Topband: Fw: Noise Limiter Help - Thanks

Tom Rauch w8ji at
Sun Oct 1 22:03:58 EDT 2006

> If I am correctly reading the text you quoted, it appears 
> that DXE has
> decided to sidestep this problem rather than solve it by 
> not providing T/R
> switching. That is unfortunate. Lots of pretty good radios 
> (for example, the
> TS850) lack a separate input for a receive antenna.
> While I'm glad to see DXE come out with what looks to be a 
> better engineered
> version of a very useful product (I trust Tom to do a top 
> quality design),
> I'd be happier if it worked with my radios and the way I 
> use them.

Hi Jim,

The NCC has two identical channels that are designed to have 
low noise floors with high dynamic range. The output TOI of 
both channels is identical at about 35dBm. It doesn't have 
one channel designed to take a 100-200 watt transmitter for 
a few milliseconds while waiting for relays to switch and 
another channel designed to be used with a very small noise 
antenna that hardly picks up any strong stations.

There is too much variation in transceiver interface timing 
to guarantee RF will not be applied to input circuits. The 
risk of bad interface timing ruining an attenuator or input 
filter isn't worth it. The additional expense to eliminate 
the risk isn't worth it. Most people willing to spend extra 
money for a high quality two channel unit won't be phasing a 
TX antenna against a noise antenna. It was never intended to 
compete with the ANC-4 or MFJ-1025, which are devices for 
casual use where a TX antenna with gobs of signal is phased 
against a small noise antenna that has almost no signal 
level. (One of those units has around 70dB less dynamic 
range than the NCC on the noise antenna port!)

It's more for a balanced system with two nearly equal very 
good receiving antennas.

Now that certainly doesn't stop the user from using an 
external T/R relay (DXE is actually releasing a sequencer 
that controls delay up to 30mS and even delays the CW output 
to the rig), or using a line tap to sample small amounts of 
TX signal to null the TX out of a RX antenna, or any of the 
other applications. It's just the risk or cost of fitting 
the NCC safely into every possible market didn't go hand in 
hand with the product goal ....phasing two receive antennas 
without seriously hurting dynamic range.

73 Tom 

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