¦ Article ¦ HF-antenna's ¦ Tribander ¦
Last review: 22-11-2003
CONCEPT.
Due to the availability
of modern modelling software, creative HAM's have the ability to devellop
and construct Yagi-Uda antennas with a bigger complexity than one might
think. I tried to make a tribander for 10, 15 and 20 meters which fits
in the average budget and in the average garden. The antenna uses a
6 meter (19 ft.) boom.
Commercial manufacturers have spend
years of investigation, testing and modelling to devellop modern tribanders
which are build out of several monoband yagi's placed on a single boom.
The most recent designs do not use any coils or traps in neither the
driven elements or director-reflector groups. The use of full size
elements has the advantage that the efficiency is always superior to
designs with
coils. An optimized 2-element design can outperform a 3 element shortened
antenna in real life competition. It is also easier to design a computer
model with full-size elements. Coils can be modelled but are much more
difficult to construct as accurate as modelled. For example: If you
model a short 2 element yagi with 20uH coils you can expect the VSWR
and optimal
Front to Back to shift 10 KHz if you're able to construct the coils
within 1% tolerance. If you're goal is a DX-segment it may be difficult
to hit
the target ! Imagine the challenge if we reflect this into a multiband
homebrew design. For the following design I looked to the well known
Force12 C-3E element placement. I followed a "try and error" method
to place a 2 element 20 meter Yagi, a 2 element 15 meter Yagi and a
4 element 10 meter Yagi on a 6 meter (19 ft.) boom in an order for
acceptable
GAIN,F/B and VSWR. All elements are metric and I used F22 AlMgSi 0,5
aluminium tubing. As modelling engine EZNEC 3.0 by W7EL was used. http://www.eznec.com
Only
one coaxfeed is used to feed the antenna. The coax is attached to the
20 meter driver and both the 15 meter and 10 meter driver are excited
through the 20 meter driver. By tweaking the distance between the coupled
drivers and the 20 meter driver it is possible to adjust the VSWR-curves.
Also the lengts of the drivers have to be adjusted in the modelling program
to obtain good curves for VSWR on all bands. To raise the impedance near
50 Ohm I used a B-match which was actually 210 cm RG-213 cable. All elements
are isolated from the boom and a split element is needed for the driver.
| Since this antenna is working so well I fail finding the time to finish this article, although I will continue soon... |