Vectronics HFT-1500 User Manual download pdf (Page 8)

From February 2003 QST © ARRL

Antenna Tuner Testing Methods vs Accuracy

In the last round-up of manual antenna tuners, all of the

test data was created using the test methods devised by

Frank Witt, AI1H (described in depth in April and May 1995

QST

). This method uses an antenna analyzer and measures

changes in SWR with changes in loads on the tuner. That

change in SWR can be used to calculate the tuner losses.

Following the Product Review, we received some correspon-

dence that suggested that the method was insufficient to

produce “laboratory grade” accuracy in measurements. At the

time, this prompted some further investigation on the part of

ARRL Lab staff. The results of this study indicated that AI1H

method was reasonable over the

useful

range of the tuners

(the conditions for which the loss was reasonable). The Lab

concluded at the time that the accuracy decreased as the

tuner losses increased.

When the Product Review Editor brought the Lab another

collection of tuners to test, it was decided to employ a more

direct method of loss measurement, and to perform many

measurements by the AI1H method as well, so as to put the

issue to rest. By comparing the two sets of results, a reason-

able conclusion could be drawn about the correlation of both

methods used.

In this review, the method used to measure the loss of the

tuners was as follows. Two test fixtures were built which would

hold combinations of high power 50-Ω “non-inductive” carbon

resistors (one fixture for parallel combinations and one for

series). Even with the non-inductive resistors, some net fixture

inductance was apparent at some frequencies, so a variable

capacitor was used to compensate. The fixture accuracy was

measured using the Lab’s vector impedance meter.

Each fixture used an input connection (for the tuner) and

an output connection. The output connection went to a 50-

Ω

input power attenuator, which took the place of one of the

resistors in the load (for the series loads, it was always the

one on the ground side of the network). The output of the

power attenuator was connected to a high accuracy laboratory

wattmeter, and the actual attenuation was measured for

each frequency. The tuners were matched at low power, then

100 W of RF was applied at the input, with the output being

measured by the Lab’s wattmeter.

Because of the extra steps involved, the process was more

time consuming than the AI1H method. Therefore, to reduce

the required test time to a reasonable amount, the number of

tests were reduced by eliminating the 16:1 SWR tests (out-

side of the claimed matching range of most tuners) and by

eliminating the balanced output tests (non-trivial to perform

by the direct measurement method).

The results of these “direct method” tests appear in the

tables in this review. However, that still leaves the question of

how the older test method compares with these results. Be-

fore getting into the nitty-gritty, it should first be noted that a

variation of a few tenths of a decibel (excellent RF measure-

ment precision indeed) translates to a significant difference in

loss percentage for relatively low losses. That is to say that if

one method indicates a loss of 3 percent and the other indi-

cates a loss of 6 percent, a good portion of the difference is

in the limits of the measurement accuracy.

In comparing the results for both methods using the Lab’s

test equipment, when the tuner losses were less than 20

percent, there was excellent correlation between measure-

ment methods (loss differences of 2-3 percent). For losses

between 20 and 40 percent, loss differences

generally

ranged

around 5-6 percent, with a couple of measurements that

differed by 7 percent. (An accuracy difference of 7% is about

0.3 dB.)

In a “big picture” examination, the raw data seemed to

suggest that one of the sources of error was related to the

measurement of reactive components in an impedance mea-

surement. This is part of the 2:1 SWR measurements made in

the AI1H method. It appeared that for higher reactive values,

the measurements were not being accurately reported by the

antenna analyzer being used for the test. Given previous

tests of antenna analyzers, this is not entirely surprising. The

current spate of SWR analyzers in use by hams do a good

job at measuring the SWR and impedance of antenna sys-

tems. However, expected variations in the manufacture of

these types of equipment can lead to significant variations in

the results, especially with resonant loads as found in

matched antenna tuners. With the luck of the draw—as ap-

parently happened with the ARRL Lab’s instrument—the

accuracy can be quite good, but the next meter off the shelf

might give different results under the same test conditions.

Although the differences in terms of percentage might be

relatively minor, reading 90% loss under circumstances

where the loss was really 80%, this “10%” difference does

mean a 3 dB change in the amount of power the tuner can

safely dissipate under those conditions, possibly a consider-

ation for the high-power operator.

The bottom line is that the original test method was rea-

sonably accurate, but not necessarily reproducible. To the

extent practical, the Lab will continue to use the more-

accurate “direct” method for future testing. The AI1H method

will still be used for some testing, but only with careful cross-

checking of the instrumentation used.—

Michael Tracy,

KC1SX, ARRL Lab

This is how each tuner was connected for testing in the

ARRL Lab. A 100 W RF source (off picture rear) fed the

input of the tuner. The resistive load was connected to the

antenna output. The load was connected to a 50-

Ω

ΩΩ

Ω power

attenuator (off picture right), which took the place of the

final resistors.

This is a close-up of the parallel load test fixture. The variable

capacitor was used to compensate for fixture inductance

encountered at some frequencies.

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