Getting a balanced feed into the shack is usually not a big problem. First
are your planning on using "real" open wire or the brown vinyl covered
stuff? Real open wire presents a wee bit more of a challenge because it is
not insulated and the spacers typically will allow it to abruptly twist and
thus may short one side to the other. The vinyl line is a bit more robust
mechanically but does suffer from a bit more loss, although not near as much
as coax, and will change somewhat more with accumulation of moisture or ice
on the outside. I use the brown vinyl line from the wire center feed point
at the top of the tower, standing off of the tower some 18" and supported
every 10 ft with 1/2" PVC pipe stand-offs. The line is twisted 5 times
between the stand-offs. It enters the house at the wooden eve vent,
supported through the attic near the roof rafters and drops through the
ceiling via 1" slit to the operating position. Keep it away from metal
object that are parallel to the run. Metal at right angles to the run are
of little concern. Measured balance in the system is better than 5%. That
is clearly good enough for ham purposes. For insulation consideration,
under most situations the extreme value of voltage on a open or balanced
feed line is about 3000 volts max. Give this consideration when bringing it
through windows or other openings. This only requires about 1/2" of
insulation properties.
As to using a balun, most baluns in the tuners are basically 4:1 toriod
units supposedly rated at some value of power. My experience in measuring
baluns in general is that they are all over stated in terms of true power
handling ability. Truly a balun rated at legal limit begins to suffer
compromised performance in typical configurations at somewhere near 600
watts. For legal limit operation use a balun that is rated at a power level
of 5 KW or more. Core saturation causes the heat to build up and heat is
power loss. Keep in mind that the rating of a balun is stated in power
handling into some resistive load of the correct Z. I've never put up an
antenna that met those requirements. Case and point: a 4:1 balun would then
be used between 50 ohms on one side and 200 ohms on the other side,
resistive values, no reactive values considered.
The center Z of a 1/2 wave dipole located 1/4 wave above ground is about 75
ohms. The same dipole at 1/8 wave above ground is about 30 ohms and at 5/8
wavelength above ground is about 90 ohms. A balanced feed line of open wire
construction will have lower loss than any form of coax for the same given
length. Regardless of open wire line Z, it is not important unless you
intend it to serve as a 1/4 wave matching section. The feed point of the
antenna is most important in choosing a 1:1 balun or a 4:1 balun.
Again take a 1/2 wave antenna located about 1/4 wave above ground thus it
has a load Z of some 75 ohms. Attach a 4:1 balun to the feed line and the 75
ohms is now transformed down to 18.75 ohms. That is the Z value the tuner
must handle. The same antenna at 1/8 wavelength height and the center Z is
about 30 ohms, attach the 4:1 balun and the tuner must deal with 7.5 ohms.
My point is this, tuners don't like very very low Z values to deal with. It
is much better to deal with higher values from a loss point of view. Thus a
1:1 balun is more suited for this application.
Should you choose to operate the antenna on a harmonic then the center Z
becomes quite high. In practice in the neighborhood of 3000 to 4000 ohms.
The text books and the NEC programs show the values higher but these
typically may not take in to consideration the end effects and insulators
and etc. Here the application of a 4:1 balun is better suited.
In all cases I have found that 1:1 current baluns of KEW to 10KW rating for
a frequency of 1.8 to 30 MHz will do the job nicely for most legal limit
operations. I do not suggest running any significant length of coax between
the tuner and the balun. If at all possible, bring the balanced feed to the
operating position, put in the balun of your choice, connect it to the tuner
by a 12" to 18" run of RG-213 and let the tuner do its job of transforming
and matching the required load Z. (Comment: most tuners that are built-in
to radios won't handle the range of Z required for this configuration and
all band operations.)
Since I am on the subject, I'll ask and answer a question. Where and why
did the 4:1 balun originate? In years past hams used 1/2 wave folded
dipoles often made of open wire line or the brown 300 ohm TV line. These
antennas in normal proximity to earth then presented a feed point Z of about
200 to 300 ohms. A 4:1 balun then was used to transformed the antenna feed
Z to the 50 to 75 ohm range which was suitable for most transmitters.
Today, still the folded dipole is a very efficient and broad banded antenna.
However, it won't work on even harmonic bands, thus it is not a desired
choice for a "all band" antenna.
Paul, in conclusion, use the feed line of your choice, use the balun of your
choice and let the tuner deal with it. If it doesn't work to suit you then
change something like the balun ratio or feed line type or length. That is
what ham radio is all about.
73
Bob, K4TAX
----- Original Message -----
From: "Paul DeWitte K9OT" <k9ot@mhtc.net>
To: <tentec@contesting.com>
Sent: Tuesday, January 31, 2006 11:49 AM
Subject: [TenTec] openwire feed OT
>I would like to reopen the discussion on getting open wire into the shack.
> A while back there was quite a discussion on wheather to use a 4-1 or a
> 1-1
> balun when going to coax from open wire outside the shack and then running
> coax in to a tuner.
>
> No one mentioned just hooking the coax direct to the open wire without a
> balun.
>
> First I realize that you are going from balanced to unbalanced. How would
> this differ from feeding a dipole direct with coax and not using a balun
> at
> the feed point?
>
> I would like to put up a temporary loop and it is not feasable to run
> openwire in to the shack. But I would like to feed it with open wire from
> the loop to where it comes into the house (cheaper than buying more coax).
> So I am looking for suggestions.
>
> Since I am all T T it is only partly off topic.
>
> Thanks
>
> 73 Paul K9OT
>
>
>
>
> --
> No virus found in this outgoing message.
> Checked by AVG Free Edition.
> Version: 7.1.375 / Virus Database: 267.14.25/246 - Release Date: 1/30/06
>
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|