Topband: DX Engineering 4-Square RX Antenna: Element Amplifiers?

[Tom W8JI]

~  .5 dB might make as much as a change from 25 dB F/B to 20 dB. I wouldn't 
expect it to be noticeable.


----- Original Message ----- 
From: "Jeff Maass K8ND" <jmaass at k8nd.com>
To: "Topband Mailing List" <topband at contesting.com>
Cc: "'Jim Galm'" <jim at w8wts.com>
Sent: Tuesday, September 22, 2015 9:15 PM
Subject: Topband: DX Engineering 4-Square RX Antenna: Element Amplifiers?


> 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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