The Opek HVT-600 is a 200 Watt 10-band (1/4 λ) ham radio antenna (80m - 2m). Manufacturer Opek of Taiwan also offers marine, CB, cellular, LAN, and commercial radio antennas (https://www.opekantenna.com). I purchased mine through Walcott Radio of Iowa in the USA (https://www.walcottradio.com). The antenna consists of a matching coil with nine taps (and jumper cord), a bottom fixed section, and a telescoping top section. The bottom has PL-259 connector. The telescoping top is held in its various positions by a hexagonal socket-head screw (key is provided). It's a tall antenna, 52-½" at shortest and 71" at maximum extension of the telescopic section.
I paired the Opek antenna with a Comet CM-5M Magnetic Mount (4-½" diameter magnet) with the required SO-239 base and 13 feet of built-in RG-58A/U coax (https://cometantenna.com). I purchased the magmount through Radioworld in Toronto (https://www.radioworld.ca). Coax from the magmount runs into the car through a window left open a crack (downwind side works better!).
The antenna manual consists of one side of a single sheet of paper listing the antenna's general particulars and giving a frequency chart for correct jumper position and overall length to use for each of the bands. Unfortunately, OPEK provide only one length per band. Knowing that some bands are wider than others, and knowing that VSWR would be very dependent on exact configuration of installation, I took those dimensions only as a rough starting guide and checked them with an antenna analyzer, placing the magmount in proper position on my specific car.
The lengths for the higher frequencies worked fairly well. For example, the recommended length for the 2m band provides a VSWR of 1.5 or less from 144 to 148 MHz. In addition, and despite not being advertised as being a 70cm antenna, the telescoping section can be extended to achieve a VSWR of a little less than 2.0 in that band, making it usable there, with a tolerant radio or bit of help from a tuner.
On 10m again a single antenna length suffices for the whole band. VSWR is below 1.5 from about 29.1 on down, meaning one can work AM and USB without a tuner. Highest VSWR is a bit under 3 at the top end of the band (FM zone).
The antenna was not envisioned for use on the 11m band, but as I want to work CB as well, I found a workable combination of jumper and length to get a VSWR below 2.0.
Below that, though, things get complicated. On the 20m band, OPEK suggested overall length of 1610mm for that band, i.e., 63.385 inches, but I found that 65.5 inches worked better, though the best VSWR was a little under 3.5.
It got even more complicated for 40m, where it initially seemed that no length would bring the VSWR under 5.0. However, the product page for this antenna at https://www.americanradiosupply.com/opek-hvt-600-multi-band-mobile-antenna-40-meters-mod-hack-david-vk4ice/ recommended placing a hose clamp on the outside of the matching coil section just above the 2nd to last tap on the bottom. This enabled the antenna to achieve a VSWR to about 2.5 for a portion of the 40m band, though the bandwidth is rather narrow and one would have to change lengths at least once to be able to work the whole 40m band, even with a tuner. It gets even worse for the 80m band, where the bandwidth is so narrow that one effectively needs a different length of telescopic section for each frequency worked!
In practice, it is a somewhat fussy process to change bands. The antenna has to be taken off the roof and the hex key used to loosen the telescopic section. One needs to carry a tape measure to get the telescopic length exactly right (this is quite critical at the lower frequencies). Plugging in the jumper is easy; no tools required. Keeping the bottom of the magmount clean is important, lest one damage the paint on the roof of the car. Also, if one happens to be a bit on the short side, it is helpful to also has to carry a step-stool for easier access to the roof.
The antenna is provided with only one key for the hexagonal socket head screws. The exact size of the Allen key is not given anywhere in the "manual" and by measurement seems to be neither a standard metric nor SAE size. My vendor, Walcott, does not carry spares and the manufacturer did not even answer my email! It turns out a 5/64 SAE size will work, but is a little tight. Replacement jumper wires are available at https://www.americanradiosupply.com/replacement-jumper-for-opek-hvt-600-antenna/
My plan for the future is to explore better grounding than the magmount alone can offer, with hope of making the lower bands more accessible. Working when parked means that one or more counterpoise wires, either hanging or even grounded via a metal tent peg are potentially practical options. Other suggestions from reviewers at https://www.eham.net/reviews/view-product?id=10937 included replacing the hex head socket screw with a thumb screw, making whip length adjustments faster and eliminating the whole issue of the Allen key, and another substituted a longer whip from a Wilson 1000 to achieve made for better and broader tuning on the lower bands. Finally, another suggested was to add a capacity to improve bandwidth on lower bands.
73, Kathleen, VA3WEX
Appears in: Rambler Vol. 55, Issue 5 January 2023