[Antennas] More Ladder Lines and Beams

[Don Havlicek]

Comments inserted below:


Sandy and Kees Talen wrote:
> 
> Thanks, lots of good information coming in.
> 
> One solution would be to lower the impedance of the ladder line to
> more closely match the beam antenna. The only way I know how to
> do this is by decreasing the gap (will increase the capacitance and
> loss) or increase the diameter of the conductors (I don't have a desire
> to run 4" or whatever parallel aluminum tubing). Parallel ladder lines
> would work too ....or stacking several ladder lines spaced apart by ?

I still wonder why you would rather deal with all the problems of:
1. interaction with surrounding objects
2. impedance bumps/changes with variations in line and 'wind sway'
3. matching to the antenna
4. loss of antenna discrimination by having the feedline 'part of the
antenna'.

> How about things you could do with open line that are not possible
> with coax ....like progressively narrow the spacing on the transmission
> line starting out with 2" (about 600 ohms) and ending with the equivalent
> 
> of twisted pair (about 100 ohms ....maybe less) which is one heck of
> a lot easier to route around the rotor. 

Easier?  but not as easy as coax!

>The low Z end would be lossy but it's only that small portion. 

Oh?  Since when, and how, does the loss 'localize'?

>Or things you can do with the beam since you DON'T intend to drive it >with coax ....like what is required to
> make the beam look like 400 ohms, or at least higher than  50 ohms ?
> I see antenna articles all the time with ladder line tied directly to
> dipoles, etc. 

You COULD use a T-Match or Delta-Match, both of which would require you
to eliminate the open feed point at the boom, grounding the driven
element there.  The Delta-Match would pair up with your open-wire line
better than the T-Match.  Read the info in any antenna handbook about
those two matches.  [I use T-Matches, adjusted for about 200 ohms, and
matched to 50-ohm coax]

>I haven't seen problems discussed there......maybe an oversite.
> One of the beams I've had had one side grounded to the boom, 

OH?  Can't say as I have seen this in over 45 years of
designing/building/tearing down and rebuilding LOTS of beams!  If you
were feeding a dipole, would you ground one side of it to the tower at
the feedpoint?  Essentially, that's what you're inferring.

>this one does not and nowhere does it tell you to ground one side ...so this is
> essentially a dipole. The only thing it tells you is to add a choke (8
> turns
> of xx" diameter....or a ferrite bead choke) at the beam to force current
> off the outside of the coax since the beam IS a balanced load.
 
That 'choke' is a BALUN [of sorts].

> Brings up another point, how many people ground the coax at the tower ?

Depends on how you're feeding the driven element!  IF the connection is
directly to the driven element .. NO .. if through a balun, then the
GROUND [common] of the Balun is connected to the boom of the antenna
VERY close to the feedpoint. [some people do NOT do this, but simply run
the coax down to the shack - you'll have to ask them why!]

> Seems like creating a ground loop to me since the only "connection" you
> want from the center conductor to the shield is the load itself.
> 
> Please, any comment on : Another question:  how much loss can
> you expect across a 4:1 balun, say the typical small ones in an antenna
> tuner ? I've always assumed it to be 10%+/-. 

In my experience, the loss in the baluns I have built [some from Press -
the Wireman's - kits] runs in the neighborhood of 2% or LESS.

>Why do most manufacturers assume a 4:1 is "good enough" for matching 50 ohms to 450 ohms ?

Not aware that ANY manufacturer does.  What they do is provide a 4:1
balun to give balanced feed.  The TUNER provides the match to WHATEVER
impedance shows up on the TUNER side of the balun!

>  I replaced mine with a 9:1 balun (three? cores which, of course costs more
> ...that may be the answer). Loss effect of using three cores ?

Why three cores?  The impedance transformation is NOT dependant on the
number of cores, but on the number[s] of TURNS in the BALUN!

> effect of core material selection or just use the powdered iron "red" 3-30Mhz ones from Amidon.

Any GOOD core material that will NOT saturate under use will do.

> Ferrite vs powdered iron in this application ?

I use ferrite, when I can afford it, but have used powdered iron cores
for smaller, lower power baluns.

> Power handling ?

Check with :   http://www.thewireman.com  for Press's
prices/specifications on baluns.  I am in no way connected with his
business, but find he provides quality items at a good price.  His balun
kits are excellent, in my opinion.

Don - N8DE
> 
> 73s  Kees K5BCQ
> - - -