Antenna Feedline Question

[Tom McCulloch]

Hi all,
  I am running a barefoot K2 with various options, including the auto-tuner
and am getting ready to put up a new antenna.  I've decided on a 40 meter
dipole that I will feed with 450 ohm 'window' line.  This will be my only
antenna, at least for a while.  I operate mostly on 40 meters (CW) but also
a bit on 20 (psk31) and 15/10 (also CW) when they are open and hope to tune
the 40 meter dipole to those bands with the auto-tuner.

 I am planning to run the 'window' line all the way into the shack to the K2
(which is about 5 or ten feet from the outside wall).  My question is should
I also use the BL1 at the K2 and will I have any trouble tuning to the other
bands.  I guess I don't understand what 4:1 means, in this case as I'll be
using 450 ohm feedline to the K2.

Thanks in advance for your comments.

Tom
WB2QDG
K2 1103


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[Ron D'Eau Claire-2]

Tom WB2QDG wrote:
I am running a barefoot K2 with various options, including the auto-tuner
and am getting ready to put up a new antenna.  I've decided on a 40 meter
dipole that I will feed with 450 ohm 'window' line.  This will be my only
antenna, at least for a while.  I operate mostly on 40 meters (CW) but also
a bit on 20 (psk31) and 15/10 (also CW) when they are open and hope to tune
the 40 meter dipole to those bands with the auto-tuner.

 I am planning to run the 'window' line all the way into the shack to the K2

(which is about 5 or ten feet from the outside wall).  My question is should

I also use the BL1 at the K2 and will I have any trouble tuning to the other

bands.  I guess I don't understand what 4:1 means, in this case as I'll be
using 450 ohm feedline to the K2.

----------------------------

The balun does two things: it provides an impedance transformation and it
helps reduce "common mode" currents on the feedlines. You may find one or
both of these functions useful, or neither of them may be needed. That is,
you may not need a balun at all.

First, the 4:1 is the impedance transformation ratio into a resistive load.
For example if you had a folded dipole it'd show about 300 ohms at the
center. A 4:1 balun there would provide a 75 ohm termination (300/4) for the
rig. That is it would provide you with an SWR of about 1.5:1 at resonance.

That holds true in general for such baluns, but not always, when you connect
them to a non-resonant antenna system. The 4:1 ratio can be useful if your
tuner can't find a match on some bands since the balun will change the
impedance it "sees".

Your dipole would be 66 feet long if cut for 40 meters. The window line is a
decent feedline when operating the feeder at a high SWR. On any frequency
between 7 and 30 MHz, the SWR shouldn't exceed about 10:1 (if you tried
using 50 ohm coax it'd probably exceed 50:1 on some bands). It's that lower
SWR that makes the higher-impedance lines more efficient than coax and other
low-impedance lines.

Some folks will say that the antenna will be badly unbalanced if a balun is
not used. That's the other function of the balun. It provides "push-pull"
currents to the feedline and it prevents non "push-pull" currents from
flowing through the balun. The non "push-pull" currents are called "common
mode" currents - they are identical or common to both wires. It works like
this. If, at some point along the feedline, there is a current of 1 amp of
RF current flowing up one side of the feeder, with an ideal system there
will be exactly 1 ampere flowing down the adjacent wire at that instant. Or
if there is exactly +50 Volts RF to ground on one feeder, there'll be
exactly -50 Volts RF on the opposite feeder at that instant. That's the
ideal way for a balanced feedline to operate since those opposite and equal
currents (or voltages) produce equal and opposite electric or magnetic
fields that cancel each other out. At a short distance from the feedline the
RF field produced by those opposing currents or voltages is zero. So no RF
is radiated (or picked up) by the feedline  - it all flows to the antenna.

In the real world that is seldom, if ever, the case. First, the load at the
far end of the feed line must also be balanced. You get pretty close feeding
your dipole in the center, but proximity effects of the two ends as they
pass objects unbalances the system, so even with a balun there will be some
unbalance. Most dedicated balun users would be astonished to see just how
little the balun actually affects the balance of currents in the feedline,
especially at the antenna end. The length of the feeder itself acts as a
"balun" to ensure balanced currents at the feed point.

