Hi Jim,
Happy Holidays!
Hope you are not affected by the fire.
You are an expert on theory but you have often said that you do not like
openwire and openwire fed dipoles.
I am assuming you have not used them very much for that reason.
I doubt all of this theory will help Bob in his immediate quest to find a balun
for his matchbox, though it is certainly good advice to keep learning.
Having used openwire-fed dipoles for about 50 years, here is some of my
practical experience:
Erect a dipole of any convenient length.
Feed it with openwire (I prefer homebrew ladderline over WIREMAN windowline).
Using an ASYmmetrical matchbox, such as the Model 238-x, or any of the
multitude of T-network matchboxes, insert a balun between the feedline and its
coaxial output.
Key-down 1kW of power (after first matching, of course), and then simply touch
the PL-259 on the matchbox side of the balun... or the case of the matchbox,
whatever.
Do this for each and every type of balun you can build, borrow, or buy.
You soon learn what works and what does not. You 'FEEL' the difference.
OR.. after 15 years of running this with OMNI V/VI/VI+, you soon see which ones
trip the circuit breaker and which ones don't.
I blew a small fortune on high-power commercial baluns which not only did now
work, but often burned up.
Eventually I quit buying, read RF-Ham, studied Steve's Rainbow Chart on RF
Chokes and began building my own.
Since then I have never burned up another balun.
What works (as tuner balun):
A 1:1 Guanella balun.
What kinda works:
A very good 1:1 Maxwell balun. Depends on how well the balance is on the
feedline/antenna. If poor, it won't work.
A very good dual-core 4:1 balun, if and only if you have high impedance at the
matchbox end of the feedline and a good balance in the antenna system.
What never works:
A voltage balun, whether 4:1 or 1:1 - except of course at low power.
A 4:1 Guanella balun wound on a single core. This one has no CMI and should
never be used in RF antennas.
(Definition of "works" is not just the ability to make QSOs but also the
ability to do so without getting RF burns.)
Bob asked what kind of balun to use with his matchbox.
I have given him clear instructions on how to build one that I 'KNOW' works
because I've use it often.
It will only cost about $15 to $30, depending on how much power he plans to run.
That's not theory. It's experience.
Steve suggested I should use #52 instead of #43.
It is probably a good suggestion. I will try this... in the field, not in the
lab.
I spend a lot more time on the low bands than on the high bands and most of the
time I have used a beam on the high bands anyway, so I have always placed more
focus on 80/40. Thus my choice of #43.
One thing we should always do when using openwire is twist the feedline about 1
turn per meter/yard, being careful not to short the two lines together if the
feedline is not of insulated wire. This helps reduce common mode current (CMC)
pickup on the feedline. It won't help if the imbalance is in the antenna
itself, but now days the feedline almost always picks up CMC from consumer
products in our neighbourhood so we should always twist the feedline.
In 2013 I spent a few hundred hours running CMC tests on dipoles (in the field,
not in the lab).
I only took measurements on 40m and above. About 1000 measurements. All well
documented.
I did this on standard dipoles, as well as OCF-dipoles.
I tested several kinds of baluns.
At the end of the test I concluded there was more CMC on the feedline on 40m
than any of the other bands, but with good baluns or combination of bad balun
and good choke, I was able to reduce the CMC to noise level on my RF Ammeter.
I also found more CMC on the feedline on odd-harmonic bands than on
even-harmonic bands.
In 2015 I repeated the test using 80m antennas. The antenna was at the same
physical height (but only half the electrical height).
All of the baluns that had worked quite well in the 40m dipole test failed
miserably on 80m.
There was no way to master this with any kind of transformer balun, except with
the addition of a good Guanella choke.
At the end of the test, I concluded that the electrical height above ground
played a major role in the amount of CMC on the feedline.
I also concluded that with antennas close to the ground, as most of us city
dwellers have, more CMI is required and thus chose #43 for my preferred balun
and choke mix. If I were only building 20/15/10, of course I would choose
another mix.
For baluns at the feedpoint of a low band dipole, I use #43; if I need 200
Ohms, then I place a 4:1 Ruthroff on #61 core between the 1:1 Guanella and the
antenna. Steve has me curious to try #52. I did try it in a 4:1 dual-core
Guanella and it failed miserably in an 80m OCFD. I have yet to build a 1:1
choke or balun with it.
