VHF/UHF Antennas - Using 75 Ohm TV coax instead of 50 Ohm Coax |
If you ask 10 hams about using 75 Ohm cable, you will probably get 10 different answers. The answers will be from one extreme to the other. Some use 75 Ohm coax exclusively and others never use it. Even though high grade 50 Ohm cable can be expensive, and 75 Ohm cable is readily available and cheap. With the proliferation of cable/satelite TV, often times, free. Some are afraid that it will somehow damage their equipment. Mistuning your transmitter might hurt something, but that has nothing to do with the type of cable you use.
Coaxial cable is simply used to convey a signal from one place to another. Generally there is a driver (transmitter) and receiver (antenna). When receiving, the antenna is considered the driver and the radio is the receiver. A transmitter and a antenna can be designed to use just about any impedance imaginable, but has been standardized over the years to 50 Ohms.
There are basically two types of coaxial cable normally used for amateur radio service. 50 Ohm and 75 Ohm. There are coaxial cables with other impedances, but they are seldom used for amateur radio service. For example, RG-62 which is 92 Ohms, and was normally used with IBM 3270 terminal connections. A 62 Ohm coaxial cable is often used by older equipment for interconnects. I think the old Heathkit SB-101 specified 62 Ohm cable for it's connection to the SB-650 Digital Display unit. But the RF Output was still 50 Ohms.
But within each impedance range, there are several different coaxial cables available. For example, in the 50 Ohm variety, there is RG-8, RG-8X, RG-58, RG-174, RG-213, RG-316, etc. They all exhibit a impedance of approximately 50 Ohms but differ in Velocity Factor, Attenuation, Power Handling, Insulation type, Diameter, Wire type, Shield type, etc.. The same holds true for 75 Ohm cable. The most common variety of 75 Ohm cables are RG-59, RG-11, and RG-6. Again, like with 50 Ohm coax, the physical differences change the properties.
One of the things you should look for in a coaxial cable is "Loss" or "Attenuation". Loss is generally rated in dB for 100 foot lengths of cable and is strictly the loss due to the resistance of the wire and operating frequency. Higher frequencies will experience more loss due to "skin effect", than lower frequencies. As an example, the loss in RG-59 is 1.0 dB/100', at 7 MHz (40 Meters). But the loss in RG-59 is 6.0 dB/100', at 144 MHz (2 Meters). 6.0 dbof loss means you will be losing three quarters (75%) of your signal just in 100 feet of RG-59 cable. And that is assuming you have a perfect match on the transmitter and antenna ends.
A similar table can be show for 50 Ohm cables.
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VHF/UHF Antennas - Loss Due To SWR |
| Line Loss, Due To SWR, Calculator | |||
|---|---|---|---|
| Set Parameters as Desired | Results | ||
| Line Type: | Matched Loss: 3.409 dB |
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| Line Length: | |||
| Frequency: | MHz | SWR Loss: 0 dB | |
| Load SWR: | : 1 | Total Loss: 3.409 dB | |
| Power In: | W | Power Out: 45.613 W | |
The charts in the previous section only deal with the ohmic losses. Those losses are mostly due to the size of the center conductor. It is important to note that the losses increase as the frequency increases. This is partially due to what is know as "skin effect". Alternating current signals, from audio and RF, tend to flow towards the outside of a conductor. This isn't as pronounced at low frequencies but still exists. As frequency increases, the flow goes further and further towards the outside of the conductor. This provides even more resistance to the signal. Cable with less loss usually has a larger center conductor.
But there can be other reasons for signal loss in a coaxial cable. There are losses caused by the transmitter-to-coax mismatch and the coax-to-antenna mismatch. The calculator on the right will give you and idea of what to expect in losses. While these losses are not as bad as the basic cable losses, they still add to the overall system losses.
In the calculator (above right), first select the kind of coax you have. There are many different types so just select one that is close to yours. The designators in parenthesis are the equivalent designators for some specific types. Then enter the length of your coax, the frequency of operation, the SWR you obtained at the transmitter, and the approximate power output of your transmitter. Your effective output power will be listed under Results.
The equations listed in the box on the right, are the equations used in the calculator. The reference to MIL-C-17, in the equations, is a link to the PDF of the document where the K1 and K2 values are obtained.
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VHF/UHF Antennas - 75 Ohm Coaxial Cable Identification |
Usually, the cable number is printed or embossed on the outside of the cable. Sometimes it is easily readable and sometimes not. If the number is embossed, you have to look very carefully with a good light reflecting off of the cable. The number should be listed every 2 or 3 feet.
But there are other ways of identifying the cable type. The physical differences that drive the properties can be used to ID the cable. It helps if you have a dial caliper. A manual caliper will do, but is more difficult to read.
RG-59 is the thinnest of the three and nominally measures
0.240 inches (6.15 mm). This is about the same size as RG-8X and has about the same flexibility.
The inner conductor is made of copper-plated steel. The dielectric is 0.145 inches (3.7 mm)
in diameter. The outer conductor is a braid of bare copper wires which covers 95 percent.
Note: There are two types of RG-59, RG-59/A and RG-59/B. While they are the same impedance, the
RG-59/A type has a slightly smaller outside diameter and slightly higher attenuation (dB/100').
