Hi Bob,
I made a number of experiences on 160/80m with commercial and home made
null steerers, but finally I decided that commercial ones, whitout a
number of modifications or adds (i.e. filters) were not satisfactory on
low bands and I made one, my project, that's totally passive.
It introduces some loss (like all steerers, don't be fooled by amplifiers
and S-meter readings, what counts is S/N ratio), partly reduced by the
use of a 90deg delay line along the sense antenna, but doesn't suffer any
intermod problem like it always happens on 40-80-160 in the commercial
ones I tested.
The attenuation of a well designed passive steerer is not too big or
detrimental, at least for low bands, if the sense antenna delivers a
signal that's not more than 20DB down the main antenna.
In my steerer I added a 90deg delay line (missing in the commercial ones
I tested) because on 80m and 160m sense and main antenna are always very
close in term of phase distance, and without the delay, the phase null
would be critical to obtain since phase control would be at the edge to
produce a null.
In such situation, tipical to commercial null steerers, to avoid a great
attenuation there are variable amplifiers, ahead of main and sense
antenna. The amplifier, is a cheap solution but suffers overloading and
introduces noise.
I found null steerers working best when both antennas have the same
polarization and if the noise is radiated by a single source. In a power
line noise they are less effective if the radiation of the noise is along
a big portion of the line and not confined in a specific point.
The null steerers are top performer to avoid overload by local huge
signals on same or another band, i.e. nearby contester pileupping or rag
chewing even few KHz apart.
Altough less effective against a specific noise, when using two non
directional antennas I found my steerer (it should apply to commercial
ones too) able to give a pattern on reception (but antennas have to be
enough separated and same polarization), this was found useful to improve
reception reducing sky/atmospherical noise.
Of course, to have an effective array the second antenna has to be more
than a sense one (mine is -15db compared to the transmitting one for 160m),
but often some 20 Db of attenuation on 160m are nothing, receiving.
73,
Mauri I4JMY (one of IR4T)
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