duoband antenna for 80 en
(published in Electron #7-1999, Radcom #10-1999 and Antenne Compendium #8)
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.
For this exercise the antenna has been
When building the antenna in your
environment, please make sure the antenna height is conserved as on the
At designing the antenna, it appeared that
changing antenna length (horizontal wire) is most influential at
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
The V-type again consists of a center-fed
horizontal wire of
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 (
Table 1: Results for
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.
Also for the
Table 2. Results on
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 %.
A duo-band antenna has been described for the
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 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.
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.
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
Bob J. van Donselaar,