[RFI] RFI With Smoke Detectors

[Jim Brown]

On Fri,8/21/2015 8:31 AM, Greg Troxel wrote:
> Have you tested/evaluated the RadioWorks T-4G+ line isolator, or the
> similar things from MFJ?

No. Looking at the photo and lack of specs, I wouldn't expect them to 
work very well.

>   I know you recommend many turns around
> specific mixes, but I'm curious what the shield impedance is on these.

See my measured data, and method of measurement. k9yc.com/RFI-Ham.pdf

The problem with all-band non-resonant antennas fed with open wire line 
is that it is not practical to choke the feedline effectively. Dean 
Straw, N6BV (retired ARRL Antenna Book editor), showed this quite 
clearly at the Pacificon Antenna Forum several years ago.  To be 
effective, the choke must be at the antenna feedpoint, the choke must 
have a very high RESISTIVE choking Z. The only practical chokes I know 
of are low impedance lines -- either coax or a short section of parallel 
wires wound on a toroid.

Close-spaced enameled wire yields 50 ohms, close-spaced THHN (house 
wire) about 90 ohms. Jerry Sevick published this work 20 years ago, and 
I've confirmed it in my own lab. Those "all-band wonders" operate with 
very high VSWR on most (all?) bands, and the resulting mismatch fries 
any choke that might otherwise be useful.

73, Jim K9YC