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 (Marquette, MI) This is an update on my original vertical
doublet design.
The main antenna I'm using at my Michigan location is a 44 ft horizontal doublet running north/south
in an inverted-V configuration. The center mounting mast is a 30 ft aluminum telescoping pole
and the feedline is 36 ft of 300 ohm ladder-line.
The antenna actually does a great job on 20 and 30 with good performance on 40, but now that I'm dug
in, I've decided to put my old Texas vertical doublet in service as a second antenna.
I've always wanted to try linear-loading this vertical doublet to make it more useful as a DX antenna
on 30, 40 and possibly 80 meters, but never got to it until now.
Here are a few things to remember while reading my notes.
1) The reason for this experiment was to find a small footprint, good DX antenna for 30, 40
and 80.
2) There ain't no antenna test range in my backyard.
3) All comparisons are against my 44' horizontal doublet.
4) The basic antenna started in life as a 32.5', 20 meter homebrew vertical doublet.
5) Radials? Don't have room for them and never used them on a vertical doublet.
6) No attempt was made to resonate the antenna on a specific frequency, but wanted to get it
close to 40. And, here is a good one.
7) Dipole balance is difficult to maintain, because the elements above the feed point tapper
more than the elements below the feed point, and the distance from the linear-loading wires to the
aluminum is slightly different. Oh well!
Number six may sound strange, but I didn't want to pull the vertical down to do any trimming of the
linear-loading wires once in place. I just wanted to effectively lengthen the antenna and move
the resonant frequency toward the 40 meter band. I had a good idea of what resonant frequency
to expect based on prior results of others. Since I am feeding this antenna with ladder-line,
the exact final resonant frequency was not important.
 According the the ARRL Antenna Handbook, linear-loading
introduces little loss and has a low Q, which allows good bandwidth (up for debate by a few).
There are a couple ways to linear-load an antenna. I selected the one shown to the left, which
would shorten the overall length of the antenna the most. I elected not to run the
linear-loading wires all the way to the doublet extremes.
So, how long should the linear-loading element be? I picked 12.5 feet', simply because I wanted
to run the wire 3/4 of the way up and down each element. (Each loop used 25' of wire.) I
figured this would resonate somewhere between 7 and 10 MHz, and that was good enough for me.
(Final "system" resonance turned out to be 7.5 MHz.)
So, what should the distance be between the linear-loading wire elements and the wire/aluminum
elements? 3" between the wire elements looked good to me, and I'd live with the distance
created between the wire and aluminum elements. How's that for solid engineering?
Some have used 450 ohm ladder-line for linear-loading elements with good results. I haven't
heard of anyone using 300 ohm line. I'm going to assume that the open-wire design I'm using
isn't as sensitive to rain and snow, thereby maintaining resonant stability.
Overall length: 32' 6"
Each element length: 16' 3"
Each linear-loading element length: 12' 6" (25' in each wire loop)
Feedline: 100 ft of 300 ohm ladder-line
Vertical doublet is isolated from ground.
More construction information is included with the
photographs below.
73 and have fun with your construction projects...
Joe ( AJ8MH )
aj8mh@arrl.net
*L. B. Cebik, W4RNL ~ 1939 - 2008 ~ SK as of April 2008
(October 7, 2009) This is
the second configuration of this vertical doublet. The guy configuration has been changed and
the linear-loading element length has been changed, as well as the spreader mounting design,
so we're at day one establishing the antenna's performance. There are a couple reasons for
making these changes.
The guy changes were needed to simply keep the antenna in the air. The linear-loading
length was changed to lower the resonant frequency to try and get it closer to 40
meters. At the same time, I wanted to see if there was a change in 20 meter performance
thinking that prior poor performance was being cause by a tuning stub effect attributed to
the old linear-loading wire length. I was also interested in seeing if lengthening the
linear-loading wires noticeably change the take off angle. At this time, I don't know
if this is even possible to check, but it's on my list.
So, now I am back to checking on-air performance. I'll add my observation during the
next several weeks.
(October 10, 2009) Remember, this is the second configuration of this linear-loaded
vertical doublet design. Performance is still noticeably poor on 20. On 30, the
antenna has lost gain compared to the original design. It seems that lengthening the
linear-loading elements caused a drop in performance on frequencies higher than the new
resonant frequency.
I can only speculate that this is caused by multiple lobes in the pattern or the
linear-loading elements could be acting as tuned stubs attenuating the signals, but have no
way to prove that theory. I would like to hear your theory, especially if you design
antennas. E-mail me.
