[Heathkit] Re: Broadband dipoles

[Robert & Linda McGraw (K4TAX)]

On the topic of broadband dipoles, for years we find and hear of several 
antenna models that claim to be great with regard to bandwidth.  The coaxial 
sleeve, the bazooka, the extended Zepp just to name a few.

>From some real field experience, thus lacking antenna modeling laboratory 
facilities, some of these seem to perform better as poor dummy loads.  They 
are reported to be very broadband, very quiet and such.  So is a dummy load. 
In reality, they do radiate, some better than others; they do receive, some 
better than others.

With the modern transceivers today, those that employ power cutback circuits 
when SWR values rise, and those which truly like nothing better than a clean 
50 ohm load, I transverse to the good old center fed wire which employees a 
balanced feedline.   Now before you get the flame throwers lit, understand 
that the balanced feed systems are not near as difficult to institute as the 
tales that were spun of hams long ago.  Today's feedline material is quiet 
robust physically, it tolerates wind, weather and twisting and will handle 
large amounts of RF with very little loss.  The historical idea of keeping 
it away from metal is a tale that I find generally carried to extremes.  The 
rule is simple, feed line width times about 5 to 10 and that's it.  For 1" 
wide line then 5 to 10 inches of space is adequate.  Make some PVC 
stand-offs and bring it down the tower or mast, applying a few twists 
between stand-offs.  I improves the balance and reduces the wind whipping. 
As to getting it in the house, no big deal.  With wood frame or vinyl frame 
windows, just run it under the sash and close the window.  Connect it direct 
to the balanced output of the tuner and enjoy the fine art and performance 
of a balanced fed antenna.  Oh you say, what about feedline radiation and 
TVI?  Well if it is balanced it doesn't radiate as the currents in each side 
are opposite and thus the fields cancel.  If it is unbalanced it will 
radiate.  What about a ground?  Again, if it is balanced no ground is 
necessary.  If you have a "hot" mike or key then it isn't balanced.

Check the sources and various studies relating to balanced and open-wire 
line vs. coaxial line with high SWR values.  Hint: open wire and balanced 
feedlines win with lower loss every time.

There are several companies that make tuners which have true balanced 
outputs.  Rich Measures has detailed writings on this topic.  Then there are 
those that use some type of internal balancing methods.  Unless excessive 
power is applied, they work with very little loss.  Yes, I know what happens 
when the core of a balun saturates, it get hot, then very hot, then smokes. 
This is not a fault of the balun, it is the fault of the amount of power 
applied.  Power ratings are everything and you can't get one too large.  I 
do find that several companies make tuners with baluns that are under rated, 
to very under rated in terms of their power handling abilities.  Personally, 
all of my baluns are external and are rated at 5KW or more.  If you wish to 
build your own and one that is inexpensive and one that will handle all the 
power you can legally generate and work 160 M through 10 M, the "ugly balun" 
made of some 25 ft of RG-213 carefully wound on a piece of 3" PVC will do 
the trick.

Happy Antennas to each and every one.

73
Bob, K4TAX