[Antennas] Balanced line construction

Sat, 31 Jan 2004 18:40:05 -0500

Dear Sandor,

You are indeed lucky to have an excess of materials and time for building
your own balanced line. We hope that you also have the good WX for that
purpose - Hi!

A few points are worth mentioning:

- having the wire is the least of the problems - building the individual
insulators and applying them can be quite a task. First, there is the
question of sensitivity to dampness and sunlight. The only material that has
been used successfully is porcelain or ceramic. Everything else degrades
with time or is problematic in the wet. Needless to say, building insulators
of porcelain or ceramic is not a trivial matter. They are also prohibitively
expensive.

- Applying the insulators to the wire itself. Most commercial insulators are
provided with either a hole or a notch for the wire. The hole requires that
the wire be threaded through the insulators. Think about that before you go
to far! Once the wire is threaded through the insulators, the  insulators
must now be "fastened" to the wire. Typically, this is accomplished by using
small lengths of the same wire twisted around the main wire on each side of
the insulator, thus keeping it in its original position. This suggests that
the wire be solid, rather than stranded, in order to have the requisite
stiffness.

- Home constructed insulators can be made from almost any material, but the
material should NOT take on water. That means that any use of wood requires
treatment with a sealant as a minimum. For example, soaking in hot wax.

- Almost any plastic is better than wood, but remember degradation in
sunlight. Most plastics have a limited life in direct sunlight.

- Fastening home constructed insulators is often accomplished by a notch
cutout from the tip of the insulator into the wire hole. This allows you to
press the wire into the notch and then into the hole, where it is then
captive - to a certain extent. Fastening such an insulator is typically done
in a similar fashion to that mentioned above. However, an additional option
for home constructed insulators is to carefully drill the wire hole to
provide a snug fit for the wire. This has disadvantages - not only in terms
of getting just the right size hole, but also in terms of any unplanned
change in wire size. Once the holes are drilled, it is impractical to switch
to a smaller wire size and tedious to re-drill the holes for a larger wire.

- Recommended material for insulators includes vinyl moulding sold in
hardware stores as 1/4 round moulding or better yet, 1/2 round moulding.
This material is easy to cut and drill and is reasonably long lived in
direct sunlight.

Additional Thoughts:
- The impedance of existing balanced line materials is only of  academic
interest. Learn to live with what you've got. There is virtually no value in
knowing what the impedance actually is. For example, unless you are
operating on one frequency only, there will be a substantial SWR and there
will be a significant mismatch at the transmitter. Using a 4:1 balun to
transform a 450 ohm balanced line to 110 ohms is a myth. The only way that
is going to succeed is if the load at the end of the balanced line is 450
ohms, which is almost never the case. Having said that, there is,
nonetheless, a range of values which have proven to be useful, and it is
quite reasonable that if one uses a line with a characteristic impedance
outside of that range, he is asking for deeper problems. The range of
materials commonly available and useful is from 72 ohm twinlead to 450 ohm
"window" line.

- There is no really good basis for insisting on one impedance or another
for a balanced line. If you can't get a decent match with impedance Z1, what
makes you think that it will improve with impedance Z2? The length of line
involved and its characteristic impedance only provide a transformation -
nothing more and nothing less. If the transformation to too extreme, and you
conclude that you need a higher or lower characteristic impedance, think
hard about what the situation will be on another frequecy or another band.
Typically, use of plug-in matching sections, such as fixed capacitors or
fixed inductors, designed to be used either in parallel or in series, will
do a better job of bringing the impedance into range than trying to find a
single characteristic impedance that will work on all bands with a given
antenna.

I strongly recommend the web site of Cecil Moore for a good understanding of
the entire antenna system and the part played by the balanced line.

Please do not consider my suggestions as being "against" balanced line. On
the contrary, I personally prefer balanced line over coax for several
reasons. Not the least of which is the power handling capabilities under
high SWR. I have built my own balanced line and I have used commerical 450
ohm window line. The wet WX advantages of the home-made line are hardly
worth the effort, in my opinion.

I find that most users do not know enough about transmission line theory to
get themselves out of trouble, and I also find that misunderstandings and
myths about transmission lines lead people down the wrong path more often
than not. See my Web site for additional comments in that vein.

Good luck,

and 73,

Bob Lay (W9DMK), Dahlgren, VA
[email protected]
http://www.qsl.net/w9dmk
----- Original Message -----
From: <[email protected]>
To: <[email protected]>
Sent: Saturday, January 31, 2004 3:00 PM
Subject: [Antennas] Balanced line construction

> I have been unable to locate much info on the construction of balance line
> and was wondering if maybe some of you have a link or two you wouldn't
mind
> passing on.  I want to "roll my own" because I have so much scrap wire and
so
> little funds that building may be more of a pain then buying but it's the
> satisfaction of knowing you did it yourself, even in something as small as
balanced
> line that gives a certain amount of satisfaction.  I also need to find a
site or
> some help in determining impedance of stuff I already have on hand.
> Thanks to all who do reply, God Bless
> de KG4FET Sandor
>
>
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