Hi Stefan,

> I suggest to discuss along a concrete schematic that you could present 
> along with the question(s).

Here is a simplified description of the setup. RX antenna (at the
moment, but soon to change or have more than one choice)

http://www.n1bug.com/lnv.jpg

Then there is a two way splitter. One output feeds the LF RX system:
band pass filter, preamp, band pass filter, receiver. The other
output has the same line up but for MF.

The preamps, with diodes added at the input

http://www.n1bug.com/w1vd-pre2.png

Finally the receivers (one for LF, one for MF)

http://www.n1bug.com/srlite2.png

These receivers may look too simple but with the filters and preamps
have done quite well for me!

I was hoping for something better than the diodes to put there at
the preamp input. Something which would limit to a much lower level.

For HF, a company called ICE used to make RX protectors (ICE-196)
which use some kind of very small transformer on a ferrite core. I
think the idea was the core saturates to provide limiting. But there
were other parts, including diodes, resistor, capacitor. I have two
of these, one is permanently connected to the RX antenna input on my
FT-2000 transceiver. Unfortunately these do not work below 1 MHz.
The insertion loss becomes very high at MF and LF.

Probably the best, as many people have said, is to give up this
silly idea of limiting at the RX front end and use a fast relay to
disconnect RX antenna and put instead 50 ohm resistor on RX input.
This we can say is quite safe for the receiver. :-)

It's just some work. I have a FT-2000 which I use with home built
down converters for CW and digital QSOs on MF and LF. I think there
is 15 ms between FT-2000 PTT output going low and the beginning of
RF output, so with a fast relay it should be OK.

I also have a Ultimate 3S beacon transmitter. It provides +5V output
for external T/R switching, supposedly with a programmable delay. I
have it set to 50 ms now. I am sorry to say I do not completely
trust this as there are some things which do not work quite as they
should with this device. I don't know if the T/R delay is correct
and reliable. I will try to measure it.

The FT-2000 and all LF/MF receiving stuff is in one room, the ham
shack. MF and LF down converters, amplifiers, and U3S are in another
room, the lab or work shop. There was no more space in the ham
shack. ;-) It's really quite silly to call that little room the lab
or shop, because there is a ongoing radio construction project in
every room of my house! The whole place is a work shop!

Some time in the future I may be able to use my HP 3325B synthesizer
with a linear PA to TX Ebnaut. I don't have any idea how I will
provide proper timing to switch the RX input relay with that, but  I
will worry about it later.

For now I think I should just make the decision and do the work to
make both the FT-2000 and the U3S (if its T/R delay is reliable) to
switch a relay disconnecting the RX antenna during TX. Unless
somebody has a better idea about serious limiting at the RX...

>> I need to stop being lazy. ;-)
>>    
> Oh oh, i know what you mean. Good point :-) I can observe the same 
> behaviour here. Sitting in front of the monitor, watching grabbers, 
> watching the WSPR map, programming some Linux stuff, playing with 
> SpecLab can be quite comfortable and relaxing. When looking back, 10 
> years ago i have been more spontaneous in doing portable experiments.
> But can this tendency be stopped or is it hopeless? It could be helpful 
> to analyse the causes instead of 'fighting' against oneselve.

Sounds like fun! I'm a little bit tired from building. I built all
the LF and MF RX stuff including receivers, preamps, many filters,
splitters and so on. I built the U3S, TX converters, TX antenna,
variometers, transformers. I built a few LF and MF PAs before
finding success... and success came after many weeks of tracking
down trouble. Now I'm just finishing up the build of a power supply
for the new LF PA, also doing a lot of work to make new and better
cables to connect all this stuff together. I enjoy building stuff,
but I am ready for a little break. All this has been a bit of a rush
trying to get it all done as quickly as possible. After I finish the
power supply, RX protection, and new RX antenna maybe I can rest and
do some operating... ;-)

73,
Paul