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Old July 3rd 11, 01:39 AM posted to rec.radio.amateur.antenna
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Default Does anyone have plans for a 4:1 air core balun that actually works?

Michael wrote in
:

On Jul 2, 7:05*pm, Owen Duffy wrote:
Michael wrote
innews:7c2889df-092d-4471-bfb2-6db

:
...

* So how do I go about feeding a Lazy H with ladder line back to
the tuner for high power without over heating the balun? *The tuner
is a Dentron MT-3000a. *The built in balun is not balanced at all
at 10


I will respond in a couple of hours, but in the meantime, please
clarify that everything between the ATU and LazyH is up for
consideration.

What impedance line are you using?

Do you feed the antenna midway between the elements of at one element
(branch or distributed feed)? What do you think the feed point
impedance is?

What band(s)?

Owen


I am using 450 ladder line. The Lazy H is center fed with 1/2 wave
spacing between the top and bottom elements and the elements are 1/2
wave elements. 450 ladder line is connecting the top and bottom
elements. I am not using the expanded Lazy H version. This is the
classic center fed Lazy H design as shown in the ARRL antenna
handbook. I only intend to use the antenna on 10 meters. I don't
know what the exact impedance is (sorry). The Lazy antenna is about
50 feet off the ground at the top wire. The amplifier I want to use
with the Lazy H is a Drake L4B with two Eimac 3-500z tubes.


Ok, if we define the feed point to be half way between the dipoles, you
ought expect that the feed point impedance is relatively low, some where
of the order of 50 ohms, give or take some reactance and it is a fairly
balanced / symmetric load. Note that the feed system to each dipole is a
tuned length of transmission line, that makes this a narrowband feed
system.

So, your challenge is to deliver the transmitter a nominal 50+j0 load.

Re the ATU's integral ATU, always regard them as likely to be
unsuitable.

Let's regard that, integral balun aside, your ATU is probably capable of
matching a fairly wide range of impedances at 10m with reasonable
efficiency.

If you objective in using a balun is to minimise feedline direct
contribution to radiation, your objective is to try to force equal but
opposite currents in each conductor.

An ideal voltage balun will approach that objective ONLY with a
perfectly symmetric load. If you think you have a perfectly symmetric
load, it is probably because you haven't measured it.

An ideal current balun will approach the curent balance objective
irrespective of load symmetry, so it is a better choice for your
application.

Why do you need a 4:1 transformation? Depending on your feedline length,
the impedance presented to the ATU may be as low as somewhere round 50
ohms, and perhaps as high as several thousand ohms. It might seems good
to transform several thousand ohms by a factor of 4... but transforming
the lower impedances by a factor of 4 exacerbates ATU losses.

Since wavelength is short, I would be inclined to try to use a feedline
length of an integral number of half waves, and an effective 1:1 current
balun at the ATU. This minimises working voltages within the balun and
ATU (reducing risk of flashovers), without driving high losses in the
ATU are very low impedances.

If you think about your own requirements, and finding a solution that
fits, you will do better than Googling up HamUniverse articles and the
like.

Owen


 
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