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From: David Brodbeck on 2 Nov 2009 00:18
I have an old MFJ-202B RF noise bridge I picked up used a while back.
I've been attempting to use it with my 10m transceiver, but I can't seem
to get any useful results. No matter what I connect to the UNKNOWN
jack, the noise in the receiver is lowest with the RESISTANCE knob
turned to the 0 ohms position. It never nulls completely, but as I turn
the knob clockwise the noise increases steadily from about S3 at 0
ohms to S7 at 200 ohms. I tried connecting a resistor to the output
jack, just in case the antennas I was trying to test were out of range,
and I still got the same result.

I can't figure out what I'm doing wrong, or what could be wrong with the
unit. It's a fairly simple circuit, and it does make white noise...
From: Jitt on 2 Nov 2009 10:41
In article <TuGdnfQ5KqsH9nPXnZ2dnUVZ_oWdnZ2d(a)speakeasy.net>,
gull(a)gull.us says...
> I have an old MFJ-202B RF noise bridge I picked up used a while back.
> I've been attempting to use it with my 10m transceiver, but I can't seem
> to get any useful results. No matter what I connect to the UNKNOWN
> jack, the noise in the receiver is lowest with the RESISTANCE knob
> turned to the 0 ohms position. It never nulls completely, but as I turn
> the knob clockwise the noise increases steadily from about S3 at 0
> ohms to S7 at 200 ohms. I tried connecting a resistor to the output
> jack, just in case the antennas I was trying to test were out of range,
> and I still got the same result.
>
> I can't figure out what I'm doing wrong, or what could be wrong with the
> unit. It's a fairly simple circuit, and it does make white noise...
>
One of the steps in setting up a (Heathkit) noise bridge
is to short the "unknown" jack with a short jumper inside
the unit. Maybe check if this was left in!
From: Baron on 2 Nov 2009 15:51
Meat Plow wrote:

> On Sun, 01 Nov 2009 21:18:17 -0800, David Brodbeck
> <gull(a)gull.us>wrote:
>
>>I have an old MFJ-202B RF noise bridge I picked up used a while back.
>>I've been attempting to use it with my 10m transceiver, but I can't
>>seem
>>to get any useful results. No matter what I connect to the UNKNOWN
>>jack, the noise in the receiver is lowest with the RESISTANCE knob
>>turned to the 0 ohms position. It never nulls completely, but as I
>>turn
>> the knob clockwise the noise increases steadily from about S3 at 0
>>ohms to S7 at 200 ohms. I tried connecting a resistor to the output
>>jack, just in case the antennas I was trying to test were out of
>>range, and I still got the same result.
>>
>>I can't figure out what I'm doing wrong, or what could be wrong with
>>the
>>unit. It's a fairly simple circuit, and it does make white noise...
>
> Do those things actually work? I've never had hands on experience with
> one because my rigs have built in DSP. I'm sure what you have was
> designed to work on narrow SSB signals not voice as you might be
> thinking (although you made no mention as to what mode you are
> operating in).

The receiver is used as the detector for the noise level.

--
Best Regards:
Baron.
From: Jamie on 2 Nov 2009 20:27
Meat Plow wrote:

> On Sun, 01 Nov 2009 21:18:17 -0800, David Brodbeck
> <gull(a)gull.us>wrote:
>
>
>>I have an old MFJ-202B RF noise bridge I picked up used a while back.
>>I've been attempting to use it with my 10m transceiver, but I can't seem
>>to get any useful results. No matter what I connect to the UNKNOWN
>>jack, the noise in the receiver is lowest with the RESISTANCE knob
>>turned to the 0 ohms position. It never nulls completely, but as I turn
>> the knob clockwise the noise increases steadily from about S3 at 0
>>ohms to S7 at 200 ohms. I tried connecting a resistor to the output
>>jack, just in case the antennas I was trying to test were out of range,
>>and I still got the same result.
>>
>>I can't figure out what I'm doing wrong, or what could be wrong with the
>>unit. It's a fairly simple circuit, and it does make white noise...
>
>
> Do those things actually work? I've never had hands on experience with
> one because my rigs have built in DSP. I'm sure what you have was
> designed to work on narrow SSB signals not voice as you might be
> thinking (although you made no mention as to what mode you are
> operating in).
Yes they work, normally only if the noise is local..

From: Dave Platt on 2 Nov 2009 21:35
In article <IrLHm.40$XH1.5(a)newsfe22.iad>,
Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_(a)charter.net> wrote:

>>>I have an old MFJ-202B RF noise bridge I picked up used a while back.
>>>I've been attempting to use it with my 10m transceiver, but I can't seem
>>>to get any useful results. No matter what I connect to the UNKNOWN
>>>jack, the noise in the receiver is lowest with the RESISTANCE knob
>>>turned to the 0 ohms position. It never nulls completely, but as I turn
>>> the knob clockwise the noise increases steadily from about S3 at 0
>>>ohms to S7 at 200 ohms. I tried connecting a resistor to the output
>>>jack, just in case the antennas I was trying to test were out of range,
>>>and I still got the same result.

>>>I can't figure out what I'm doing wrong, or what could be wrong with the
>>>unit. It's a fairly simple circuit, and it does make white noise...
>> Do those things actually work? I've never had hands on experience with
>> one because my rigs have built in DSP. I'm sure what you have was
>> designed to work on narrow SSB signals not voice as you might be
>> thinking (although you made no mention as to what mode you are
>> operating in).

>Yes they work, normally only if the noise is local..

Y'all are confusing a "noise bridge" with a "noise canceller".

The MFJ-202 is one of the former. It's a type of impedance measuring
device. It generates broadband RF noise (via a zener diode),
amplifies it (a three-transistor circuit) and then feeds it to a
bridge circuit. An unknown impedance (typically an antenna) is
connected to one side of the bridge, an adjustable impedance is
connected to the other side of the bridge, and your radio receiver is
connected to the center of the bridge. You listen to the hissing
noise from the receiver, tweak the R and C controls, and when the
noise level drops (ideally, is completely nulled out) you can read the
unknown impedance's R and X values from the adjustment knobs.

It's a substitute for (e.g.) an MFJ-259 impedance meter (much less
expensive).

It's also usable as an "antenna tuner-tuner". Connect it between your
rig and your antenna tuner / transmatch, set the knobs to "50 ohms R,
zero ohms X", and adjust your transmatch to null out the hissing.
Just MAKE SURE that you disconnect / bypass the noise bridge before
transmitting, or you'll burn it up for sure!

It's not an active noise-cancellation / nulling device for over the
air signals, like the MFJ-1025 and -1026 are. Those do an entirely
different job, in a rather different way.

--
Dave Platt <dplatt(a)radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
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