RG-6 is the standard that cable and satellite companies utilize. The inner conductor is bare copper. The dielectric is 4.6 mm with an outer conductor made of aluminum polyester foil surrounded by a tin copper braid. The outer conductor coverage is 100 percent for the foil and 61 percent for the braid. the outer diameter is 6.9 mm.
RG-11 is used for when the cable tap is a very long distance from the residence. It is easily identifiable as it is a very thick cable (and hard to work with). The insulation diameter is 0.285 inches (7.24 mm) and the outside diameter is 0.405 inches (10.3 mm). The inner components are generally made of the same material as the the RG-6.
There may be other variations.
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VHF/UHF Antennas - Antenna Orientation |
A simple dipole can be oriented horizontally or vertically. This is also known as polarization. Generally, a antenna with horizontal elements, like a dipole, is horizontally polarized. Polarization is more important for VHF and UHF antennas (6m, 2m, 70cm), and less so for HF antennas. This is mainly due to the fact that VHF/UHF is generally line-of-sight and polarization effects are preserved. Using the same polarization on both sides of a communication reduces losses. Losses in the range of 3-6 db can be experienced if one station is using horizontal and the other is using vertical polarized antennas. That can mean the difference between making a contact or not.
On HF (3-30 MHz) signals generally travel over long distances and encounter ionization layers, mountains, and many man made structures. This twists and turns the signals so that initial polarization effects are mostly lost.
But on the VHF/UHF bands the type of polarization used depends on your mode of operation. For example, if you are on 2 Meters (146 to 147 MHz) and working FM (simplex or repeater), you would want to have a vertically polarized antenna. However, if your trying to use CW or SSB in the lower end of 2 Meters, a horizontally polarized antenna would be best. 6 Meters is also very similar. Vertical antenna for FM and a horizontal antenna for CW/SSB.
Antennas like the Ground Plane have both vertical and horizontal elements. But this type of antenna is vertically polarized and mostly omni-directional. The main radiating element is vertical and the horizontal elements are there to simulate a ground. The horizontal elements are the counterpoise.
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VHF/UHF Antennas - Building an Antenna for 2 Meters |
| 300 Ohm Twin-Lead, 2M Antenna | |
|---|---|
| Coax Type: | |
| Frequency: | MHz |
When I first started as a ham (1965), I used a very simple antenna on 2 Meters. It was a Folded Dipole made from 300 Ohm Twin-Lead and RG-58 coaxial cable. I taped it up on the wall over my transceiver (Heathkit Twoer) and had a lot of great contacts. But times were very different then. I was running 4-5 Watts AM. FM and repeaters were a thing of the future. However, what hasn't changed is that the same simple antenna will still work. It might he and advantage to orient it vertically when running FM simplex, or throuth a repeater, but it will still work. If you do turn the radiating element vertically, make sure that the coax comes away from the antenna element horizontally for 3 or 4 feet before going down vertically to your transceiver.
The small calculate (above left) and the diagram (above right) work together. The calculator provides you with the capability to select the type of coax your are using and the frequency of operation. Changing anything in the calculator will adjust the dimension on the diagram. The type of coax is really only applicable to the 1/2 Wavelength Phasing Section. The length of the coax down lead should not matter. Note that, even though the 300 Twin-Lead is 1/2 wavelength and the 1/2 Wavelength Phasing Section is 1/2 wavelength, their dimensions are different. This is because a Velocity Factor, which can range from 66 to 80 percent depending on the type of coax, is applied to the 1/2 Wavelength Phasing Section, making it shorter. Higher quality cables tend to have better Velocity Factors, making them longer. You should find that the dimensions are not critical. From one end of the 2 Meter band, to the other, may only mean a difference of 1 inch or less.
Making the antenna is simple. Common 300 Ohm Twin-Lead uses #20 - #22 stranded wire for the conductors. The conductors are separated by about 0.3 inches (7.5mm) and are kept in place by a plastic sheath. For 2 Meters (146 MHz) you would need about 3' 2-1/2", (0.977 M). Note: Cut a section of Twin-Lead about an inch longer on either side. Then remove that extra inch of insulation from each side and expose the wires. The end wires are then twisted together and soldered.
Now find the exact center of the Twin-Lead. You can use measurements or carefully fold it in half and mark the exact center. Then cut one of the conductors and strip about 1/2 inch of insulation from each side. This should give you two pieces of stranded wire about 1 inch apart and will then be the feed point.
The coaxial cable to your transceiver can be almost any length. But when you are using a high loss cable, like RG-58 or RG-59, keep it as short as possible. Otherwise you will more power in the feedline than necessary. At the feed point for the 300 Ohm Twin-Lead, attach the coax that goes down to your transceiver and the loop of coax. The loop of coax is known as a BALUN and is for interfacing the BALanced antenna element (the 300 Ohm Twin-Lead) to the UNbalanced coax.
At the point where the coax connects to the antenna element, note how the coax center conductors and shields connect. All of the shields are simply soldered together by wrapping a piece of small gauge bare wire around each of the shields and then soldering to the shield. Keep your heat low on the soldering iron and be careful not to melt the insulation between the shield and the center conductor. After soldering the shields together, you should have three wires from the center conductor of the coax. The center conductor from one side of the loop is soldered to one side of the antenna element feed. Then the center conductor from the other side of the loop, and the center conductor from coax going to your transceiver, are twisted together and connected to the other side of the antenna element feed
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