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The Three L-Antennas

Wide – Equal - Tall



A space saving antenna in the form of an upright L has been around the amateur radio world for a long time.  References are found back to a QST article in the 60’s 1, the ARRL Antenna Handbook in the 70’s through 90’s 2 and to L. B. Cebik’s 3,4 and Craig LaBarge’s 5 writings in recent years. Commercial designs are seen in the Buddipole TM 5, MFJ-1790 6 and the CrankIR7 antennas. By the use of antenna modeling software10, this article is to expand our general knowledge relative to angle-of-bend and off-center-feed (OCF) configurations of the L-Antenna.

The L-Antenna

So simple.  So easy to tune.  Just hang up a wire or put up a pole and run another adjustable length wire or pole off to the side.  Connect your coax at the bend.  It really does not matter how you connect the coax. It really does not matter how long is the vertical arm. Simply tune by adjusting the angle and length of the side arm. The total length of the two arms will end up around a half wavelength. 

L-Antenna Angle vs ImpedanceAny L-antenna can be looked at as a center-fed dipole bent in the middle or… a vertical monopole with a single radial.

At an elevation one-half wavelength a straight vertical dipole has about 1.6 dBi gain (deciBel isotropic). The L form however has gain around 4 dBi in the direction where the radial is pointing.  Depending on ground conditions and mounting height, the impedance is around 40 ohms. This makes the configuration a decent match for 50 ohm coaxial cable.


Since the impedance of a vertical dipole is around 70+ ohms, there obviously is a 50 Ohm point somewhere between that and the 40 Ohms of an L-dipole… usually near 75 (or 15 below horizontal).  Gain at this point is around 3 dBi.                   

Interestingly, as the arm/radial continues to be raised  above horizontal, the impedance declines to around 30 ohms, the Standing Wave Ratio (SWR) approaches 2:1 but the directional gain increases up to 5 dBi….  several times the gain of a vertical dipole.


Off-Center-Feed (OCF)

In computer modeling or practical construction of a horizontal half-wave antenna, it is observed that impedance rises as the feed point moves away from center.  Resonant frequency and gain remain the same.   This characteristic is used when matching low-impedance antennas like a J-Pole or when using a Gamma Match or Delta Match on multi-element beams. Commercial ground plane antennas with short radials are also off-center designs.

Figure 2 following is a generalized impedance graph of what happens when feeding a dipole off-center.

Fig 2 OCF_ 2x Rel Imp
A little Off-Center there is little change in Relative Impedance. 

At 0.4 Feedpoint Ratio the Relative Impedance is 1.09 greater.

Example: 37 Ohms at Center rises to 40.3 Ohms
at 0.4 Feedpoint Ratio.


Farther away from Center the rate of rise increases.  At 0.3 Feedpoint Ratio the relative impedance is 1.44 greater. At this ratio 37 Ohms at Center becomes 53.3 Ohms.

Note: The farther off-center, the greater the current imbalance between arms, the greater the “common mode” current on a coaxial feed line.  Common mode current problems cause a change in tuning if you touch the cable, sometimes tingling or “bites” to face and lips when transmitting.  RF radiating from the coax can couple with nearby electronics or power lines to affect TV and radios as well as control devices like alarms, thermostats and monitors.

Recomendation: Use a choke balun at the feedpoint.

Wide, Equal  and  Tall  L-Antennas

The Equal L-Antenna
With “equal” arms, a perfectly tuned horizontal arm is slightly longer due to the effect of ground. The arm-to-arm ratio therefore is not exactly 0.5.  Impedance is around 40 ohms.  SWR can be as low as1.2 to 1.3:1.  Horizontal and vertical RF polarization is equal.

The principles of OCF apply very well for L-antennas.  The Wide or Vertical characteristic can emphasized by making OCF ratios smaller or larger.

The Wide L-Antenna
As the off-center feedpoint ratio goes less than 0.5, the L-antenna becomes wider and the impedance rises.  The antenna hears and talks more like a horizontal dipole as the SWR approaches 1:1.

The Tall L-Antenna
As the off-center feedpoint ratio goes greater than 0.5, the L-Antenna becomes taller and the impedance rises.  It hears and talks more like a vertical antenna as the SWR approaches 1:1.

Based on antenna modeling at one-half Wavelength Height,  Figure 3 below summarizes the SWR and Impedance as the feedpoint is changed by 0.05 (5%) increments around the center 0.50 ratio. 

L-Dipole Ratio study
Figure 3

It is clear that an L-Antenna is easy to tune at any up-bend ratio between about 0.30 and 0.65; and with effort, out to a 0.25 or 0.75 ratio.   For hams wishing to fine-tune their L-Antennas, 0.40 and 0.60 are the magic points.

