Saturday, June 8, 2013

Linear-Loaded Vertical Doublet (Dipole)

Linear-Loaded Vertical Doublet (Dipole)

[ Marquette, MI ]  This is an update of my original vertical doublet design.
In Texas, I used a 32 ft vertical doublet and fed it with 300 ohm ladder-line.  I 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 isn't an antenna test range in my backyard.
3)  All comparisons were against my 44' horizontal doublet when it was in use.
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 I wanted to get it close to 40.
7)  Vertical doublet balance is difficult to maintain for many reasons.
8)  Any antenna that isn't full size is a compromise.  How much of a compromise can be debated by the engineers.
9)  Antenna modeling wasn't used, but would have been nice.  Antenna theory was thought about for a very brief moment.  Remember, contacts can be made using a wet string in a stiff breeze on a good day.
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 to me.

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 below, 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.  To bring this antenna into resonance at 7 MHz, the main element would have to be lengthened a couple feet.  It may be beneficial to add a top radial hat and bottom radial shoes.)
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
Radial System:  None
Matching:  Homebrew balanced tuner
See the original vertical doublet design for tube size and lengths.


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.


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.


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.

Performance notes:
(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 caused by a tuning stub effect attributed to the old linear-loading wire length.
So, now I am back to checking on-air performance compared to my 44' horizontal doublet at 32'.  Yes, it's like comparing apples and oranges, but I'll add my observation during the next several weeks, anyway.
(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 as to why, so I would certainly 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.  (It's not terrible.  It just doesn't favorably compare with the horizontal doublet.)  The current design should only be considered if your focus is 40 and maybe 80 or you don't have enough room for any other type antenna.
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 to work running 100 Watts on 40.  They include PP5KR-Brazil, LX/PA6Z-Luxembourg, OK1XC-Czech Republic, TX5SPA-Austral Islands, VK6AA-Australia and F5CWU-France.
Just listing my DX contacts doesn't prove much, so I'd like to comment for the record that I switched between antennas to confirm the performance of one antenna over the other.
(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 the 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.  Germany and Hawaii were worked.  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.  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.  That would be too easy, but it sure would save a lot of time and allow me to add to my country totals quickly.
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.
73 and have fun with your construction projects,
Joe (AJ8MH)

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