CAROLINA WINDOM® Compact™
| The "Compact" version of the CAROLINA WINDOM® provides all of the best features of the standard CAROLINA WINDOM®, but with an overall length that is only one-half of the needed support length of the standard CAROLINA WINDOM®. Performance is nearly identical. The power rating is the same. Only, the length and physical layout are different. History: I have been working on a shorter version of the CAROLINA WINDOM® for years. The main focus of the effort was to maintain the performance of the CAROLINA WINDOM®, while reducing its length. Compromising the incredible performance the CAROLINA WINDOM simply was not acceptable. The CAROLINA WINDOM® Special series was the first reasonable attempt. Performance was compromised only the lowest frequency band. On all other bands, full performance and power rating was maintained. This was a good compromise, but I wanted to improve the compromise required on the "bottom" band. The CAROLINA WINDOM® Special turned out to be very popular very popular antenna and provided a way for a lot of folks to have an effective signal on bands that would not normally be available to them due to space limitations. Over the years, various techniques for reducing the length of the CAROLINA WINDOM® was attempted. Even though I am not a fan of trap antennas, even that technique was tried. Loading coils were also tried in an attempt to shorten the antenna while permitting use of the entire wire length on all bands. Linear loading proved to be very promising, but most common methods adversely affected the multiband qualities of the CAROLINA WINDOM®. I was able to achieve significant length reduction. It was possible to reduce the length of the antenna by 50% or even more with some linear loading geometries and bending techniques. You will probably see some of these techniques in future projects. Unfortunately, the best linear loading techniques have difficult assembly issues and building them is very labor intensive. One day I just stumbled across a simple by an elegant length reducing technique that worked well with the CAROLINA WINDOM®. I had been bending up the active elements in various ways and making lots of measurements and recording the data for later optimization. I was a bit tired of the entire project when an idea from one of my old notebooks came to mind. Several years ago, I had drawn up a simple change in antenna geometry but never got around to testing it as a computer model or as a real, physical, antenna. So, before I finished up the testing series I was working on, I decided to give the idea a try. I wasn't expecting much, but it was a simple, reproducible, configuration. It was worth a few hours effort to get some data on paper to see if the idea merited further investigation. The problem was that the technique is one that I have recommended against in the usual installation of an antenna. This technique involves bending the antenna back on itself and is one of my cardinal, absolute no-no, rules. However, this was different than just bending the antenna in some arbitrary way. The idea was to make the bent or folded part of the antenna as electrically invisible as possible. There are several technical reasons that make this necessary, but I'll save that for later in this article. To make a long story short, I followed the idea I had sketched in my notebook years ago, and I gave it a try. The initial test was a success. The CAROLINA WINDOM® continued to function correctly, provided multiband coverage, and presented a decent SWR curve on most bands. It was late in the day and starting to get dark outside, so I attached one of my station's feed lines to the experimental antenna so that I could listen to band activity later that evening. Generally, a good listening test is a good indicator of performance if you have several other antennas in the air for comparision. On the air testing and power testing come later. If an antenna can't "hear" well, testing doesn't proceed. Usually, I have a CAROLINA WINDOM® 80 and a SuperLoop 80 in the air at all times. In place of the CAROLINA WINDOM® 80, I had a G5RV temporally in the air. I have used vertical antennas of various types to avoid the complication of comparing antennas with different azimuth radiation patterns. Unfortunately, the verticals typically fall short of the performance of the antennas I'm testing, so I rely on the CAROLINA WINDOM®, SuperLoop™, and common dipoles for testing and making comparisons. Of course dipoles are the best choice for testing since they are used as standards. However, putting six or eight of them in the air at the same height above ground and with the proper orientation is a difficult proposition. Thus, I rely on antennas that I a very familiar with. The SuperLoop™ 80 is a very high performance antenna. Mine is nearly 100 feet in the air, nearly twice the standard 50' test height that I use for experimental antennas. The performance of the SuperLoop™ 80 at 100 feet and gives me something to shoot for that is seldom achieved by any antenna half the height above ground. When testing and comparing antennas, I often use a G5RV since it is a very popular antenna and nearly everyone is familiar with it. It's a good choice for a performance bottom line. Mine is supported the 50 feet above ground, the same height as the antenna I'm testing. Any new antenna I make must perform better than the G5RV and approach the SuperLoop™ as closely as possible. Later in the testing process, a CAROLINA WINDOM® is substituted for the SuperLoop™. When testing, I can't have both the SuperLoop™ and a CAROLINA WINDOM® in the air at the same time because the CAROLINA WINDOM's space is used for the test antenna. Anyway, back to my story; when I got into my radio room later that evening, I listened around the bands, comparing the three antennas. As expected, in most directions, it was the usual order of performance. The G5RV was at the bottom, the test antenna was in the middle and the SuperLoop™ was at the top. Just for reference, when this article was written in June of 2009, we were at the bottom of the sunspot cycles. There was a little activity on 20 meters, 40 was OK and 75 was full of the usual signals. The "DX window" on 75 meters was not producing any challenging signals, but 20 meters produced some interesting results. In some directions, the new version of the shortened version of the CAROLINA WINDOM® was nearly as good as the SuperLoop™. In other directions, it was the SuperLoop's equal or better. This was most unusual because the SuperLoop™ is supported much higher in the air the test antenna. This performance equivalence definitely captured my attention. What I was observing was a CAROLINA WINDOM® that was folded in half but with performance that was similar to a standard CAROLINA WINDOM®. The G5RV was down quite a bit in these tests but I wasn't taking radiation patterns into account during this casual observation. After further observations on 20 meters, I turned my attention to 40 meters. Comparing the two antennas on 40 meters produced the same results as on 20. Performance was just what I would have expected from a full size CAROLINA WINDOM®. | Then, it was back to 75 meters. It was
late enough for something to show up in the DX portion of the band and sure
enough there was a small pileup in the DX window. I'm not one to pass up a pile
up, so I listened for the fellow's call and prepared to give him a call. I
didn't have a linear amplifier turned on and I was running less than 100 watts,
so, I didn't expect much. Moving off frequency to let the automatic tuner do
its thing, I immediately moved back on frequency and gave the DX a call when he
signed with the station he was working. Like I said, there was a small pileup
and I was running barefoot, so I didn't expect much. He came back immediately
and give me a good report. As you know, most reports are 5 and 9 no matter what
signal report you really deserve, so I asked for a critical report, telling him
that I was running 100 watts. The report came back as 5-9 plus 10 which is a
nice report under the prevailing conditions. So, I logged the station and
marked the antenna used as the SuperLoop™ which is the antenna I though I
was using. Was I surprised when I looked at the remote antenna switch and
realized that I was using the experimental, shortened CAROLINA WINDOM®,
which would soon be named the CAROLINA WINDOM® 80 Compact™. What a
nice surprise that was. I usually don't apply power to a test antenna until
much later in the testing process. That happens after I tweak things a lot and
optimize the SWR curves. Power testing is a very carefully conducted test
procedure. But, then, running 100 watts isn't a challenge in the power
department and the SWR was in the range of the rig's automatic tuner., so all
was well - especially since the first contact with the antenna was the result
of breaking a pileup on 75 meters.
