Ultra-compact Multi-band Wire Antenna

Live in an area that prohibits external antennas?  Maybe your lot is just too small to put up one of the popular multi-band wire antennas.  If you have an attic, garage, workshop, or deck that is a little over 12’ in length you can operate on 40 meters through 6 meters with a relatively cheap and easy to build wire antenna thanks to WB2JNA.  An article describing his antenna design was published in the ARRL Antenna Compendium, Vol. 6  (ISBN: 978-0-87259-743-3).  The antenna design is also included in Simple and Fun Antennas for Hams (ISBN 978-0-87259-862-1), Chapter 12,  pp. 7-8.

This article discusses my implementation of the WB2JNA antenna.  Figure-1 is a diagram of the antenna.

Figure-1. Cheap & easy to build  multi-band antenna diagram

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ANTENNA MODELING

If you have the capability, it’s a good idea to model an antenna before you build and evaluate it.  EZNEC+ models for the 40-meter and 20-meter bands are available at the link below.  Thanks to AC6LA for the tips to improve the models using AutoEZ.

If you are an EZNEC user, you should seriously consider adding AutoEZ to your toolkit.  AutoEZ is an attractively priced add-on that interfaces with EZNEC+ and enables you to easily refine your models.  AutoEZ includes a powerful optimizer feature that can be used to find optimum radiator lengths, coil inductance values, and many other important aspects of an antenna.  I used the AutoEZ optimizer feature on this project to find the loading coil inductance values and counterpoise lengths that produced the best predicted SWR curves for the 40-meter and 20-meter bands.

NOTE: AutoEZ indicates that the SWR curves for each band can be significantly improved by adjusting both the inductance of the loading coil and the length of the counterpoise.  You may want to consider using optimum length counterpoise for each band of interest.   A rotary switch could be used to switch between counterpoises   Another possibility is to use a retractable steel tape measure for the counterpoise  Using an antenna analyzer, you can compile a table of the optimum coil tap positions and counterpoise lengths for each band of interest.

WB2JNA EZNEC Models (zipped)

CONSTRUCTION

The radiator is 12’ long.  The counterpoise is either 16′ or  32’ long.  Read about counterpoises. The term counterpoise in this context means a length of insulated wire with one end attached to the shield of the coax transmission line at the antenna feed point.  The counterpoise for this antenna is situated above physical earth ground and is made of black #14 AWG copper stranded THHN insulated wire.  The counterpoise has a SPST switch in the middle (16’ point) to facilitate band switching.  The loading coil is wound on a short piece of Schedule 40 2” ID PVC pipe.  Twenty feet of black #14 AWG copper stranded insulated hookup wire was used to wind the coil.  The coax connector block for the transmission line was made from a short piece of 1 1/4” ID schedule 40 PVC pipe, (2) couplings, (2) plugs, and a SO-239 connector.  The coil tap lead (red) has an alligator clip on one end.  The other end of the coil tap lead is connected to the coax center at the connector block.  The counterpoise is connected to the coax shield at the block.  The connector block is attached to the coil form with a plastic zip tie.  Figure-2 shows the coil and coax connector block.  The coil form has eyebolts in either end for suspension purposes.  The connector block has an eyebolt in the top so the block can be attached to the end of the coil form.

NOTE:  You may want to add a few more turns to the loading coil to provide a wider range of inductance in case it is needed to for your implementation.  You can also buy loading coils and tapping clips from Barker & Williamson.

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Figure-2. Loading coil and coax connector block

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LOADING COIL MEASUREMENT

The coil was analyzed on the 40 meter and 20 meter bands with an AIM-4170C.  For the 40 meter band,  the end-to-end coil inductance measurement was ~23.7 uH.  Figure-3 shows the 40 meter analysis.  Figure-4 shows the 20 meter analysis

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Figure-3. 40 meter band coil analysis

The 40 meter coil inductance curve is relatively flat across the 40 meter band.

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Figure-4. 20 meter band coil analysis

The 20 meter coil inductance curve varies significantly as the frequency increases.

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40 METER BAND SWR CURVE MEASUREMENT

The antenna was hoisted to a height of approximately 10’ and the SWR curves for different bands were plotted using the AIM-4170C.  The SWR curves for all bands analyzed were significantly improved by coiling 10 turns of the RG-8X coax transmission line at the source end to form a choke.   The coil was secured with plastic zip ties.  The tap of the loading coil was moved around to find the best SWR curve for the 40 meter band.  The counterpoise length was set to 32’ (SPST switch closed).  Figure-5 shows the 40 meter band SWR curve.

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Figure-5. 40 meter band SWR curve

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20 METER BAND SWR MEASUREMENT

For the 20 meter band SWR analysis,  the counterpoise length was reduced to 16’ (SPST switch open) and the loading coil tap was moved around to find the best SWR curve.   Figure-6 shows the 20 meter band SWR curve.

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Figure-6. 20 meter band SWR curve

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17 METER BAND SWR MEASUREMENT

The 16′ foot counterpoise was used and the loading coil tap moved around to find the best SWR curve.  Figure-7 shows the 17 meter SWR curve.

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Figure-7. 17 meter band SWR curve

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6 METER BAND SWR MEASUREMENT

The 16′ foot counterpoise was used and the loading coil tap moved around to find the best SWR curve.  Figure-8 shows the 6 meter SWR curve.

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Figure-8. 6 meter band SWR curve

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15, 12, 10 METER BANDS

By adjusting coil tap position,  length of the radiator,  and length of the counterpoise, it is possible to achieve acceptable SWR curves for the 15, 12, and 10 meter bands.

OPERATION

NOTE: This is a compromise antenna and will not provide the performance of a properly configured full-size antenna.  It’s purpose is to get  you on the air in a very limited physical space.  The use of an automatic antenna tuner is recommended due to the significant variations in impedance presented by the antenna at different frequencies. You can expect some differences in SWR curves from those included in this article due to construction variances and environmental factors at your QTH.

Some reasonably priced automatic tuners:  LDG AT100ProII MFJ-925; SG-239

The antenna performed reasonably well on 40 meters.  I was able to check into several regional 40 meter nets with signal reports ranging from 55 to 59.  Although I didn’t transmit on 20 meters and 17 meters, I could hear plenty of stations.   It’s certainly an interesting antenna and well worth building especially if you need a ultra-compact multi-band antenna.

SAFETY NOTE:  The counterpoise may radiate on some bands.  Be sure to take steps to ensure no one comes in contact with the 12′ radiator or the counterpoise when transmitting.  Keep the transmitter power to 100 watts or less if you’re using the antenna indoors.

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