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
|Description||Insulation||Nom. Imp. OHMS||Nom. Vel. of Prop.||Nom. Atten. @3OMhz. Per 100|
|RG-8X type||Cellular Polyethylene||50||78%||1.75 db|
|RG-8-A/U or RG-213/U||Polyethylene||52||66%||.93 db|
|Belden 9913 or JSC 3500||Semi-solid Polyethylene||50||84%||.65 db|
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
First 468 ÷ 27.045 = 17.305 Then .84 X 17.305 = 14.535 Feet
Approximately 14.5′ is your multiple
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′
High Frequency 27.905
Low Frequency 26.185
Bandwidth In MHz 1.720
1.720 ÷ 2 =.860
Center Frequency 27.045
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.