| KC6STT |
Rating:  |
2010-07-14 | |
| Flimsy, but it works! |
Time Owned: 3 to 6 months. |
PRODUCT REVIEW:
DIAMOND MODEL
CP-62 SIX-METER
GROUND PLANE
ANTENNA
Review by
Stacy Mc Intyre
(KC6STT)
This is my review of the antenna after I’ve built, installed, and had time to use it for about three months.
OVERVIEW:
This antenna is designed as a 5/8 over 5/8 wave collinear ground plane. The design uses a vertical radiator approximately 22 feet in length, and three ground-plane radials of 54 inches. It is designed to give about 4.5 Db or so of gain over a quarter-wave ground-plane, and a radiation pattern that will be directed at the horizon, and close to the ground. The vertical radiator has a phasing coil in the center and the bottom section is adjustable for best SWR over any 2 MHz section of the band.
The design is an improved version of the old Diamond and Comet models that were originally introduced over ten or so years ago (I also owned the original Diamond version as well). The improvements were made in the base section of the antenna. The original design had non-UV-stabilized plastic in the ground-plane section that would deteriorate with exposure to the sun over just a few years. This would, of course, cause the antenna to fail mechanically in a strong wind at its weakest point (the base). It also had a mast-mounting design that was somewhat more cumbersome to assemble and install. The new antenna now uses a base design just like many of their VHF and UHF antennas, with the cable connection concealed inside the mounting tube for weather-proofing, and easier-to-use mast-mount hardware. All hardware is stainless steel.
This new design also has an improved power rating versus the original design. In the original design, the antenna was only rated at about 250 watts SSB. Many users discovered that they could modify the coil assembly in the base to increase this to 500 watts. The downside was that the modification caused the antenna to lose some bandwidth, as I found out after performing the modification on my own antenna. The new design has a larger capacitor in the base for improved power-handling without loss of bandwidth.
The antenna’s light-weight design will not hold up well in severe winds. It is not suitable for mountain-top use or locations prone to winds over 75 MPH unless the vertical radiator can be supported somehow. And it won’t hold up well in severe ice, either. Such a shame! It would probably make a good low to moderate-elevation repeater or remote-base antenna otherwise.
The antenna is made in Japan, and is sold by RF Parts Company in San Marcos, California. The cost is about $200.
ASSEMBLY AND INSTALLATION:
For a seasoned antenna builder such as myself, the antenna went together quickly. I only looked at the 3-page instructions to verify that I had all the parts I was supposed to have, and, once I unpacked everything, basic assembly took all of 25 minutes, with a break. But I did discover a serious issue that needs to be carefully watched for (I’ll discuss that in a moment).
But even for someone who has not assembled all the antennas I have, the instructions would be easy to follow, as long as you can overlook the notoriously bad ‘Japan-to-America’ grammatical issues. You also need to be aware that all of the measurements given in the manual are metric. Not that there’s a whole lot of measuring to do during assembly. Only when you get to setting the bottom length for the correct SWR would any real measuring required (and I did not even do that!).
The first thing to do is put together the base section, including the support tube, radial ring, feed-point unit, and mast brackets. Make sure you route your cable through the tube and connect it to the feed point unit before doing anything else! After the base section is assembled, install the three radials into the radial ring, and tighten the lock nuts.
Next you’ll slip the bottom radiator section onto the feed point unit and align the bolt holes (there are two of them).
Now here’s the part I warned you about: When you install the two hex head bolts into the base, do not over-tighten them! I cannot stress this enough! I accidentally stripped one of the holes in the aluminum using only light force. The bolts are not a good fit in the holes (meaning they fit loosely). I had to hunt around for another piece of hardware to replace the original bolt that would no longer tighten. The company should have come up with a slightly larger bolt, or tap a slightly smaller thread, in the feed-point unit. This is the only major issue I have with the antenna so far.
The vertical radiator is assembled using self-tapping screws, two for each joint. As my original version of this antenna had a problem with water getting into the phasing coil, and causing the SWR to spike during storms, I sealed all the joints, and the top coil cover, with ‘liquid tape’.
Finally, I slid the assembled radiator into the bottom tube. As I mentioned earlier, I did not do the measuring they recommended in the manual. I simply took an educated guess, and slid the radiator into the bottom section about 10 inches, and tightened the clamp. Since I was shooting for best match in the SSB portion of the band, the longer antenna dimension was called for. And figuring antenna work is always ‘hit-and-miss’ to some extent, I would adjust it later if there was a significant problem.
The antenna was then put on 20 feet of mast and fed with about 35 feet of 8-X cable.
ON THE AIR:
An SWR check found that my ‘educated guess’ was pretty close to the mark. At 50.120, I had a flat SWR. And at 52.54, the meter showed just under a 2:1. That’s perfectly usable for FM work, local or distant. I have not bothered to re-adjust the antenna.
On the air reports are somewhat compromised as far as DX work is concerned because of my local terrain conditions (I have some mountains in the directions most skip comes from). This is not a fault of the antenna. It is because of the flat radiation pattern you get with this antenna design. It just does not climb over nearby hills well, even at 100 watts. I simply don’t hear skip that others are hearing most of the time. A horizontal loop, simple ¼-wave ground plane or ‘Ringo’ might work skip better here.
(When I had the original version of this antenna, I lived on a hill, and I was hearing and working skip that many other local stations could not hear. The antenna even worked Japan. Twice!)
On the other hand, the antenna performs very well locally. It penetrates into the fringes nicely with only about 25 watts. No one has much trouble copying me Q5, FM or SSB.
As I mentioned earlier, I had a problem with water in the original antenna. This one has gone through a few rain events. I made sure to check the match during one of these storms, and so far, I have not had any problems.
FINAL THOUGHTS:
Except for its somewhat flimsy mechanical construction, and the price, I would highly recommend this antenna. It is best suited for a QTH with slight elevation or on flat terrain with few nearby obstacles. The price could be justified if you lived where a beam antenna is out of the question for serious skip work.
As an interesting aside, the vertical radiator does come close to an electrical quarter-wave on 20 meters. So if someone were to run the proper length ‘radials’ from the bottom of the antenna, you could possibly end up with a center-loaded 20-meter vertical, usable with a tuner. And it does make a good SWL antenna, too!
See you on Six!
73 de Stacy
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