TopBand: Transformers for pennants and flags

[w8ji.tom]

Hi Peter,

What a well thought-out post with good analysis! 

I'm not quite  ready to agree with the assumption common mode excitation
directly relates to common mode impedance and feedline impedance, the issue
is almost always more complex. Some factors make the antenna LESS sensitive
to common mode problems than impedances alone indicate. One is the loop
impedance of the Pennant, since it is not infinite some common mode current
will harmlessly balance itself out. This is what happens in a small loop,
where severe unbalance can be tolerated without ill effects IF the loop is
isolated from earth. The larger the loop, the more important balance
becomes.  

Let's just assume for now the common mode impedance requirements are
complete.

> Considering that the gain of a quarter wave monopole over similar earth
is
> about +1 dBi, one begins to see the magnitude of the problem. In other
> words, to avoid filling in the rear null to any significant degree, it is
> necessary to attenuate the common-mode gain by some 50 to 60 dB!
 
> This is a simple potentiometer problem. To obtain a loss of 17.5 dB,
> Xc must be at least 7507*10^(17.5/20) = 56295 ohms. From this, we can
> calculate the maximum allowable interwinding capacitance as 1.54 pF at a
> frequency of 1.83 MHz.

Here is the problem I see. We'll never achieve the required amount of
common mode coupling capacitance with a single transformer. The ends of the
cable and antenna will have more stray coupling capacitance than 2 pF, even
a perfect transformer would not be good enough.

Assuming the common mode isolation requirements are correct (I need to
think about it a while, but my gut instincts tell me the requirement is
much less stringent), the only real world system that would decouple the
antenna and feedline properly would be a series of resonant common mode
chokes along the feedline. In effect the solution would involve breaking up
the feedline with resonant common mode chokes, and they would have to be
high Q resonant chokes! 

A second problem is most people will install these antennas near other
"stuff", and have coupling into other conductors. This antenna very well be
site limited, and there may be MUCH better solutions. That's the problem
with models, sources, antennas and even grounds are all perfect. We can
design antennas that will be almost impossible to actually make work like
the model predicts! 

> I would avoid the use of a manganese-zinc ferrite like 73, as it appears
> far too lossy for this sort of transformer at 1.8 MHz. The fact that it
> works well for transmission line transformers up to 30 MHz (or beyond) is
> not really relevant, because the application here relies on magnetic
> coupling, whereas transmission line transformers do not.

That was a point I tried to make earlier. Most transformers are somewhere
between the two types, and the mix selected really depends on the
application. With transmission line transformers, the core material loss
tangent isn't critical. All we want is a high impedance. With isolated
windings, loss tangent becomes a factor. The more isolated the windings,
the more critical loss tangent becomes.

The core I suggested works very well for general applications, in
particular Beverages and loops with better balance close to earth. With a
"loop" like the Pennant balance issues become a real headache, especially
if the antenna is elevated above ground with vertical feedline.

The transformers I wound have a primary to secondary capacitance of about
18 pF on 160 meters. That is nothing when feeding a Beverage or "balanced
shape" loop, but it is a problem with some "bigger" small antennas like
these. 

> At this point, it must be emphasized that a transmission line transformer
> SHOULD NOT be used for this application, as their end to end isolation is
> much too low. They are really only suitable for low impedance circuits.

IMO, based on 25 years of antenna experiments and RF design, they have very
specific applications where they can should be used. Antennas are NOT one
of them, unless it is matching a near-PERFECT unbalanced antenna (like a
vertical with many many radials) to a coaxial line. For Beverages, elevated
radial verticals, and almost anything else additional common mode isolation
is advisable.

They are not only suitable for low impedance systems, they are limited to
certain applications
where unbalance is near-perfect.
 
> My choice is an FT140-43 toroid with 8 turns on the secondary, and 34 or
35
> turns on the primary (for 900 or 950 ohms respectively). Sorry to specify
> the larger and more expensive core, but one needs the higher AL value to
> get sufficient inductance (2). With this core, the shunt reactance will
be
> +j689 ohms, leading to a nice conservative design. This larger core will
> also make it easier to keep the windings separated.

I agree. But I think in this case the solution is to augment the matching
transformer with a few common mode chokes along the feedline, so the
feedline is "broken up" where it approaches the antenna. We may well be
trying to "fix" and antenna that will never be predictable, because of its
inherent sensitivity to 
common mode.

These problems have always kept me away from loaded large
ground-independent loops, that use the vertical wires as antennas. It's
often better to use two small verticals or two small loops phased.   

73 Tom

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