Recently I posted a request for information on any experience people have
had with the new AEA VIA Bravo analyzer
(http://www.aea-wireless.com/viabravo.htm). I got no responses, probably
because it's still quite new and also quite expensive (about 5x the price of
the popular AEA CIA-HF analyzer). Since I do a lot of impedance
measurements, I decided to get one for myself.
I didn't realize how new this product is until I discovered I had serial
number 17! It also came with a note saying that the PC interface software
wasn't ready yet and would be sent to me later. The software package, that
allows the instrument to download and plot data on a PC, is one of the
strong points of the AEA analyzers.
The popular antenna analyzers from MFJ, AEA, and Autek are all more than
adequate for the kinds of SWR measurements most amateurs require. I was
more interested, however, in the accuracy of the complex impedance
measurement capability of the VIA Bravo. I have an old General Radio 1606-A
R-X bridge that has long been a reference standard for antenna measurements.
The GR is quite tedious to use, requires an external signal generator and
receiver, has no PC interface capability, but, being entirely passive, is
immune to RFI overload. Its specified accuracy is very good--generally on
the order of 1 percent for resistance and 1 to a few percent for
reactance--although the readings come off an analog dial.
To test the complex impedance measurement capability, I connected the VIA
Bravo, the GR, and also my CIA-HF analzyer to various loads and compared the
readings. Here are the results for some selected loads.
First, I did some measurements on a real antenna:
VIA Bravo CIA-HF GR-1606-A
60 ft vertical @ 1830 kHz 10.5-j235 51.8-j222 8.4-j243.2
60 ft vertical @ 3500 kHz 30.3-j8.1 33.5+j7.3 31.0-j9.4
60 ft vertical @ 7000 kHz 45.8-j194 65.8-j200.6 46.0-j201.4
The VIA Bravo showed a small bit of instability on the 160 meter measurement
whereas the CIA-HF did not. It's possibly the effect of a local low-power
BCB station 3 miles away. The Bravo does have a built-in spectrum analyzer
that allows you to determine if there are strong RF signals present that
might interfere. The analyzer did show the local BCB station strength
at -25 dBm against a background of weaker stations, but that doesn't seem
strong enough to present a problem.
Next I connected various discrete component loads to the different
instruments and measured them at 1830 kHz:
VIA Bravo CIA-HF GR-1606-A
50 ohm precision dummy load 49.8+j0 49.6+j0 50.5-j0.5
180 ohm resistor 183-j4.0 186+j1.5 188-j7.7
470 ohm resistor 478-j45 490.8+j1.5 490-j50.8
470 pF 10% cap 3.4-j170 30.0-j168.1 0.5-j180.3
7500 pF 5% cap 0.3-j7.8 3.34-j4.85 0.1-j8.5
100 pF 2% cap 17.0-j729 588.2+j519 1.3-j770.5
large toroid coil .01+j280 89.6+j289.8
0.8+284.7
air wound coil 0.5+j27.5 7.04+j27.7 0.2+j26.8
small toroid coil 0.4+j12.3 3.9+j9.5
0.1+j10.4
51 ohm in series with
air wound coil 53.9+j28.8 54.8+j27.1 57.0+j27.3
270 ohm in series with
470 pF 258-j186 296+j130.3 261-j196.7
18 ohm in series with
small toroid 19.1+j13.6 20.2+j9.8 19.0+13.1
The discrete components have up to several inches of wire leads connecting
to them, which does influence the measurements a bit.
There's a lot of data here, but you can start to see some trends. The VIA
Bravo and the "reference" GR bridge track each other pretty well. The Bravo
does tend to overestimate somewhat the resistance component of a load which
is primarily reactive, but the CIA-HF is much worse here (for example, in
measuring the 100 pF cap). The CIA-HF sometimes gets the sign of the
reactance wrong, when the reactance is either very small or very large.
AEA claims the accuracy of the VIA Bravo as follows:
+/-1 ohm at 10 ohm load
+/- 1.5 ohm at 50 ohm load
+/- 4 ohm at 100 ohm load
+/- 35 ohm at 500 ohm load
"for indication only" above 500 ohm load
I'm not sure if these tolerances are for resistance and reactance separately
or total impedance, but I suspect it's the latter. There is no accuracy
spec given for the CIA-HF, but it's intended primarily to measure load
impedances accurately only in the vicinity of 50 ohms, which it appears to
do. The VIA Bravo is much more capable at measuring much smaller or large
impedances, but it should be for the asking price.
Unlike the CIA-HF, the Bravo has built-in non-volatile memory (24 memory
locations to be exact). This allows you to take data in the field, store
it, and retrieve it at a later time. That can be very useful.
The LCD display of the VIA Bravo appears to be identical to the CIA-HF,
which means it's sometimes hard to read because the characters are small and
the contrast is not great, especially in low lighting situations. Also,
like the CIA-HF, the user keypad interface is not very intuitive, only worse
because the Bravo has a lot more features. However, it's not hard to learn.
The MFJ259B, on the other hand, could almost be used without reading the
manual.
In summary, the Bravo appears to be fine instrument at a premium price. For
typical amateur work, however, it's probably hard to justify, considering
the cost, and the more inexpensive MFJ, AEA, or Autek units will do just
fine. For almost all the measurements I'll be making (except for the most
critical ones), I'll be using it in place of my GR bridge.
73, John W1FV
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