A dipole antenna is a good antenna for single and multi-band use. The dipole is simple and inexpensive to make.  While is possible to install a remote tuner in the attic and bring coaxial cable down from the antenna system to your station for a 1:1 SWR on all bands consider the tradeoff between power lost, overall antenna/tuner costs, and the antenna radiation efficiency.  

Keep in mind that physically short antennas, those whose length is 1/8 wave length or less are extremely inefficient.  For example, a 20 foot long non-resonant dipole may present a 1:1 SWR with a remote tuner at 80 meters but the actual signal radiated is only around 10% of the delivered power.  In other words, an SWR of 1:1 does not mean that your antenna radiates well.  A 50 ohm resistor placed at the end of a piece of coax will show a 1:1 SWR but will radiate very little RF.

As an alternative to a remote tuner, consider using RG8 or LMR400 coax cable rather than RG58 or even RG8X.  For feedline lengths of under 100 feet, the loss due to high SWR is minimal at HF.  For example, the additional loss in a 50 foot length of Belden 9913 (RG8) due to a 3:1 SWR is 0.14 db.  The total feedline loss is 0.37 db.  Thus a 100 watt output from a transmitter is reduced to 91.766 watts at the antenna which is insignificant.  Any antenna tuner, either remote or the one in your radio has a loss of about 10 percent.  In other words, even at a perfect match, you will lose 10 watts in the tuner in your rig or 10 watts in the remote tuner. 

Band       MHz        Length
10m           28.4        16' 6"
15m           21.1        22' 2"
20m           14.1        33' 2"
40m             7.1        65' 10"
80m             3.6       130' 0"

The pattern of any HF dipole becomes less and less directive as the antenna height is reduced below 1/2 wave length above ground.  In addition the elevation pattern approaches that of a sphere – the antenna approaches an NVIS antenna colloquially known as a “cloud warmer”. See radiation elevation plot for ¼ vertical vs. dipoles.

If the attic area is not large enough to accommodate an almost full-size dipole, simply erect as much of the antenna as possible in a straight line, then bend the ends of the dipole up, down, or sideways from the main portion of the system   For a balance dipole, attempt to maintain symmetry in the system by bending the ends of the antenna in equal amounts.

Shortened Dipole Antennas

A physically shortened dipole is a practical alternative to a full size antenna for the upper HF bands. There are several alternatives for shortening a dipole antenna to fit in the available space;

• 50% linear loading line in each leg of the dipole without loss.
• double bazooka with the same results as a linear.
• install a loading inductor in each leg of the dipole with some loss. Here is a link to information on loaded dipoles including an interactive calculator:  https://k7mem.com/Ant_Short_Dipole.html

The typical 2 car garage is about 20 foot by 20 foot with an attic storage space above.  A loaded inverted V dipole for 20 meters can be hung from the top of the roof ridge truss with the dipole legs extending down to the attic floor along the truss legs.  By adding legs of 22 feet and 16 feet the dipole becomes a fan dipole and covers 20, 15, and 10 meters with a single feedline.  Coverage of 17 and 12 meters can be obtained by using the tuner in your rig or an external tuner.  

Off Center Fed Dipole Antennas

A variation of the basic dipole is the OCF or off center feed dipole.  A 68 Foot antenna that can provide a reasonably good match across multiple bands which are even harmonically related, such as, 80, 40, 20, 10 and 6M. The OCF dipole is like a standard horizontal dipole but fed at a position other than the center typically at the 1/3 – 2/3 position. The OCF antenna gives lower SWR with wider bandwidth over more bands than the small length center fed dipole. Remember the OCF dipole has an impedance of 200 to 300 Ohms so you will need a 4:1 or 6:1 UNUN if you do not use a remote auto-tuner. See SWR plot for center vs. OCF feeds.  Click here for detail explanation of how the OCF Dipole works.

BALUNs and UNUNs

A BALUN is a simple device that converts a balanced source/destination to an unbalanced destination/source.  Coax cable is unbalanced.  A dipole is for the most part a balance antenna.  By placing a 1:1 BALUN as the feed point of the antenna, we convert the unbalanced feedline into a balance load and keep the outside of the coax braid from acting as part of the antenna.  This reduces RF current flowing on the outside of the coax shield back into the shack.  A folded dipole has a feed point impedance of 200 ohms which requires a 4:1 BALUN.  The BALUN simultaneously converts from balanced to unbalanced and transforms the 200 ohm feed point impedance to the 50 ohm coax feedline impedance.  An UNUN on the other hand is a device that maintains unbalanced to unbalanced matching but has the primary purpose of preventing the RF from the antenna flowing down the coax and back into the shack.  It is primarily an RF choke.  An UNUN can be placed at the shack end of a feed line to effectively block any RF current that may be coupled into the feedline from the antenna from flowing back into the radio.  It should also be placed at the feed point of any unbalanced antenna such as a vertical.  How can you tell if you need an UNUN?  Set your transmitter internal meter to SWR, key your transmitter, and note the SWR.  While keying the transmitter, place you hand on the feed line or move it around.  Does the SWR change?  If so then your feedline is behaving as part of the antenna and could benefit from an UNUN at the transmitter end. 

If you would like more information on deed restricted antennas other interesting presentations click here.

More information on dipole antennas click here