PARALLEL DIPOLE

The concept on which this is based is that with antennas at widely different frequencies each will be resonant within its band and will have sufficiently high impedance on other bands so it doesn't readily accept power from the transmission line.  The arrangement is often successfully employed, however there are two pitfalls that can limit the usefulness of this approach:

1.  In addition to the connected parallel impedance of the additional dipole(s), there is significant mutually coupled impedance that may be hard to predict.  It will range from negligible to significant as the direction of the second dipole is changed from (its azimuth) being perpendicular to, toward being parallel to the first.

http://www.qsl.net/ta1dx/amator/practical_dipole_antenna.htm
Wires in close proximity tend to couple and produce mutual inductance.  In parallel dipoles, this means that the resonant length of the shorter dipoles lengthens a few percent.  Shorter antennas don’t affect longer ones much, so adjust for resonance in order from longest to shortest.  Mutual inductance also reduces the bandwidth of shorter dipoles, so a Transmatch may be needed to achieve an acceptable SWR across all bands covered.  These effects can be reduced by spreading the ends of the dipoles.

If the wires at the center feed point are separated by at least 5 1/2 inches vertically and the ends separated by 38 inches in the 2 to 18 MHz range, it is possible to accurately cut the antenna element lengths for given frequencies using the formula 

	Length in feet = 234 / frequency in MHz  

(With some consideration of:
Lowest freq antenna  468/freq X .96
Middle antenna 468/freq X 1.04
Lowest antenna (highest freq antenna) 468/freq X 1.04}

 and eliminate the need for pruning.

2.  While the impedance (of non-resonant elements) may be high, except at (odd or 3rd) harmonic frequencies, it will generally be reactive resulting in a need to retune the antenna lengths compared to a single-band dipole.

You must do some trimming to make everything work. The good news is that any energy that goes into the off-band dipole will be radiated, although not necessarily in the desired direction.

Using soft copper wire that significantly stretches overtime due to wind loading will eventually de-tune your longest elements.   Copper-clad steel wire for the longest element will assist in overcoming this.

http://www.eham.net/articles/12399
Tune in VERY small increments, check each band before and after each adjustment. Above all, be patient and do not be surprised if you have to make lots of adjustments to get it right.

http://www.eham.net/articles/10974
The adjustment method is to adjust the highest band (longest/lowest frequency antenna) to match at the bottom of the band, and then adjust the next shorter (higher frequency) band.  Then check for how much the resonant point of the first antenna has changed (as long as it has not moved above the middle of the band, you should not need to re-adjust this one yet).  If you had to re-adjust that band, re-adjust this band. When the highest band (longest/lowest frequency antenna) is still below the middle of the band and the next band is at the bottom of the band, move to the third and repeat the steps done before. Repeat all these steps until you have all your bands done.

http://www.eham.net/articles/21407
Optional: Cut the wires to the bottom of the band, and leave (1’ to 4’) dangles hanging from the ends. When tuning, all you have to do, is fold back the ends, and twist them together. This makes tuning easier, and actually adds a little end effect (a good thing).  Increasing the dangle length of a lower frequency element  appears to lower the resonant frequency of higher frequency elements; dangles of shorter elements may also raise the resonant frequency of that dipole element.  Attach small non conductive weights at the drop wires after final tuning. (see: Loose Ends below.)

It maybe necessary to make one leg of an antenna shorter or longer than the other leg, due in part to stray capacitance from a rain-gutter, etc.  The lowest SWR will occur when one leg is a slightly different length than the other leg.

http://www.qsl.net/ta1dx/amator/practical_dipole_antenna.htm

The length of a half-wave in feet is 468/frequency in MHz, but because of interaction among the various elements, some pruning for resonance may be needed on each band.  Therefore, the length of each quarter-wave element may be roughly calculated by length in feet = 238 / frequency in MHz.

How much to prune: When assembling the antenna, cut the wire 2 to 3% longer than the calculated length and record the length.  When the antenna is complete, raise it to the working height, and check the SWR at several frequencies.  Multiply the frequency of the SWR minimum by the antenna length and divide the result by the desired f0.  The result is the finished length; trim both ends equally to reach that length and you’re done.

Loose ends: Here’s another trick, if you use nonconductive end support lines. When assembling the antenna, mount the end insulators in about 5% from the ends. Raise the antenna and let the ends hang free. Figure how much to prune and cut it from the hanging ends. If the pruned ends are very long, wrap them around the insulated line for support.

Altering the element "droop" between straight and inverted-vee style is helpful in fine tuning each element after you get them cut pretty close. Straight gives higher impedance (toward 73 Ω), dropping the ends (inverted-vee style) lowers the impedance (toward 50 Ω) so you have a useful range of adjustment available there.   The characteristic impedance of a (straight) dipole antenna is normally 73 ohms.  Raising the antenna above ground lowers its impedance. 

An harmonic of a particular frequency (the fundamental frequency f) is another frequency that is an integer multiple of the first one. For example:
 
    if f = 125MHz, the harmonics would be: 2f = 250MHz, 3f = 375MHz, 4f = 500MHz ..... etc

Resonances repeat, for half­wavelength-long dipoles, at odd multiples of the fundamental-resonant frequencies. For instance, a dipole resonant at 2.5 MHz is also resonant at 7.5 MHz (3 * f), 12.5 MHz (5 * f), and so on. These higher-frequency resonances are known as harmonic resonances.

