[Top] [All Lists]

Re: [Antennaware] modeling feedlines- coils - traps

To: "Jim Miller" <JimMiller@STL-OnLine.Net>, "AntennaWare Reflector" <>
Subject: Re: [Antennaware] modeling feedlines- coils - traps
From: "Guy Olinger, K2AV" <>
Date: Fri, 18 Apr 2008 13:43:12 -0400
List-post: <">>
This is actually a fair amount of work to come up with 6 models and make sure 
they are "clean" at the frequency used and represent specific components. 
Also you will find that generalities do not always map well to individual 
ham's specifics.

Modeling feedlines correctly is only exceeded in complexity and gotcha's by 
modeling dirt. Here's a how to for a dipole fed with coax.

To model a coax feed for the *essential* effect  (not exact) break the dipole 
into two wires. Run a vertical wire down from the junction to represent the 
coax. Just use #12 for everything to start with. To begin, set the segments to 
be 1 segment per foot everywhere and wires even foot lengths. Only change to 
non-multiples of a foot and mixing shorter and longer segments when you can 
see if it changes anything vs equal. If it changes, take it back to equal 
segment lengths.

Break the coax wire up as needed to show sloping, running along the ground, 
etc.  Where the coax is laying on the ground or buried, place the wire .01 
feet above ground. Take it all the way to the house. Where the coax is 
grounded (as where your coax enters the house?) run a one foot wire from .01 
ft above ground to 0.0 height, the length equal to one segment of the wire 
coming to it.

Place the dipole source in either dipole half in the SECOND segment from the 
center junction. This avoids problems from placing a source in a segment 
adjacent to a complex junction. You can play with the % from end to make this 
happen. This will not be an exact dipole impedance point, but it WILL place 
power in the dipole, show the resultant conductor currents to a couple 
significant digits, and allow you to move placements and view conductor 

It FREQUENTLY happens in explicitly modeled dipoles INCLUDING nearby 
conductors, and slopes, that having the shield on one side or another of the 
center insulator makes a big difference. Flip the coax shield by moving the 
source to the other side of the junction.

If the land is sloped, model the situation to place the slope in the 
conductors, as if you had tilted the land to make it level and left the wires 
as-is.  Sloping land unbalances a dipole.

Wires coming into complex junctions need to have the same segment length and 
diameter at the junction.

To put a choke on the coax add a load into any segment (except next to the 
complex junction at the dipole) and set the load to produce the blocking 
impedance specified for the particular device at the frequency.  If you don't 
have that information, you will have to go MEASURE something, or pick a 
reasonable value that COULD be constructed. There is NO one value for a choke, 
only what is constructed/used.  You can put multiple chokes in various places 
on the wire representing the coax and see what it REALLY takes to get common 
mode current off the shield. Having a single choke at the feed point only 
lessens common mode noise getting into the feed

If this doesn't seem like enough information, you have a sense of how goofy it 
can be to model this stuff.

Once you have this done, start adding passive conductors for your tower, other 
antennas, nearby metallics like gutters and downspouts. Watch values change as 
you add them.

73, Guy.

----- Original Message ----- 
From: "Jim Miller" <JimMiller@STL-OnLine.Net>
To: "AntennaWare Reflector" <>
Sent: Wednesday, April 16, 2008 9:44 PM
Subject: [Antennaware] modeling feedlines- coils - traps

My current thought obsession :

Here is your assignment for next week :  L.B., where is this in your modeling 

I guess I am looking for an example of a simple model with the feedline 
modeled. Is there one in the examples that come with eznec?  Name?

Can somebody send my a simple dipole with :

1) a coax direct fed,
2) with a 1x1 balun,
3) with window line

And a model with a coil :

4) at the feedpoint
5) somewhere out toward the middle

6) Also something like a trap dipole for 40 and 80 for instance (showing the 

Thanks es 73, de Jim KG0KP
Antennaware mailing list

Antennaware mailing list

<Prev in Thread] Current Thread [Next in Thread>