Hi Joe,
I often remind hams that coaxial cable is a three conductor cable when
connected to the feed point of an antenna. Two of the conductors are
the obvious ones, but the outside of the shield forms a third conductor.
The velocity factor of this "third conductor" is approximately 0.95,
but in this application its length is not critical, any length approximating
1/4 or 3/4 wavelengths will work very well.
I'm sure you know that 1/4 wave vertical has very high impedance
at the top, the same applies for a 3/4 wavelength vertical. When
you ground a coaxial feed line 1/4 or 3/4 wavelengths from the dipole
feed point, you're essentially making it into a vertical antenna with high
impedance where it connects to the feed point of the dipole. Very little
current will flow onto the high impedance presented by the outside of
the shield.
If the coax drops straight down from the dipole it will carry negligible
RF current; however, parasitic currents will be induced into the coax
shield if you pull the coax to the side, destroying its symmetry to both
sides of the dipole. By the the way, exactly the same symmetry
requirement exists for coax isolated from the antenna by a choke...
If you pull the choke isolated coax to the side it will also carry
parasitically induced RF current.
We eliminate cross-station RFI at W3AO by using only flat top
horizontally polarized antennas. All antennas for the same band
are aligned tip-to-tip with several hundred feet of spacing which
dramatically reduces their coupling. We have no RFI at all when
operating four transceivers (CW, SSB, digital and GOTA)
simultaneously on the same band.
73
Frank
W3LPL
----- Original Message -----
From: "Joe" <nss@mwt.net>
To: donovanf@starpower.net, rfi@contesting.com
Sent: Thursday, July 18, 2019 2:35:17 PM
Subject: Re: [RFI] Fair-Rite Common Mode Chokes
Thanks Frank,
Now 1/4 or 3/4 I'm assuming free space not figureing in velocity factor?
I am old school when I came to dipoles been a ham since 1975 and field day
since 76. and most dipoles have been the simple split the coax and solder. bo
baluns no chokes etc.
This year we actually did some antenna placement thought to lessen multi
station interference, and we had a 3 foot vertical and then placed a regular
dipole so the vertical was off the ends of the dipole, and we were actually
amazed a FT-8 station and CW station had no real issues being on like 20 meters
at the same time.
So thinking if we are gonna re do the dipoles to light weight ones. mat as well
do it right and help minimize interference as much as possible. and hence the
thought of the balun to choke off the coax, so I am to assume the 1/4 tor 3/4
is acting like what to keep rf off the braid?
Joe WB9SBD
Sig
The Original Rolling Ball Clock
Idle Tyme
Idle-Tyme.com
http://www.idle-tyme.com
On 7/18/2019 9:04 AM, donovanf@starpower.net wrote:
Joe,
We use a fifty foot length of RG-8X on each of the nine dipoles used at
the W3AO mega-Field Days. We then transition to hundreds of feet of
RG-213 to reach the centrally located tent on our 1000 foot x 200 foot
Field Day site. We use lightweight Budwig insulators and lightweight
dacron rope.
A choke is unnecessary on a monoband dipole, simply grounding the
coax shield approximately 1/4 or 3/4 wavelengths from the feedpoint
is completely adequate. Only a short ground is needed.
73
Frank
W3LPL
----- Original Message -----
From: "Joe" <nss@mwt.net> To: "Roger (K8RI)" <k8ri@rogerhalstead.com> ,
rfi@contesting.com Sent: Thursday, July 18, 2019 1:15:25 PM
Subject: Re: [RFI] Fair-Rite Common Mode Chokes
To change the subject some here,
After a slight mishap at field day, a Dipole fell down in a storm, and a
ceramic insulator hit a cars window and broke it.
So the plan is to make an ultra light set of dipoles. Plastic
insulators, coax fed with 58 size coax. etc.
Now what cores, how many, etc. should I use instead of a heavy balun to
make the choke? Low barefoot rigs 100 watts. so trying to keep things as
light as possible.
Joe WB9SBD
Sig
The Original Rolling Ball Clock
Idle Tyme
Idle-Tyme.com http://www.idle-tyme.com On 7/18/2019 12:22 AM, Roger (K8RI)
wrote:
<blockquote>
I (so far) use ferrite cores only for common mode chokes and I've made
and used many particularly for 160. 75, and 40 center fed slopimg
dipoles and half slopers.
73, Roger (K8RI)
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