The 200D appears in the accompanying photographs identical! to the 200-watt model that appeared in the 1984 QST review. Note the toroid and several other devices, which are almost certainly power resistors. Max-corn's reply ("Correspondence/' June 1985 QST) to Hall's review indicated That the "fer* rite transformer is connected directly 10 the S0*239 input.,T Further, "the resistors in the unit .are connected to the output of the transformer to create additional RF loading." That arrangement was likened to a "dummy dipoie" by at least one writer (M. Slavin. "Technical Correspondence," April 1985 QST)f but some letters of recommendation included with the units I received ind^ated that Maxcom has some satisfied customers, including the U+S> Army, f resolved to remain neutral until I had my own results.
Maxcoms installation instructions consist largely of a listing of the features of their units The instructions briefly describe dipoie. inverted vee, and longwire installations. There are also a few simple illustrations provided. The instructions could be somewhat more detailed and explicit, especially for non-amateur users.
Maxcom recommends a minimum length of 35 feet per leg "or a dipoie Fifty feet per side are preferred, especially on the lower HF bands. Changing the installation from a dipoie configuration to that of an inverted vee, according to the directions, produces little or no effect on the SWR.
I erected the Maxcom and ail comparison antennas in the inverted-vee configuration. The Maxcom units were used with one of two antennas: dipoies 70 and 128 feet long overall, All of the antennas tested had their centers 30 feet above ground, with the ends a minimum of 6 feet above ground. The antennas were mounted in the clear, with the receiving position located several hundred feet away. I made signal comparisons using an attenuation pad calibrated in 1 dB steps placed in line with the distant receiver's feedline. The test arrangement assured a constant level ot input power to the tuner regardless of SWR. All SWR measurements reflect the actual SWR at Ihe input to the antenna.
1 first checked the SWR response of the Maxcom units using the? 128' dipoie. Measuring at 50 kHz intervals, the 200D showed! a maximum SWR of 1.5:1 over all Ihe US amateur bands from 3,5 to 29.7 MHz. This dipoie actually resonated ai the low end of the 80 meter band, but surprisingly the SWR was the highest on that band, ranging from 1,4:1 to 1.5:1 across that band, On the other bands the SWR never exceeded 1.3:1.
The XLD-150 did not perform as well in this situation, showing an SWR of up to 1.8:1 on 80 meters and as high as 1,6:1 at the upper end of the 10 meter band. Elsewhere the SWR was Jess than or equal to 1 4:1
I next measured the SWR for the two Maxcom matching devices when used with the70h dipoie. With the 200D, the SWR on 80 meters dropped to a maximum of 1.2:1 on 80 meters, but was a flat 1.6:1 across the 40 meter band It ranged from and from 1.7:1 to 1.8:1 on 15 meters, but elsewhere it met or surpassed the specified 1.5:1.
The XLD-150 also gave a low SWR on 80 meters with the short antenna, but again the SWR on 40 was relatively high (1.7:1). The 10 meter measurements were consistanily 1.8:1 across ihe band. Failure to correct the SWR measurements for feedline toss would have shown incorrect measurements better than 1.6:1 over the frequency bands tested, regardless of which antenna was used,
Low SWR present at the transceiver output doesn't mean the antenna radiates well. Therefore, for the next phase of the review I compared Signal strengths on various bands, with and without the Maxcoms in fine. Using the 200D Maxcom, I found the received signal strength (measured as noted above) at opposite ends of the 80 meler band to differ by up to
"The Maxcom units initially impressed me as well-made devices.
4 dB when using the 128' antenna. I was concerned that something was amiss with my measuring technique, but a check ot ihe same antenna with direct coax feed showed at most a 1 dB variation in received signal across the band. Repeating the measurements with the 200D confirmed the original results.
With the Maxcom at the feedpomt, signal strength graduaily dropped by 4 dB as the frequency neared 3.5 MHz. The XLD showed a similar response over that frequency rangeSignal strength measurements on the other bands demonstrated a more umtorm response.
Now for the big question: How much signal strength gain (or loss) can a Maxcom antenna matcher provide when compared with the same antenna fed with coax? Again with the caveat that these tests were not done on an antenna range, here's what 1 found.
With the 128 foot inverted vee operating on 80 meters, the signal from the Maxcom was typically 9 to 10 dB down from the same antenna directly fed with coax On 40 meters, this antenna acts as two half-waves in phase and has a theoretical gam of about 1.9 dB over a 40 meter dipoie. When fed through the 200D. the signal from the long antenna was 2-3 dB down from a 40 meter dipoie fed directly with coax. Allowing for the gain of the longer antenna, it appears the Maxcom is actually 4 to 5 dB weaker. Measurements using the 128 antenna on 10 MHz showed the Maxcom to be approximately 12 dB down, My experience shows that resonant dipoies outperform the Maxcom by at least 4 dB.
Other antenna arrangements will probably have different results.
Are there any situations when the Maxcom system wins out? Yes, there are. In some instances the 80 meter dipoie fed with a Maxcom matcher will give stronger signals than a monresonant dipoie fed directly with coax. Such a condition causes a high SWR to exist on the feedline. and that in turn causes feed-line iosses to rise, especially on the higher frequencies.
Suppose an antenna has a 31 SWR at the transmitter at 21 MHzf and the feedline has a nominal toss of 2 dB at that frequency The high SWR will cause an additional ioss of 3 db for a total of 5 dB feedline loss Here, the Maxcom may very well outperform the non-resonant dipoie fed directly with coax, especially considering nearly ali solid stale transceivers will operate only at reduced power output, if at all, when faced with a 3:1 SWR.
The Maxcom matcher in this case may still have relatively high losses, but with a low SWR present on the feedline other iosses will be low, and power output from the transceiver will remain at maximum. The result is more radiated power is radtated with the Maxcom.
Maxcom Automatic Antenna Matchers are not Cheap. Retail price for the 200D is S599. The XLD-150 runs $299. The buyer should balance the units* cost with other considerations such as power ratings and installation requirements. If the Maxcom has a place in the amateur market, it is for folks who like the convenience of coaxial feediines, have room for one antenna- and primarily work stations with strong signals."
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