Thanks to everyone for the feedback regarding antenna transformers, which
is very encouraging. Several people have since asked about transformers for
beverages, and since the answer is probably of general interest, I'll post
it here.
Why beverages need good transformers
------------------------------------
The requirements for a beverage transformer are less onerous than for a
pennant or flag, because the antenna already operates in the unbalanced
mode. Therefore there is no other mode of operation to suppress, in which
the antenna might receive signals from all directions, and fill in the
pattern nulls.
However good common-mode isolation between the feedline and antenna still
pays dividends. Once again Larry W7IUV's observations are pertinent:
"After investigating, I came to the conclusion that I had a
ground loop occuring in the coax shield. Using the same
transformer construction as I did for the pennant, I floated
the shield connection at the antenna end of the line. i.e.
coax goes to the primary, antenna and ground to the secondary,
no local ground to the coax.
This did not help the F/B of my poor unterminated buried
Beverages a bit, but it did reduce the power line noise and
computer birdies a bunch!"
Over the last couple of years, I have found on my own antenna (a
bandswitched elevated vertical), that a tremendous amount of local
electrical noise can be coupled into an antenna via the outer conductor of
the coax. This is despite baluns (ununs) at both ends of the feedline,
several clip-on ferrite suppressors, a separate ground stake and thick
ground lead at the antenna, and the shack equipment all shielded and bonded
to earth.
Now consider a low output antenna like a beverage; just how much more
susceptible it will be to conducted noise! If a high output antenna like a
vertical is affected, a beverage will be history unless common-mode noise
is eliminated.
I believe baluns and clip-on ferrites are ineffective simply because so
much inductance is needed at low frequencies. If one is unlucky, they could
even turn the coax into a resonant length, making the problem worse. What
is needed is complete isolation between the antenna system and the
coax/shack system, and a transformer with a separate primary and secondary
is ideal. To maintain this isolation, as Larry says it is essential to NOT
ground the outer conductor of the coax at the antenna, but leave it floating.
Good isolation would probably benefit most low-band antennas, not just
beverages. If the antenna is also used for transmitting, tuned link
coupling at the antenna should be seriously considered, due to the limited
power handling capability of available ferrite cores (remember we are
talking about normal transformers, not transmission line types). This is
definitely one of my next projects.
Transformer construction
------------------------
The highest permeability ferrite I would use for a beverage transformer is
#43, with a permeability of 850. This is actually very high for a
Nickel-Zinc ferrite, most of which are around 50-250. It will work well at
1.8 MHz, and should be usable at 3.5 MHz. The Q=1 frequency is 5.5 MHz.
For a 50:450 ohm transformer, the following turns are recommended:
Core 50 ohm 450 ohm
Type winding winding
------------------------------
FT114-43 9 27
FT140-43 8 24
The shunt reactance will be +j550 and +j690 ohms respectively for the two
types.
Materials
---------
I know people often lean towards high permeability ferrites (>1000), even
at HF. Sometimes such ferrites work but often they don't, contributing to
the aura of mystery. The recent discussions about the pros and cons of
u=10,000 cores is a good example. We tend to forget that the Q1 and Q2
materials, which remain so popular amongst amateurs, were amongst the first
ever ferrites to be commercially released. The technology has since moved
way beyond these early types, and I can't think of any reason why one would
want to use such old fashioned materials these days, in preference to more
modern ones.
There's really no mystery, and it's worth taking the trouble to select the
right ferrite. I strongly recommend the Amidon book, which apart from a
comprehensive set of graphs, has some good applications information as well.
I hope the above is of some use. Happy experimenting to all!
73,
Peter VK3APN
pnesbit@melbpc.org.au
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