Notebook Series - Multi-band Twin-T Capacitive Loaded Vertical Doublet (Dipole)
[ Marquette, MI ] My linear-loaded vertical doublet (dipole) 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 (dipole) 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 (dipole) 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 that it will be usable as a multiband antenna with 40 meters being the main band of
interest.
Once again, here are a few items to remember while reading this page.
1) The reason for this experiment was to find a small footprint, good DX antenna for 30, 40 and
80.
2) There isn't an antenna test range in my backyard.
3) The basic antenna started in life as a 32.5', 20 meter homebrew vertical doublet. It was
then modified for 40 meters by adding linear-loading.
4) Radials? Don't have room for them and never used them on a vertical doublet.
5) No attempt was made to resonate the antenna on a specific frequency, but I wanted to get it
close to 40.
6) Vertical doublet balance is difficult to maintain for many reasons.
7) Any antenna that isn't full size is a compromise. How much of a compromise can be debated
by the engineers.
8) Remember, contacts can be made using a wet string in a stiff breeze on a good day.
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 12 feet.
(24' total across the top and 24' across the bottom.)
Feedline: 100 Ft of 300 ohm ladder-line
Radial System: None
Matching: Tuner
The linear-loaded vertical doublet (dipole) has been taken down, and the loading elements have been
removed returning the vertical doublet (dipole) to its original 20 meter configuration. Now for
the conversion to a twin-T capacitive loaded vertical doublet (dipole).
This conversion is 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. Each wire was cut to 12 feet, but they should have been shorter. Cebik's design
shows 10 feet for each side.
Using 100 feet of 300-ohm ladder-line, the lowest feed point "Z" inside the shack is around 5.8 MHz
with the longer elements. I do use a 1:1 current balun and a tuner with this antenna.
I modeled this antenna over real ground using MMANA-GAL Basic with the lengths used including the
feed line. The model shows the following:
1) 7.05 MHz, 0.82 dbi, 25.2 Elev
2) 10.12 MHz, -0.19 dBi, 22.2 Elev
3) 14.05 MHz, -1.38 dBi, 21.0 Elev
If you look closely at the photograph, you can see the small insulators on the top hat. You can
also see the guy-rope lines and the feedline.
In this picture, 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 (dipole) 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.
Performance notes:
(September 25, 2011) As mentioned, this antenna is resonant around 6 MHz, but easily handles 80
through 17 meters with a balanced tuner. I'll report on how it performs on each of the
bands. I'll be running 100 watts.
40 meters wasn't all that great tonight, but I did work E79D (Bosnia and Herzegovina) and DK3FW
(Germany) with a 579. He was running 1.5KW and a two element yagi. I also worked Miquelon
Island just off the coast of eastern Canada. Later in the evening, I worked HA6OA (Hungary) on CW
and HA8RM (Hungary) on SSB.
(September 27, 2011) I haven't been doing much operating. Called CQ on 30 and worked K5FA in
Mississippi. I was S9 plus 10.
(September 28, 2011) Worked PR7AB (Brazil) on 40. Got him on first call. Antenna
tuning changes in rain. Really should use true open-wire line with doublets!
(September 29, 2011) Worked 3D2R (Rotuma Island - A Fijian dependency) in 40 meter pile up.
(October 03, 2011) T32C (Kiritimati - Christmas Island) in the log. Worked on 30 meters.
(October 06, 2011) Still not working much on 40 and 30, but H77REX is in log for a 40 meter
QSO. Been on 10 with the yagi!
(October 08, 2011) This one was difficult because of weak signals in both directions. In
fact, he may not have my call correct, but he's in the log (8:30 pm local) as 9L1BTB (Sierra
Leone). A bit of a struggle, but I worked DL60HSC (Germany). I called an EA3, but he didn't
hear me. Not much DX on the band for a weekend.
(October 09, 2011) Got F5IN (France) on first call, but had problems with UT7UJ (Ukraine).
UT7UJ did confirm. Also logged OK4RQ (Czech Republic). All contacts on 40 CW.
(November 9, 2011) My vertical doublet (dipole) came down this afternoon in an early snow
storm. BIG surprise. It broke at the center insulator. It was made from 1 inch
diameter Acetron rod and was supposed to exhibit high strength and stiffness, be porosity free, with low
moisture absorption and good wear-resistance. I've been using it for at least ten years. Not
being a mechanical engineer, I don't fully understand the stress at the center feed point. The
vertical doublet (dipole) was guyed at both the top and bottom elements. Ice on the top capacitive
loading wire may have been an issue. I will try and fix this before winter is here to stay.
(November 10, 2011) I got the antenna on the ground and the insulator rod broke at the top
feed-line screw. I had drilled holes to run the bolts all the way through the aluminum tube and
rod, so that created a weak point. The old insulator is Acetron/Acetal and it's like Delrin/Acetal
which is more commonly used as an insulator on antennas. Tensile strength specifications are
confusing, but it appears to me that Acetron and Delrin are much weaker than Fiberglass. A new
solid rod has been ordered from Max-Gain Systems, but this time it's Fiberglass. I also ordered
clamps from DX Engineering that will allow me to connect the feedline without drilling holes through the
Fiberglass or aluminum tubing.
(November 14, 2011) Parts have arrived. I had to replace the 1-1/8 inch aluminum tube below
the center insulator, because it was slightly bent. I happened to have one in the shed from
another project. Today, I rebuilt the guy-wire supports to mount on the 1-1/8 inch aluminum tubes
above and below the center insulator.
(November 15, 2011) I didn't want to drill holes through the new Fiberglass center insulator for
the feeline connections, so I bought hose clamps with studs to connect my feedline. To take strain
off the connections, I fabricated a strain-relief made from polyethylene. To hold it in place
between the two elements, the Fiberglass center insulator rod slips through it. I added the new
guy wire connection plates and hoisted the antenna back in place. Now to wait for the next snow
storm.
(November 18, 2011) Here is a look at the log for the past few days after the antenna was put back
up. 40 meters: PJ5/DL7VOG (Sint Eustatius), JA1NUT (Japan), SP3GXH (Poland), OK1CF (Czech
Republic) and UW2ZM (Ukraine). 30 meters: FG/DK9PY (Guadeloupe), ES5QD (Estonia), and R7MA
(Russia). It's snowing and the antenna is still up...
(November 27, 2011) I operated 40 meters exclusively during the CQ-WW contest this weekend, and
managed to work all continents and 41 entities in 19 zones in about 5 to 6 hours of operating. All
with 100 watts. I was just listening much of the time. Contacts included VK2IM, ZM1A, KL7RA,
6V7V, EF8M, JA3YBK, C5A, C4N, PI4DX, 4O3A, NH2T, KH6LC, CR2X, VP9/N3AD, etc.
(December 3, 2011) I tied the two leads of the feedline together this afternoon, and used a tuner
to get the antenna on 160 meters. I made three contacts (2 states - 1 province) during the ARRL
160 meter contest with W8JI, Georgia being the furthest. Not bad for a little after three in the
afternoon. ...I dropped back in on 160 tonight and made 17 more contacts in 14 more states and one
additional Canadian province. So, the wet noodle works on 160 in a pinch. Now, back to
40.
73 and have fun with your construction projects...
Joe (AJ8MH)
ex: WPE8EUM, WN8AQL, WB5FCO and WJ5MH
*L. B. Cebik, W4RNL ~ 1939 - 2008 ~ SK as of April 2008
