This article explores use of RG−6/U coaxial transmission line for transmitting applications.
RG−6/U (RG6, RG−6, RG6/U) type coaxial transmission line is widely used for receiving applications. RG−6/U is available in a wide range of forms:
Make no assumptions about its velocity factor, measure it for length critical applications.
Fig 1 shows the loss of common low cost coax cables.
Fig 1 is a loss model that is based on the following assumptions:
These are reasonable assumptions for most practical transmission lines with homogenous conductors down to about 100kHz. Transmission lines that use conductors that are not homogenous, eg copper clad steel, silver plated copper clad steel (CCS), will not conform to the loss model at low frequencies where the outer layer of the conductor is less than a few skin depths in thickness. For more information on the loss model, see the Transmission Line Loss Calculator.
Some types of RG−6/U use a CCS centre conductor and will have higher loss at low frequencies that shown in Fig 1, depending on the thickness of the copper cladding which may vary from cable to cable.
RG−6/U happens to be the lowest cost cable and it has loss comparable with the most expensive cable in the group, being RG213.
Coax has two limits to its ability to handle power:
Voltage breakdown is near instantaneous, and operating voltage is higher with VSWR>1. Nevertheless, for most applications, voltage breakdown is less limiting than overheating.
The breakdown voltage of RG−6/U is typically around 2.7kV, which would be reached at a power level of nearly 50kW in a matched 75Ω system.
The maximum power will also be limited by the power lost in cable loss and which needs to be dissipated without raising the cable above its maximum operating temperature of typically about 80°C.
Fig 2 shows the average power handling capability of common coax cables operating in still air at 40°C and with VSWR=1 and based on the loss assumptions stated above. Fig 2 is reverse engineered from a table in the ARRL Antenna Handbook using the loss model used for the Transmission Line Loss Calculator, and figures for RG−6/U and LMR240 estimated considering the dielectric type.
Operation of a cable at VSWR>1 reduces it power handling capability. The effect is that power handling is due:
On the assumption that most power that is lost is lost as I^2*R loss in the R element of the RLGC model, average power handling capability is reduced by a factor equal to the VSWR, so for example if the capability of RG−6/U at 7MHz is 1580W with VSWR=1, it would need to be de-rated to 1580/2 or 790W if VSWR=2.
Voltage breakdown is an instantaneous effect and so limits the peak power at which a cable may be operated.
Because of thermal inertia, the cable temperature increases as a result of power averaged over time. The duty cycle (transmitter on / transmitter off cyle) and modulation type may reduce the average power for a given peak power.
For example, considering only the modulation type, the peak to average ratio for compressed SSB is about 15dB, for a factor of about 30. Considering again the example of RG−6/U at 7MHz is 1580W with VSWR=1, it would withstand 30*1580W or 47kW of SSB telephony from a thermal limit perspective, but for A1 type Morse code with a peak to average ratio of about 2.2, the thermal limit would be 2.2*1580W or 3.5kW.
If the period of on/off duty is short compared to the thermal time constant of the cable, duty cycle can also be considered in rating the cable for the specific application.
Connectors for RG−6/U are usually specific to the type of cable construction. Connectors are readily available for F, RCA and BNC. BNC connectors are the most interesting for transmitting applications. F connectors have a significant disadvantage in that the connector shield connectivity is very depend on the tightness of the nut (like some other kinds of connectors like UHF connectors) and is less suited than the BNC where both the inner conductor and shield connectivity are independent of the mechanical retention.
Fig 3 shows a Compression type BNC connector for RG−6/U before installation and the cable trim dimensions for the BNC connector. Sixteen mm of cable jacket should be inserted inside the un-compressed connector for proper termination, and it is worth marking the cable with tape to visually ensure full engagement.
Fig 4 shows the cable preparation detail and assembly for compression BNC connectors. The picture is over simplified, in that the plastic sleeve that is 'compressed' is actually slides into the connector body taper, pressing the coax outer conductor against the barbed tube that slides between the dielectric and the shield, and the final step is the concertina of a section of the plastic sleeve into the jacket as shown.
