Notes on Stacking an open sleeve feed antenna and a direct dipole feed monobander

By Dan Levin, N6BZA and Marty Levin, W6BDN

This article first appeared in, and appears here courtesy of,

antenneX Online Magazine
http://www.antennex.com
Issue Number: 33, January 2000

In October of 1999, my dreams of global domination were crushed. I had been planning my first ever contest expedition - to FG for the ARRL 10 meter contest. The 10 meter contest has always been my favorite, and my family desperately needed a vacation. Guadeloupe seemed perfect - sunny beaches, good food, and to top it off, FG5BG's DX Shack beckoned. Unfortunately, the best laid plans of this man were torn asunder when my boss intervened. I could still operate the contest, but it was going to have to be from my home station - I couldn't take the week off work that I had planned. My mind immediately turned to ways that I could improve my home antenna situation for the contest.

My home station is a single radio medium pistol operation. A TS-850S/AT drives a W6BDN 4CX1500B amp. Last year I installed a 70 foot crank-up tower, and earlier this year I replaced my olde TET tribander with a Force 12 C-3E. I've been very happy with its performance, but of course I wanted more more more. If I was going to operate the contest from home, at least I could make the most of my antenna situation. I started thinking about rebuilding the old antenna (which was still on the roof, awaiting disassembly and storage) into a 10 meter monobander, and stacking it with my C-3E. 3 dB of stack gain sounded mighty good, given that the gain of the C-3E is only 7.2 dBi or so on 10 meters. In addition, I wanted to be able to beam in more than one direction at a time - given that we usually have openings to both the U.S. and either EU or JA simultaneously. I had read horror stories about trying to stack dissimilar antennas, and even worse ones about stacking tribanders and monobanders, but I thought that I would at least give it a try.

Out came the trusty antenna modeling software, and with advice from internet mailing list denizens on TowerTalk and antenna guru L.B. Cebik, I designed a direct dipole feed 3 element yagi for 10 meters (based on a W6SAI design, modified for slightly more gain at some cost in SWR). The yagi is on a 12 boom, has about the same gain as the C-3E, and the direct 50 ohm feed avoids any phase issues that might be induced by a beta match. I modeled it using the old elements from my TET as the source of tubing. This didnt result in the best taper schedule ever conceived, but would be easy to construct, and time was of the essence.

Everything was fine, until I started modeling the stack of the two antennas. I quickly realized that the computer wanted me to feed the antennas almost completely out of phase in order to get the stack to behave "normally." As you can see below, feeding the antennas in phase resulted in a pattern that looked like what you would expect to see if the antennas were, in fact, out of phase - a very high angle of radiation and very poor gain.

If I modeled feeding the antennas 180 degrees out of phase, the results were better - low angle of radiation and high gain. I was perplexed.

I sent off an email to L.B., who quickly confirmed that the open sleeve feed system on the Force 12 antenna causes about a 160 degree phase shift on 10 meters. As you may know, the C-3 family (at least the single coax versions) excite only the 20 meter driver directly. The 10 and 15 meter driven elements are parasitically excited. This phase shift will exist on any similar open sleeve feed antenna but the amount of the shift will vary depending on the open sleeve geometry.

Back to the computer. Now that I knew I was dealing with a phase shift, I wanted to determine as closely as I could what the exact phase delay was. L.B.s excellent article on stacking points out that a phase shift of as little as 30 degrees can cause a loss of .4dB of gain. I carefully modeled my exact physical layout, including the fact that the driven elements of the two antennas would not be precisely aligned one above the other. I produced the following table of modeled free-space gain vs. feed angle delta:

Phase (degrees of delay at the monoband feedpoint)

Gain (dBi in free-space)

-110

10.19

-120

10.36

-130

10.47

-140

10.52

-150

10.52

-160

10.46

-170

10.35

-180

10.19

While L.B.'s models show a 160 degree shift caused by the C-3 feed system, my models showed that for my physical installation and the two actual antennas that I was going to stack, about 145 degrees of delay was more accurate. With that delay in the model, I got much better results:

The stack was optimized for gain, not front-to-back but this run looked much better than the original one shown above! Confident that the situation was under control, I built and installed the monobander on my tower. I was using an Array Solutions Stackmatch to stack the antennas, so I added an extra 11.3 feet to the 9913 feed line on the monobander (984/28.400 = L = 34.65'. L * (140/360) * .84) = 11.3'). I put the stack on the air, and everything seemed great. A quick check with four stations resulted in unanimous agreement that the stack was louder than either antenna by itself.