Another thing your balun can do for you is to help isolate the antenna from
rig for common mode currents. Your feedline is in a high RF field, since it
runs right up to your antenna. Unless it comes away from a perfectly
balanced dipole at exactly right angles, RF currents will be induced in the
feed line by the strong field around the antenna. These currents flow along
both wires simultaneously in addition to the RF currents flowing from the
rig to the antenna. To these induced currents, your two feeders just look
like one "fat" wire, so the same voltage and current will be present at all
points along both wires; they are "common mode". The balun only allows
balanced or "push pull" currents to flow through it. So it will isolate your
rig from these common mode currents. Sometimes that's important. For
example, your feed line may be just the right length so the common mode
currents produce a high RF voltage loop right at the rig. In that case your
rig may tend to be "hot" with RF, producing bites when you touch it and
perhaps erratic behavior caused by RF voltages getting into the circuits.
The most immediate clue that you have a voltage loop at the rig is if your
tuning changes when you touch things. If that happens you can try changing
the length of the feedline, but that may not be convenient and it may
produce a voltage loop at the rig on a different band. So most operators go
for the "one size fits all" solution and put a balun at the rig end to help
isolate it from the common mode currents. And the balun can help keep the
currents balanced at the rig end, reducing pickup and radiation near the rig
end of the feed line.

So, if you experience a rig that seems "hot" with RF on some bands, try a
balun at the rig end of the feed line. Generally that seems to be a serious
problem only among those who must use a compromise installation with some
coax running from the end of the open wire feeders to the rig. That length
of coax often passes through the house where it needs to provide shielding
to prevent pickup or radiation of RF that may interfere with household
electronics. The coax shields the transmit RF currents, but he problem is to
keep common currents on the outside of the shield from causing trouble. The
balun will help a lot with that. Such installations work because they keep
that length of coax very short, but they still suffer significant losses in
most cases on the bands where there is a high SWR on the coax link.

The bottom line is that you may not need a balun at all. That's because you
probably do not need any impedance transformation and you may not need the
isolating effects of a balun, so you eliminate one device in the antenna
system. Everything in the system has some loss. A balun does not radiate, so
its losses are not contributing to anything.

That said, typically a good balun like the BL1 will have low losses in most
cases, so it won't hurt.  

Ron AC7AC


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[Joe-aa4nn]

Hi Tom,
Go ahead and cut your dipole for 7.000Mhz.
Feed it with 450 ohm ladder line.
By all means, use the BL1 connected to the 450 ohm ladder line in the shack.
Use a short coax jumper, 3 or 4 feet, between the BL1 and the K2 auto tuner.
You're in business, 40m-10m.
73 & gl,
de Joe, aa4nn

----- Original Message -----
From: "Tom McCulloch" <[hidden email]>
To: <[hidden email]>
Sent: Sunday, May 14, 2006 12:17 PM
Subject: [Elecraft] Antenna Feedline Question

>
> Hi all,
>  I am running a barefoot K2 with various options, including the auto-tuner
> and am getting ready to put up a new antenna.  I've decided on a 40 meter
> dipole that I will feed with 450 ohm 'window' line.  This will be my only
> antenna, at least for a while.  I operate mostly on 40 meters (CW) but
> also a bit on 20 (psk31) and 15/10 (also CW) when they are open and hope
> to tune the 40 meter dipole to those bands with the auto-tuner.
>
> I am planning to run the 'window' line all the way into the shack to the
> K2 (which is about 5 or ten feet from the outside wall).  My question is
> should I also use the BL1 at the K2 and will I have any trouble tuning to
> the other bands.  I guess I don't understand what 4:1 means, in this case
> as I'll be using 450 ohm feedline to the K2.
>
> Thanks in advance for your comments.
>
> Tom
> WB2QDG
> K2 1103

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[Don Wilhelm-3]

Tom,

In any antenna system where the feedline operates with an SWR other than
1:1, the transmission line acts as an impedance transformer, and the
impedance seen at the feedpoint of the transmission line will depend on the
feedline length.  The feedpoint impedance is not to be confused with the
characteristic impedance of the line.  A 450 ohm ladder line may have a
feedpoint impedance ranging from very low to very high.

You 40 meter flattop antenna should have an feed impedance of about 70 ohms,
and if your feedline is an electrical halfwave (or a multiple) on 40 meters,
the shack end will also have a 70 ohm feedpoint impedance, but if it is an
odd multiple of a quarter wavelength, the shack feedpoint impedance will be
quite high.

If you want a bit more information on the transformation properties of a
transmission line (in plain language with no math), take a look at the
antenna, feedline, tuner article on my website http://w3fpr.qrpradio.com

This same length antenna on 20 and 10 meters will have a high impedance at
its midpoint because each side is a half wavelength or full wavelength
long - again, this will be transformed to a low impedance at the shack end
if the feedline length is an odd multiple of a quarter wavelength at these
frequencies.

Be aware that the K2 autotuner has a bit of trouble loading a very high
impedance, so I recommend that you extend the length of your antenna a bit
to keep away from the high impedance points and make the transmission line
some multiple of a half wavelength for all the bands of interest.

The use of a balun is optional, but if you do use one, the current type
(like the BL1) is preferred over a voltage balun and whether or not a 1:1
balun or 4:1 balun will handle the line better depends on the feedpoint
impedance at the shack end - some experimentation may be in order (or
transmission line measurements and analysis).

