[TowerTalk] 40M rotary dipole

[Jim Thomson]

Date: Thu, 21 Sep 2017 11:06:58 -0700
From: "Doug Ronald" <doug at dougronald.com>
To: <towertalk at contesting.com>
Subject: Re: [TowerTalk] 40M rotary dipole

<The RF current your receiver responds to flows on the inside of the outer
<shield of the coax transmission line, so what difference does it make what
<currents may be flowing on the outside of the shield?

<Doug
<W6DSR 

##  Install a dipole..with no CM choke at the feedpoint,....and the outer shield of the coax now
becomes  part of the antenna.   On TX, with no choke, feed power to the shack end of the coax...
and once it gets to the feedpoint, current will..spillover onto the outside of the braid, flowing down
the outside of the braid.   So now you have 3 currents flowing.  Center conductor,  inside of braid...
and outside of braid.   On RX you get the same effect. 

##  On a 40m rotary dipole, with no CM choke at feedpoint,  your  FS ratio is degraded. 
On a  yagi,  with no CM choke, both the FS  and also FB + FR will be de-graded. If you
have a lot of local man made noise, within 1000 ft radius, and no CM used, the coax now picks
up loads of noise.   Coax is typ routed vertical, at least for the last portion of the total run.   You end up
with a big vert antenna,  picking up noise on RX..and also radiating on TX.   If coax bonded to top and bottom
of a tower...and no CM used, the tower becomes the radiator.   You just reduced the effectiveness of the 40m dipole.
Then folks wonder why they have RFI in their homes.  Stuff 1.5 kw into the coax....and watch it radiate on the outside,
as into your home..and your neighbours  etc. 

##  The clamp on rf ammeter on the coax, with and without the CM choke is like apples and oranges. 
Depending on coax line length, a CM choke is not always required, like when using .25 or .75 wave, or
any odd multiple of a qtr wave coax.   This is phy length, not electrical length.  Its  common mode current,
not differential mode current.  Odd qtr wave length lines  typ result in a high Z  at the feedpoint.   Even with the
base of the tower grounded,  it may be grounded for DC and lightning events,  but are still not a good  RF ground.
So using odd multiple of a qtr wave is a crap shoot at best.  Besides  with 10 x ham bands.... and if more than one ham
band is being shared on any one run of coax, you will have a tough time  trying to obtain the correct length. 

##  http://www.karinya.net/g3txq/chokes/  heres  why you dont want reactive chokes. 

Loads of info on K9YCs  site. 

Jim   VE7RF