In article , Jerry Avins wrote:
Tweetldee wrote:

...

Now, as to your concern to keep 100W of RF from blowing up your noise
canceller. That's what antenna changeover relays are for. They normally
connect the antenna to the receiver, but when the transmitter is keyed up,
the relay connects the antenna to the transmitter output, and disconnects it
from the receiver. Simple, but effective, and has been used for many many
years in amateur as well as commercial radio equipment.

The line from the change-over relay to the receiver should be shorted
when the receiver is disconnected and either short or a half wave, or
else open when the receiver is disconnected and a quarter wave long.

Jerry


It's important to protect the both the noise antenna input and the main
antenna inputs on the noise canceller from RF - but trying to figure out if
you should short or open the receiver input based on the wavelength is a
little difficult when the frequency range covers a continuum of multiple
octaves. Not to mention the problem with putting a quarter or half wave chunk
of feedline between the changeover relay and the receiver input when the
wavelength is 160 meters. That technique works well for any single frequency
(or very narrow frequency band as a percentage of the center frequency) and is
commonly used at VHF frequencies and above with diode and relay switching
circuits for the additional isolation it affords.

An important consideration is the voltage on the changeover relay and the
components connected to it. The obvious part is the 100 W and 50 Ohm
situation, but consider what happens to the voltage when you have a high (SWR
for the hams / VSWR for the engineers). Jason, since this is your project,
I'll leave you with the research to figure out the potential range of reactive
loads with various antennas as well as the calculation of the resultant
voltages. Remember a good simulation models the real world, not just a
convenient part of it. I will tell you the result if you don' t give this
part of your design its due. Think about PCB traces vaporized, capacitors
shorted, transistor "fuses", relay housings melted and what we used to call
"essence of Allen Bradley" from the burned resistors. It's about the same as a
lightning hit or running 1000 watts through a 100 watt unit! This can be a
safety issue as well as causing unncessary delay and cost.

One of the other people noted that it was important to do a lot of reading,
research and analysis of your own - I'll second that!

If you haven't run across it yet, start with the term "Null Steering".

'nuff said for now,

Randy Gawtry
Timewave Technology Inc.