Several folks wrote and asked.
Typically, how many -- on which bands? Thanks.
And.....
How many beads do you use??? How many on each band???
I got the clue on what beads to use and how many from W0UN.
Both W5KFT and I emailed John and asked him the
above questions and more - so here is his message back.
(I knew I was saving this for some reason.)
From: John Brosnahan <broz@csn.net>
Subject: BALUNS
>I have a question - I asked you awhile back about what
>torroid to use for a choke on rg-213 and I bought and used
>the ones you told me to get - they have worked great as
>far as I can tell. ( how do you tell?)
You need to be able to measure the RF current flow
on the shield to know if they are doing the job.
There are currents probles--MFJ used to make
one and maybe still does. But there are other
factors involved, like losses that show up as heat.
RG-213 is not too heat resistent, so the net
effect is melted coax. Something that happened
to me using a DX Engineering balun on 10M.
Near the end of a contest the coax shorted out
while running 1500 out from an 87A in high
duty cycle CQing. This caused me to investigate
the choke balun a little more. I built a test setup
that terminates the coax in two, series-connected
25 ohm resistors, and grounds the coax on the
TX side of the choke. With short leads on the
resistors the SWR should look like 1-1 whether
or not the center tap of the two resistors is grounded
or not. If the ferrite cores are not doing their job
at all then the SWR will look like 2-1 since one
resistor is shorted out by the center tap to ground
and the coax shield connection to ground.
I run this test on an HP-4195 network analyzer
that I have here at home--every ham station
should have one of these $25,000 beauties!
>Is there anyway to "know" how many to use for a given freq/band?
The above test tells you how many you need.
I just install a few beads and sweep the frequency
band of interest and see what happens to the
SWR. As long as it stays low everything is
OK. As it starts to rise then the choking action
is inadequate.
>Can you use too many? Why?
The only limit to the number is financial. But the
ones further from the load work less and less
until the ones 7 or 8 removed from the load
are really doing nothing. This is not a simple
series inductance problem where the currents
are equally divided. The losses reduce the
currents to a point where any more than just
enough are wasted. No harm will come, other
than to your budget.
>
>What is the diffrence in using 43 or 77 or ??
On paper 77 material looks better for HF. It has
more mu and looks like it will require fewer beads,
but this is what the DX Engineering balun used
(until I convinced them to change to 43) and what
melted my coax. The 43s will require a few more
beads but it will not smoke your coax. BTW
The reason my DX Engineering balun smoked and
other users had no problems is that I forced the
balun to do its job by grounding the center of
the hairpin for DC (noise reduction and lightning
protection).
<These questions came from W5KFT>>>>
>John, Geo WB5VZL and I were talking after the contest this past weekend
>about the
>baluns that we made up per your specs. They have all worked great and
>performed
>without a hitch. Thanks for the help and input on them.
>
>We were talking about using the #43 ferrite beads to make additional baluns
and
>wondered about making a single style or type of balun that could be used on
all
>bands 160-10 or even 80-10. George seemed to remember that the number of #43
>beads used was critical for the individual frequency or band. I was
>wondering if
>we could just make up a single type using XXXX number of the cores and have
>them
>available to work on anything. Would appreciate your input on this.
You can make a general purpose balun for 160-10 with no problems
except that if it is only used on the higher bands you are wasting you
money on some of the cores. I don't recall my exact test results and
will have to redo and expand them for this book I am working on, but
it seems like 5 beads will work for 20-6 meters. Probably use 6 or 7 for
40-6 meters. I think 8 is adequate for 80-6 and I would probably go with
10 for 160-6. They don't "fail" below a particular frequency, they just
get less and less effective. So you could use a 5 or 6 bead balun on
80 meters but it wouldn't reduce the shield currents as much as an
8 bead one would--but it is much better than nothing! This is why
the current style balun made of chokes is so great. There is nothing
critical about them--you can't use too many and if you use too few they
just become less effective.
>
>Also, I was curious as to the use of the #43 core instead of the #77 core.
The
>#77 has better specs in the less than 30mhz range, however I am not sure if
>it is
>enough better to make appreciable difference. You might comment on this.
>
HEAT (as I mentioned in the Geo part of the note) is the only reason. If you
are a exciter only type of guy then the 77s will work even better than the
43s. But if you like to warm the ionosphere(but not your coax) you had
better use the 43s.
(Gee, I gotta stop helping these guys with better propagation to
make their signals even better. What am I thinking about!)
73 John W0UN
John Brosnahan
La Salle Research Corp 24115 WCR 40 La Salle, CO 80645 USA
voice 970-284-6602 fax 970-284-0979 email broz@csn.net
So there you go.......
--
George Fremin III
Austin, Texas C.K.U. "It is hard to be in one place at the same time."
K5TR -- Overheard at the TR-Log booth
512/416-7010 at the Dayton Hamvention
geoiii@bga.com
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