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.
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
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.
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.
ex: WPE8EUM, WN8AQL, WB5FCO and WJ5MH
*L. B. Cebik, W4RNL ~ 1939 - 2008 ~ SK as of April 2008