73,
Don W3FPR

> -----Original Message-----
>
> Hi all,
>   I am running a barefoot K2 with various options, including the
> auto-tuner
> and am getting ready to put up a new antenna.  I've decided on a 40 meter
> dipole that I will feed with 450 ohm 'window' line.  This will be my only
> antenna, at least for a while.  I operate mostly on 40 meters
> (CW) but also
> a bit on 20 (psk31) and 15/10 (also CW) when they are open and
> hope to tune
> the 40 meter dipole to those bands with the auto-tuner.
>
>  I am planning to run the 'window' line all the way into the
> shack to the K2
> (which is about 5 or ten feet from the outside wall).  My
> question is should
> I also use the BL1 at the K2 and will I have any trouble tuning
> to the other
> bands.  I guess I don't understand what 4:1 means, in this case
> as I'll be
> using 450 ohm feedline to the K2.
>
> Thanks in advance for your comments.
>
> Tom
> WB2QDG
> K2 1103
>

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[K. Rice]

You've gotten plenty of good advice, and one of the best pieces of advice
was Don's suggestion to make the antenna a bit longer than the ~66 ft
required for a resonant half-wave dipole on 40 meters.  As Don mentioned
making the antenna longer will make it easier to tune on 20 and 10
meters where a 66-ft antenna has an extremely high feedpoint impedance.

Exact length is not critical as long as the two legs of the antenna are
of equal length.  If you have room, you could try an end-to-end length
of 87 +/- 3 ft  or 105 feet +/- 5 feet.  These two lengths are not
magically efficient or any such thing, but a lot of us have found them
easy to tune on multiple bands with simple tuners.

The two lengths will look familiar to users of the W3EDP and G5RV
antennas.  Both lengths tune well as a center-fed doublet on non-WARC
bands from 40m on up.  Both can be pressed into service on 80m if
need be.

73,
Ken
K3VV
Coopersburg, PA

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[Ron D'Eau Claire-2]

Ken K3VV wrote;

...one of the best pieces of advice was Don's suggestion to make the antenna
a bit longer than the ~66 ft required for a resonant half-wave dipole on 40
meters.  As Don mentioned making the antenna longer will make it easier to
tune on 20 and 10
meters where a 66-ft antenna has an extremely high feedpoint impedance.

----------------------------------------

Avoiding a close encounter with 1/2 wavelength is important, Ken, but
cutting the antenna to a length other than 66 feet won't necessarily help
and might make it worse. That's because the feed line acts like an impedance
transformer. The impedance at the feedpoint at the antenna will appear at
the terminals of the feed line *only* if the feed line happens to be exactly
1/2 wavelength long, electrically.

It can be rather difficult to predict the actual feedpoint impedance without
some careful measurements and calculations. Usually it's easier to see if
the ATU doesn't have enough range, then adjust either the feed line length
or the antenna length a bit until it works on all the bands.

Ron AC7AC


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[N2EY]

In a message dated 5/14/06 1:08:55 PM Eastern Daylight Time,
[hidden email] writes:

> 40 meter
> dipole that I will feed with 450 ohm 'window' line.  
>
>
> I am planning to run the 'window' line all the way into the shack to the K2
>
> (which is about 5 or ten feet from the outside wall).  My question is should
>
> I also use the BL1 at the K2 and will I have any trouble tuning to the other
>
> bands.

There's no magic answer to that question. It all depends on a bunch of
factors, such as the length of the window line.


 I guess I don't understand what 4:1 means, in this case as I'll be
>
> using 450 ohm feedline to the K2.

In the case of the BL1, 4:1 means that the BK1 not only converts from a
balanced line to an unbalanced line, but also changes the impedances by a factor of
4. A 200 ohm impedance on the balanced side will be transformed to a 50 ohm
impedance on the unbalanced side, etc.

However it should be remembered that baluns, even the BL1, are not without
their limitations. If the impedance appearing at the shack end of the line is
very high (thousands of ohms) or very low (a few ohms), or very reactive, the
BL1 may not be   able to do the job very well.

A useful tool for situations like these is antenna modeling software. A good
modeling package will give you an idea of how the antenna will work *before*
you put it up. Note that no modeling package is perfect, and none can be any
better than the information you give them. But they're a starting point.

For this specific application, I suggest downloading the program DIPOLE3 from
here:

http://www.btinternet.com/~g4fgq.regp/page3.html#S301"

DIPOLE3 is a freeware DOS executable that will give you some info on how your
proposed dipole will work. It won't tell you patterns or elevation angles,
and won't store the data, but it will give you an idea of what the feedpoint
impedances will be, SWR, and approximate system efficiency.

The only catch is that you have to enter the data in metric units. That's not
really hard once you know the conversions, and you really only do it once for
each antenna configuration.

Fool around with the values and get an idea of what goes on.

73 de Jim, N2EY
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