W3DZZ - a classic dipole heavily modified, now 5 bands & no tuner !
The classic W3DZZ by charles buchanan.
Never has there been a more true, tried and tested design for 40/80m than the w3dzz.
When doing fieldday and /P operation, it is sometimes useful to have a
decent antenna which will do nicely on both short and long skip.
i tend to always bring my verticals, but they are no good on low-band
short skip out to lets say, 5-800km. So i really needed a design which could
do both, and i never could be bothered with all the work connected to
multiband dipoles. you have to arrange masts and lines and spikes and all
sorts of stuff. i like to play radio, not erecting towers and pulling lines.
SO, i needed a design which would be easy to put up, could be used on
multiple bands, and do well on 40/80m short skip (local stuff).
I thought thru the designs i knew, or could come up with, but none
really seemed to do the job. then i came across the W3DZZ .
The original work was done by Finnish amateur OH2EC and the
collective modifications would prove to be rather successful.
All in all it is still a simple dipole with only 2 traps, BUT the LC values of
these traps, together with a quarterwave transformer for the 20m band,
a halfwave transformer for the 80m band, and 2 pieces of wire 1.5m long
for bringing in the resonance point on 15 and 10m, has allowed this dipole
to be used on 80-40-20-15-10 with very good efficiency, low return loss,
and no need for an internal/external tuner.
Below is an image of the completed design with dimensions.
I decided i was going to build this as light as possible for fieldday and /P use with only
1 fibreglass mast, configured as an inverted vee ( /|\ ) so i would only need one mast
with the apex at around 10 meters.
The choice of cable would be RG179 for the quarterwave transformer,
connected to 7 meters of RG316 before reaching the ground.
Like this, the top section is very light, with not much load on the mast compared
to fabricating it all in RG58/RG59. I also made the traps with silver-mica capacitors
good for at least 100w, instead of coax capacitors to save some weight (and loss).
2 x 120pF 500v mica's in series equals a fine 60pF 1kV capacitor.
you COULD be lazy and make coax-traps, but you will find it hard to match the
exact values given and they are very lossy compared to teflon wire and mica's.
If you choose to make traps from coax, you should as minimum use good coax
with teflon dielectricum, like RG316 or RG400, or RG303 for higher power!.
Heres a couple of shots of my traps just before the mica's are soldered in place.
Always use hardware with the poorest magnetic properties you can find to avoid
heating from RF in the screws, bolts & nuts.
Brass are number 1, stainless steel number 2, NEVER use iron or other ferrous materials.
The heat can be so intense it will melt the coil-former around the bolt.
And heres a couple of shots of the lower feedline, choke, and quarterwave transformer.
The reason my image says 3.65m is that the velocity factor of RG179 is different from
RG59 which i used originally, so i had to recalculate the length needed.
The choke is placed in the junction between the 75ohm and the 50ohm downline
it is made of 5 turns of the RG316 on 5x FT50-43 cores and 1 EMI core from a USB
cable, giving a total inductance of 125uH, which equals around 3K ohm on 80m.
NB! the red line in the top image is the placement of the balun/choke !
This is because the upper 3.5m of the feedline acts as a impedance
transformer. This transformation from the higher impedance of
the dipole on 20m, to the 50ohm's of your coax, will not happen
if you stick the balun/choke in the feedpoint of the antenna
like you would on a normal dipole antenna.. this is important.
The boxes i have used are very small and light, and will not add much weight to the final design.
The antenna-wire i use is a very light and strong multicore "Litze" from DX-Wire in Germany.
Heres a couple of images of the completed antenna system. the total weight of
the entire package is a mere 730 grams !, well under 1 kg and very easy to put
up. even with a small pulley at the top of the 10 meter fibreglass mast there
was only slight bending of the top section when raising the antenna & coax.
Last but not least, heres a couple of plot's from initial tuning of the antenna system.
As you can see, very reasonable results for a multiband antenna like this.
These measurements are without the 26m extra 50ohm cable from the end
of the feeding harness. So the plots are taken with only the top 1/4 wave
transformer and the 7 meters RG316 below, giving a feedline total of 10,6 meters.
If you add the extra coax, the results will look even better due to cable losses :-)
I have set the antenna up several different places and the changing ground conditions
have had next to no influence on the readings. the /P performance has been great.
I can only recommend this antenna, relatively easy to build, consistent results
and good performance are trademarks of this great 5-bander.
Now go build one for yourself and save your hard earned cash :-)
UPDATE May 24 2015
i decided i needed some new low-loss traps capable of handling more than
the standard 100w. Since i have build a small PA, i wanted to be able to run
at least my 400w into the antenna, but i might as well upgrade with the future
in mind, so let's make it more suitable for running just around 1kw.
This increased power handling sets some pretty high demands for the components
used in the design. The wire has to be well insulated, and by that i mean a very
good and stable insulation from materials like teflon. You will also need a couple
of very high voltage transmittig capacitors capable of handling both high voltage
as well as high RF current.
I sourced a couple of new-old-stock russian RF ceramic cap's 60pF 3.5kV from
ebay, and some silver plated teflon coated litze wire, aproximately 2 mm2
from Peter at DX-Wire in germany. I highly recommend Peter for sourcing
good quality antenna litze and most other antenna hardware items.
I also decided to try and fabricate the coil formers from polycarbonate
and place the cap's inside the tube. just as i have done before with the
low power version. the old ones were made with PVC formers.
I have used 110mm long x 40mm diameter tube for the traps.
heres a couple of shots of the fabrication. These traps should
be able to handle QRO without any problems.
I glued in some end-caps to keep the capacitors dry.
i will say that due to the very high Q of these traps, they are a bitch to
tune up but i got there in the end. They resonate nicely around 7060kHz.
Time for some real life testing :-)