[TowerTalk] NCC and phased Active verticals follow up

[Tod -ID]

Since the original posting I have made some additional measurements and wish
to comment on the 1850 kHz spur I observed.

> At my location with the installation parameters I used I 
> found that there is a significant signal at 1850 kHz where I 
> can hear the AM content the local stations at 590 kHz and 
> 1260 kHz. This was of no concern during the Stew Perry 
> warm-up since it was much above the recommended CW section of 
> the 160 meter band plan for the US. 


This spur is apparently caused by the following:

The DXE Active Vertical is designed to cover a very wide range of
frequencies. It has available shunt inductors and capacitors across the
input from the whip that can be selected by jumpers. The default is no
jumpers and there is a fairly constant power gain of -6dB from input [2500+
ohms] to output [75 ohm] when used in the default mode. At my location the
BC band 1260 and 590 kHz stations are relatively strong. Using my TX
vertical as the antenna the daytime signal of 1260 is S9+60 dB or higher
{FT1000MP].

I inserted the jumper designed to enhance 160 meter operation and measured
the BC band signals and also the 1850 spur at the station end of the active
vertical. Here is what I found:

590  kHz = -35 dBm
1290 kHz = -18 dBm
1850 kHz = -80 dBm

For a little perspective if S9 is -73 dBm then -18dBm is S9+55 dB !

I inserted a four pole 160 meter band pass filter [W3NQN type] in series
with the feed line and got the following measurements:

590  kHz = -70 dBm
1290 kHz = -70 dBm
1850 kHz = -80 dBm

I next ran the feed line into the preamp I have been using. I did it with
and without the band pass filter ahead of the preamp. Without the band pass
filter the spur was S9 on the FT1000MP. With the band pass filter in line
the spur was barely audible. This means that the preamp is the place where
the mixing I had observed was occurring. The mixing occurred because the
ARAV does exactly what it is designed to do -- it passes a very wide band of
frequencies and provides power gain to those signals.

For those of us interested in 160, the fact that the ARAV works so well is a
potential problem because downstream from the active whip some piece of our
equipment is likely to be unable to cope with the very strong out of band
signals. The answer is to insert a band pass filter or a very good high pass
filter that has high rejection of frequencies in the BC band in the output
feed line of the active whip to reduce the out of band signals.

In my measurements there is still a signal at 1850 kHz at -80 dBm. That
should not be there. It is possible that something external to the active
whip is doing mixing of the BC frequencies and radiating the 1850 signal. It
is also possible that something in the electronics of the ARAV is doing some
mixing producing that rather low amplitude signal. Either way, the
electronics of the ARAV were not producing the S9 signal I was hearing last
weekend, my preamp was doing that.

I wrote this note because I wanted to be sure that no one reading the
original comments would be left with the opinion that I have found some
defect in the ARAV2-1P unit or the NCC unit-- that is not the case. My
problem was entirely attributable to the preamp I had in the output of the
NCC.

Tod, K0TO