Wire Gauge Standard VS American Wire Gauge

Standard VS American Wire Gauge

SWG	Diameter (inch)	Nearest AWG
12	0.104	10
14	0.08	12
16	0.064	14
18	0.048	16
20	0.036	19
22	0.028	21
24	0.022	23
26	0.018	25
28	0.148	27
30	0.0124	28
32	0.0108	29
34	0.0092	31
36	0.0076	32
38	0.006	34
40	0.0048	36
42	0.004	38
44	0.0032	40
46	0.0024	--

Standard vs American Wire Gauge.

Antenna Insulators

Common materials that you can use for insulating hardware when building antennas. Homemade insulators are inexpensive. I encourage you to make them, especially for your dipoles.

High-quality insulators are mandatory for good performance. A half-wave dipole has very high RF voltage at the outer ends. This means that your end insulators must have a high dielectric factor (high breakdown voltage and infinite resistance). I suggest that you use insulators that are made from polyethy- lene rod or tubing (available from industrial plastic vendors). High-impact polystyrene rod or tubing is also good. Plexiglass T™) is also a good insulating material, but it is brittle and shatters easily, especially during cold weather. It is best to avoid this material for long dipoles. You should also avoid nylon insulators, as they may heat and burn when subject- ed to high RF voltage. PVC pipe and tubing is similarly poor in the presence of high RF voltage.

If you purchase commercially made insulators, try to obtain glazed porcelain ones. Another acceptable commercial insulator is made from molded polyethylene. Radio Shack stores. stock this type of end insulator. They are available also from farm stores that sell electric-fence components. Fig 4 shows how to fashion your own insulators from tubing and solid rod.



Fig 4 - Examples of homemade antenna insulators. The unit at A is made from solid plastic rod. Grooves may be cut at the ends, as shown, with a router. This relieves the stress on the antenna wire and helps to keep the wire from shifting position on the insulator. Suggested dimensions are given for developing a strong insulator. Example B shows how to make an insulator from plastic tubing. A rectangular plastic block (C) is also suitable as an insulator. Grooves can be added to the ends of this unit also.





If you are willing to spend additional money when building high quality homemade insulators, please consider the use of Delrin or Teflon rod. These materials are also available from industrial plastics dealers.

For short-term emergency situations you can make your dipole end insulators from 4-inch pieces of 3/4-inch dowel rod. Drill the holes in the wood, then boil the wooden insulators in canning wax for 10 minutes. The wax will impregnate the wood, which will prevent it from absorbing moisture and becoming lossy. Alternatively, you may soak the dowel-rod insulators in exterior polyurethane varnish for 24 hours. Allow them to dry thoroughly before using them.

Dipole Conductors

The general rule is to use wire for HF-band dipoles -- no. 12 or 14 being the most popular size. Aluminum tubing is used for HF, VHF and UHF Yagi antennas because this material is strong enough to be self-supporting. HF-band Yagis are equipped with elements that telescope. This reduces the overall weight by permitting us to use small-diameter tubing for the outer ends of the elements. The telescoping antenna elements also allow us to make easy adjustments when tuning the system for low SWR and maximum forward gain.

I suggest that you use stranded no. 14 copper wire for your HF-dipole elements. This wire is easily available and it does not cost a great deal of money. It is sufficiently flexible to endure under the stresses of wind and moderate icing, assum- ing it is supported properly. If the span of the dipole is greater than 130 feet, try to have a center support where the feed line is attached. RG-8 coaxial cable is quite heavy, and this places considerable stress on the dipole. RG-8X, on the other hand, is smaller and lighter, and may not cause too much stress at the center of the dipole. RG-8X will safely handle up to 1000 watts of RF power if the SWR on the line is less than 2:1. It is approximately the same diameter as 75-ohm RG-59 cable.

