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.
Performance notes:
(September 25, 2011) This antenna easily handles 80 through 17 meters with a balanced tuner, but
efficiency will be off on some bands. I'll report on how it performs on each of the bands 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
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