Use of fishrod antennas is getting more and more common to cover all bands 7 ...
28 MHz, including WARC. The typical length of the radiating wire is about 25
feet, that is about one quarter wavelength at 10 MHz.

On lower frequencies (e.g. 7 MHz) that antenna shows a low radiation resistance
and a strong capacitive reactance. On higher frequencies resistance goes up and
down with frequency, showing very high peaks, and reactance does the same,
though obviously with a different pattern.

Typical configuration used is:
- 1:4 transformer (balun or unun) at the antenna
- 300-ohm or 450-ohm flat twin-lead
- tuner at the transmitter

SWR is generally high, except on certain specific frequencies (which may not
fall in the amateur bands).

I would like to hear some opinions on the usefulness of using the transformer
and the twin-lead. My arguments a

- above 10 MHz, where impedance is generally fairly high, the transformer could
(??) be helpful to reduce SWR, but below 10 MHz it should be harmful, as its
transformation goes on the wrong side.

- use of the twin-lead is usually justified by its low attenuation, that limits
the extra attenuation caused by the high SWR existing on the line. In my opinion
the 300-ohm twin-lead attenuation is reported to be low mainly because it is
measured in presence of a 300-ohm load, that draws low RF current. But in the
subject application, where the antenna impedance is uncontrolled, what should
count is the ohmic resistance of the twin-lead conductors which is not
particularly low due to their fairly small diameter. Wouldn't an RG-213 do
better than the twin-lead?

Thanks for your comments & 73

Tony I0JX
Rome, Italy

On Sat, 3 Oct 2009 21:08:57 +0200, "Antonio Vernucci"
wrote:

I would like to hear some opinions on the usefulness of using the transformer
and the twin-lead. My arguments a

- above 10 MHz, where impedance is generally fairly high, the transformer could
(??) be helpful to reduce SWR, but below 10 MHz it should be harmful, as its
transformation goes on the wrong side.

Hi Tony,

Good of you to notice that the 1:4 can also be viewed as 4:1 (or did
you notice?) with it reversed. This is not strictly so, as the
classic BalUn consists of wound transmission lines with characteristic
Z at the geometric mean of the load and source Z. However, lacking
this doesn't always stop the experimenter.

- use of the twin-lead is usually justified by its low attenuation, that limits
the extra attenuation caused by the high SWR existing on the line. In my opinion
the 300-ohm twin-lead attenuation is reported to be low mainly because it is
measured in presence of a 300-ohm load, that draws low RF current. But in the
subject application, where the antenna impedance is uncontrolled, what should
count is the ohmic resistance of the twin-lead conductors which is not
particularly low due to their fairly small diameter. Wouldn't an RG-213 do
better than the twin-lead?

If your BalUn has already done the bigger job of turning a High Z to a
modest one, the common logic for the need for twin line has also been
diminished.

Working with, designing, and building BalUn applications demands a
good tool for validation. Do you have something that will measure Z
with accuracy?

73's
Richard Clark, KB7QHC

Good of you to notice that the 1:4 can also be viewed as 4:1 (or did
you notice?) with it reversed.

That is obvious, but in common designs there is no provision (e.g. a relay) for
reversing the transformer when operating on frequencies where the antenna
resistance is lower. So the transformer remains there as it is, doing the
opposite job of what it should do.

Working with, designing, and building BalUn applications demands a
good tool for validation. Do you have something that will measure Z
with accuracy?

No, I am not building that antenna or doing any experiment. I am only trying to
understand the rationale behind what people proposes and I am seeking advice
from people having specific experience on the issue.

73

Tony I0JX.

"Antonio Vernucci" wrote in
:

Use of fishrod antennas is getting more and more common to cover all
bands 7 ... 28 MHz, including WARC. The typical length of the
radiating wire is about 25 feet, that is about one quarter wavelength
at 10 MHz.

By fishrod antenna, I assume that you mean a telescopic fibreglass pole
of about 10m height, used to support a straight or approximately straight
(including a very coarse pitch helical) vertical wire of the same length.

This has a lot in common with the popular 43' vertical, just the lengths
are different, and the frequency coverage will be different.

The use of a 4:1 balun on all of these things seems inspired by one
antenna manufacturer's recommendation and supply of 4:1 voltage baluns
for the application. Their site shows testimonials, and claims thousands
sold. Eham reviews abound with glowing testimonials.

However... the application of a 4:1 voltage balun seems to me not only to
lack design rationale, but to be quite undesirable in driving common mode
current on the coax feedline, and potentially very lossy in
configurations where the feedline is buried.

I discuss the use of an untuned vertical as a multiband antenna, and
raise the insanity of the voltage balun application at
http://www.vk1od.net/antenna/multiba...ical/index.htm .

Owen

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 G3TXQ

In message
,
steveeh131047 writes
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.

You might like to refer to the thread "UNUN Cores?How To Wind?",
started on 1 August. In particular, the last posting (by 'UK Monitor')
suggests a link to this website:
http://g8jnj.webs.com/currentprojects.htm

--
Ian

steveeh131047 wrote:
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 G3TXQ

Some time ago I made a series of careful measurements of a transformer
which was at the feedpoint of a multiple band antenna. At frequencies
where the feedpoint impedance was very much different from the (purely
resistive) design impedance, the transformation wasn't equal to the
design transformation, and the transformer introduced both series and
shunt impedance. At some frequencies, these effects were extreme, and
the transformer acted nothing at all like an ideal transformer.

Modeling a system like this with an ideal transformer might be an
interesting intellectual exercise. But that's all it is -- the real
system won't behave anything like the model. You can extend a
transformer's range of impedances and frequencies by using great care in
the initial design and construction, then adding compensating circuitry.
The job gets more difficult as the transformation ratio increases. I
seriously doubt you'll ever come close to making a transformer anything
like the one described in the last paragraph.

The amateur way is to build a system with a transformer, then figure out
how to live with whatever you get. An engineering approach usually
involves designing a system with predictable and repeatable performance,
and that precludes depending on a transformer over a wide impedance range.

Roy Lewallen, W7EL

On Oct 4, 7:59*pm, Roy Lewallen wrote:

Modeling a system like this with an ideal transformer might be an
interesting intellectual exercise. But that's all it is -- the real
system won't behave anything like the model.

Roy,

I didn't express myself well - the final paragraph was meant to be a
rhetorical question which cast doubt on the validity of the
conclusions!

73,
Steve G3TXQ

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

steveeh131047 wrote:
On Oct 4, 7:59 pm, Roy Lewallen wrote:

Modeling a system like this with an ideal transformer might be an
interesting intellectual exercise. But that's all it is -- the real
system won't behave anything like the model.

Roy,

I didn't express myself well - the final paragraph was meant to be a
rhetorical question which cast doubt on the validity of the
conclusions!

73,
Steve G3TXQ

Sorry, Steve. It's really hard to express subtlety or sarcasm in this
sort of written venue -- as I've found out so many times myself.

Roy Lewallen, W7EL