[600MRG] 630M Noise Reduction

[Murray Greenman]

Paul and others,

This note is aimed at those of you who use an active whip antenna 
(PA0RDT, AMRAD etc) on receive. It's an aspect of noise reduction that 
many of you many not have considered.

One thing I learned from a VK ham, with extensive LF/MF experience, is 
that a significant proportion of the noise you receive on an antenna 
such as this is actually radiated into the antenna off the feed line. 
The source of much of this noise is the house wiring 'earth', system, 
which is far from a real quiet earth.

There is a very effective cure for this.

1. The first thing to do is use a 1:1 isolating transformer at the 
receiver end, to prevent noise appearing on the feed line from the mains 
earth system (after all, your receiver will be 'grounded' to the mains). 
A simple method is to replace the power feed choke at the receiver end 
with a 1:1 transformer, or you could simply add a transformer between 
the power feed box and the receiver. A suitable transformer would be a 
12 mm binocular core with a 10-turn bifilar winding. Of course you could 
also power the antenna from a battery to be sure noise doesn't get in 
that way.

2. Fit a really decent common-mode choke in the antenna feed line, and 
ground the antenna side of the choke to a real earth: the best would be 
your transmitting ground plane star point. The design described here is 
effective upwards from about 100 kHz. I constructed the water-proof 
common-mode choke as follows:

     a. Stack 12 x 25 mm mains suppression grade toroidal cores in two 
rows of six, securing them with electrical tape.
     b. Wind as many turns as possible of RG174 or other small diameter 
coax through these cores (you should be able to manage 10 turns).
     c. Mount the assembly in a die-cast box. The receiver end of the 
choke connects to an RG58 feed cable that passes through a nylon gland.
     d. The antenna end of the choke connects to a coax connector (BNC or 
UHF) which passes through and is connected to the die-cast box case.
     e. Fit a ground bolt and wing nut to the case, and run a short lead 
to the main antenna earth.

3. The die-cast box should sit next to the main transmit antenna earth, 
and the coax run from the other end of the box to the antenna (now 
quiet) should be no longer than necessary, and preferably above ground. 
Note that the receiver end of the choke is not connected to the die-cast 
box case.

73,
Murray ZL1BPU/ZL1EE
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