[TowerTalk] help with interference problems

[Stone, Gary R.]

Jim and others,

Thanks for the info.  OK - another question.  Regarding the FT-240
suggestion.  Is there a suitable snap on type that would help - even if
I used several of them in a row?  My reason is the CAT5 coming from the
tower is cut to the exact length and no room to wind any on a toroid.
It would mean replacing the cable.  So, is there any help from many of
the snap on type available?  I do have some of them installed already
near the router and on one end of a cat5 run from the router to my shack
(about a 50 foot run) and I don't seem to get any help from them but
maybe I am not using the correct one?  And the size is also a little off
- they fit loosely on the cat5.  So, any chance of a snap on anything
that might help?

Gary, N5PHT



Gary Stone, BA, LCDC  J

     Case Manager, DSAP

       O  903 - 583 - 6411

-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Jim Brown
Sent: Tuesday, July 17, 2007 10:32 AM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] help with interference problems

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





_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk at contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk