The following traces provide some
performance comparisons of serveral "off the shelf" VHF/UHF splitters, used as combiners.
Details of the measurement setup are provided here.
The bottom trace is resonse with a single signal source, and the unused combiner input terminated.
The top trace is response with the same signal source connected to both combinter inputs.
It should be noted the interconnecting cables from signal sources to combiner were
both Eagle Aspen 39B2 cable, and were 19", within .1". Both increasing and
decreasing the lengths of these cables will move peaks and nulls up or down in frequency.
Shorthening or lengthing just one cable can also move a null, and/or introduce additional
Perfect Vision 22-233
Pico Macom MS-2D
Ace Hardware 3107612
If you want to combine multiple antennas, a spectrum analyzer can easily become
your best friend. It will help you to understand the real response of the specific
physical combination of components you've selected, and at the specific frequencies of
Not surprisingly, all splitters are not created equal. Both electrical and
internal physical layout/design will affect performance parameters (loss, input
port to input port crosstalk, input port to output port crosstalk, etc.).
It is possible to pick very unfriendly combinations of interconnecting cable lengths
and combiners, as in the Pico Macom MS-2D and Arista 54-1920. They actually result in about
a 1dB and 2 dB of loss, respectively, at about
690 Mhz (Ch 50). But please note this only occurs for this particular
choice of coax length. It is possible to adjust the length of either or both
interconnecting cables to move this null somewhere else in the spectrum or eliminate it altogether.
Overall, the Perfect Vision appears to have the best UHF band performance, being
2.5dB++ across most of the range. Most others average around 2 dB. Again,
note that dip at 700 MHz can be moved.
A Case AGAINST Combining
In general, combining two antennas can be a good bit of work, and
you're never going to get more than that 2.5dB gain. One might argue that if
you're going to go through that much work, you might as well go for combining 4
antennas and hope for about 5 dB of gain, than to just stop at two.
Signal strength varies dramatically both short term and long term. First of
all, there is consistent diurnal variation. Over the course of any given day
variations of 6 dB are quite common, and variations in excess of this (8 to 10 dB)
have been measured with some regularity. Seasonally, environmental conditions in many
areas also vary dramaticaly and bring more or less adversity with them. In urban,
hilly, and/or mountainous locations reflected signals can arrive at an antenna at
power levels close to the direct path, resulting in inter-symbol
interference that eventually results in intolerable error rates. This is known as Multipath,
and its different
paths can be affected differently by time of day and environmental (weather) conditions,
further exaserbating the extremes between good and bad reception. In mountainous regions the presence/absence of snow, and depth of snow pack
during winter/summer can affect different signal paths differently.
Reception reliability depends on margin, defined as the amount of signal
you have over and above the minimum you need to
receive a station. Reliability through adverse environmental conditions depends
on that margin. If your signal is just barely on the edge of being receivable (on the "cliff"),
2.5 dB isn't going to buy you much margin, 5 dB might buy you a little. If you are
at a point where dropouts or pixelation occur relatively infrequently, 2.5 dB
may very well reduce the incidence rate and thus improve reliability. If you are
at a point where you can never get a particular station, not even 5 dB is likely to
produce 99.9% reliable reception. Consider also that it is quite
possible to have perfectly ample signal level and still experience frequent
pixellation or dropouts because of multipath. Where that is the case
simply increasing signal level is likely to be
As a rule of thumb, if you don't have at least about 10 dB of margin during the
best of times, you are not likely to withstand the worst of times.
The arguement against combining is that 2 to 4 dB of additional field strength can often be found
by repositioning, with considerably less effort and engineering complexity (read:
"trial and error"), than combining. Especially in those cases where there are multipath contributors,
moving an antenna a few meters one way or the other, increasing its height by a few
meters, rotating it slightly off axis to
avoid an offending contributor, or elevating it (pointing it upwards) 5 to 10 degrees to avoid ground reflections
can produce noticable improvements.
Where transmit towers are at a common location, the use of Yagi based designs versus
bow tie designs can help to overcome multipath. Their narrower beam
width can help to reject reflected signals arriving off axis (from the sides).
In summary, the only time 2.5 dB of increased signal level is likely to be rewarding
is in those scenarios where a station is already "almost" reliable. In those
scenarios, there are often more manageable alternatives.
A Case FOR Combining
Where combinning antennas can be rewarding is in cases
where multipath is a primary factor. Combining antennas vertically or horizontally
has the affect of increasing its directivity vertically or horizontally, respectively. That
is, increasing gain in the directly forward direction versus off-axis gain.
But one of the biggest issues is determining
the source of the interfering signal. It may be ground reflections coming
from below, or off some terrain or structure to the side.
In the case of ground reflections, upward tilting of an antenna can help.
Improvements of around 10 points have been oberserved on receivers with 0-100 scale "quality"
indicators. If you've determined there is sensitivity to upward tilt, but need further improvement (read: "reliability"), two vertically stacked antennas
will result in less sensitivty to power coming from below or above the line of sight.
Reflections from the side are commonly associated with buildings, hills, or terrain
in general. In these cases, horizontally mounted antennas will narrow the beam
width horizontally. Again, increasing the sensitivity directly forward, and
reducing sensitivity to the sides.
There is a significant amount of just plain luck involved when combining
antennas, especially without adequate instrumentation
to characterize your combining network response.