I know you are no fan of the OCFD antenna, Jim. Neither is Dean Straw and I
respect both of you guys quite a lot.
But I do know how to measure antennas, how to use them in contests and analyse
my results.
I can show you my field measurements on real antennas and show you just EXACTLY
what affect common mode current has on the 'shape' of the SWR curve as well as
its ultimate level of SWR. It can cause significant skewing of the curve and
even double-dip in SWR.
One of my OCFD antennas has been under review by a local German magazine for
about 3 months now. It uses a hybrid balun that I designed.
The antenna was used at full legal power in CQWW SSB and CW contests. The test
review will be printed in the February or March issue of the magazine.
I am very pleased with the tester's ongoing feedback.
BTW, he also has a PHD in EE and is chief technical editor of the mag.
So yes, there is theory, but the rubber meets the road ON-THE-AIR, not in the
lab and not in the text book.
Bob asked a simple question.
I answered it.
Steve answered it too.
Jim, tnx for the data-point on cross-winding producing less capacitance than
continuous winding.
I failed to pick up on that point, but have always used cross-winding anyway,
just for its mechanical benefit.
Happy New Year!
73,
Rick, DJ0IP
(Nr. Frankfurt, Germany)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Jim Brown
Sent: 28 December 2017 19:11
To: tentec@contesting.com
Subject: Re: [TenTec] 238 "Tuner" Balun
On 12/28/2017 2:46 AM, Rick@dj0ip.de wrote:
> The problem that we are fighting is common mode current
One fact that is being missed in this discussion is that the transmission line
as a common mode element of the antenna system, and, like any antenna, current
varies along the length of every element based on the boundary conditions. The
most common "boundary conditions" are the open circuits at the ends of
conductors that force the current to near zero (limited by capacitance at the
ends to free space and the antenna's surroundings). When we add a common mode
choke with a high choking Z, we force current to near zero, creating another
boundary condition AT THE POINT WHERE THE CHOKE IS PLACED. A choke at the
antenna feedpoint forces near zero current there, but the feedline may still
connected (as common mode element) via the tuner to ground. A choke at the
tuner can another near-zero condition.
With a choke at the tuner and no choke at the feedpoint, the feedline is still
part of the antenna, and is vulnerable to picking up noise on receive. This can
be clearly seen (and studied) by a simple NEC model of the antenna that
includes the feedline and the choke(s) using the Load function in NEC. My
tutorial on chokes shows how to determine the parallel RLC equivalent circuit
of a choke based on measurements of its impedance vs frequency. NEC computes
currents vs length for every element of the antenna, and can display them
graphically and as table.
> In all of the tests that I have run on dipoles and OCFD antennas, the lower
> bands, being electrically closer to the ground, have significantly more
> problem with CMC than the higher bands.
> Therefore I chose to run with #43 when 80m is required.
#31 is several dB better on 80M and a lot better on 160M.
> I agree the point of using bifilar.
If the device in question is operating as a transformer (that is, two windings
coupled magnetically but with no DC connection), from a common mode point of
view, that's a BAD idea, because it increases the capacitance between turns,
and reducing the common mode impedance. Without that capacitance, they have the
same effect as a good common mode choke. Transformers are fundamentally
different from common mode chokes in that the core carries the entire signal.
For receiving only, loss in the core may not matter, but for transmitting, loss
can burn a lot of transmit power. At low power levels, this simply makes the
signal weaker. At high power levels, losses in the core are likely to cause it
to self-destruct.
It should be obvious that cores having low loss at the frequency of interest
should be used for transformers. Likewise, the core should provide a magnetic
path (dimensions) capable of handling the flux density associated with the
power level. Fair-Rite #61 is quite efficient below 10 MHz, with losses
beginning to increase above that frequency. Fair-Rite #67 handles more power
to higher frequencies.
N6RK, a pretty smart EE retired from HP's instrumentation group, uses
#67 for high power transformers in his antenna system.
The only advantage of a bifilar-wound transformer is slightly greater mutual
impedance between windings, which increases efficiency and power handling.
Separating the windings (that is, placing them on opposite sides of the
core) reduces (greatly) the capacitance between windings, increasing its common
mode impedance.
73, Jim K9YC
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