[TowerTalk] Fan Dipole

[Don W7WLL]

When W5PR noted research by the military on fan dipoles, a light went on in 
one of the few remaining brain boxes. Back among the old musty TM's from the 
Signal Corps courses I took back in the 50's I seemed to recall I'd tucked 
something away about multi-dipoles that I had forgotten about.

Found it (them). There are two. I have no idea where I got them, but 
probably off the internet someplace! The first is titled "Use of the Cobra 
Head to Connect Multi-element Dipole Antennas", by a Tim Shaw and reflects 
some Ft Sneeze USACEEIA work done in the mid to late 60's. With it is a 
paper titled "Multiband HF Dipoles" also by Tim Shaw but with added detail 
written by a George Lane (also at Ft Sneeze) but after Tim Shaw had retired.

The original short article by Shaw with diagram shows the elements center 
connected with zero spacing. His conclusion is that end separation is 
important and should be at least 5 feet. No mention is made of center 
spacing.

The Lane article, done sometime in the early '70's it appears, references 
the original Shaw work but also a Stanford Research Institute report. The 
SRI report (I don't have a copy) apparently indicates the center separation 
on a multi-element dipole should be 14 cm (5.5 inches) and the ends 1 meter.

The Lane article does not indicate any tests done comparing the two methods 
of center connection and resulting differences, if any nor a reason for his 
selecting the 5 inch center connection separation so I assume he did it 
based on the SRI report.  I just did a quick internet search and couldn't 
find anything there on any such tests either, but did notice there are two 
amateur radio camps on center separation, zero and 5 to 6 inches. Obviously 
a zero connection point for the wires is the easiest. From Jim's PP and a 
couple of other hams pics of a separated center connection, the work done to 
separate the wires at the center point looks harder to construct and 
maintain.

The Lane article by the way also addresses results of putting the low freq 
element on the bottom vs the top (flipped) and gain differences seen with 
some detail on the reasons for the differences.

I am a little surprised there is no comparative test data on such an antenna 
in the two center configurations. Would have thought either Fort Sneeze or 
SRI would have done that. Can someone lend an opinion (or result of a 
comparative test) of some separation vs no separation at the center point? 
The end element separation I fully understand.

I am going ahead with the 160/80/40 fan but with zero center separation and 
will see what happens. Have just to add the 160 wires and trim the antenna 
up after I fully raise the tower. Maybe over the winter I can build one 
using the center separation numbers noted by Lane based on the SRI report 
and do a swap out next spring or summer.

Appreciate the feedback and continue to learn from the practical experience 
and wisdom present on this site.

Don W7WLL



-----Original Message----- 
From: Doug Smith
Sent: Saturday, September 30, 2017 12:02 PM
To: Towertalk Reflector
Subject: Re: [TowerTalk] Fan Dipole

I am familiar with that research.  The bottom line was that by spacing the 
feed points of each set of dipoles approximately 6 inches from each other 
*and* by spacing the ends of each of the shorter dipoles approximately 3 
feet below the longer dipole above it, interaction would be minimized.

I believe that the term “interaction” covered both resonance and bandwidth 
of the higher frequency dipoles but the bandwidth issue might be based on my 
own experiences or preconceived notions.  Not sure at this point — it’s been 
many years since I read that report..

73,
Doug, W7KF
http://www.w7kf.com <http://www.w7kf.com/>



> On Sep 30, 2017, at 7:56 AM, Chuck Dietz <w5prchuck at gmail.com> wrote:
>
> With a fan dipole, research sponsored by the military showed that it helps
> if you can spread the dipoles apart about 8 to 10 inches at the center. I
> have a piece of plastic about two feet long and about 8 inches wide that I
> drilled holes in for each wire and for a u bolt for the balun. A wire runs
> between the holes on each side to connect the dipoles.
>
> Chuck W5PR
>

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