Note that there is an impedance effect caused by ground coupling as the horizontal arm gets longer. The Wide L-Antenna can not quite get to a 1:1 SWR.

Of particular interest is that L-antennas have both vertical and horizontal polarization. The ARRL Antenna Book says: “Some immunity from fading during reception can be had by using two receivers on separate antennae, preferably with different polarizations9. 

In this case there are different polarizations on one antenna.  The resulting effect of reduced fading can be heard on DX signals by switching between an L-Antenna and a conventional dipole or ground plane.  For nearby communications, an L-Antenna makes all the difference in hitting repeaters and talking to hams using both verticals and horizontal dipoles or yagis.


The Tall-L Antenna

When the vertical arm is tall, the length of a 90 side arm length can be adjusted to resonate the antenna to the desired frequency.  The SWR match of a Tall-L antenna can be better than the Equal-L Antenna.  Depending on elevation, ground and diameter of the wires or tubes used, there will be some ratio between the length of the vertical and horizontal arms that will give the best match to a 50 ohm coaxial cable. According to Figure 3, with a 90 side arm: start with 0.6 Tall Ratio.

This tall L-configuration is quieter than a conventional ground plane and performs well for DX.  It can be a good choice when listening around the band.

It has another notable feature.  It develops a horizontal component from the side arm. This gives some directionality and gain over a vertical antenna.    The directionality is a bit more broadside to the side arm but generally semi-circular as seen in Figure 5.  The angle of the side arm can be used for tuning OCF ratios as tall as 0.75 up-bend . 

Modeling

Antenna modeling software can optimize a vertical dipole  into the tall OCF form.  Using that capability, Figure 4 below shows what happens to a Tall OCF dipole as the shorter lower arm is swung upwards in 15 increments from 0 (down), to 90 (horizontal), to 150 (60 up).  Standard conditions are: 2/3-1/3 ratio, #14 AWG, wavelength feedpoint elevation, over “real ground”.


Tall L-Antenna Gain Graph

Figure 4
  • The lowest SWR occurs with the variable arm at 105… 15 above horizontal.    
  • The gain of the Tall-L Antenna is over twice that of the OCF vertical dipole at 0.

Model Example (Figure 5)

Conditions:
4NEC2 software10 model of a Tall-L Antenna, fed at wavelength over real ground using #14 wire, optimized
at 28.4 MHz .
Predicted dimensions: Vertical arm: 3.147 meters tall; Horizontal arm: 2.08 meters long.
Total length: 5.227 meters at 60-40 ratio.
Impedance: 50.5 –j0.43
; SWR: 1.01; Gain: 2.69 dBi

 

Below is the 3D view and horizontal/vertical polar graph produced by the antenna model.

Tall L-Antenna 3D Graph

Figure 5  Tall L-Dipole

This is essentially the radiation pattern of a vertical antenna.

Note: the radiation pattern is 2.69 dBi stronger on the hemisphere towards the side arm. The opposite side has 0.01 dBi gain therefore signals are about half as loud from the back hemisphere. Compare this with an omni-directional 1.5 dBi circle which is the norm for a vertical antenna.

Observe the 10-degree low angle radiation for DX and the slightly stronger signal at a 40-degree upward angle.  No energy is wasted skyward. 

The take-off angle from 10 througn 40 is a good configuration for general, all-around band scanning. In addition it will hear polarized signals that might otherwise be too weak. 

If a second horizontal arm is added at 180 opposite and both shortened to tuned to the desired frequency, the radiation field balances.  The radiation pattern becomes circular (no gain) and the antenna becomes a tall ground plane vertical.

The MFJ7-1790 10 meter antenna is an example.  It has an 11-foot vertical and two 6-foot radials to give “Low Radiation Angle for outstanding DX”.




The Wide L-Antenna

When the vertical arm is short, the length of a 90 side arm can be adjusted to resonate the antenna to the desired frequency.  The SWR match can be better than the Equal-L Antenna.    Depending on elevation, ground and diameter of the wires or tubes used, there will some ratio between the length of the vertical and horizontal arms that will give a nearly perfect match to a 50 ohm coaxial cable.

This Wide-L configuration has the characteristic of being shorter than a halfwave dipole with nearly the same gain.  Having both horizontal and vertical polarization, it is less effected by undulations in DX skip reflections (QSB) than a conventional dipole.

Antenna modeling software can optimize for the best wide OFC ratio at 1/2-wavelength feedpoint elevation over ground.
Using that capability, Figure 6 below shows what happens with a 2/3-1/3 ratio as the longer lower arm  is swung upwards in 15
increments.


Wide OCF L-Antenna Gain Graph
Figure 6

Note that the best SWR and Gain combination occurs at around 105, or 15 above horizontal, whereas, the shorter 60/40 ratio occurs at 90 horizontal.
Gain compares favorably with the gain of a center-fed dipole using the same wire and fed at the same elevation over the same ground.  SWR is lower.

