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A Simple Multi-Band Wire Antenna for HF

Ladderline

Ladderline. By Gerry Ashton (Own work) [GFDL, CC-BY-SA-3.0 or CC-BY-SA-2.5-2.0-1.0], via Wikimedia Commons

Few things are more versatile than a simple wire antenna when working HF. Presented below is the Ladder Line Fed Doublet Antenna, or simply, “doublet antenna”. This antenna is one of the most popular wire antennas since it is so simple to build and it’s usable on practically any band on which an antenna tuner can provide a match. And although this antenna requires the use of an antenna tuner, it is very efficient so long as it is at least 0.4 wavelengths (also noted using the λ symbol) or longer at the lowest desired frequency of operation. Feeding of the antenna is accomplished through the use of 300Ω ladder line. Finally, a 1:1 current balun1  along with a short run of high quality coax (LMR 400 or RG-213) allows for an easier entry to the shack and connection to your antenna tuner.

Formulas

First, determine the lowest desired frequency of operation. Typically this is either the 160 meter, 80 meter, or 40 meter band. Once the lowest operating frequency is decided, use the standard 1/2 wavelength dipole formula to determine the overall length:

Length = \frac{468}{Frequency}

It should be noted, however, that length is not critical here. So long as the overall length is > 0.4λ the antenna will work well.

In addition  it is important to keep the ladderline feed to odd multiples of 1/8λ. Doing so will prevent an easier match for your antenna tuner. This may be calculated by:

Length = \frac{123}{Frequency}(Velocity Factor)

Where the velocity factor may be obtained from the specs of your chosen feedline. 0.88 is a typical value for 300Ω ladder line.

Multi-Band Ladderline fed Doublet Diagram

Multi-Band Ladderline fed Doublet Diagram

Example

Suppose we wish to construct a doublet and our lowest frequency we wish to operate is 3.5 MHz.

The overall length of the antenna is:

Length = \frac{468}{3.5} = 133.7'

And rounding the length up to 135′ may be more convenient for measuring purposes.

The feedline should be an odd multiple of 1/8λ at the lowest frequency. So if our feedline has a velocity factor 0.88, the 1/8λ is therefore:

Length = \frac{123}{3.5}(0.88) = 30.9'

So if the required feedline distance is 75′, we should use 92.7′ of feedline since the 75′ distance from antenna feedpoint to the tuner is not an odd multiple of 30.9′. The excess ladderline may be “zig-zagged” or run in a non-direct route in order to use up the extra length. But under no circumstances should the ladderline be coiled or left to lie on the ground. Also, it is important to keep the feedline at least 6″ or more from nearby metallic objects.

So in this example, a doublet antenna that is operable on every band from 80 meters – 10 meters with a feed point located 75′ away from the radio, we need 135′ of wire and 92′-8″ of ladderline.

But before building the antenna, we must have a way to connect the feedline to the wire and hang the wire between a convenient set of trees.

Center and End Insulators

Ladder-Loc by WA1FFL

Ladder-Loc by WA1FFL

Electrically connecting the antenna wire to the feedline is simply a matter of soldering the wires to each side of the feedline. However, we must have a way to remove strain from the soldered connection or the antenna will fail mechanically in a very short period of time. You can either consruct a simple center insulator from plexiglass or PVC or purchase a specially made center insulator just for this purpose. Many choose to use the “Ladder-Loc” by WA1FFL which is available at many dealers.

DX Engineering Center and End Insulators

DX Engineering Provides a Kit Consisting of both Center and End Insulators

In addition to the center insulator, means of connecting a rope to the ends of the antenna for support is required. Again, this device may be constructed out of PVC or a purpose specific part may be purchased. These items are also available from nearly every dealer. DX Engineering also offers a kit that includes both the center and end insulators.

Balun

Once the antenna is built, we will need some way to feed it into our radio room and into the antenna tuner. While antenna tuners exist that can accommodate ladder line, most do not. Plus, feeding ladderline into a house is a more difficult than using coax. Thus a balun (short for BALanced UNbalenced) is useful. Simply connect the ladderline to one side of the balun and coax to the other side of the balun and connect to your tuner. For this antenna, a 1:1 current balun is recommnded in order to use coax as a means of ingress to your ham shack.

Note that you will want to use the highest quality coax possible. LMR-400 or RG-213 at minimum. Smaller diameters such as RG-8X, RG-58 and similar will simply have too much loss owing to the very high SWR present. The advantage of the ladder fed doublet is that ladderline has extremely low loss even in the presence of very high SWR. The same cannot be said of coaxial cable. Like the insulators mentioned above, baluns and coax are available at nearly every dealer.

Recap

Despite the rather verbose explanation above, a ladderline doublet is really quite simple. Using our example above, constructing a single antenna usable on every amateur HF frequency from 80 meters and higher consists of only:

  • 135′ of antenna wire
  • 92′-8″ of ladderline (or whatever amount you need keeping in mind the 1/8λ caveat
  • Center Insulator and two end insulators
  • Balun
  • Coax
  • Support rope

Where can you get all this stuff? Dealers abound and while CARC doesn’t endorse any specific dealer, here are a few links to get you started:


  1. See a very detailed treatment in this paper by W8JI for DX Engineering

Permanent link to this article: http://w4cae.com/a-simple-multi-band-wire-antenna-for-hf/

5 comments

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  1. mike

    Very nice simple explantion to set up a very good antenna i made one and it works superb g0pxg mike england uk.

  2. Dave

    It is a great ant design. I was corrected by someone smarter than me that this design is not a dipole or doublet but simply a center fed 1 wavelength antenna. Put two of these up and spaced 1/2 wave length apart, Feed them in phase, mount 1/8th wave above earth and you have an 8db gain antenna with take-off angle of 75-90 degrees. Great for AUXCOMM / EMCOMM or just for great close in signal coverage with DX broadcast station attenuation; great for nets and working mobile stations. Add reflectors 5% longer on the ground or buried a few inches in the sole and improve the transmit efficiency. Close to earth wires do suffer from earth losses.

    73,
    Dave
    Wa3gin

  3. Gary Glintenkamp

    Just to clarify, did you mean 300 ohm i.e. TV ribbon or 450 ohm window line as shown in the pictures for the matching section?
    Thanks, 73, Gary KD2IBQ

    1. KN4QD

      300 ohm was used as the basis for this article but you could also use 450 ohm if that’s what you have on-hand. Just be sure to adjust the velocity factor of your chosen feed line accordingly.

  4. Gary

    I really like your doublet antenna design, and am planing to build one, however I do have a question, and I want to make sure I’ve got the numbers correct (as I am a new ham & still have a lot to learn). Due to house & tree placement, wind etc I plan to use 12 gauge, stranded, insulated copper wire (for the 135′ antenna, between trees, 60′ off the ground) to the same wire, ladder line,(no solder joint) 450 ohms 1 3/4″ wire spacing, to the house and a 1:1 balun connecting to 55′ of LMR-400 going in the atic and through a medal pipe to the basement ham shack & tuner/ radio.
    My question is how long (or short) can the ladder line be from the center antenna insulator to the 1:1 balun? Ideally for my location it would be between 30′ and 40′ long w/o being too short or laying on the roof etc. (I’m thinking VF of about 90) so 30.9 ft?
    Thanks for the help! 73 Gary KD2IBQ

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