Index

 

V & H type

Results on 80

Results on 20

Definitions

duoband antenna for 80 en 20 meter

(published in Electron #7-1999, Radcom #10-1999 and Antenne Compendium #8)

 

 

Introduction

 

The other day a presentation on antennas was held in the local radio club and a coil-shortened antenna was presented to comply antenna size with local real-estate requirements. This adaptation comes at a prize as such inductor will limit the usable antenna bandwidth within SWR = 2 limits while lowering antenna gain. It occurred to me, that end-capacitors might be able to perform the same antenna shortening 'trick' with the penalties.

 

End-capacity or 'top-loading' has the effect of enlarging antenna current at the end, effectively lengthening its electrical size. At the lower HF bands this type of lengthening is not frequently applied since capacities to have any effect will have to be not particularly small at these frequencies.

 

Top loading of an antenna has the effect of introducing a second parameter to the antenna design next to antenna length, so it should be possible in principle to design an antenna that will resonate at two different frequencies. We will investigate this premises in the next paragraphs, designing an antenna for the 80 and 20 radio-amateur bands as these are popular locally for local respectively DX operation.

  

 

Antenna modeling

 

For this exercise the antenna has been modeled at 10 m. above average ground with a conductivity of 5 mS/m. and a dielectric constant ε = 13. Wire diameter is 1,8 mm. as this is close to solid and multi-stranded wire often just for these purposes. Nice thing with modern antenna design programs is that you may vary your design more or less 'at will' so many configurations may be investigated before the 'hard-labor' parts have to be faced.

When building the antenna in your environment, please make sure the antenna height is conserved as on the 20 m. band an few meters is easily a substantial part of a wavelength and will therefore influence parameters as antenna gain, antenna impedance and elevation angle.

At designing the antenna, it appeared that changing antenna length (horizontal wire) is most influential at 20 m. and end-capacity (vertical wires) on 80 m. This may also be effective when pruning the antenna to your preferences.

 

 

Antenna types

 

As may be expected there are more designs based on the two-parameter model that fulfill the requirements of resonance at two frequency bands. In the end I selected two for further investigation that were simplest from a constructive point of view. Both types connect to 50 Ohms, using a (preferably current) balun in case of coaxial transmission line.

 

The H-type consists of a center-fed, horizontal wire of 21,5 m. at both ends connected to the centers of vertical wires of 11 m. This set-up may be realised in various ways one of these as in the stylized drawing of figure 1. A simpler way may be to use an upper rope for mechanical support and helper mid-lines to pull tight. The lower vertical wires may be left dangling or tied to ground fixing point. Many more solutions may be imagined.  

 

 

 

        Figure 1. H-type design

 

 

 

The V-type again consists of a center-fed horizontal wire of 21,5 m., this time with each end connected to vertical wires again 5,5 m. long at 90 degree at each other and at the antenna, see figure 2.

 

 

 

                 Figure 2. V-type design

 

 

 

Results on 80 meter.

 

To get an impression of antenna performance I calculated a number of relevant electrical parameters and put these next to comparable information of a 'standard' dipool, cut for resonance (38,5 m.) at the same frequency and an coil shortened dipool of the same radiator length of both top-loaded designs, with two inductors of 19 μH, Q = 200 at each side of the feed-point. Results are in table 1.

 

 

 

 

azimuth 0 degree

azimuth 90 degree

impedance

SWR

Band-

 

gain

elevation

gain

elevation

resonance

re. 50

width

 

 

(dBi)

(degree)

(dBi)

(degree)

(ohm)

ohm

(KHz.)

 

H-type

6.13

55.5

6.13

38

29.9

1.67

141

 

V-type

5.82

53.2

5.82

38

31.9

1.6

151

 

Dipole

6.33

57.5

6.33

38

49.2

1.02

202

 

Coil

4.55

55.5

4.55

38

15.9

3.2

47

 

 

Table 1: Results for 80 m. band

 

At this height above ground the antenna is radiating at an elevation of 90 degree and gain figures are given at this maximum. The width of this lobe is showing (small) variation, therefore the - 3dB elevation figures are in the table. As may be found in the DX probability and elevation angle the elevation figures of all types at an azimuth of 90 degree allow for some DX traffic as well.   

 

Gain figures for the H-type and V-type are close to the 'standard dipole', but the latter at almost double size! SWR of the new design is low enough for direct connection to the transceiver, for a large portion of this amateur band.

Bottom line, both top-loaded designs show better characteristics when compared to the coil-shortened design of the same length.

 

 

Results on 20 meter

 

Also for the 20 meter band the antenna parameters on antenna impedance, gain, elevation and bandwidth have been calculated. Again results have been compared to a 'standard dipole' this time of 10,4 meter for resonance at band center. Because the antenna size relative to wavelength is different for both, a diagram has been made relating antenna gain and elevation to azimuth. Lets start of with the results in table 2.

 

 

 

impedance

resonance

(Ohm)

 

SWR

re 50 Ohm

 

Bandwidth

(kHz)

 

 

 

 

H-type

32

1,56

226

V-type

49,8

1

362

Dipole

73,1

1,46

1402

 

Table 2. Results on 20 meter

 

Again SWR is within range for a transceiver without tuner, for V-type even best. Bandwidth for both H-type and H-type is smaller than for dipole, but for V-type is still covering almost the entire amateur band, both for phone and morse.

 

In the graph below gain and elevation of maximum radion have been presented, relative to azimuth. To get an idea on DX performance, I have added the line angles for communication for 75 % and 25 % of time; best conditions have a probability in between these lines see DX probability and elevation angle.

 

 

 

 

 

In the graph it may be noticed that H-type gain is highest, although V-type is also very useful at 5,5 dBi over a large azimuth. Although differences may be noticed between the various antennas, all show a nice gain over wide area of azimuth between the angles for whith DX is probability is between 75 % and 25 %.

 

 

Concluding

 

A duo-band antenna has been described for the 80 m. and 20 m. radio-amateur band, that is about half the size of  a  resonant dipole on 80 m. yet has the same characteristics. Also on 20 m. band, this duo band antenna is showing comparable characteristics to a dedicated, resonant dipole. For this new design, no coils, traps or special transformers have to be made, so a local experiment to check characteristics may be quickly set-up.

 

 

Appendix.

 

Bandwidth

In this article, bandwidth is defined as the difference between two frequencies for which the real part of the impedance is equal to the imaginary part; SWR on these frequency point will be slightly larger than 1 : 2.

 

Antenna gain

Antenna gain is always given in dBi for the lowest elevation maximum. More maxima may be present with deep 'nulls' in between and/or in a different direction. Check the total radiation pattern for you particular set-up and radiation preferences.

 

Elevation

For the elevation angle I took the lower -3dB point of the lowest elevation maximum. Presenting maximum radiation angle only does not yield information on the width of the lobe and for DX purposes one usually would like to know the lowest angle for which still enough energy is available.

 

DX probability

Rhode & Schwarz regularly presents information on reception probability depending on communication distance and elevation angle. The probability angles in the graph have been taken from such presentation, simplified to the radio-amateur frequencies in the  20 m. bands and for the 75 % and 25 % probability only. More on these probability graphs may be found in DX probability and elevation angle.

 

 

 

Bob J. van Donselaar,

mailto:on9cvd@amsat.org