[TowerTalk] Dipole questions for the Liberal Arts Major

[Jim Brown]

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