Following such a positive experience, I decided to model the
antenna on the computer to see how the computer predicted the comparison
between the experimental CAROLINA WINDOM® and the standard version. The
difference was about 2 dB which is perfectly acceptable for an antenna that is
only one-half the length of the standard CAROLINA WINDOM® antenna. Moving
on to computer models for 7 and 14 MHz produced some interesting results. 40
meter performance was about the same as with a standard CAROLINA WINDOM®.
20 meters brought a couple of surprises. In some specific directions and
takeoff angles, the experiential version of the CAROLINA WINDOM® actually
produced a small advantage (3 dB) or so over the standard version. Changing the
shape (geometry) of the CAROLINA WINDOM® Compact™ produced better
results in terms of radiation patterns and improved SWR curves. I had
predicted, in my original notes, that the geometry would be critical and would
have to be optimized, especially if the particular geometry of the CAROLINA
WINDOM® Compact™ was to work as conceived and achieve minimum
interaction between the various parts of the antenna except where it was useful
to have element interaction. I will say that I always take computer modeled
results of CAROLINA WINDOMS® and similar antennas with a grain-of-salt.
That's because it's very difficult to model a CAROLINA WINDOM®. There are
just too many variables involved. The major problem with the computer models is
the effect of the matching unit which acts as a series inductance with the
Vertical Radiator. The Line Isolator is another complication with the computer
models. Still, it did show that there were dimensions and geometries that would
produce better results. This encouraged me to really take a critical look at
this new CAROLINA WINDOM® variation. The computer models were converted to
physical antennas for testing. Literally, a notebook full of data and weeks of
experimentation followed which resulted in the CAROLINA WINDOM®
Compact™ in its current configuration. There will definitely be a
permanent CAROLINA WINDOM® 160 Compact at my QTH.
The secret is in the geometry It looks like we have another winner. After optimizing the distance between the main and folded elements and optimizing the droop angles, droop depth, and offsets, maximum performance was achieved. To further enhance the performance of the CAROLINA WINDOM® Compact™, the matching unit, Line Isolator and length of the Vertical Radiator had to be optimized. As is usual with the CAROLINA WINDOM®, for best matching and maximum radiation at low takeoff angles, each model of the antenna requires special modifications of the matching unit and Line Isolator. All the design effort was rewarded with performance from a half-size antenna that is amazingly similar to the full size version. In some cases, since the patterns of the CAROLINA WINDOM® and CAROLINA WINDOM® Compact™ are somewhat different, the "Compact" version actually outperforms the standard CAROLINA WINDOM® in some directions and specific takeoff angles. In general, the radiation pattern of the standard CAROLINA WINDOM® is nearly ideal and this configuration is recommended if you have the room for the full-size version,. That said, there is nothing wrong with the radiation pattern of the CAROLINA WINDOM® Compact™, it is just a bit different from the standard CAROLINA WINDOM®. The takeoff angle is still very low, but the CAROLINA WINDOM® Compact's™ radiation pattern is not as omnidirectional as the standard CAROLINA WINDOM®. It's about the only compromise the CAROLINA WINDOM® Compact™ makes. Since the difference is only marginal under actual operation conditions where radiation patterns are affected by just about anything metallic in your vicinity, I doubt if you will ever notice the difference while operating on the air. You'll still be working just about everything you hear and as with all CAROLINA WINDOMS®, you hear and work just about any station that anyone else is working, even when other stations have expensive beams in the air. Of course, I'm not saying that the CAROLINA WINDOM® Compact™ is a "beam beater," but it is very competitive. If you can afford and want to spend the several thousand dollars and put in the weeks of hard work to put a Yagi on a tower, by all means do so. You'll still need the CAROLINA WINDOM® or CAROLINA WINDOM® Compact™ for the lower bands. On the other hand, if you want an unequaled general purpose antenna or an antenna that makes a DX "big signal," then you need to look no further than the CAROLINA WINDOM® or the CAROLINA WINDOM® Compact™. Of course, you should start planning where you're going to store all those new QSL cards from around the world. |
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