|Short Multiband Dipole|
A simple and effective multiband antenna array can be made by connecting a number of parallel dipoles, spaced a few inches apart, to a single low-impedance transmission line at a common feed point. But you may not have space for a full size dipole on each band of interest. So some of those dipoles may need loading, coils so that they fit in available space and resonate on a particular band of interest. An antenna like this will perform very well, at modest heights, as long as the distance space available (A in the drawing below) is longer than 0.2λ, for the lowest band of operation.
The drawing below is an example of what can be done. The drawing shows three dipoles. Two of the dipoles are dipoles with loading coils, to fit in available space. The other dipole is just a full size for a band where a 1/2 λ dipole will fit. This can be expanded to include any combination of shortened and full size dipoles that you wish. Just note that, as you add dipoles, things start getting complicated and heavy.
As an example, you might want an antenna array that covers, the low end of 80, 40, and 15 meters (old Novice CW area). But you only have a space of 60' (18.3m) to work with. A full size dipole for 80 meters and 40 meters will not fit in that space, so loading coils will be needed. But the dipole for the 15 meter band will easily fit in the available space.
Another example might be an antenna array for the WARC bands (30, 17, and 12 Meters), but you only have 40 feet of space to work with. While the 30 meter band antenna would require a loading coil, the 17 and 12 meter bands would not.
A good place to start might be the center feed-point. The drawing on the right shows one possibility. Its made from a 18" (45.7cm) section of 2" PVC pipe with end caps. It includes one Eye-Bolt, as a hanger, and three pairs of Eye-Bolts for the dipoles. On the bottom end cap, a coaxial connector is installed. The dimensions are not critical. However, the spacing between the dipole Eye-Bolts should not be less than twice the diameter of the loading coils. So if you are using loading coils with a 2"-3" diameter you should space the Eye-Bolts about 4"-6" (~10.2cm-15.2cm) apart. For a three-dipole setup, as in the drawing on the right, you might start with a 18" (45.7cm) length of PVC pipe. Extend that by 4"-6" (~10.2cm-15.2cm), for each added dipole beyond three.
You can, of course, customize the feed-point. While the drawing shows the Eye-Bolts wired internally, for each of the dipoles, they can also be wired externally. You might also use two binding posts instead of a coaxial connector at the bottom. This will allow you to attach a Balun for better feed-line isolation.
Listed below is a combination antenna length table and loading coil calculator. On the left side of the table, under Freq. (MHz), is a listing for each amateur band from 160 to 10 Meters. The frequencies shown are for the center of each band, but they can be adjusted to meet your needs. What you will find out, however, is that the loading coil calculations will not change very much. Next to that is the Dipole Length. For dipoles that are shorter than the Available Space (A), the require required lengths are listed. But if the Available Space (A) is less than required, for a particular dipole, only the available space is listed for the Dipole Length.
|Efficiency||Calculator for xxx Antenna.|
|160m||160m||None||Excellent||Available Space (A) =|
|80m||80m||None||Excellent||Distance (C), as a Percentage of B: %|
|60m||60m||None||Excellent||A = xxx|
|40m||40m||None||Excellent||B = xxx, C = xxx|
|30m||30m||None||Excellent||Required Inductance for LC1/LC2: xxx uH|
|15m||15m||None||Excellent||Coil Form OD: xxx|
|12m||12m||None||Excellent||Coil Length: xxx|
|10m||10m||None||Excellent||Turns: xxx, TPI: xxx, App. Q: xxx|
Next to the Dipole Length is the Efficiency Threshold. The length listed is 1/5 of a λ (20%) for the band of operation, for each band that will not fit in Available Space (A). Within the calculator, this length is used to determine the Efficiency. This is a rating as to whether a particular antenna is considered Good or Fair. A rating of Good indicates that there is enough wire in the air so that the antenna should provide good performance. While a rating of Fair means, don't expect too much. That antenna is just too short for good operation.
To the right of the Efficiency Threshold is the Loading Coil Inductance. If a antenna for a particular band will fit in the Available Space (A), "None" is listed.
- The Efficiency Threshold is 1/5 of a λ (20%) for the band of operation. If the Available Space (A) is less than this threshold, the antenna will still work, but its efficiency will be low and it will be rated as Fair. Antennas with a length (A) above the 1/5 of a λ, but less than a full 1/2 λ, are marked Good.
- If you select a band that is marked Full Size, 1/2 λ, no Loading Coil is required and ignore the Loading Coil information.
- The default Length/Diameter Ratio is set to 1.5:1. This will create an inductor with high Q
- The Loading Coil Outside Diameter (Coil Form OD) reflects the closest PVC and CPVC Schedule 80 pipe diameter. Schedule 40 pipe diameters are the same, and can be used. However, the wall thickness of Schedule 40 Pipe is less than Schedule 80 Pipe.