The Noise Bridge

There is another instrument that you may use for resonance tests. It is known as a noise bridge. It is connected between your receiver and the feed line to the antenna. Your antenna will be purely resistive at resonance, so the null in the noise from the bridge will be the deepest at the frequency of antenna resonance.

The reactance controls on the noise bridge will be at zero when this null occurs. The bridge will provide a resistance reading when fully nulled, and this will indicate not only antenna resonance, but allow you to read the feed-point impedance (resistive) of the antenna. In order for this instrument to be accurate, you must use an electrical half wavelength of coaxial feeder between the antenna and the bridge.

The instrument generates white noise, and this is heard in the receiver output (speaker of phones). The bridge controls are adjusted for minimum noise to indicate antenna resonance. You will need to tune your receiver to various frequencies in the ham band of interest in order to locate the resonant frequency of the antenna.

The noise bridge is readjusted for a null at each of these frequencies until you find a frequency that yields a deep null with the reactance controls at zero. This may seem complicated now, but if you study the operating booklet for your noise bridge, things will fall into place easily.

source : Novice Antenna Notebook by DeMaw, Doug

Finding the Correct Dipole Length

We have discussed the standard formula for cutting a dipole to length [L = 468/f(MHz)]. First, you may wonder why 468 is used instead of the free-space factor of 492 for a half wavelength. This is a good question: If our dipole was in free space and with no feed line, the length would be greater than that obtained from the 468 factor. The end effects of the insulators and guy lines, plus antenna proximity to trees, the earth and other influences, causes the antenna to have stray-capacitance effects that tune it lower in frequency. 

You may think of the dipole as an inductance, and stray capacit- ance appears in parallel with it, just like in other tuned circuits. These effects are compensated for by changing the length factor from 492 to 468.

Let's assume that you have cut your dipole wires in accordance with 468/f, as discussed earlier. How can you be certain that this is the correct length (antenna installed) for your chosen frequency of operation, say, 3.7 MHz?

 First, you need to know that the lowest SWR occurs at the resonant frequency of your antenna. It may not be the ideal 1:1 ratio we seek. Rather, it may be 1.5:1 or even 2.3:1. If the SWR is high at the reson- ant frequency of the dipole, you will know that the feed line is not matched to the antenna feed point. The SWR will rise either side of the resonant frequency as you change the operatYou will need an SWR indicator or bridge, as they are called, in order to check your antenna resonance and SWR. 

The SWR indicator must be designed for the impedance of your feed line, such as a 50-ohm bridge for 50-ohm coaxial line. The SWR instrument is installed between your transmitter and the feed line. 

Next, check across the amateur band for which your dipole is cut, observing the SWR reading every 25 kHz or so. Note the frequency at which you obtain the lowest SWR reading (minimum reflected power). 

This will occur at the resonant frequency of the antenna. If the lowest SWR occurs lower in the band than your design calls for, remove small amounts of wire from the ends of the dipole until the lowest SWR is noted at the preferred antenna resonance point. If the lowest SWR reading is observed higher than your chosen frequency, you must add a small amount of wire to the ends of the dipole. Continue this process until the lowest SWR occurs at the chosen operating frequency.

You may use a dip meter for checking the resonance of your antenna. You may do this by connecting a three- or four-turn small coil between the shield braid and inner conductor of your coaxial cable (antenna erected, and feed line attached). A solid-state or tube type of dipper may be used. Insert the dip-meter coil into the small coupling coil on the feeder. Adjust the dipper through its range and locate the frequency at which a deep dip is noted on the meter. Listen to the dip- meter operating frequency by means of your receiver, and note the frequency. Adjust the dipole length until the dip meter indicates resonance at your preferred operating frequency.

We will discuss methods for mismatch correction, later in in this notebook. A minor mismatch (1.5:1 SWR or less) is not of concern to us for routine operation, provided the trans- mitter is designed to deliver normal output power at low values of SWR.

source : Novice Antenna Notebook by DeMaw, Doug