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

Tom W8JI w8ji at w8ji.com
Sun Aug 30 19:53:46 EDT 2015


> That's similar to mine, but mine has two 1N4148 diodes in series,
> back-to-back, total of four diodes.
>
> However, after I did that, I got to thinking that it would be better with
> only two. Or that there was no need for four. (Don't ask me to explain,
> it's been a long time since I've thought about that.)

You probably used four diodes for a good reason.

20 dB is 2.73 volts RMS across 75 ohms. That is 3.86 volts peak.

If you use back-to-back diodes, the system clamps at .7 volts peak and mixes 
at lower levels. That's like 5-10 dBm or less for the sum of all signal 
power.

Remember that clamp is seeing the vector sum of voltages from ALL the 
signals on the RX antenna, not just signals on the band you are using or the 
signal you are listening to.

I can't use diodes here because of IMD, so I use a hard limiting clamp that 
is preset by a Zener diode.

Why would anyone want a clamp system that reduces the IM DR of a modern 
receiver? If you pay all that money for an RX, why not use it? I would think 
your 4 diodes are the absolute minimum to use almost any receiver's full 
dynamic range.

73 Tom



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