Introduction
Co-ax, short for coaxial, and not meant to be confused with the verb coax, is
the means by which radio transmissions are conducted from an external antenna
to your scanner/receiver.
As such, it represents one of THE most vital links in your monitoring station.
People go out of their way to spend good money on a scanner/receiver and
antenna but hamstring their investment by using inadequate (read cheap)
cabling!
Most times this results from simple ignorance in thinking all co-ax cables are
created equal. Far from it!
Hopefully this page can remedy such problems and help people "tune-up" their
systems to get better performance!
Cable Tips
For the most part try and remember these simple rules : thicker, more rigid
cables give better results.
If it's about a half-inch thick (take a look at your pinky finger) and likes to
hold a bent shape, you've probably got a winner!
If it's only about a quarter-inch thick or less and flails about easily, you
could be doing better!
You get what you pay for. Usually priced in cents per foot, the cheaper cables
will run you 20 cents/foot while the more expensive ones run 70 cents/foot and
more. And of course there's savings advantages in buying longer lengths or
pre-cut lengths.
The longer the cable, the more it attenuates (weakens) the signals as they
travel from the antenna to the receiver. For lengths of 25 feet and less, it's
not a BIG concern as to which cable type you use. Beyond 25 feet, you should be
paying more attention. If you want the best rececption, you'll use the best
cable you can regardless of length!
The higher the frequency, the more a cable will attenuate it. HF frequencies
(below 30MHz) are not affected very much so almost any cable will do. If you
are listening to VHF frequencies (30 - 300MHz) then it becomes much more of a
concern. For UHF (300MHz and up) you had better make cabling a big priority!
Since most scanners/receivers go over 1GHz these days, you can see the
importance of choosing the right cable.
Cable Types
The basic co-ax types you run across in our hobby are RG-58, RG-59, RG-174,
RG-8, RG-6, RG-11 and 9913 (which is really a form of RG-8).
These types basically differ in diameter and stiffness which is due to
differences in types of outer jacket material, braiding, center conductor type
(solid or stranded), shielding (used or not), dielectric type (air spacing and
gas injections) and materials used (copper, aluminum, etc.).
These differences in properties determine the capicitance and velocity factor
which are terms used to indicate how efficiently various signals are conducted
through the cable.
For our purposes, we want efficient cable so that the signals received at the
antenna make it down to the receiver with as much strength as possible.
If a really weak signal is received at the antenna but a poor choice of cable
is used, it's possible that it may attenuate the signal such that the receiver
is unable to detect it! So if you use good cable, you WILL hear more signals.
Be aware that you will see some funky notations for cables at times. Usually
these take the form of suffixes like '/U', 'A/U', '/CU' and '/X'. These
represent slightly different constructions of the same cable type which means
they'll have different efficiencies. So take heed! A /U cable is usually a much
better choice than the same cable in its /X form.
You are also going to find that cables come in two different types of
resistances : 50- and 75-ohm. RG-8/58/174 are all 50-ohm while RG-6/11/59 are
75-ohm.
While most scanners/receivers specify a 50-ohm load at their antenna jack,
either type cable can be used. Why? Because the impedance of the entire
scanner/cable/antenna system as a whole changes radically over the wide range
of frequencies that such radios cover.
If we were just listening to one narrow range of frequencies then we would
worry about matching the cable/antenna impedance to the receiver impedance to
get the most efficient signal transfer. Since we listen to signals from 25MHz
to nearly 1GHz it's just not practical so pay it no mind.
Cable Choices
Most cabling used in our hobby and supplied by manufacturers is the cheaper and
less efficient RG-58 and RG-174. These are OK for very short cable runs and
HF/VHF-lo frequencies but you can easily get more performance by replacing them
if possible.
To compare the different types, we'll need to use a reference of some sort. For
our purposes, let's use figures that tell us how much of a 100MHz signal is
lost in 100 feet of cable. These are usually specified in dB (decibels). Keep
in mind that a 3dB loss represents 1/2 the original signal strength (50%
transmission) while a 6dB loss represents 1/4 the original signal (25%
transmission).
