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Technical Discussion Topic:

"AA-908 Operation in the Presence of High RF Fields "

FROM: Paul WN7T ...

I am using the new improved SP2 version of the 908AA.  I have a problem accurately measuring the VSWR of my 160 meter antennas.  I have a full size dipole for 160 meters up about 120 feet up on a mountain at an HAAT of 1000 feet.  There are a few local AM stations below me in the city.  I have a remote controlled double-L tuner to feed the antenna with ladder line.  I can not, for the life of me, get the 908AA to give me a decent reading of VSWR.  I can tune the antenna absolutley flat on all bands 160 through 15 meters.  However, measuring the tuned antenna with the 908AA usually gives a 2:1 VSWR at best.  Today I tuned the dipole flat for 40 meters (using power from an TS-830s and a good Tokyo High-Power meter).  When I measured the resonant system with the 908AA it showed a VSWR >10 with Z = 0 + 51j.  I suppose this is due to AM BCB energy causing problems with the wheatstone bridge circuit.  I have used a ICE 402X high pass filter in line to help with this problem, but this is somewhat irksome.

I have the same problem on a shortened 160 meter vertical at another QTH which is located in Seattle in the heart of the city.  Despite knowing the antenna is perfectly flat with my Bird 43 and a 50H slug, the 908AA will at best show about a 2.0 VSWR on 160 meters.  When I use the ICE filter with the 908AA, the measurement accuracy improves.

Is this a problem encountered by others using this instrument?  I am considering incorporating a small elliptical filter inside the 908AA to improve performance in hot RF environments.  Any suggestions, people?

FROM: Joe N2CX ...

Indeed the AA908 shares some of the same shortcomings as other inexpensive antenna analyzers when used with large antennas in high field strength environments.

The common issue with these analyzers is that they use untuned detectors. Thus any strong RF energy picked up by the antenna will be fed back to the detectors and will be indistinguishable from the reflected energy from the internal signal source.  Expensive high end commercial measuring instruments use tuned detectors to prevent this shortcoming.  However incorporating tuned narrowband tuned tracking phase-compensated detectors in simple instruments is not terribly practical. A single detector would approach the complexity of the whole AA-908 and three or four would be needed for the measurements made by the 908.

So the answer is to somehow keep the induced RF from affecting measurements. One way is to increase the signal generated by the analyzer to the a level where the interfering signal is swamped by the desired signal.  This means pumping some real power into the antenna.  For example with a level of 1Vrms into the antenna from the instrument, a reflected signal (or any RF on the antenna) of 1/3 volt will result in an indication of 2:1 SWR.  At 1.2:1 SWR the reflected signal is only 1/10 volt.  In a past lifetime I had to tune LF transmit antennas with a GR bridge.  We had to use several watts of power to get reasonable readings.  This, too, is impractical for the AA908 and would require sending station identification to satisfy FCC requirements.

Some sort of filtering such as you suggest is probably the simplest way to go.  I note that some time back MFJ sold filters to overcome interference from nearby RF sources though I believe they do not do so any more.  One reason is that any filter in line between the antenna and the analyzer will introduce impedance errors I the measurements.  The low pass filter you mention might be usable though its high power rating may be overkill for measurement use.  A low pass filter that cuts off well below 160 meters is usable and its effect can be minimized by noting the impedance errors it adds when connected to a good 50 ohm dummy load then compensating for these errors in impedance readings with a "real" antenna.  The errors need to be measured and compensated for at each in-band frequency of interest.

Alternatively if you know the frequency of the signal causing problems you might be able to use a high-Q series-tuned circuit across the feedline to remove the interference.  If it has high enough Q and if its tuned frequency is far enough away from your measurement frequency you may add only small measurement errors.  Even so, you might be able to do some compensation as described for the low pass filter.

Please feel free to discuss this further if you wish since you are by no means the only one with this situation! 

 
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