Single and Multi Band Dipoles:

Looking for a simple, reliable afforadable multi-band antenna? This is it! This is without a doubt the best antenna for the money. It offers flexible construction options and the parts are within any ham's financial reach. An added bonus with any horizontal antenna is reduced noise. Most naturally-occurring noise is vertical and as much as a 25 dB reduction can be found simply by operating horizontally. An additional reason to use a dipole over a vertical is that many less expensive verticals require several radials of 1/4 wavelength. A couple of those and you have a dipole anyway, only without the noise reduction. So why not just operate a dipole?

The Basic Dipole
Using the initial principle for a 1/2 wave dipole, the length is determined from the free space wavelength, then, shortened to account for the velocity factor of the wire. Free space wavelength is found from the formula C/f, speed of light/frequency. C = 300,000,000 meters/second. So let's use 10 meters for an example. 300x10^6/28x10^6 = 10.7 meters. To convert meters to feet multiply by 3.278 feet/meter. This yeilds 35 feet for a full wave. Thus a 1/2 wave is 17.5 feet. Taking all of this together yeilds a formula for a half-wave (free space) in feet of 492/Freq(MHz) or 492/28=17.5'. Simple so far? But when you want to make a piece of wire resonant, the velocity factor of the wire must be considered. In this case we find that the radio wave travels a little faster in wire by about 5%. So the velocity factor is 0.95. Multiply the (492/f)x0.95 and you now have 468/f(MHz) 468/28= 16.7'.

So, for any frequency, a half-wave dipole using bare wire can be estimated by this formula, 468/f(MHz). Of course, there are other factors that you will need to consider. The area in which you string up the wire will likely have effects that make the dipole more inductive or capacitive depending on the material and proximity of nearby objects. So some tuning will be required.

Feeding the dipole
A simple dipole will have a feedpoint impedance around 90 ohms when stretched flat over nominally conducting earth at a height of at least a 1/2 wavelength. You can lower it's impedance nearer to 50 ohms by lowering the ends to form an Inverted V with an enclosed angle between 120 and 90 degrees. Doing this will also give you some vertical radiation in the plane of the dipole as well as some broadside to the antenna. So this helps provide a more omni-directional pattern.
Feeding this with coax is an easy and effective method. Do not listen to all those who say this won't work or is very lossy. Providing that you operate this antenna nears it's resonant frequency(ies), it will not present sufficient high SWR to be lossy at HF using coax.At HF frequencies the losses are minimal if you cut the antenna to resonance. Even if you opertate a little off the resonant frequency a good grade of RG-8 or RG-8X will not have significant loss. However, if your intention is to operate all bands with a tuner, than by all menas, consider using open wire line, as the SWR will start at about 6:1 (on the line) and get worse from there.
If you wish to feed this with coax you will need some form of balun (balanced to unbalanced) transformer. There are three ways to go about this. The idea behind the balun is simply to isolate the currents flowing on the antenna from flowing down the outside of the coax. The simplest form of balun is a "choke". This can be most easily made by winding several turns (about 10) of the coax into a coil about 6" in diameter right below the feedpoint. A better "choke" type balun uses ferrite beads. These are sold commerically as the W2DU balun and comes with an SO-239 for the coax on the bottom and connections for the dipole wires and a supporting rope on top. These too are quite inexpensive and very effective. Both of these choke type baluns work simply by presenting an inductive reactance to the current on the shield. The currents flowing in the center conductor and on the "inside" of the shield balance each other out and do not create any inductive effect in the choke or surrounding ferrite sleeves. Essentially, these currents are in balance and the current which seeks to flow on the outside of the shield remains alone to be inductively impeded by the choke. Neat, huh? And lastly, there is the more classic transmission line transformer usually wound on a ferrite toroid. I use this type of balun and have been very happy with it. It works eseentially identical to the ferrite bead type of balun but I prefer it for it's smaller size and "self sheilding" properties.

If you choose to use balanced line you'll still need something to transform the radio's unbalanced feed to the ladder line. Preferrably this would be a BALANCED tuner, NOT A T-MATCH TUNER WITH A BUILT IN BALUN! If you already have a good T-match tuner that you'd like to use a simple 1:1 antenna balun will work as well. Why 1:1 when going to a higher impedance line? Because the antenna impedance is what is ultimately being matched, not the impedance of the line. You could choose to match the line to the antenna with a 4:1 and a 1:4 at each end of the transmission line but it's not necessary. To delve into this subject further here would be best left for another writing.

Multi-band dipoles
Now here is where these antennas really are cool. You can use the same feedline and balun to connect additional dipoles for additional bands. I've made several of these over many years and they work much better than the so called multi band antennas like the G5RV. G5RVs are NOT multi band antennas! See THE G5RV Mystique for more info.
I have two antennas like this that I use primarily. My home station uses 160, 80 & 40 meter antenna elements with the radiating elements spaced apart 5 1/2" apart using drilled spacers of 1/2 PCV pipe. My portable antenna covers 80, 40 (15), 20, and 10 meters. If you want to do this you should trim the wires to resonance starting with the highest frequency first. You'll notice there's no 15m element. That's because the 40 meter wire will resonate on 15 using its third harmonic.

Different wire types
I started this using bare wire as the basis for design. Although many people like using readily available, cheap electrical wire which is insulated. This works fine but may stretch (or even break) after many years. I use stranded copper #12 wire with THHN insulation and it has stayed up for 10 years so far. Also, the insulation tends to capacitively load the wire making the resonant frequency drop. So these antennas should be cut shorter than the 95% figured above and (use 0.9 instead of 0.95) then cut to resonance once in place. But cutting to resonance should be done for any antenna anyway.

Now, if you want to get fancier, or make your antenna shorter you can add coils to load the antenna for resonance at frequencies far below what the length would normally resonate at. The ARRL antenna book gives a handy chart showing length versus coil reactance and location to make a variety of these designs. Or you could add traps to make a shortend 80m antenna (100') resonante on 40m. There's lots of design variation to accomodate any location and operating frequency.

For the money you'll invest in this antenna, and the performance you get once tuned to resonance, you'll likely never consider spending the hundreds of dollars these all-band wonders claim for their wire antennas. But don't take my word for it, just listen to the signals on the band of stations running wire antennas versus a psuedo all-band wire.
Just remember, an antenna does NOT need to be resonant to radiate effectively. The purpose of using resonant antennas in this application is to provide a reasonable match without requireing a tuner and thus permitting the use of coaxial cable.