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 – 6 meters with a relatively cheap and easy to build wire antenna thanks to WB2JNA.  An article describing his antenna design was published in theARRL 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.

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Figure-1.  Cheap & Easy multi-band antenna diagram

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CONSTRUCTION

The 12’ radiator and the 32’ counterpoise [1. 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 feedpoint.  The counterpoise for this antenna is situated above physical earth ground.] are made of black #14 AWG copper stranded THHN insulated wire.  The counterpoise has a “spade” electrical quick disconnect pair [2. Spade electrical disconnect pair. spade electrical disconnect] installed 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.  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 coil form.

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Figure-2.  Loading coil and coax connector block

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LOADING COIL ANALYSIS

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 increases significantly as the frequency increases.  However, the increase in inductance poses no noticeable problem because only the first couple of coil turns are used for tuning above the 40 meter band.

NOTE:  Take a look at the article by VK1OD on tapping an air-core inductor.

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40 METER BAND SWR CURVE ANALYSIS

The antenna was hoisted to a height of approximately 7’ 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 up 10 turns of the RG-8X coax transmission line at the source end to form a choke [3. If a balun is going to be used with the antenna,  it should be a current type to provide counterpoise isolation.  Due to the relatively high impedances on some of the bands, a 2:1 or 4:1 current balun may provide a better impedance match. An antenna analyzer will indicate which balun provides the best overall impedance match.]  The coil was secured with plastic zip ties.  Placing the tuning tap at the midpoint of the coil provided the best SWR curve for the 40 meter band.  The counterpoise length was set to 32’.  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 ANALYSIS

For the 20 meter band SWR analysis, the tap was moved to the second coil turn from the radiator end and the counterpoise length was reduced to 16’.  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 ANALYSIS

The 20 meter coil tap and counterpoise settings were used for the 17 meter band SWR analysis.  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 ANALYSIS

The 20 meter coil tap and counterpoise settings were used for the 6 meter band SWR analysis.  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.

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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 using a very limited physical space.  A tuner should be used with this antenna.  You can expect some variances in SWR curves from those included in this article due to construction variances and environmental factors at your QTH. You may want to install an inline SPST switch in the counterpoise at the 16′ point to facilitate band switching.

The antenna performs 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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