Coax Types & Lengths
Did you know there are many types of RG-8/U coax? This is what most people refer to as the heavy stuff. RG-8/U coax
is heavy but there are a number of different grades and if you get one of the cheaper grades you might as well get the
thin stuff. Belden has about 4 grades of 8/U and the RG-213/U which is a military spec. cable equivalent to RG-8-A/U.
Belden coax shields range from 95% to 97% and 100% on the 9913 their low loss 8/U.
Below is a table of Belden specifications on the coax I normally use or recommend. These are not all the specs. available
but are the most important for our use in this article.
Belden Coax Specifications
Nom. Vel. of Prop.
Nom. Atten. @
3OMhz. Per 100'
RG-8-A/U or RG-213/U
Belden 9913 or JSC 3500
Other manufacturers make 8/U type cable with shields that range from 66% to 80% and you can't tell without stripping
them back to look at the shielding. The worse cables I've found are the pre-made type. You're better off making your own
or having cables made from known high quality cable. I've had experience with JSC Wire cable and found their RG-58-
A/U, RG-213/U, and their 3500, a 9913 equivalent to be an excellent low cost alternative to the Belden counterparts.
The insulation column indicates the center conductor insulation material. Mil. spec. cable such as the RG-8- A/U, RG-
213AJ, and RG-58-CIU use the solid polyethylene insulation. These cables have a nominal velocity of propagation
rating of 66% and a shield consisting of 95-97% copper braid. This is an important number in calculating coax lengths,
as you will see. The RG-8/U type, RG-8/X type, and RG-58/U type have a cellular polyethylene insulator and are
normally referred to as foam. These less rigged foam cables, have a slightly improved attenuation figure and a velocity
factor of 78% and a shield consisting of 95-97% copper braid. The 9913 and 3500 have a semi-solid polyethylene
insulator. A spiral air space is created between the center conductor and the insulator making it semi-solid. These cables
have a velocity factor of 84% and a shield consisting of 100% aluminum foil plus 90% tinned copper braid.
As you can see the nominal impedance is 50-52W. Don't worry about the 52W impedance, you'll never see a difference
on your SWR. As mentioned earlier the nominal velocity of propagation for polyethylene is 66%, cellular polyethylene
is 78% and semi-solid polyethylene is 84%. It is used to determine the length of cable at a particular frequency to keep
from making your coax a factor in your SWR. You determine the length and use that multiple to get the total length you
need. If your antenna is resonant and your SWR doesn't show 1:1 it most likely is the coax or the coax length.
Mobile and base systems use different formulas. Base antennas work best using multiples of 1/2 wave lengths of cable.
Mobile antennas work best using multiples of 3/4 wave lengths of cable. You may recall that one antenna that seemed
impossible to match, this could have been the problem. I've had truckers come in and say they've tried everything, even
new coax, and nothing works. The first question I asked; How long was the coax cable? The response usually was 6-12
feet and once it was replaced with an 18 foot piece of RG-58-A/U the match came down, usually below 1. 5: 1.
Here are the formulas used to calculate the coax length:
Base Coax = 468 + Freq. in Mhz x Velocity of Propagation
Mobile Coax = 702 + Freq. in Mhz x Velocity of Propagation
Now say you have 50 feet of coax cable in your base station and want to replace it with 9913. First you must decide
where on the band you spend most of your time or if you bounce around a lot take the lowest frequency and subtract it
from the highest frequency then divide that number by 2 and add it to the low frequency. This will give you the center
frequency you use.
Formula with your information plugged in is: 468 ÷ 27.045 X.84
Bandwidth In MHz
First 468 ÷ 27.045 = 17.305 Then .84 X 17.305 = 14.535 Feet
Approximately 14.5' is your multiple
1.720 ÷ 2 =
To determine how many multiples you need 50 ÷ 14.5 = 3.448
So you need either 3 X 14.5' = 43.5' or 4 X 14.5' = 58'
Now you can calculate for any other type of coax by plugging in the velocity factor from the coax table and use the other
two figures as before. For mobile using RG-8/X coax, just divide 702 by 27.045 then multiply that figure by .78. 702 +
27.045 X .78 = 20.25' Using RG-8/X cable your mobile multiple is 20 feet 3 inches. For most mobile installations I
multiple is enough.
Use these formulas for all types of coax cables and frequencies. The correct length of cable is much more critical on
mobile installations. I've seen a 4:1 match corrected with the proper length of coax. Base installations are less critical,
but keeping the length close will give you an acceptable SWR across a broader bandwidth.
Coax cable types can be mixed as long as the correct multiples are used for each length. Maybe you want to use 9913
outside your house and RG-58-A/U inside because it's easier to run thinner cable in the house. For 27.000Mhz you
would use 14.5' multiples of 9913 and you would use 11.5' multiples of RG-58-A/U. If you're replacing a single 50'
piece and 11.5' is enough to reach your equipment, then you'll need a 43.5' piece of 9913 and a 11.5' piece of RG-58-A/U
coax. Use a double female or barrel connector to splice the two together. This will work fine and won't create any
problems as long as you wrap it with coax seal. Coax seal is a plastic putty that can be molded to form a weather tight
seal that will never leak. It stays soft and pliable and connections can be unscrewed at any time.
The placement of and the length of coax to the SWR meter has an effect on the accuracy of your readings. If a linear
amplifier is used, the meter should be in line with the antenna side of the amp, otherwise it should be after the radio. A
very short piece (6-12") of cable should be used between the radio and the meter. If you require more than that, use a 1/2
wavelength of cable. Use the formula in this article or an easy to find 12' pre-r made RG-58-A/U cable. This will give
you the most accurate reading. There's nothing worse than troubleshooting a problem that may not be there. If you start
with known test equipment your project will take less time and fewer patients.
