1/2 wavelength. RG-8X would need to be 14 feet.
Question:
Ok, seriously nerd, when I trim my coax it changes my SWR. You
can't tell me it's not good to lower my SWR from 1.5 to 1.2
by taking off a few extra feet of coax.
Answer: Hey, I'm not a nerd! Go ahead, change your coax length.
If you change coax length and it affects your SWR in minute
amounts, everything is working fine. If your SWR was 2.5:1 and
putting in a 4 foot jumper brought it down to 1.3:1, this large
change indicates you have real problems...i.e. common mode currents
(see above). Really, you should be changing the antennas length
to alter SWR. NO special length of 50 ohm coax is going
to fix or lower your SWR signficantly and/or boost performance.
Period.
Question:
I notice when I change coax length, my "modulation" needle jumps
more / harder / faster when I talk. I get more watts out of
the radio (verified by a watt meter) with certain lengths of
coax. Is there a certain length that will allow my radio to
put out the most power?
Answer: No, there isn't a magic certain length that will do
this. Certain lengths of coax will allow your radio to "see"
a load that it can couple with better which results in more
power out of the radio / amp. Unfortunately, there isn't a good
way to determine what length you need to allow the radio to
put out the max wattage its capable of. I am only refering
to newer solid state radios with transistors. Old tube radios
usaually have devices built into them to tune the radios finals
impedance to match that of the input end of the coax. If you
want to accomplish the same effect with your solid state radio,
pick up a device known as a "antenna tuner". The term antenna
tuner is misleading because it doesn't actually tune the antenna
- or take the place of tuning the antenna - it simply lets the
radio couple to the antenna system with better efficiency. Other
more appropriate names for the antenna tuner are the transmatch
or feedline flattener. If your SWR is low (below 2:1), don't
expect to notice a (performance) difference from using an antenna
tuner.
Question:
Why do so many people recommend using 1/2 wave mutiples of coax?
Will it really hurt me if I take the time to measure a 1/2 wavelengh
multiple of coax?
Answer: The idea of using 1/2 wavelength multiples of coax comes
from the fact that the antennas feedpoint impedance is "mirrored"
at the input of the coax when using the said length. Many operator
make / made the assumption that was a good thing because it
was just like having the antenna hooked right to the radio /
SWR meter. If anything other than a 1/2 wavelength mutiple is
used, the impedance the radio / SWR meter sees is the antennas
feedpoint impedance transformed to some other value of impedance.
So, if your antenna has a feedpoint impedance of 25 Ohms and
you use a 1/2 wavelength wave length of coax, the radio will
"see" 25 Ohms in the input end of the coax. If you were to use
some other length, say a 1/4 wavelength of 50 Ohm coax, the
radio would "see" an impedance of 100 Ohm. What consequences
does this have? None. Whether the impedance is 25 Ohms or 100
Ohms, the SWR is STILL 2:1. No matter what the 50 ohm
line length, the resultant SWR is still 2:1...at the antennas
feedpoint, at the input end of the coax and at any and every
point along the coax line.
Many
operators take half the truth of transmission line theory and
make up their own rules. If you have been reading my page since
its inception, you know I used to be "uneducated" when it came
to transmission line theory. Sorry to admit I thought coax length
was important. It was drilled into my head by somebody I respect(ed).
This
isn't the easiest part of CB to wade through. Hopefully I've
covered this with enough detail to set everyone straight. Many
beginner amateur radio operators and students have misceptions
and make false extrapolations in tranmission line theory. There
are many conditions that must be stated when simplifying things.
I have made one assumption here. I have been assumming the coax
loss is negligible. At CB frequencies this is a pretty safe
assumption to make.
Question:
Ok then, why is the length of 75 Ohm coax line important? If
coax length doesn't matter, why is 75 Ohm coax different?
Answer: I've heard this a question more than once. The fact
your radio has a 50 Ohm jack on it is the reason you use 50
Ohm coax. The antenna is designed to have a feedpoint impedance
of 50 Ohms. When all the impedances match, maximum power is
transfered from the radio, through the coax, out the antenna.
Using coax with a (characteristic) impedance of 75 Ohms can
potenially transform the antennas feedpoint into another value..another
value such that the resultant SWR will vary with line
length. This function is handy for matching antennas to the
feedline that do not have feedpoint impedances of 50 Ohms. This
is beyond the scope of this section, but antenna makers can
specify certain lengths (1/4 wave) of 75 Ohm coax to achieve
the proper match between radio and antenna. Again, this form
of matching is not possible with 50 Ohm coax.
