NO NO NO NO NO...............

LISTEN to TIM!!!!!!!!!!!!! Tim is correct, you are WRONG.

While the diodes might be benign in the presence of small and medium
sized out of band signals, they do exactly what Tim said in the
presence of large input signals like you might find near transmitters.
Remember, the 200 KW FM broadcast station at the end of the block, or
the 5 KW AM broadcast band transmitter down the road from you. Even
though these signals are no where near the frequency you are trying to
listen to, the diodes will see them and act as a non linearity and
produce IMD all throughout the radio spectrum.

Remember, these diodes are not protected by a frequency selective
tuned circuit.....so any big signal that comes down the antenna is
gonna make a problem for you , especially if you're trying to listen
to a weak signal.

Tim is correct.

Years ago I was working for a tv station as a chief engineer. I had to
oversee the installation of the new 900 foot tower and make sure the
tower monkeys aligned the dish properly for the STL. So, I spent quite
a few days at the site. The guys putting up the antenna had just
purchased new ICOM HT's and had them modified for out of band
operation so they could use them for communication between the tower
people and the ground crew running the wench.

I told them I had the same radio as they did, (which was true). When I
showed them my radio, there were no odd audio sounds coming out of the
speaker. When they turned on their radios, all they could hear was the
overload from the FM station sharing the same tower.

The owner of the tower company was a ham and modified the ICOM's with
back to back diodes at the antenna to keep the front ends from blowing
out because they often worked within 10 or 20 feet of big powerful
antennas. He told me without the diodes he added, he would lose one
radio per day to rf overload and he used to carry spare front end
transistors. Part of his evening ritual was to sit down and remove the
blown front end transistors and replace them. He had done it so many
times he could do it in 5 minutes! But, after he put the diodes in,
the front end didn't blow anymore....but they always had interference
from other transmitters as a result of the diode mod!

He learned the hard way not to introduce non linearities before the
front end tuned circuits! I think you need the same lesson Allison.

I took one of his radios home that evening and modified it. I
basically removed the gasfet front end transistor and threw his back
to back diodes in the trash. I added a 10 db pad to attenuate the
incoming signals across the entire spectrum. They spent about 3 weeks
doing the tower job, and by the end of the job, I had modified all his
radios. They never had another interference problem and they never
lost a single front end. The owner was a good guy and offered to pay
me dearly for tweaking his radios.

With all due respect, the diode capacitance is IS NOT the problem.
It's the non linearities associated with the rectification in the
presence of big out of band signals.

Listen to Tim, he's telling you the truth!

T


The diode capaitance is low and really not a factor in preamp gain or
noise. They are non conducting and the capacitance is easily absorbed
into fees and matching systems.