RadioBanter - View Single Post - Fishrod anìtennas - transformer and twin-lead

Owen Duffy · October 4th 09, 10:38 PM posted to rec.radio.amateur.antenna

steveeh131047 wrote in news:5e53bd91-a69c-452f-9dab-
:

It's interesting to model this sort of arrangement, note the wide
range of feedpoint impedances that will be seen on the different
bands, and observe the feedline losses that might be incurred when it
is directly fed with 50 ohm coax. If you then introduce an ideal
transformer at the feedpoint and repeat the exercise you will
generally find that feedline losses increase on some bands and
decrease on others.

With the vertical length I tried, the effect of a 1:9 transformer was
to limit the more extreme losses at the cost of making some very low
losses higher. I guess over the several bands I tried you could say
there was a net improvement with the transformer.

But one question would be how to build this ideal 1:9 transformer
which maintains its transformation ratio and exhibits zero loss across
the wide range of impedances and frequencies involved.

Steve,

My article on the unloaded vertical includes a discussion of the unun /
balun thing. I did run models incorporating an ideal 4:1 current balun,
and found that coax loss is better on some frequencies and poorer on
others.

The model is not directly applicable to a generic end user installation
because the coax loss depends on line type, length etc, and the ideal
balun assumption is not a good estimator for practical baluns with
extreme loads.

I haven't published an article on the unun model, I should one day
perhaps.

Practical baluns are likely to have higher losses under extreme operating
conditions, and that will result in lower VSWR than otherwise, so the
added complexity of a real world balun is that at those extremes, it will
tend to be lossier, its transformation departs from ideal, and line
losses will tend to be lower.

I have created a model of a practical ferrite cored 4:1 unun, and explore
it with different core materials, dimensions, and windings. The models
reconcile well with G8JNJ's experimental ununs and my own prototypes...
but reconciliation on extreme loads taxes both Martin's and my own
measurement capabilities.

I cannot guess what inspired the application of 4:1 voltage baluns to
these antennas, much less why a reputable manufacturer would recommend
the configuration which to my mind defies sound principles. Nevertheless,
it does appear that thousands are successfully in use, and many hams have
the QSLs to prove that an antenna that lacks sound explanation "works
real good" anyway.

I do think there is good reason to apply a common mode current choke to
such antennas, not to recommend them as a matter of routine (though they
won't hurt much), but in some implementations one could expect a
significant common mode current problem, and a commom mode current choke
may be part of an effective mitigation.

As to whether a 4:1 transformation is universally better than 1:1, ideal
or otherwise, I doubt it. If the coax loss is a problem, and it will be
for all but lowest loss configurations), an ATU at the feedpoint seems
the better solution.

In making the observation that loss helps to reduce line VSWR, which in
turn reduces line VSWR, perhaps the only sensible design rationale behing
the 4:1 voltage balun applied on the subject antenna is that it drives
common mode current on the feedline, and in the case of a buried feedline
(as they often are), the power lost in heating the soil tames the
feedpoint impedance, reducing line loss. But, does that maximise system
efficiency? (Though it may work in that way, I doubt the 'designers' had
that in mind.)

Owen