Fat Dipoles

Started by linkclan, May 02, 2015, 11:11:10 AM

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linkclan

I've been pondering a fat dipole as a replacement for my 135ft all band. With two properly spaced elements they are supposed to increase bandwith even enough to cover all of 75 and 80M.

Now, the motivation was found with the appearance of the new KX3 and the need for an antenna on the deck. With the wife complaining the whole way, I constructed a Fat 30 resonant dipole with the apex extending 12ft above the crows nest which is already 25ft above ground. It is up with just a couple carpenters clamps to satisfy the temporary requirements.

The results were fantastic! Even as it was a bit short, min SWR 1.04@ 10.160, it is flat across the whole band with 1.16 @ 10.100. First contact was 1732 miles with 10 watts! Outside on battery, the noise floor was only an S unit.

Only complaint is the lack of cushions on the deck chairs ;D

73 K6PLE



BrassPounder

I designed a bow tie dipole antenna for 40m several years ago.  Each side of the antenna is composed of two wires, each 30 feet long, separated 48 inches at the outside ends and tied together at the center.  There's a 6 inch wire pigtail that connected the to the balun terminal.  It was erected as an inverted V with an angle of about 120 degrees.  The 2:1 SWR bandwidth was from 6850kHz to 7350kHz with the center frequency around 7150kHz.
I used a 3/4" inch piece of PVC pipe for the spreader.  Had better than 1.5:1 SWR over the whole 40m band.

I scaled it to 30m to see if the scaling would work.  Worked great and had nearly a MHz of bandwidth.

Here are the formulas to calculate the wire lengths for any band.

One side of the dipole = 214.5/F (in MHz)
distance between the ends = 28.6/F (in MHz)

If anybody builds one, let me know how it works out.

73,
John, WN4OFT

linkclan

This is exactly what I did. My formula was:

Length= 442.5/f
Width= 11.25/f this is also the gap spacing between the first spreader.
Height= 112.5/f

I fed it with coax via a commercial adapter and am very happy with it.
Also found a new assembly device.... Residential ground wire crimps work great for stops on either side of the PVC spreaders and the end insulators.

I am very curious about how well this would work in the flat-top configuration and fed with 600ohm ladder line to a matching device. My limited brain sees some logic in it as 1. there would be more wire up in the air, 2. bandwidth should increase at the resonant points resulting in a lower Q and less L/C to match resulting in more radiated power. I remember reading somewhere that this was done with a 1/2 wave 80m loop with surprising results. Would be interesting to see a model of the possibilities.

Paul-K6PLE