Taking a look at some representative numbers for our 100MHz/100ft cable, here
are the figures for cables that you're likely to come across :
| Cable | dB loss |
|---|
| RG-174/U | 8.4 |
| RG-58 | 4.2 |
| RG-58/U | 3.8 - 4.5 |
| RG-8/X | 3.4 - 3.7 |
| RG-59 | 2.9 |
| RG-8A/U | 2.8 |
| RG-59/U | 2.5 - 4.0 |
| RG-6 | 2.3 |
| RG-213/U | 2.1 |
| RG-6/U | 2.0 - 2.1 |
| RG-11 | 1.4 |
| RG-8/U | 1.3 - 1.9 |
| RG-11/U | 1.2 - 2.0 |
| 9913 | 1.3 |
| LMR-400 | 1.2 |
| LMR-600 | 0.9 |
| LMR-900 | 0.6 |
| LMR-1200 | 0.45 |
Looking at the dB losses, you can see I've ranked the table from worst down to
best. Makes it pretty clear why you should try and avoid RG-58 & RG-174 type
cables for most situations!
For most purposes, the large LMR co-ax is not suitable. While the LMR-400 & 600
have 0.5 inch diameters, the 900 is 0.9 inches and the LMR-1200 is 1.2 inches!
Pricing on the LMR series is high as well. 400 is $1.50/foot, 600 is $3/foot,
900 is $8.50/foot and 1200 is $11/foot.
And just for another emperical reference point, if we looked at the dB losses
at 400MHz instead of 100MHz, the values in the above table can be roughly
DOUBLED (remember that doubling a dB value will scale it logarithmically;
doesn't take long to kiss most of your signal good-bye!) I think you can
project what will happen at 800MHz and therefore the importance cable length
and type plays on how well you receive the higher frequencies!
Connectors
Along with cables come the connectors needed to hook everything together.
The connectors used in our hobby basically consist of BNC, PL-259, F-type,
Motorola and in some cases RCA. If you see SO-239, it's a chasis-mount version
of the PL-259 female connector.
BNC is the connector seen on most scanners and lab test equipment. PL-259 (or
SO-239) is usually found on larger antennas and commercial equipment. F-type
are used for most cable TV fittings. Motorola is used on some mobile antennas
and commercial equipment. RCA types are usually used for TV and stereo
equipment connections but can be found on some scanner gear as well.
Most of your smaller width cables (RG-6/58/59/174) will use BNC, F-type,
Motorola and RCA. The thicker cables (RG-8 & 9913) usually use PL-259.
That's why it's always a good idea to have several adapters handy so you can
cross-connect various equipment, cables and antennas to experiment with. Radio
Shack and radio dealers carry a wide variety of such connectors.
When ordering cable from a radio dealer, they can often put different type
connectors at each end alleviating the need for additional connectors to match
the cable to your antenna and scanner/receiver. So be sure to know which type
and gender you'll need on both ends.
With a little experimentation, you can buy your own cable and connectors to
build your own. With a soldering iron or crimp tool, you'll be able to make a
cable at a moment's notice for that emergency situation. Just the length you
need with the proper connectors!
The golden rule is to use as few connectors as possible. Most good quality
connectors attenuate the signal very little (0.3 dB) but some can be quite poor
(1-3 db)! So if you use more than one, it could be as bad as selecting a poor
type of co-ax! Or in other words, you could loose half your signal, or more,
simply by using bad or multiple connectors.
As connectors age, are plugged and unplugged repetitively and exposed to the
elements, they will attenuate more and more. Be sure to seal those connectors
which are outside and give everything a good check annually to assure proper
operation.
Web Links
- AES cable
- Belden Attenuation vs. Frequency table
- Durham Radio cable
- Grove Antenna Adapters
- Grove Coax Cable
- HRO cable
- Radio Shack RG-6
- Radioware co-ax cable
- Radioware co-ax connectors
- Radioware attenuation table
- R.F. Connection co-ax cable
- R.F. Connection attenuation table
- SSB Electronic cable
- Texas Towers LMR page
- Universal Radio Co-ax
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