Message 26517 in TECH.WW
From: G8MNY
Date: Fri, 24 Jul 26 06:49:00 Z
Newsgroups: TECH.WW
Subject: RF Noise Bridge for LF,MF,& HF
Message-ID: <141_GB7CIP>
Path: N2MH4|GB7BED|EI2GYB|GB7CIP
R:260724/0653Z 34282@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260724/0653Z 14776@GB7BED.#21.GBR.EU LinBPQ6.0.25
R:260724/0652Z 56766@EI2GYB.DGL.IRL.EURO LinBPQ6.0.25
R:260724/0649Z @:GB7CIP.#32.GBR.EURO #:141 [Caterham Surrey GBR] $:141_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Sep 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Revisited from an old 2nd hand project.
SCHEMATIC
Z Screen
CIRCUIT
BRIDGE AMP AMP AMP NOISE SOURCE
THE CIRCUIT
The noise from the zener (use the noisiest U have) is amplified by the 3 RF
amp stages using RF transistors, this provides about 1V peak of noise (as seen
on a scope). The 3 transistors are any good small signal RF NPN types e.g. BF
series. Layout is not too critical, but small is normally best for RF. Feed the
transformer with a coax lead to keep the noise well screened from the Rx port.
THE TRANSFORMER
This is the heart of the bridge, a well made one with give good nulled match
VR & connections are ALL critical, if a good 50ohm non inductive measurement is
going to be any good over all HF bands.
Screens
ANT ___ | | ------------
NOISE | | -------------
Wind the 4x 4 turns on thin enamelled copper wire in parallel flatly (no
twisting) around a 1-2cm dia ferrite ring. Identify the 4 wires to form the 2
centre taps ct. and clean off the enamel, twist the ct pairs together & solder.
crosstalk from the primary winding, leave this end a few cm long so it can be
moved around for precision nulling.
Place a piece of tin can (or drill in earth tags on die cast box.) under the
closely located output sockets, transformer, & up to the VR 100R centre slider,
this all helps provide a low ground Z. Keep all connection leads as short as
possible!
TESTING & CALIBRATION
1/ With a Scope or capacitor to Rx check the RF noise increases down the amps.
2/ Using a digital ohm meter measure & make a temporary scale (on paper) for RV
e.g. stop, 0, 20, 30, 40, 50, 60, 70, 75, 80, 100, stop.
3/ With No ANT or a Shorted ANT, there should be lots of noise on the Rx port.
4/ With a good 50ohm dummy load the noise should null out with the pot at
around the 50 ohm mark, check this on all bands. If it varies try moving the
nc wire end to equalise the crosstalk to the Rx port on the highest bands.
Nulls of 40dB should be possible on most bands!
5/ Modify the scale to precisely agree with the RF impedance matching point.
IN USE
You can use with Rx to match any aerial with a suitable ATU.
It can be used as a noise source for checking filters and Traps etc.
With a spectrum analyser the multiple resonances of any aerial system will be
seen. As a noise source the spectrum response of any filters can be measured.
See also Tech bul "A Versatile Pulse Tester"
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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