#### Notebook Series - Multi-band Twin-T Capacitive Loaded Vertical Doublet (Dipole)

[ Marquette, MI ] My linear-loaded vertical doublet has been doing an
excellent job the past two years, but it's time for a change. I've wanted to play with a twin-T
capacitive loaded vertical doublet design since looking at the Force 12 Sigma series and the TransWorld
TW2010 Adventurer vertical, so now is as good a time as any to try and build one.

The Sigma and Adventurer series use T-bars at each end of the vertical doublet as an effective loading
technique. On the lower bands the Sigma (and I believe the Adventurer) also incorporate center loading
coils. The late L. B. Cebik, W4RNL, discussed this type of antenna, but without center loading, and
that's the antenna I will be building. My antenna will be fed with 300 ohm ladder-line, so it will be
usable as a multiband antenna with 40 meters being the main band of interest.

See the original 20 meter vertical doublet
design for tube size and lengths.

Current configuration:

Overall length: 32' 6"

Each element length: 16' 3"

Top and bottom wire elements that form the Twin-T are 30 feet across.

(15' each side of the vertical radiator on the top and bottom.)

Feedline: 100 feet of 300 ohm ladder-line

Radial System: None

Matching: Tuner

The linear-loaded vertical doublet has
been taken down, and the loading elements have been removed returning the vertical doublet to its original
20 meter configuration. Now for the conversion to a twin-T capacitive loaded vertical doublet.

This conversion was much easier when compared to linear-loading the old antenna. First, an eyehook
was installed at the top of the vertical and the top-hat was installed. To insure good electrical
contact, a short jumper wire was run from the top-hat wires to a solder lug under the eyehook. The
top and bottom "T" sections were cut to 20 feet (10 feet each side) per Cebik's design.

(Updated 11/05/2012)

When I started to use an automatic "L" tuner with this antenna, I noticed that the tuner didn't like the
impedance presented on the lower end of 40. After modeling the antenna, I discovered that the
resonant point wasn't in the 40 meter band, so I ran the model with different lengths of "T"
sections. 30 foot "T" sections (15 feet per side), seemed to model much better, so I changed the wire
lengths. Now, the tuner is able to handle the antenna on 40 and 30 without any issues. I use
100 feet of 300-ohm ladder-line, a 1:1 current choke, and 6 feet of coax to the shack.

I'm not really sure why my antenna was so
different from Cebik's, but I suspect that the angles I used for the "T" sections caused the resonant point
shift. I suggest you model your configuration or simply start with 32 feet total length for the top
and bottom "T" sections and trim from there. This is much more important if your plan is to feed the
antenna with 50 ohm coax.

If you look closely at the photograph above, you can see the small insulators on the top-hat. You can
also see the guy-rope lines and the feedline.

In the picture to the right, you can see the yagi, which is about 90 feet from the vertical. You can
also see the close proximity of the fence and in-ground basketball hoop. I'm sure there is some
interaction taking place.

Of course, the guys for the vertical doublet are made from rope. I use 2 sets of three guy-ropes to
keep the vertical stable. The addition of the top-hat also helps stabilize this antenna.

The feedline runs to the second floor of the house, to the tower and down to a feedline junction box.

Here is a picture of the lower hat.
You can just see the black wires running up at an angle from the base of the vertical. I know, you
shouldn't have any obstacles around your vertical, but I'll live with this installation.

There you have it. Nothing to it.

Please remember to review the original 20
meter vertical doublet (dipole) design for tube size and lengths. You can also review the 40
meter linear-loaded modification to the
original antenna.

73,

Joe (AJ8MH)

ex: WPE8EUM, WN8AQL, WB5FCO and WJ5MH

*L. B. Cebik, W4RNL ~ 1939 - 2008 ~ SK as of April 2008