[TowerTalk] Increased bandwidth of *fat* radiators

[Robert Chudek - K0RC]

The discussion about cage dipoles and increased bandwidth is quite timely for my QTH. Two days ago I found 3 feet of 1/4" aluminum tubing with a tiny hose clamp attached to one end laying in the back yard. It had dropped off one end of my 40m dipole. Something was obviously wrong... it was 70 degrees, calm, and sunny outside. My normal luck is 20 below zero, 35 mph winds, and sleet.  :-)

I hooked up my AEA VIA Analyst and swept the 40m band. The dipole was now resonant about 7.4 MHz instead of the normal 7.1 MHz. This horizontal dipole is up 65 feet and is side mounted to my tower. It's a single element Telrex and starts out 2.25" OD and tapers to 0.25" tips. It's a split element, direct fed, and over 70 feet long.

Today I reinstalled the tip (nothing was broke, it only vibrated loose) and ran another sweep with the Analyzer. It went back to 7.1 MHz on the nose.

I also have a 40m sloping dipole off the same tower which is probably at a 60 degree angle. This antenna is made from #12 solid copper with a Teflon-type black coating. It also resonates around 7.1 MHz. Both antennas are fed with RG8x coax, maybe 80 or 90 feet long. My measurements were made at the end of the coax, not at the antenna feedpoint so there will be a bandwidth *bias* (towards wider) in these readings.

I posted the antenna graphs as JPG files on my website here:  http://tinyurl.com/dzqpl6  

The *Return Loss* graph will show the precise resonant frequency of both antennas. The *VSWR* graph will show the "apparent" bandwidth of these two antennas. What will jump out at you is the "fat" dipole has an unmistakable wider bandwidth than the #12 wire. Comparing the 2:1 VSWR values, the wire is about 480 KHz and the Telrex is over 1 Mhz. On a basic level this supports the idea that a *fatter* radiator provides more bandwidth.

On many levels this comparison is "apples to oranges", but I thought some of you would like to see *real world* values from antennas I use at my QTH. If I get ambitious I might round up some more wire and buy some of those spreaders. In that case the comparison will be "more equal" because I can compare an antenna in the same position and orientation.

73 de Bob - KØRC in MN