If I might interject a brief side-note - while I was at the computer, I decided to model the gain of the stack vs. the distance between the two antennas. The following results were very educational.

The gain increases rapidly and significantly, until you get within a couple of feet of the optimum distance (23 feet, in this case). But it degrades quit slowly as separation increases beyond the optimum distance. In fact, while decreasing the separation only 5 feet resulted in a decrease in gain of three tenths of a dB, increasing the separation by 10 feet cost only half as much gain. This suggests that stacking multiband antennas is easier than it otherwise might be, because at least in this particular case, the slow degradation in gain as separation increases beyond the optimum point means that we can stack multiband antennas at near the optimum separation for the lowest band without hurting the higher bands too much.

But back to our story! I am a careful guy (at least most of the time), and I wanted to be sure that I hadn't made a bad mistake. After all, I wasn't going to have too much time to experiment with my new toys on the air - and I wanted the best results possible for the upcoming contest. Besides, I work in the computer business, and I have learned not to trust software further than I have to! Luckily, I have access to plenty of high tech toys at the office and good elmers in the form of my dad, W6BDN, and a neighbor about 1000 feet away, W6TC. The three of us designed an experiment with advice from Jay, WX0B of Array Solutions, to test my installation.

I had the parts lying around to build a 10 meter band-pass filter (an in-process project for my future 2 radio contesting setup). I finished up the filter, and then using my MFJ-259B as a signal source and a 500 Mhz dual trace digital oscilloscope from work, I verified that in the center of the passband, the filter didn't introduce any phase shift. I did this by running the output of the '259B into a UHF 'T', and then into the scope first through equal length coax jumpers (as a control), and then inserting the filter in-line with one of the jumpers. The scope clearly showed no phase shift at the center of the passband and significant shift as I moved outside the passband. Confident that my equipment was ready, I packed up the 'scope, filter and assorted coax connectors and adapters, and headed home.

Nine o'clock that night found me on my roof in 45 degree weather, juggling about $9,000 worth of electronics! I put the bandpass filter in line with the C-3E's coax (I wanted to make sure that out of band signals from 15 or 20 meters weren't contaminating my measurements), and then fed both antennas into the borrowed 'scope. In 50 ohm mode the 'scope was quite capable of showing the RF waveforms coming off the yagi's. W6TC gave me a nice clean FSK signal at 28.300 (I had identified this as the center of the passband of the filter during my previous measurements), which showed up as a beautiful pair of sine waves on the 'scope's display. In about 2 minutes I had verified that my delay line was working perfectly, and that the two antennas were in phase to within 5 degrees or so (basically to within my margin of error). Then just to be sure my equipment was working, I put a spare 12' piece of RG-8X in line with one of the antennas - and sure enough they now read significantly out of phase. I packed up, and was quite pleased to make it down the ladder in the dark without dropping anything!

The next weekend was the 10 meter contest. I let the magic smoke out of the amplifier power supply on Saturday morning, which severely limited my score but the results from the stack were excellent. The combination of a very noticeable increase in gain when the two antennas were both pointed the same direction with the ability to beam in two different directions at the same time made me feel like a big gun. Even with 7 hours of low power operation and 3 hours off on Sunday morning to fix the amp, I still managed 1750 QSOs (SSB only), thanks in no small part to my new stack.

There are two morals to this story.

  1. Open sleeve antennas induce a significant phase delay on the parasitically excited bands. You need to take this delay into account when stacking them with dissimilar antennas, or you will get a very poor result. Computer models using MININEC based software are capable of modeling these situations quite accurately, so do your homework before you build.
  2. Killjoys who insist that it is impossible or impractical to stack non-identical antennas are wrong. It took me less than 15 hours of work, including building the monoband yagi, to model, install and verify my stack (including time to deal with the whole phase issue that is the point of this article!). It clearly helps that my monobander is of a very simple design with direct 50 ohm feed but in theory the same approach should work for any antenna that can be accurately modeled in software. My results were excellent, and yours can be too if you take the time to do it right.

 

References:

I Want to Build a 3-Element Yagi Part 2: How Wide-Band Shall I Make It? L.B. Cebik. http://www.cebik.com/3lyg2.html

Stacking What difference does difference make? L.B. Cebik. http://www.cebik.com/stphase.html