[RFI] Fair-Rite Common Mode Chokes

[donovanf at starpower.net]

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 at mwt.net> 
To: donovanf at starpower.net, rfi at 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 at 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 at mwt.net> To: "Roger (K8RI)" <k8ri at rogerhalstead.com> , rfi at 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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