Note that 6mm of exposed braid and shield should be folded back over the outside of the jacket, see Fig 5. With quad shield cables, the outer foil can be cut off and discarded, but the inner foil should be folded back as shown to facilitate correct engagement of the connector. If the inner foil is glued to the dielectric, leave it in place and just fold the outer layers back. Cable preparation is made easier using purpose designed tools that are quite inexpensive. Without rushing, and taking time to check the work, it is quite easy to terminate a cable in less than a minute.
Fig 6 shows one type of tool that can be used for applying compression connectors. The BNC connector is shown prior to compression.
Fig 6a shows an inexpensive type of tool that can be used for applying compression connectors. There are similar tools that do F connector only, but this one can do F, BNC and RCA (see chromed adapters screwed into the side of the tool).
Connectors can be obtained with colour coded compression sleeves and / or colour coded identifying rings which install over the connector body. Most connectors of this type are designed to be waterproof when coupled to a BNC socket. Compression connectors are also available for RCA and F connector.
|RG−6/U dual shield||$37.00/100m|
|Cable trim tool||$13.00|
Table 1 shows the approximate cost (A$) of materials from my trade supplier, other suppliers may have different prices.
Price comparisons are difficult when budget value cables are not all available from the same supplier. Figure 7 shows the relative cost of the cables used in this article. The prices for most are from a trade supplier, retail prices are typically 30% higher. The supplier does not sell RG8/X type so no price is given, but it seems to sell in the US for around US$1/m (about A$1.10/m though it would probably sell for more than A$2/m in Australia). The LMR240 type price is from a retailer. These prices are for budget versions of the cables, but cables that are of adequate quality for the task.
The ability to make a soldered connection to the shield is a distinct advantage for terminating cable other than on compression or crimp connectors.
RG−6/U cable often uses a shield that is a combination of foil and braid. The foil may be aluminium, or more commonly alumininised plastic film. The braid may be made of copper wires, aluminium wires, or a combination of both. Copper wires used in the braid are often plated and have a silver colour, combination copper wires / aluminium wire braids are almost always plated copper.
Copper wires are solderable, some combination copper wires / aluminium wires are solderable. If a solderable shield connection is important, a cable with copper braid or copper wires / aluminium braid should be selected, and a sample tested for solderability.
For most HF transmitting purposes, there is no real need for the quad shield types of RG−6/U, a dual shield cable will be cheaper and easier to terminate. Single shield cables are somewhat rare, but with adequate braid covering are fine for HF.
CCS centre conductors are often used for RG−6/U. If the copper cladding is not sufficiently thick, skin effect losses on the centre conductor will be higher than indicated in Fig 1 at low HF frequencies. A cable with hard drawn copper centre conductor avoids this issue.
Some applications may use 50Ω antennas and may require the transmitter be presented with a 50Ω load. This section canvasses some techniques for using RG−6/U (Zo=75Ω) in such a system.
A broadband transformer can be constructed using a ferrite or powdered iron core to effect a transformation from 75Ω to 50Ω, the turns ratio would be (75/50)^0.5 or 1.23.
A quarter wave transformer is a narrow band impedance transformer, and depends on a quarter wave transmission line of Zo=(50*57)^0.5 or 61Ω between the 50Ω load and 75Ω line (and vice-versa). Unfortunately, 61Ω line is not readily available, and so needs to be constructed.
The twelfth wave transformer is a narrow band transformer, It is a series cascade of a section each of 75Ω line and 50Ω line of length nominally 30° each between the 50Ω load and 75Ω line (and vice-versa).
Fig 7 shows the length of the nominal twelfth wave sections for different impedance ratios. For example, the case of 75/50 is 29.3°.
V1.07 20/02/09 09:39:08 -0700 .
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