Saturday, April 23, 2016

KG4GVL Skeleton Sleeve Pt 1

In my quest for finding an antenna that gives me two bands without a tuner, one of the most interesting candidates so far is the "Skeleton Sleeve Dipole" featured in the Appendix of Small Antennas for Small Spaces.
This antenna fascinates me because it's one of the physically simplest multiband antennas I've seen. I know an EARCHI type endfed could be called "simplier", I also think they could be called "lossier" too. I've built a couple Half-wave end fed antennas, and while they are impressive, I am not really sure about how good they are. the SSD has no tuner, no traps, no matching section. When deployed to the proper height, they can exhibit gain when compared to a dipole of similar height, especially on the higher frequency band.

I wanted to build an antenna that was easy to operate on bands that it's hard for my W3EDP to tune, namely 30m and 17m. Especially 30m. With the the solar minimum upon us, and the potential for a larger Maunder minimum type event, I figured it would be wise to have something dedicated to 30, that may be the best daytime DX band.

The Original 40m/20m Folded skeleton Sleeve Antenna Courtesy W1ZR

I started, by taking a spreadsheet and calculating the length of the antenna depicted in the book as a percent of a standard dipole in feet ie: L/(486/f) * 100%. L = total length of the folded skeleton sleeve in feet, f = Frequency in MHz.

After that, I calculated the similar percent needed for 30m and 17m, and soon discovered that if I was going to keep what I thought was similar spacing and construction as the original, 17m would not fit! I decided to go with 30m and 15m instead, figuring it would be a better fit. I came up with the model mentioned in a previous blog post. It didn't take me long to figure out that the model didn't quite work the way I expected it to work. I had some questions about why that could be, so started out by fiddling with the lengths of the antenna, and even putting it in freespace. Eventually, I emailed the expert, W1ZR, the original designer of the antenna. He sent me some wonderful information and tips about modelling these types of antennas I want to share.

First off, here's what a generic image of the design looks like:

I plan on building the top style. The bottom one has some potential, too, but the top style is the one I want to build because it's over all length is shorter than a traditional dipole.

The particular dimensions I'm after

Above is some of the excellent information W1ZR sent me. Notice the dimensions for most band combinations. I asked him if he used any formulas to come up with the numbers, and he said no. He created the model with trial and error, and then built it out to see if real life fit the model. That's dedication the ham radio way! I circled the information for the 30/17 model so you can get a rough idea of the dimensions. I got all this information last fall, and over the winter I began building the antenna with the idea that I would have it up in time to catch some winter QSOs and Spring E-Skip on 17. Turns out, I busted my timetable all up by going back to school! 

To be continued...

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