The Array Solutions device also uses the transformers to increase the
voltage at the diodes then steps it back down which means that the diodes
are not doing their limiting at 50 ohms. Therefore, your set of 2 series
diodes or even one diode each direction is limiting at a higher power
level
than the Array Solutions device.
From the QST article.
"The transformer increases the voltage level to allow limiting by a pair
of
back-to-back diodes and then another transformer matches the output to a
50
ohm receiver input."
Here are the problems:
1.) The peak voltage at 75 ohms and 100 mW (20 dBm) is almost 4 volts.
2.) Receiver impedances are all over the place. I've seen them as low as 20
ohms, and as high as 150 ohms. Most of those I measured are closer to 40-80
ohms.
3.) The voltage at the diodes is the vector sum of all signal voltages. If
you have a wide band antenna, there can be considerable net voltage from
many hundreds of small signals summing. I can light a 12V filament lamp dull
red off my Beverages at night, and I am 30-40 miles from the closest active
AM BCB station. It is the sum of hundreds of signal from hundreds of miles
that is the problem.
4.) Receivers limit the signal range to something centered around the
selected band, so they don't see that wide swath of summed voltages. The
diodes in a limiter do.
5.) The miniciruits transformer mix and create IMD, and are very sensitive
to dc current, even at pretty low levels. The point where they add IMD is so
unpredictable compared to limiting, they are not a good choice in receive
systems. This is especially true when you have no idea how many hundreds of
signals that transformer has to process at the randowm receive systems in
the field. (I tried them for antenna and amplifiers and abandoned them back
in the 70's. My eight element look antenna array initially used them, but
the LORAN signals and AM BCB signals killed them).
The last thing in the world useful for RX protection is a soft limiter. It
has to be a hard clamp, set just safely below whatever RX port voltage might
threaten equipment. No clamping or distortion until that point. Premature
limiting does absolutely no good, and potentially many bad things.
If you pay thousands of dollars for a receiver that has a wide spaced
dynamic range of over 100 dB, why would want to make it into 1970's Yaesu
FT101 performance with a limiter? Remember, this is not a few signals in the
passband. The diodes are pre-filter, and they clip at the sum of all the
hundreds of small signals across them.
If you use diodes, they become more acceptable is a modest amount of
pre-filtering is used to keep needless signals (especially the AM BCB) out.
73 Tom
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