Index 
duoband antenna for 80 en (published in Electron #71999, Radcom #101999 and Antenne
Compendium #8) Introduction The other day a presentation on antennas was
held in the local radio club and a coilshortened antenna was presented to comply antenna size with local realestate 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 endcapacitors might be able to perform the same antenna
shortening 'trick' with the penalties. Endcapacity or 'toploading' 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 radioamateur bands as
these are popular locally for local respectively DX operation. Antenna modeling For this exercise the antenna has been
modeled at 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 Antenna types As may be expected there are more designs
based on the twoparameter 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 Htype consists of a centerfed,
horizontal wire of
The Vtype again consists of a centerfed
horizontal wire of
Results on 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 Htype and Vtype 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 toploaded designs show
better characteristics when compared to the coilshortened design of the same
length. Results on Also for the
Table 2. Results on Again SWR is within range for a transceiver
without tuner, for Vtype even best. Bandwidth for both Htype and Htype is
smaller than for dipole, but for Vtype 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 Htype
gain is highest, although Vtype 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 duoband antenna has been described for the
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 setup 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 radioamateur frequencies in the Bob J. van Donselaar, 
