Wire antenna lengths and patterns


All antennas are a compromise. As you only have a given amount of power you can focus all of it in one pinpoint direction at the expense of all other directions or spread it out equally in almost all directions. Gain in one direction is always at the expense of signal strength is other directions.


I wrote up some info that may help explain how wire antennas work. The lengths are just ballpark numbers as the exact length depends on many variables such as exact frequency, size of wire, distance above ground, type of insulation and other factors.  Use these lengths as a guide. 


Quarter wave wire.

Approximate lengths are:

130 feet on 160 meters

65 feet on 80

33 feet on 40

16 feet on 20

10 feet on 15

8 feet on 10 


This length wire has zero current at the open end. ( there is always zero current at the open end of a wire)

Maximum current at input.

Low input impedance.



Half wave wire.

Approximate lengths:

270 feet on 160

135 feet on 80

67 feet on 40

33 feet on 20

21 feet on 15

16 feet on 10

Zero current at both ends.

Maximum current is at center which is quarter wave from open ends.

If fed at end input impedance is about 2400 ohms.

If fed in center input impedance is low (good match to coax)

Maximum radiation is broadside to wire. 


Slightly longer than one quarter wave long. About 3/8 wave long:

175 feet on 160

90 feet on 80

50 feet on 40

20 feet on 20

15 feet on 15

12 feet on 10

When fed at the end has about a 50 ohm impedance plus some inductive reactance as it is not resonant. A capacitor can be used to tune it to resonance resulting in a near perfect match to 50 ohm cable. The high current point is one quarter wave back from the far or open end. Maximum radiation always comes from the high current areas on the wire. 


Wires a full wave long.

Approximate lengths:

520 feet on 160

270 feet on 80

135 feet on 40

66 feet on 20

44 feet on 15

32 feet on 10 


The current distribution and resulting radiation pattern is different depending on whether the wire is end fed or center fed.

If end fed the impedance is very high similar to a half wave which is end fed. The pattern does not have a peak broadside but has more of an X pattern. Current on each half wave section of the full wave wire is opposite in phase resulting in a null directly broadside as well as off the ends. 

If center fed the impedance is also very high but the current in the two half wave sections is in phase and results in peak radiation broadside. Nulls will still occur off the end.


With inverted L configurations where the feed point is at the bottom of a vertical portion, the overall length in wavelengths will determine where the current maximum  or maximums will occur. If there is only one maximum current point, whether it is on the vertical or horizontal portion of the wire will determine the dominant polarization, directivity and the radiation angles. With multiband use this will usually be very different on different bands.


For a 175 foot long 160 meter inverted L for example, the high current portion will be on the vertical portion and this will result in vertical polarization that is essentially omnidirectional.

There will be significant horizontal radiation from the horizontal wire. The amount depends on the length of the horizontal wire. This antenna is very good for both DX and domestic communications on 160 meters. The longer the vertical portion the better for DX. The limit being a straight 175 foot vertical. This gives better performance than the standard quarter wave vertical. Since there is no horizontal portion, high angle radiation will be minimal. 

With longer horizontal portions (shorter vertical portions) the high angle horizontally polarized signals will increase and DX performance will suffer. 

I find a 40 to 60 foot vertical portion (maintaining a 175 overall wire length) makes for a very good general purpose antenna. I have worked 49 states on 160 meters and earned a 160 meter DXCC with this antenna.


Some of the antenna books have diagrams that help with understanding the current distribution and phase of that current on antenna wires.

Learning this will let you estimate radiation patterns from simple wire antennas with a minimum of work. 

The ARRL antenna book is a must for learning.  I find older editions are excellent. Not all editions are identical and it is good to have several. The ON4UN Low Band DXing book (5th edition, published by the ARRL) is also excellent.

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