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The REMBLA

from Thomas Uhlman, NJ1K on June 18, 2014
View comments about this article!

The REMBLA
(Remote Balanced Line Adjuster)

By Thomas Uhlman, NJ1K

I’ve been a fan of the 75-80 band since I first became a licensed amateur in the mid-1980s. It is a very challenging band when it comes to antenna design. I have built many different 80 meter antenna designs over the years, including horizontal loops, yagis, log periodic, dipoles, inverted vees, etc. The best performance I have found within the scope of my resources however has been the “Two Half Waves in Phase”. This is simply two half wavelength sections of wire, arranged end to end and fed at this junction with about one quarter wavelength of open wire balanced line. In layman’s terms, a “full wave dipole” with a tuned feeder.

Besides good RF performance, an additional feature about this design I liked is the SWR tuning. Trimming the feeder is all that’s necessary to bring the SWR to a point low enough for your liking. And because most amateurs don’t have the resources to place the antenna higher than about 60 feet or so, the feeder can be adjusted at ground level without lowering the antenna. I have found that in order to achieve a near perfect 1:1 match, open wire line must be used. Spacing of the two feeder wires needs to be about 6 inches center to center.

Over the years I have longed for a method of remotely adjusting this feeder length “on the fly”. After dreaming about different methods of adjusting the feeder length, I decided that a wire spool arrangement driven by an electric motor might be the most effective. I assembled from spare parts and junk, a device that I call “The REMBLA” (Remote Balanced Line Adjuster).

Scrounging through my junk box and shop I found several key components that could be of use. For the spools I found an abundance of old internet service provider software CDROM disks. I have been looking for a useful purpose for these old things that used to magically appear in my mailbox every week. I have accumulated hundreds over the years. Referring to figure one, the spools are assembled by using two disks for each side of each spool. Sandwiched between two sets of two disks is a one inch piece of 1-1/2 inch copper tube. The sandwich is held together with four 10-32 machine screws. Two spools needed to me made.

I found an old window actuator motor from my Plymouth Voyager mini-van. It’s a compact 13.8 volt DC motor/gearbox assembly. After stripping off the old pulley and cable guides, we are left with a 5/8 inch square drive output shaft. I selected a piece of ¾ CPVC pipe for the spool shaft. This pipe fit nicely over the square drive on the gearbox. I first tried to drill a hole in the square drive but found it was hardened, so the drill wouldn’t even scratch it. A second idea I used was to heat the end of the CPVC pipe with a heat gun until it was soft. I then placed it on the square drive and using two pairs of slip-joint pliers, clamped it to the square drive and held it there until it cooled and hardened. Now we have a spool shaft with a square drive on one end.

Next is fitting the two spools to the drive shaft. The center holes of the spools must be enlarged to fit the shaft. The two spools are fitted to the shaft and separated by 6 inches on center. They are attached to the shaft by drilling a small hole through the shaft close to the side of each spool. Then a piece of steel tie wire is inserted through the hole and clamped under the spool assembly screws.

The shaft assembly is set into a frame made from scrap pressure treated wood. One end of the shaft is supported by a ¾ inch CPVC coupling set into the wood. The inside of this coupling must be enlarged so that the shaft can turn freely. I did this on a machine lathe however; it can be done by hand with sand paper or with a hand drill and a sanding bit. A small amount of grease applied to the inside of the coupling provides lubrication.

The motor is attached to the frame with drywall screws. It is advisable to drill a large hole through the frame where the motor mounts so that the shaft has plenty of room.

To wind wire on the spools, drill small holes through the center of the copper spool hubs, on through the CPVC shaft and out the other side. For wire, I used a product called “Flex-Weave” marketed by Davis RF. (www.davisrf.com). Do NOT use insulated wire. Measure out two pieces about 16 feet long. Tin both ends of each piece. Feed each wire through the hole in each spool and pull it through so that exactly half is on each side. Solder each wire to each spool at the point where it exits the spool. This prevents slippage.

Lay out the spool assembly on the floor and stretch out the wires. Then run the motor a few seconds at a time until the wires are all wound up.

Now that we have a REMBLA machine, how do we install it? There are likely several ingenious ways, however this is how I did it. Referring to figure two, you will see a pressure treated 4x4x10 inserted about 2 feet into earth. I cut two 2x4’s to six feet long and hinged one end of each to the 4x4 about 2 feet up from the ground. These 2x4 arms will provide the necessary length and tension to the adjustable portion of the feeder wires.

At the end of each arm you will see an 8 inch piece of ½ inch CPVC pipe. These are the insulators for the ends of the spool wires. Each one has a small hole drilled one inch from each end. There is a larger hole drilled into the center of this insulator. The insulator is mounted to the arm by a single drywall screw. I placed two ¼ inch flat washers between the insulator and the arm so that the insulator can pivot. This is necessary to compensate for slight differences in wire length between the arm and the spools. Don’t tighten the mounting screws to the point where they’re snug. They must be loose in order to pivot.

After mounting the spool assembly on top of the 4x4 post, unwind the wires completely. Feed each wire through the small holes in the insulator. I then placed crimp connectors on each wire end and then soldered them. Once completed to this point, you are ready to attach your coax feed to one end, and your tuned feeder to the other. Normally, I found that about 60 feet of feeder is required for an element length of 127 feet to be resonant at 3.75 MHz . Now that we have up to 16 feet available and adjustable, we need to shorten the fixed feeder length to compensate. By cutting off 8 feet of the fixed feeder, we should see a resonance of about 3.75 MHz with the REMBLA adjusted about half way.