I'll make a general comment and say the antenna is usable on 20 and 30, but doesn't favorably
compare with my 44' horizontal doublet. The current design should only be considered if
your focus is 40 and maybe 80.
On 40, the antenna performs better than my horizontal doublet by 2 to 3 S-units on
receive. The way one works DX these days, you don't have enough time to ask the station
on the other end to wait while you swap antennas for a comparison, so I'm going to go out on
a limb and assume the antenna performs better on transmit, too.
So far, I'm impressed with the DX contacts I've managed on 40 running 100 Watts. They
include PP5KR-Brazil, LX/PA6Z-Luxembourg, OK1XC-Czech Republic, TX5SPA-Austral Islands,
VK6AA-Australia and F5CWU-France.
(October 14, 2009) In my opinion, 40 nighttime conditions haven't been the best, but I
worked UW7LL-Ukraine and CO6LP-Cuba this week. Today, I was able to listen to Midway
Island well into mid morning. I was actually surprised how loud he was, but I couldn't
overcome the hundreds of stations calling him.
I had a chance to drop down to 80 tonight to do a few quick comparisons with the 44'
horizontal doublet. I know this horizontal doublet isn't very efficient on 80, because
it's so short, but I wasn't expecting how bad it performed. The vertical doublet is 4
to six S-units better on receive, and seems to be comparable on transmit. 80 is not my
main focus, but I did want to mention my quick observation. This requires more
investigating.
(October 22, 2009) I've noted changes in tuner settings during wet weather. I
thought resonant stability would be better. New 40 meter contacts include KH7Y-Hawaii,
FM/KL7WA-Martinique and VP9400W-Bermuda.
(November 10, 2009) I'm still playing and just worked PJ4/DF9AN in Bonaire, Curacao,
KP2/W1EQ in the U.S. Virgin Islands and VP2MUM on Montserrate on 40 This antenna is a
keeper, but I can see that an amplifier would be nice. I just can't imagine running 800
watts (a 9 db increase over 100 watts) and working a station on the first call...maybe.
That would be too easy, but it sure would save a lot of time and allow me to add to my
country totals quickly. Again, maybe.
Note: A 9 db increase in power should only be a couple S-unit change in signal level on a
receiver, but in modern receivers, the S-meter is very non-linear below S-8, so you may see a
greater change than one would expect.
(November 18, 2009) Add OH7UE-Finland, DL4MO-Germany, EA3DTD-Spain and EA8OM-Canary
Islands to the list of worked stations on the 40 meter vertical doublet.
(November 19, 2009) Add VP2V/DL7VOG-British Virgin Islands (He got my call close
enough, AJ8MS vs AJ8MH), IK0GDG-Italy (Took him a couple tries to get my "H" right, but he
made it!) and A31A-Tonga. It would be so easy with an amp. This is my last
update.
Autek Research VA1 Analyst
data.
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SWR
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MHz (Min SWR Freq)
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SWR
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MHz (Ham Band Freq)
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1.9
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2.197
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Out of range
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3.530
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1.5
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5.400
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11.5
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7.030
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1.1
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7.500
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Out of range
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10.100
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2.6
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11.720
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Out of range
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14.030
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1.5
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15.880
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7.9
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18.080
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1.4
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22.480
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7.5
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21.030
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 Construction details and
photographs.
This is the center feed point showing how the linear-loading wires are connected.
The two closest screws in the center of the picture connect the wire to the aluminum
element. The two other screws connect the 300 ohm ladder-line to the wire.
The aluminum sections and the center Acetron insulator are hose-clamped. Each
aluminum section is slit, so they can be compression clamped.
All hardware is Stainless Steel.
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 This shows the linear-loading wires
running through one of several fiberglass spreaders. The spreaders are 4 inches
long with the holes drilled 3 inches apart. Each spreader is run through aluminum
supports that are hose-clamped to the vertical element.
The fiberglass rod is cut from the larger size driveway reflector supports.
One of the guy-ropes is also seen in this picture.
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 The linear-loading end insulators
are made from aluminum and Plexiglas. Note the hose clamp guide cut in the
aluminum.
The wire is simply looped through the Plexiglas and run back toward the center feed
point through the fiberglass spreaders.
Below is a picture showing the linear-loading wires running through the
spreaders. Shown below and to the right is the base mount.
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A few shots of the 44' long inverted-V at 30' (my
other antenna) taken during the Winter of '09.
 
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