Model Example (Figure 7, below)

Conditions:
4NEC2 software model10, Wide-L Antenna, fed at wavelength over ground using #14 wire, optimized for 90
at 28.4 MHz.
Predicted dimensions: Vertical arm: 2.083 meters tall;  Horizontal arm: 3.1596 meters long.
Total length: 5.243 meters at 60-40 ratio.
Impedance: 49.3–j0.12
; SWR: 1.01; Gain: 5.88 dBi.
 

Below is the 3D view and horizontal/vertical polar graph produced by the model.

Wide L-Antenna 3D Graph
Figure 7   Wide L-Antenna

This is the radiation pattern of a half-wave dipole.
  • The Wide-L antenna is directional to the side of the horizontal arm.
  • The radiation pattern differs only slightly from an ordinary dipole because there is no dip on one side of the horizontal radiation pattern (polar graph, red line).
  • The maximum radiation is at 30 above horizontal.  Skyward radiation is minimal if feedpoint elevation is at multiples of 1/2 wavelength.

The Tall and Wide optimized models have similar dimensions for the short and long arms.  A Tall L-Antenna basically becomes a Wide L-Antenna when laid on its side.





Summary

The following two graphs compare the Relative Gain and Radiation Efficiency of four kinds of dipoles, each as one of the antenna arms is adjusted through a range of angles.

1.      Equal-arm Horizontal (dark  line, diamond marker).  90 = standard horizontal dipole.

2.      Equal-arm Vertical (violet line, square marker).  0 = standard vertical dipole.

3.      OCF Tall (blue line, triangle marker).  90 = Tall L-Antenna.

4.      OCF Wide (red line, X marker).  90 = Wide L-Antenna

All feedpoints at wavelength elevation.
Plots stop at the point where the SWR exceeds 2:1.
OCF used is at 1/3-2/3 ratio.


4 Kinds of L-Antennas
L-Antennas Comparing Gain
                         Figure 8 - Gain vs Angle

L-Antennas comparing efficiency
                     Figure 9 - Efficiency vs angle


Conclusions

  • The Wide L-antenna is a shortened horizontal dipole that can come close to having the gain and efficiency of a conventional horizontal, center-fed dipole
  • The Tall L-antenna performs much like a vertical but with some gain towards the side arm. 
  • The Equal L-antenna is part-vertical and part-dipole with broadside gain that can be ajusted changing the angle of the side arm.

All three forms are suitable for limited space environments and have a degree vertical and horizontal polarization which helps in both local and long distance communication. 



About Wire Antenna Modeling Software

This study answers the questions of what happens when you bend a dipole at different angles and feed it at different points. 

It is unlikely an average amateur radio operator would have the time and resources to do this study by building prototypes even if he had the desire.  With modeling software a ham can do in a few hours or days what would otherwise take weeks or months, if ever. 

The results by way of software reveal underlying behaviour: how the changes in an antenna affect frequency, impedance, radiation pattern and other characteristics.  Through optimization, modeling predicts the dimensions and angles that will give the best SWR, Gain or Efficiency.

There is a catch.  Modeling software lives in its own perfect little world inside the computer. It works with the information given. The more sophisticated the program and the user, the better the results. In the real world we take guidance from the results but understand there will be differences in materials, design, construction, support structures and effects from near-by things.

Modeling eliminates dead-ends, gives ideas and guides us to a working antenna.


Tuning An OCF L-Dipole

Tuning An OCF Dipole



References:

1.      McCoy, Lewis G., W1ICP, “A Limited-Space Antenna.” QST Oct. 1960: pp. 23-25

2.      “The ARRL Antenna Book.” 1974 –1997. 225 Main St., Newington, CT 06111

3.      Cebik, L. B., W4RNL, “Whips, Tubes and Wires: Building a 10-Meter L Antenna.” QST Dec. 1999

4.      Cebik, L. B., W4RNL, “The L-Antenna.” www.antennex.com/preview/Folder01/lant/lant.htm

5.      LaBarge, Craig, WB3GCK, “The `Up and Outer` Antenna”     www.qst.net/wb3gck/up_and_outer.htm

6.      Buddipole, Inc. 3028 SE 59th Ct. #600, Hillsboro, OR 97123

7.      MFJ Enterprises, Inc., 300 Industrial Park Road, Starkville, MS 39759

8.      SteppIR Antennas,2112 116th Ave. NE, #1-5, Bellevue, WA 98004

9.      “The ARRL Antenna Book”, 22nd edition, pg. 4.2.14

10.  Free antenna modeling program by Arie Voors. www.qsl.net/4nec2/



Dick Reid, KK4OBI at QSL.net