In This Issue
© CB World Informer 1996 - 2015 Worldwide Rights Reserved
Review Of The Chipswitch
How To Make The Best Solder Connections
Slick Tricks On Microphone Wiring
Proper Base Station & Mobile Grounding
Advertising Claims...Smoke And Mirrors?
Comparison of Cobra 2010 to 2000
Power & Modulation Adjustments
New DF 10,000 Low Pass Filter
Coax Types & Lengths
Linear Amp Selection &Setup
Mobile Radio Interference
New Product Review: Astatic's EchoMax 2000
Channel Kit For PC-122 & Cobra 146-GTL
Did You Know?
Increasing Interest In CB Rumored
What's Happened To Organized CB
Santa's Best CB Gift List
Bob's CB Reopens
New Product: Midland 79-290 AM/SSB Mobile
Swap & Sell
New Product: New Anttron 305 Base Antenna
What's Happening To CB?
CBWI Proposal To Improve 11 Meters
Cobra/Uniden SSB Chassis Mods.
Review Of Midland 79-290 AM/SSB Mobile
Cobra/Uniden SSB Chassis Mod UPDATE
President Jackson Unlocked Clarifier Mod.
Cobra 148 & Uniden GrantXL Clarifier Mod.
Cobra 142GTL & Uniden Washington Clarifier
Uniden Grant Unlocked Clarifier Mod.
Uniden PCI22 PRO SSB Clarifier Mod.
Review Of The Northstar DX880HL
Big Bust At The Consumer Electronics Show
Bob's CB Has Opened
The New Mongoose Model 450 Review
Wilson Antenna Tests The Trucker 5000
A Company With Interference Solutions
Solving Telephone RF Interference
Lowpass Filters: What, Where, And How
Using Highpass Filters For TVI
How To Conduct A Noise Audit
Modern Do-It-Yourself Grounding Techniques
Using Water Pipes For RF Grounding
Using Water Pipes For RF Grounding
The New Emperor TS-3010 Review
Grounding Coaxial Cable Shields
Modern Lightning Protection - RF Entry Ports
Modern Lightning Protection - AC Power Lines
Modern Lightning Protection - Control Lines
Modern Lightning Arrestors - Polyphaser VS I.C.E.
Modern Lightning Arrestors - Alpha Delta VS I.C.E.
Modern Lightning Arrestors - Cushcraft VS I.C.E.
Galaxy DX 2547 Reveiw
Inside The DX 2547
DX 2547 Channel Mod
DX 2547 Clarifier Mod
DX 2547 Photos
DX 2547 Manual Excerpts
The Anttron Story
Anttron 305 Revisited
New Antrron Products
Aries A-SWR 460 Digital Meter
Barjan Buys Wilson Antenna
Wilson Electronic In Cell Phone Market
First Web Issue
Help Get The Word Out
Sneak Preview: The New Maverick A24
Maverick A24 Front Panel Controls
Inside The Magnum Maverick A24
Barjan Buys Francis Antenna
Wilson Antenna, 1 Year After Barjan Buyout
Solarcon I-Max 2000
False Performance Claims
CAUTION: Don't Burn Out That Radio
Magnum's Filtered Power Cord
Dragon Super Heavy Duty SO-239 Stud
CBWI...Give Us Your Opinion
Reveiw Of The RCI 2950DX
RCI 2950DX Image Rejection Modification
RCI 2950DX Coversion & Clarifier Mods
RCI 2950DX Photos
RCI 2950DX Board Component Layout
RCI 2950DX Adjustment Layout
RM-9807: Petition To Remove 155 Mile Limit
Slip-Sear Radio Box
RF Limited UTB-1 Adjustable Talkback Board
A Message From The Editor
Review Of The General Lee
General Lee Conversion
General Lee Tune-Up
Genral Lee Pot. Adjustments
Proposal For Citizens Radio Network
RF Limited EC-2018 Turbo Echo Mic
RF Limited EC-2018XTR Xtreme Echo Mic
RF Limited TRB-1 Turbo Echo Board
Santas CB Gift List
Review: RCI 6900F TB 10 Meter Transceiver
RCI 6900F TB Frequency & Clarifier Mods
RCI6900F TB Tune-Up & PCB Adj. Locations
RCI 6900F TB Photo Gallery
UPDATE: Maverick A24 Transceiver
Texas Ranger SRA-158 Stock Mic
Astatic's Final Edition D-104 Silver Eagle
Firestik's All American Limited Edition Antennas
Uniden PC 68XL Turbo 121 Combo From Mexico
The Cobra 29 Night Watch Classic
Cobra XL 450 Amplifier From Mexico
Review: The Magnum 357DX
Magnum 357DX Photo Shoot
Magnum 357DX Conversions
Magnum 357DX Adjustments
MD-4 External S/RF Meter
Adding MD-4 To The Magnum 357DX
Adding MD-4 To Other Radios
PanaVise Pedesal Mount
A Little History On The CCC People
AR-3500 Operating Manual
AR-3300 & AR-3500 Photos
Maverick A24 Technical Bulletin
Super Star Silver Salute Specifications
Magnum Delta Force 2002 Conversion
Top Gun 56 Mic
Top Gun 56 Story
Top Gun 56 Audio Test
QUAD-5 10 Meter Transceiver
QUAD-5 Magnified Photo
QUAD-5 Operator's Manual
QUAD-5 Photo Gallery
Top Gun SP-1a Speech Processor
DAS Acquires Turner
RCI Move Manufacturing