Joe,
Nice write-up. Thanks for taking the time to post your experience and
thoughts.
>It uses PTFE (instead of teflon) dielectric.
I do not understand this statement since PTFE is marketed by DuPont as
Teflon.
73, Keith NM5G
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Joe Reisert, W1JR
Sent: Friday, July 27, 2007 3:04 PM
To: n7ka@comcast.net; TowerTalk
Subject: Re: [TowerTalk] Current choke beads
Hi Arnie,
I guess there has been a discussion on baluns going on. Let me add a few
comments on the subject.
First off, there are alternatives or choices on transmission line types used
in coax baluns. RG58 is fine for low power (100-200 Watts) but for higher
power, you need larger and higher power coax such as
RG213 or teflon dielectric coax. When I wrote the first current amateur
balun article (Simple and efficient broadband balun by W1JR in Ham Radio
Magazine, September 1978) I used RG141 since it was the only high power coax
available at that time that could be fairly easily wound on a 2.4" OD
toroid. However, for the last 15 or so years I have been using the newer
RG303 coax since it is less costly (than RG141), smaller in outside
diameter, easier to use especially on toroids and will handle full amateur
power levels.
In 1989 I helped develop a low cost PTFE coax for the Cushcraft Corporation.
They needed a cost effective coax for installing mobile antennas in
automobiles that wouldn't melt if placed near catalytic converters etc.
Cushcraft marketed it as "Ultra Link" coax. Ultra Link, as I recall, is
about the diameter of RG58. It uses PTFE (instead of teflon) dielectric. The
dielectric is first covered by a foil shield which in turn is covered by a
regular braided shield but with a polyethylene outer jacket. It can easily
handle amateur high power levels. Furthermore, Ultra Link coax should be
widely available through distributors of mobile products such as Tessco and
the like and should be very reasonable in price versus RG141 or RG303.
The W1JR current balun was mainly intended for 20-10 Meters and only had 12
turns of coax but did have sufficient impedance to work from 80 meters
through 6 meters. I later tried stacking (placing two toroid on top of each
other) and that balun with 12 turns of RG303 coax has sufficient impedance
to go down to 160 meters. I see that Array Solutions (WX0B) markets both
types of the W1JR balun. I am in no way involved financially with this
design although I tested them for Jay and they worked great. Jay used a neat
layout by closely and tightly wrapping the coax which improves the upper
frequency limits.
One of the stacked (2 toroid) baluns using 2.4" OD 61 material is used on my
160/80 meter G5RV dipole and easily handles 1500 Watts even with poor VSWR
(a characteristic of the G5RV dipole). The beauty of this type of toroid
balun is that it is compact, has a good high frequency response, has low
loss and the cross over winding keeps the input and output separate
simplifying isolation and connection to the outside world. The W1JR balun is
arguably the widest distributed balun to date having been used in every R5,
R6, R7and R8 Cushcraft antenna with over 100,000 antennas sold! Cushcraft
used RG303 coax for that balun. Cushcraft uses a 4:1 balun with Ultra Link
coax in the 17B2 and 13B2 VHF high power Yagis.
Soon after the W1JR balun article was published, several individuals
published articles on the same balun but adding a tertiary wire. This in
effect made it a voltage balun, a poor choice indeed. Shortly thereafter,
W7EL wrote a paper (see ARRL Antenna Compendium, Volume
1) to correct the errors and pointed out the differences between the current
and voltage type baluns. Thus a great mystery was solved ands two new terms
(current baluns and voltage baluns) became widely used in amateur radio.
Since then, the W2DU in line coax balun has become popular due to its ease
in construction. It uses 50 to 100 ferrite beads with type 73 material. It
turns out that this ferrite material has a high loss and low resistivity in
the HF frequency range. The beads W2DU chose had a small inside and outside
diameter so the outer jacket of RG141 coax had to be removed to insert the
coax into the bead. Due to the losses and small size of these beads, the
W2DU balun can get quite hot with high power and in one documented case
(Common Mode Chokes by W1HIS) started an attic fire!
More recent W2DU type balun designs have emerged (Exploring the 1:1 Current
Choke Balun by W0IYH in June 1997 QEX). Basically W0IYH used larger diameter
and longer length ferrite beads with lower loss type
43 ferrite material. This approach seems to improve the aforementioned
problems and less chance of IMD. It uses a 36" string of ferrite beads! For
some time now, I had also been using 30 of a similar bead (see below) for my
HF line decouplers.
I used similar ferrite beads to W0IYH in my commercial antennas starting in
1992. The beads are 1.125" long, 0.562" OD with a .250"
ID with Fair-Rite 43 material. Three of these beads in a string covered with
a heat shrink tubing with a flooding agent worked great for 1:1 baluns from
100 MHz through 1000 MHz. For my commercial antennas in the 40-100 MHz
spectrum, we used 5 of these beads. At 50 MHz 5 beads in a string measured
over 1000 Ohms and over 500 Ohms at
28 MHz on a Hewlett Packard 8753C network analyzer. I believe that Amidon
part number FB-43-5621 is a very available equivalent. RG58 was used for 100
Watts and less with RG303 for higher power antennas.
This was obviously an economical choice for ease in manufacturing.
Some amateurs don't want to use the smaller RG303 coax and .562" OD beads
since it requires another set of connectors to be compatible with RG213.
Therefore, I also tested some 1" OD ferrite beads about 1.125" long with 43
material and a 0.500" ID. Five (5) of these beads in a string measured about
500 Ohms at 28 MHz and 900 Ohms at 50 MHz and would be OK if threaded on to
RG213 coax before connector installation. These beads should be equivalent
to Amidon part number FB-43-1020.
Recently type 31 material has become available from Fair-Rite. K9YC has
mentioned this in his paper on "Understanding and Solving RF Interference
Problems." This material has a higher permeability (1500 versus 850 for 43
material) and thus probably greater loss but should be better at the lower
frequencies than 43 material. I recently acquired a batch of 1" OD 31
material beads similar to the ones just mentioned. Five (5) of them in a
string measured about 750 Ohms at 28 MHz and 1,000 Ohms at 50 MHz. I don't
know if they are available yet from the typical amateur suppliers.
Finally, several have mentioned the use of clamp on beads. These are OK but
must be properly chosen for size, material, permeability etc.
I don't have any here to test. I would be worried that they stay closed and
that the outside wrapper keep them tightly bound.
Obviously clamp on beads are great if added to an already installed antenna
etc. Exposure to sunlight may deteriorate the clamping shell so attention
must be paid to installation. Also, they are probably more expensive and
require some special mounting.
So, just as amateurs chose their ideal antenna based on their individual
needs, cost, performance, XYL etc. etc. (and every ones choice is
different!), the same goes for baluns. Hopefully the above information will
be informative and complete enough to assist you in choosing the perfect fit
for your application.
73,
Joe, W1JR
[snip]
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