A few days ago we were talking about UNUN Beverage transformers, and I
showed a link to a picture of one that I make and use, which has 16 turns of
quadrafilar windings. Some comments were made regarding this, so I will
explain what's going on, and it may be of general interest to others who
tinker with MF (160 and down ) and low HF (80).
In order to match the 470 Ohm Beverage antennas to coax, I use a
conventional donut torroid, set up for 470/50 or 470/75 Ohms. When using the
type 75 core material, u=5000, it can be done easily with 7t or 9t for 50
Ohms or 75 Ohms respectively.
The windings can either be 3 or 4 wires depending on how you connect them.
I happen to use 4 wires, and "waste" one, but that gives the option of
floating the shield on the coax. See fig. 2
The 7 or 9 turns gives a good match thru 160 on the low end and around 9 MHz
on the top. If it is desirable to also use the same antennas for BCB (as a
random wire), you must have a
lower bottom end than the 7 or 9 t will give. By increasing the number of
turns by 1.414, the low end is halved. Increasing that number again by
1.414 will halve it again. The results for 50 or 75 come
out to :( 7 x 1.414) x 1.414 = 14 t and (9 x 1.414) x 1.414= 18 t.
The 16 turns is a compromise between the two which gives good performance
for either.
As far as the top end frequency, there is at least 3 octaves of useful range
in a transformer like this, more likely 4 or 5 octaves, so the top end still
comfortably covers 80 and 75 Meters.
40 M use of a Beverage is a moot point for me, since when in the HAM mode,
there must necessarily be a BCB filter in line, and it cuts of before 40M
anyway.
The same quadrafilar windings can be hooked up several ways to give
different ratios, and if you leave the taps exposed, or even centertap the
top wire, you will have an even wider range of choices.
Here is a link to a crude drawing showing some of the combinations
http://homepages.dstream.net/K0FF/HomebrewTips/CIrcuits1.jpeg
The other configuration the uses separated primary and secondary windings is
not a transmission line transformer, just a flux coupled one, using a
torroid.
Happy Building DE Geo>K0FF
PS I probably have the only BCB crystal set on the planet with an S meter,
and a volume control ( very much needed w/ 2K Ohm headset). It also has a
preselector, and rejection trap, along with variable coupling. Only thing is
it takes 3 hands top tune. hihi.
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