RF Choke for H.F M.A.Pinfold ZL1BTB
I am on the quest to reduce the receive noise level on my radio '
I have tried earth stake at the shack ( it is outside separate from the
house and used to be her "she shed" for sewing but it turned out
inadequate size wise so i moved in ) and it seemed to reduce
the background noise on 40 m by a flickering s point or so ....but "s" points
add up !!
My antenna is a full sized 80m dipole suspended 9m at each end and fed with
40m of 300ohm window ribbon. The antenna is in a field bordering on an
industrial area the feed line is connected in the shack , to a
homebrew 1:1 balun wound on a ferrite torroid core and this connects
directly with a LDG pro 2 auto tuner. A lot of the noise I think is
locally generated domestic noise which seems to reduce late at night ,
however this baseline noise can be masked by HF band noise when the
atmosphere gets somewhat energetic .
I suspect some of this local noise may be picked up via my feedline and
coming down into the shack or up to the antenna , on the surface of
the braid All I need is a choke to form a high impedance to
rf flowing on the surface of the coax braid , I did have a large red
torroid of unknown composition in the junk box so I got some rg316 and wound
about 25 turns on it and connected that in series with my ant system between the
1:1 balun and the antenna auto tuner . Its seemed to reduce the ambient noise
down by a couple of flickering "s" points.
I thought Id try one made with RG8 coax as RG213 max power rating
is 20W and the ic7300 is gonna turn that rg317 coax to a toaster element
when i hit the Tx button at 100w, so I purchased a large ferrite torroid
labeled L15 from Jaycar ( Australian based components retailer)
found a length of rg58 and rewound with 9 passes through the ring (
a much bigger torroid would have been better ) the result is the picture
above.
I leave it in the feedline , I suspect it helps a little bit the
true test would be removing it when the atmospheric noise level is lower
and see if it does make a difference on unmasking weak signals but
at the moment I look at it and consider it my " anti arthritic
copper bracelet worn on my wrist" does it really work ???
I connected the choke to the e5062 VNA tx and rx ports
connecting only the braid to the centre conductor of each of the coax leads on
the VNA and did a "21" sweep
and this was the result from 0.3 Mhz to 50Mhz . I think
need more choking impedance down below 3.00 Mhz .
Domestic "noise " radiation will be higher at 0.3 to 1 Mhz and then
will naturally reduce as one goes up in frequency towards 25 Mhz or so.
I need to try more inductance and maybe throw a hand
full of ferrite sleeves on the feedline at the antenna end ??