[RFI] RFI to clock radio

[Jim Brown]

On Wed, 07 Feb 2007 10:29:46 -0500, Alan NV8A wrote:

>We have a Sony clock radio with no separate antenna connection: the 
>power cord doubles as the VHF FM antenna, and presumably it has a 
>ferrite rod antenna for AM.

>It's located about 30ft below my 2m/70cm antenna and the feed end of my 
>long wire. It seems that no matter on what frequency I transmit, I get 
>into that clock radio.

Several questions/answers.

1) The long wire probably has something approaching a current maxima near 
the transmitter, which establishes a very strong magnetic field around 
it. Since this is also close to the radio and its wiring, this field is 
probably responsible for significant (maybe dominant) coupling into the 
radio. 

2) And what is the signal return for the long wire?  Is it the wiring in 
your house?  As Tom often observes, and Ian just reinforced on another 
post, this will happen if you don't provide a far better return for 
antenna current. 

3) The radio has multiple detectors for RF -- every semiconductor 
junction in the box, including every transistor and IC works as an RF 
detector. 

4) As solutions, I suggest three things. A good line filter on the radio, 
a good line filter on your station, and a better signal return (radials) 
for your ham antenna system. A long wire is only HALF of the antenna. 

If those don't work, replace the radio. I strongly recommend the GE Super 
Radio, which Universal Radio (a good ham and SWL supplier in Ohio) sells 
for about $45 (probably cheaper than all the filters, and a lot less 
trouble). I have two of them. It is a superb AM and FM table radio with 
good overload characteristics, good receive audio (including broadband AM 
and a good 2-way speaker system). It was designed by a consortium of 
broadcast engineers a couple of decades ago to promote AM radio. Note 
that it is a MONO radio, even into headphones. It runs on 120VAC and 
internal batteries (6-D cells). It does not use line cord as an antenna 
-- it has a single rabbit ear for FM and a hot loopstick for AM. It 
doesn't hear my KW, even when I run it on 80 and 160 into a ground-
mounted vertical that is only 25 feet away. The only things it does hear 
that it shouldn't are my Nextel phone and 2M talkie if I get them within 
a few feet. 

73,

Jim Brown K9YC