[Antennas] More Ladder Lines and Beams

[George, W5YR]

My very first beam - a four-element wide-spaced Yagi in 1947 - was fed with
300 ohm Twinlead. Impedance matching to the very low driving-point
impedance of the beam was done simply by using the Delta Match.

The Delta Match is accomplished by doing nothing more than attaching the
feedline wires to points equidistant from the center of the driven element
(which can be "plumber's delight" construction with no break in the center
and connected to the boom) where the impedance is 300 ohms. As I recall
this distance was abut 12-18 inches each side of the boom (center). The
feedline was fanned out by cutting down the center such that the entire
feed system looked like an equilateral triangle: all three sides the same
length.

In those days, little attention was paid to SWR and as an almost brand new
ham - I had been on for about a year at that time, but on 20 CW with a
Twinlead folded dipole - I had little knowledge of or concern for SWR. But
I do know that the antenna loaded very readily, would cover the entire 10
meter band and enabled me to work the world with my lite 807 rig on CW and
later on Narrow-Band FM, using a Sonar XE-10 exciter (anyone remember
those?)

So, if you want to use ladderline, just experiment with delta-matching at
the antenna and use a 1:1 current/choke balun to transition to coax into
the shack and to a tuner if needed.

I have three wire antennas fed with ladderline that use W2DU bead-baluns to
coax into the shack to the tuners. Works very well . . .

72/73/oo, George W5YR - the Yellow Rose of Texas         
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771
Icom IC-756PRO #02121  Kachina #91900556  IC-765 #02437

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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 ?
> 
> 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. The low Z end would be lossy
> but it's only that small portion. 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. I haven't seen problems discussed there......maybe an oversite.
> One of the beams I've had had one side grounded to the boom, 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.
> 
> Brings up another point, how many people ground the coax at the tower ?
> 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%+/-. Why do most manufacturers
> assume a 4:1 is "good enough" for matching 50 ohms to 450 ohms ?  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 ? effect of
> core
> material selection or just use the powdered iron "red" 3-30Mhz ones from
> Amidon. Ferrite vs powdered iron in this application ? Power handling ?