Topband: KD9SV-OK1RR relays ??? (RX Front End Protector)

[Tom W8JI]

> 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