It’s a good idea to install a balun at the feed point. I used an “ugly balun” and this can be seen in figure 2 on the left arm. Any 1:1 balun will work. You may be able to omit the balun altogether if not running high power.

If you’ve made it this far, you are ready to hook up power to it and real it in and then back out to make sure it winds up and unwinds smoothly. Note that the wires will not necessarily wind neatly or consistently. There will be minor length differences but the tension should be maintained by the weight of the arms and the pivoting insulators. This is ok and will not affect operation.

Now wire up a remote control switch. I used a SPDT switch with off position in the middle which reverses polarity in each position. I would have liked to have this switch spring-loaded but didn’t have one in the junk box. Instead I added a momentary push button switch to allow “bumping” of the motor for fine tuning. Mine will run from full extended to fully retracted in about 5 seconds.

Testing on the band showed that it can tune from 3.58 MHz to 3.9 MHz with SWR of 1:1 anywhere. Below 3.58 the SWR increases but this is only because I didn’t wind enough wire on the spools. I can add this extra range by adding some more wire and making sure that there is enough swing range of the arms.

From a performance point of view, this setup works much better than I had anticipated. With single point of control and fast tuning, I can find a near perfect match anywhere within the tuning range in a very short time. I also regularly use legal limit with this setup and have had no problems.

Enhancements that I have since included are a position indicator, and end of travel switches. I also have had a minor issue during certain weather conditions. This is when the REMBLA is adjusted very high in the band during very windy conditions. The wind can cause the arm(s) to move up higher causing the wires to slack. I have worked out a weighting system to correct this. I will attempt to detail these three enhancements in a future article.

One word of caution: please take proper measures to ensure that humans or animals do not come into contact with the exposed connections.

Most amateurs use remotely tuned LC networks (remote auto-tuners) or manual tuners in the shack. These are good solutions but can be costly. The REMBLA can likely be used on G5RV type antennas also however; I have not tried this application, so I do not know how much wire adjusting range might be needed.

So, if you have ever wanted a way to adjust open line lengths, take a look at building the REMBLA. You might like it, and you might discover a better way to build it. You also might find it works well for other applications where adjusting the open line feeder is needed.

Tom NJ1K

Member Comments:
Add A Comment
 
The REMBLA Reply
by W5DXP on June 18, 2014 Mail this to a friend!
I've thought a lot about such techniques and performed a similar ladder-line length selection function using knife switches on my all-HF-band no-tuner 130' dipole.

http://www.w5dxp.com/notuner.htm

At the bottom of the following page is a graph of the optimum matching section lengths for a G5RV. Looks like the REMBLA would have to be vertical instead of horizontal for a G5RV as it would need to be close to the feedpoint. I was going to try relays for changing the G5RV matching section length but never got a round tuit.:)

http://w5dxp.com/G5RV.HTM
 
The REMBLA Reply
by WA7PRC on June 18, 2014 Mail this to a friend!
I like my 1/2 wavelength cage inverted vee. It covers the entire 80m band under 2:1 VSWR w/o a tuner any motor-driven adjustment. QSY is instantaneous.

I later added a vacuum relay "downstream" of my entrance ground buss, and can drive it as a vertical (against ground) on 160m. I have about 1500' of radials in the ground... so far.

vy 73,
Bryan WA7PRC
http://tinyurl.com/wa7prc-80m-160m
 
The REMBLA Reply
by WA7PRC on June 18, 2014 Mail this to a friend!
I like my 1/2 wavelength cage inverted vee. It covers the entire 80m band under 2:1 VSWR w/o a tuner or any motor-driven adjustment. QSY is instantaneous.

I later added a vacuum relay "downstream" of my entrance ground buss, and can drive it as a vertical (against ground) on 160m. I have about 1500' of radials in the ground... so far.

vy 73,
Bryan WA7PRC
http://tinyurl.com/wa7prc-80m-160m
 
The REMBLA Reply
by N6GND on June 18, 2014 Mail this to a friend!
Too clever by half!

And a great inspiration. Thank you!
 
RE: The REMBLA Reply
by NJ1K on June 21, 2014 Mail this to a friend!
Cecil,

I have thought about doing what you have done with the switches or relays. It's a good way of doing it but I never tried it mainly beacuse of cost and complexity. If done with relays and a multi-position rotary control switch, it would be more compact than the REMBLA and have a wider length adjustment range.

Using 6-foot arms, the REMBLA only has about 16 feet of range, but 8 foot arms gives 22 feet. Of course one could buils a monster REMBLA with 12 foot arms and get about 33 foot range.

Now that the REMBLA has been installed for about 8 months, the only issue I have found is that the 4x4 and 2x4 arms have warped and twisted due to weather. I have had to install slotted polycarbonate wire guides near the spools so that the wires wouldn't all bunch up on the sides of the spools.

I intend on building a larger one with 8 foot arms to service my other 2-half waves on 80. I am searching out synthetic structural materials for this one (think TREX or similar). The wooden one shown in the pic above cost about $75 total not including the motor and other junkbox hardware. I may try one on a G5RV style antenna in the future but I generally don't care for multi-band single wire antennas because of pattern distortion on the higher bands.

 
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