The power-distribution mechanism requires that only one of the parallel dipoles is near resonance on any amateur band. Separate dipoles for 80 and 30-meters should not be parallel connected because the higher frequency band (30-meters) is near an odd harmonic of the lower band (80/3 ≈ 30) and center-fed dipoles have low impedance near odd harmonics. (The 40 and 15-m bands have a similar relationship.) This means that you must either accept the lower performance of the low-band (higher frequency) antenna operating on a harmonic of the lower frequency antenna, or erect a separate antenna for those odd-harmonic bands. For example, four parallel-connected dipoles cut for 80, 40, 20 and 10 m (fed by a single Transmatch [antenna tuner] and coaxial cable) work reasonably on all HF bands from 80 through 10 m.

Use a tuner at higher powers to attenuate harmonics and any possible spurious transmitter outputs.  

By using open wire or the ladder line you will not have to do any pruning of the half waves, also with the open wire you can use the 80 meter length for 40 meters and get a gain of 1.8 dB over a dipole on 40 meters. Use the 20 meter length for 10 meters and also get a gain on 10 meters of 1.8 dB gain over a dipole.

Potentials at the ends of a resonant dipole can be in the kilovolts—even with a 100-watt transmitter.  Insulate the ends of the elements well.

Parallel dipoles have narrower bandwidth than single dipoles.

Because 21 MHz is the third harmonic of 7 MHz, 7-MHz dipoles are harmonically resonant in at least a portion of 15 meters, Capacitance Hats can be used to capacitively load a 40-meter dipole to lower its resonant frequency anywhere you choose in the 15-meter band without substantially affecting resonance on 40 meters.  At a point about ¼ wavelength (at 21 MHz, or 11’) from the feed point of the 40-meter dipole solder a two-foot piece of stiff 14 AWG wire with its ends soldered together and twisted in the middle to form a figure-8, about 1/3rd of the way out from the center (placement isn’t critical).  Adjust the loop shapes to resonate the antenna on 15-meters.

Antenna height above ground:
Install a half-wavelength high or more; the higher in the air you can get it, the better.

http://www.eham.net/ehamforum/smf/index.php?topic=26799.0;wap2
Which leads us to a discussion of radiation angle in general.  When a horizontally-polarized antenna is less than about 3/8 wavelengths high, most of the radiation will be at high angles due to the ground reflection.  On frequencies below about 10 MHz, there are times when this RF will reflect back down to the earth at or near the antenna and provide useful short range communications (eg. NVIS) out to a few hundred miles.  This depends on the current ionospheric  conditions, time of day, time of year, etc., but commonly gives good coverage on 40m during the day and 80 or 160m at night.  As the antenna is made higher in terms of wavelength (or the wavelength is shorter for a fixed antenna height) the angle of radiation drops and the skip distances get longer.  Generally horizontal
antennas a half wavelength high or more will provide reasonable DX signals, while those only a quarter wavelength high much less so.  (There isn't a sharp dividing line - more of a continuous improvement as the antenna height is increased.)

Generally for heights of 1/4 wavelength or lower, vertical polarization works better for low angle radiation.  (This is due to the difference in how the waves reflect off of the Earth's surface.)  A common approach is to use the feed-line running to your horizontal antenna as a vertical wire to give you a choice of either polarization.  Another way to do this is try grounding one side of the balanced feedline below the antenna feed-point.

W4RNL's web site (www.cebik.com) article(s) on vertical radiation patterns as a
function of height above ground for horizontal antennas.  Basically, any horizontally polarized antenna at a given height above ground will have the same vertical pattern, which is determined by the height and not by the design of the antenna itself.

Avoid polypropylene rope; it rapidly deteriorates in sunlight.  Use woven Nylon (stretches) or Dacron (best UV resistance) rope.
Useful rope knots: 
    Bowline: unsliding loop

    Clove hitch: attach rope to poles

    Tautline hitch: Sliding knot for adjusting tension in a rope

    Sheet Bend: joining ends of two ropes

http://www.eham.net/articles/12399 
Parallel DIPOLE OR MULTIPLE BAND DIPOLE SPECS: Each leg in length is shown so you will need two legs.)
BAND	FREQUENCY MHZ	3RD HARMMONIC MHz	TARGET FREQUENCY MHz	WAVELENGTH FEET	
				TARGET 1/4	1/4	1/2	
6-METERS	50.000 – 54.000		52.000	4’ 6” *	≈ 4’ 6”’	≈ 9’	
10 METERS	28.000 – 29.7000	86.400	28.800	8’ 1-1/2” *	≈ 8' 4"	≈ 16’ 8”	
12 METERS	24.890 – 24.990	74.880	24.960	9’ 4-1/2” *	≈ 9' 5"	≈ 19’	
15 METERS	21.000 – 21.450	64.088	21.363	10’ 11-1/2”	≈ 11' 1"	≈ 22’ 2”	
17 METERS	18.068 – 18.168	54.417 (6-M)	18.139	12’ 10-3/4” *	≈ 12' 10"	≈ 25’ 8”	
20 METERS	14.000 – 14.350	42.863	14.288	16’ 4-1/2” *	≈ 16' 8"	≈ 33’ 4”	
30 METERS	10.100 – 10.150	30.300 (10-M) 50.500 (6-M)			≈ 23' 2"	≈ 46’ 4”	
40 METERS	7.000 – 7.300	21.713 (15-M)	7.238	32’ 4” *	≈ 32' 9"	≈ 65’ 6”	
60 METERS	5.330.5 – 5.405	16.074	5.385.5	43’ 8”	≈ 43' 7"	≈ 87’ 2”	
75 METERS					≈ 60' 9"	≈ 121’ 6”	
80 METERS	3.899 – 4.000	11.700 (30-M) 19.500 (15-M) 27.300 (10-M)	3.900	60’	≈65' 6"	≈ 131’	
160 METERS					≈ 123' 5"	≈ 246’ 10”	
137 KHz					≈ 1,708' 1"	≈ 3,416’ 2”	
* Elements for this site and General FCC license.