[TowerTalk] help with interference problems

[Jim Brown]

On Tue, 17 Jul 2007 08:12:54 -0600, James C. Garland wrote:

>Gary, the RFI is almost surely coming in on the CAT5 cable. 

YES. 

>Your first line of defense would be to use shielded CAT5 

NO. 

The most likely coupling is COMMON MODE, and shielding won't help that. The best 
and easiest fix is a common mode choke on the CAT5 tuned to your transmit 
frequency. 8 turns of that cable around a #31 or #43 2.4" o.d. toroid (commonly 
called FT-240 by the vendors who sell it at very high markups). You want two 
chokes, one at each end of the cable.  

>for the run from
>the transceiver to the router. Next, you could install a surge suppressor on
>the CAT5 cable next to the transceiver, 

NO. First, it's a common mode problem. Second, adding stuff like this to the signal 
circuits are more likely to do harm than good. Third, CAT5 cable, by virtue of its 
high twist ratio, is VERY good at rejection RF. It is the lousy COMMON MODE 
rejection of the Ethernet circuitry that is the cause of the problem. That same 
Ethernet circuitry radiates a lot of common mode trash, for the same reason. Listen 
for those birdies around 14,030 and 21,052. You'll likely hear your own and your 
neighbors. 

>Lastly, you might try bypassing the DC power supply for the transceiver. 

A choke here would be in order ONLY if the chokes on the CAT5 cable don't fix it. 

>I
>imagine is uses a POE ("power over internet") adaptor, which inserts the
>voltage over an unused twisted pair in the CAT5 cable. You could open it up
>and solder a couple of .01uF bypass caps to the output.

>Also, make sure the transceiver is well grounded to the tower. 

GROUNDING IS ALMOST NEVER A SOLUTION TO RFI PROBLEMS.

For a detailed discussion of what works and why, see the RFI tutorial on my 
website. http://audiosystemsgroup.com/RFI-Ham.pdf

73,

Jim Brown K9YC