|
Index |
duoband antenna for 80 en (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 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 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
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 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 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 %.
Concluding A duo-band 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 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 Bob J. van Donselaar, on9cvd@veron.nl |
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
