<snip>
"450ohm ladder will not radiate, like coax will, because one side
cancels out
the other side."
This is only correct if you do EVERYTHING properly. This
requires that the entire system has to be truly symmetrical.
There are two common "got-yas" here:
1.With wire antennas this is often not the case. For instance,
if one leg of a dipole passes over the roof of your house and the
other does not, then electrically speaking, you have different
lengths on each side of the feedpoint and this needs to be
compensated for (by shortening the leg running over the roof).
2. If the matchbox is a common "T" filter (like Paul's MFJ) or an
"L" filter (like the 238), then you have an unbalanced situation
that needs to be corrected. We use a balun to accomplish this,
but it is typically done in the wrong way (including with the
238). Besides the bit about not using an "Electrical Balun",
there is another aspect. If the SWR on the open wire is too
high, the balun will not be matched at its antenna side and will
therefore not properly convert the unbalanced 50 Ohm to a
balanced 200 Ohm. John (N0KHQ) touched on an important factor:
feedline length. By using the proper, calculated length for your
antenna AND any of several specific feedline lengths, you can
assure that the feedline impedance is only a few hundred Ohms
(rather than several thousand) and then this external balun works
just fine. Since the local QTH environment typically is the
determining factor for feedline length, it is typically a matter
of luck if you get good results or poor results.
<snip>
"You will still need a good quality antenna tuner. I only
recommend using a TT-238/a/b."
This is not always true, due to the reasons pointed out above.
Also, although there is certainly a lot of junk on the market,
there are a couple of other boxes available which are good as
well. The 238 is an excellent box (one of the best) for
Asymmetrical use.
SOLUTIONS:
The VERY best solution for feeding openwire to symmetrical
antennas is to use a truly symmetrical matchbox, not an
Asymmetrical matchbox. For short-term (temporary) installations
or field day work where a bit of feedline radiation may not hurt,
it's OK to use the Asymmetrical boxes and an external balun.
True Symmetrical matchboxes are difficult to find. The OLD
Johnson Viking matchboxes were Symmetrical matchboxes. A company
in Germany called "Annecke" built true Symmetrical matchboxes and
sold them for about 25 years. Unfortunately Mr. Annecke passed
away a few years ago and the company closed. "DECCA" in England
built a product called "KW E-ZEE Match" which is a Z-match and
Symmetrical. I don't think they exist anymore. Until recently,
that was about it.
New Products:
Several new products have appeared in recent years, all
"claiming" to be "Symmetrical Matchboxes".
BUYER BEWARE !
Most of these are indeed "almost" Symmetrical matchboxes and all
of them are much better than running Asymmetrical matchboxes -
but they are not perfect. What the vendors (e.g., Palstar, MFJ)
have done is to continue to use the classical "T" filter (also
known in the states as the "Transmatch") but they have placed the
balun on the TX side of the T-filter (rather than on the antenna
side). In this instance, the balun does indeed see its design
impedance on both ends and does a much better job of maintaining
symmetry. The only problem is then to be sure the matchbox is
"floating" - that is, what we normally would think of as the
"ground side" is not grounded at all. It is kept well insulated
from ground. These matchboxes do indeed work much better and are
even good enough for low power levels (i.e., 100w). However, at
legal limit power levels (for me in Germany, that's 750 watts),
even a 4 Kilowatt version of the Palstar matchbox caused
significant problems to me with RF in the shack. Running the
same antenna on my home-brew (using Annecke components) Kilowatt
Symmetrical matchbox, I get ZERO RF in the shack. AND, these new
boxes have the added advantage that they may be used to match
Asymmetrical antennas as well. So, if Open Wire is your MAIN
goal, you intend to run only 100 or 200 watts, and you would like
to be able to use them with both Symmetrical and Asymmetrical
antennas, these boxes are far better than the solutions described
at the beginning of this thread.
NEW EXCEPTION
About a year ago Palstar announced another "Parallel"
(Symmetrical) matchbox that has two roller inductors ganged
together. I have not yet tried this one but intend to do so,
soon. It appears as if it really is truly Symmetrical. If
that's the case, then it won't be very good for Asymmetrical use
but I don't care - Asymmetrical matchboxes are "a dime a dozen".
The TEN-TEC MODEL 238?
Let me assure you that this is one of my favorite matchboxes
because it is better (by design) than the "Transmatch" in most
cases. This is the box I always recommend 100% of the time when
someone asks me what matchbox to choose for Asymmetrical
antennas. By design, it has lower loss than the Transmatch.
However, using it with openwire fed Symmetrical antennas is not a
great thing to do because it suffers from the same balancing
problems as the Transmatch - it just has less loss.
HOW DO I (YOU) KNOW?
For me: 40+ years or running openwire, living in about 25
different QTH's, on both sides of the Atlantic. In addition,
being a contest freak, I've run all kinds of openwire fed
antennas at about 100 different portable / field day QTH's. I
learned back in the early 60's from the late W1ICP, and continued
following the openwire cult closely. Lew wasn't the only
expert - there were many on both sides of the pond. I don't
consider myself an expert. I'm just an experienced user.
For you: It is very easy to determine if your openwire is working
symmetrically or not. If it is not, it is radiating (which is
what it is not supposed to do). Simply attach two small lamps,
fitted with short leads (about 1 inch on one side and 10 inches
on the other side) with alligator clips on each lead and then
clip them onto the openwire feedline. Try to keep them as
parallel and symmetrical as you can (i.e., use a piece of hard
plastic or Plexiglas and some tape). If one of the lamps lights
up significantly more than the other, your entire system is not
in balance and your feedline is radiating!
By the way, I used the capital "A" and "S" throughout this post
so that it would be clearer which one I meant - its easy to
confuse them otherwise.
73
Rick
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