In January, Jim W8WTS and I were at PJ2T for CQWW 160 CW, and we both noted
that our DX Engineering 4-square RX antenna did not appear to perform as
well as usual. It is a 98-foot configuration using 102-inch whips.
It is set up "field day style" for contests when we use it (i.e. not set up
permanently), and so we thought that perhaps all the handling over the years
might have caused problems with the feed and delay cables. We had a Array
Solutions AIM-4170C VNA with us, and confirmed that all the cables were
still in good shape and were of the correct lengths.
We decided to bring home the four DXE ARAV3 active antenna amplifiers to
diagnose at home (we left the four whips on Curacao!). W8WTS has an Array
Solutions VNA-2180 2-port device, and captured curves for inputs and
outputs. His analysis is shown below. Screen captures of the resulting
curves are in an archive at:
http://www.k8nd.com/Radio/PJ2T_DXE_4-Square_amps_ photos.zip
<http://www.k8nd.com/Radio/PJ2T_DXE_4-Square_amps_%20photos.zip> .
"Each amp has a port a sweep and a port b sweep. There is also a reference
pair of sweeps with the amplifier jumpered out of the circuit, to verify
that the ports are calibrated correctly. The port a sweep shows the
impedance looking into the antenna port. We want the input impedance to be
extremely high; higher is better. There is a peak in the Z because of the
parallel LC filter on the input of the AVA-2 that rejects out of band
signals. You can see that they are all set correctly to peak on 1.8 Mhz.
"The port b sweep shows the magnitude and angle of S21. S21 is the forward
voltage gain of the amplifier. We want the forward voltage gain to be
exactly 1, which is shown on the graphs as 0 dB. The four amplifiers have
different gains at 1.8 MHz, but they are very close. The lowest is amp 1 at
1.2 dB and the highest is amp 4 at 1.4 dB. A gain difference of 0.2 dB is
very small, but it might make a difference. "
They appear to be working OK, but what we don't know is:
QUERY: How much of a gain discrepancy is tolerable in the element
amplifiers in a DXE four square?
73, Jeff K8ND
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