On Tue, 30 Aug 2005 11:53:35 -0700 (PDT), Lee Buller wrote:
>what do you do when working with dipoles to get them to the
>point where you get good bandwidth?
Here's an EE's explanation for a Liberal Arts guy. :)
First, let's look at the PERCENTAGE bandwdith that we're trying to
cover, because that's really what drives the design.
160m -- 10.5%
80m -- 13.3%
40m -- 4.2%
30m -- 1%
20m -- 2.5%
17m -- 0.5%
15m -- 2.1%
12m -- 0.4%
10m -- 5.9%
A 2:1 SWR is considered by most to be an acceptable match, and a
simple wire dipole will do that for a span of about 4% (that is,
+/- 2% of the center frequency). To cover 40, 30, 20, 17, 15, and
12, all you need to do is tune the dipole to the approximate
center of the band and you're there.
160, 80/75, and 10 meters are the difficult HF bands, but 10 is
tough only if you want to work both the low end (CW and SSB) and
the high end (FM). I've got a 3-wire dipole in my backyard that
covers 20, 15, and the bottom third of 10m with under 1.5:1 SWR on
each band.
How do you get more bandwidth? One commonly accepted way is to
increase the cross-section of the dipole, but you need to do that
on a percentage basis too. In other words, doubling the wire size
doesn't help -- you need multiple wires and spacers to accomplish
a significant improvement.
On 80, you could do that by using 4 wires spaced at 6-12 inches in
a square box arrangement. The antennna books give some guidance
about how much spreading you need to accomplish this. Folded
dipoles made from 300 ohm twinlead were used to do this when I was
a kid, but they have fallen out of favor.
W8JI has studied the length/bandwidth compromise carefully, and as
I recall, comes down in favor of cutting the antenna longer rather
than shorter, then tuning it up in frequency. I trust that he will
jump in to expand on this. A trip through his website is
worthwhile. I take that approach -- I'm a CW guy and use 75 only
for contests, so my 80m dipoles are cut to about 3570, and I use a
tuner to run them up to about 3900. I've gone higher without
smoke, and managed to work Europeans with 100w during contests.
One antenna that has broadbanded quite nicely for me on 80/75, but
that I have not had time to analyze to figure out why, is a 2-wire
antenna for 160, 80, and 40 constructed using loading coils sold
by Barry under the "HyPower Antenna Company" name. The antenna is
a classic 2-wire for 80 and 40, and is resonant on both bands.
That is, from the center insulator, there are two parallel dipoles
connected, one cut to 40 and one to about 3570. At each end of the
80m sections is added a loading coil to make the antenna resonate
on 160m (at about 1820). This antenna is hung off of the top of a
150 ft tower at W6BX, sloping down to the ground. The ground falls
away from the tower by about 20 ft, so one end of the antenna is
about 12 ft from the tower, and the other end is about 12 ft from
the ground. The antenna is fed with 75 ohm coax through a DX
Engineering dipole balun (the one they discontinued for some
reason that defies logic).
Measured at the end of something like 100 ft of coax (a VERY rough
approximation), the SWR remains well below 2:1 for most of the
band (3.5-4 MHz), and is below 2.5:1 for all of it.
Analysis? Well, it's interacting with the earth and with a tower
that has a very large cross-section, and it exhibits some moderate
directivity (I would guess 6-8 dB front to back) in the direction
that it slopes away from the tower. I don't think that the
parallel 40m dipole is broadening it, but it might be.
Hope this helps.
Jim Brown K9YC
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