Experimenting
with Chokes
<< changed 10th June 2020 to fix bad Al value for type 31 - redid graphs and tables>>
A
real antenna is never symmetrical, it is affected by nearby wiring,
pipework and pieces of metal or even conductive earth. If you use
coaxial cable then there are three paths for current. Through the
centre conductor, through the inside of the coax sheath and the
outside of the coax sheath. The two surfaces of the sheath are
independent of each other. Electricity at radio frequencies only
travels on the skin or surface of conductors – this is known as the
skin effect. (at 3.5MHz 2/3rds of the current is within a 1/30th
of a millimetre of the surface). We don’t want current on the
outside of our coax for two reasons. It makes it act as an aerial –
both for transmit and for receive and it brings RF into your shack.
When
you unintentionally transmit via your antenna feeder it may reach
nearby electronics and cause interference – obvious in the old days
because analogue televisions were so susceptible to it (TVI) but
nowadays the effect is much more subtle, burglar alarms may go off in
the next street, your coffee machine may decide to switch itself on
or it may even go unnoticed - your Wifi may drop to a snails pace or
your phone may drop from 3G to 2G. Even if unnoticed you should try
and stop it.
On
receive, interference from your home, and the homes on either side of
you may be coupled onto the outside sheath and these end up at the
input terminals of your receiver. Either by going up to the antenna
and back down the centre conductor or by lifting the earth potential
of your antenna socket at the back of your transceiver. You may not
notice this except to comment that you appear to have a noisy antenna
or you may hear harsh buzzing or whistles or bursts of noise at
specific areas of your tuning dial. Some internet lines coming into
the house are inherently noisy at HF (VDSL in particular) and modern
Wifi extenders that use the mains wiring, a lot of LED lighting and
even smart phone and tablet chargers can cause problems (modern phone
chargers use switching technology and may or may not have
anti-interference components fitted – a lot of Chinese units fit
the components to the units that are sent to get tested for EMC and
then the production line omits the components once mass production
starts. 99 times out of a 100 they get away with it.)
RF
in the shack may or may not get noticed – this year, but maybe you
change something next year and strange things happen – your
transceiver won’t switch from transmit to receive, your internet
goes slow (I had this problem and only noticed it when the spectrum
waterfall on websdr.org froze when I transmitted at 100W on 17m, it
was ok at 50W and I fixed it with a choke) or your lips get burnt
when you kiss your microphone (don’t kiss your microphone!)
The
current travelling down the outside of the coax sheath can be reduced
to nearly zero if you insert a choke or two. Usually called a common
mode choke(CMC) as it does not affect the differential current that
flows up the centre conductor and back down the inside of the coax
sheath.
If
you wind coaxial cable in a coil then the differential currents don’t
realise this as they are effectively shielded from the outside by the
thickness of the sheath but the outside of the sheath notices as it
is now having to pass through a coil – a coil that has some
inductance and hence reactance measured in Ohms. A 500 Ohm choke in
a 50 Ohm system will reduce the common mode currents to 10% of what
they were – current from antenna asymmetry elects to take the path
of least resistance (sic) and travels down the inside of the coax
instead (it acts as a balun but I don’t like the term balun or unun
– I try to use the terms chokes and transformers).
If
the current is induced onto the sheath from an external source then
the resistance causes the interference to dissipate as heat – it
doesn’t reach the shack (or the antenna)
However
a coil of coax is only effective at one frequency, it possesses
capacitance between its turns as well as inductance and this causes a
resonance – which is good at some frequencies but poor at others.
Resonance can give unpredictable results and is difficult to “tune”
to the correct frequency. The coil must not hang near a metal mast
either.
For
multiband performance, chokes should use ferrite material such as
toroidal rings or tubes. Ferrite beads are sometimes used but they
are very expensive and heavy if they are to be effective. I will
concentrate on ferrite toroids and not powdered iron toroids.
Powdered Iron should not be used as chokes, they (a) won’t have
enough inductance and (b) are low loss (high Q) and we want lots of
loss in a choke. High Q circuits make poor chokes as the resonance
peaks, whilst of a very useful high impedance is only present over a
narrow band of frequencies and that frequency moves according to
stray capacitance. Powdered Iron make good filter inductors and
narrow band transformers. Powdered Iron toroids are named with a T
such as T200-6 and they are nearly always painted, red, yellow or
glossy black etc.,
We
can use ferrites to make broadband transformers, where different
criteria apply. I am talking about chokes here.
Ferrite
Toroids are named with an FT prefix, such as FT240-43, this has an
outside diameter of 2.4” and is of type 43. Other common types
include -31, -52 and -61 as well as several in the 70’s which are
better suited to switching power supplies and frequencies of a few
hundred kilohertz. You occasionally see them used on top band
(1.8MHz).
Manufacturers
produce datasheets that can be confusing. They sometimes quote that a
given ferrite type is suitable for a certain frequency range and then
you see them being used at other frequencies. The reason is that a
ferrite can be used to build a transformer where we want low loss, or
a choke where we want high loss. Also sometimes we want resonance and
sometimes we don’t – or we want a wide band smeared resonance and
not one that is too peaky. A narrow peaked resonance does provide
very high attenuation but external reactances will detune and move
the peak.
Also
type #43 is really common and widely available – it is good enough
even if used on non-optimal frequencies. The more recent #31 is a
better choke at lower frequencies (say below 10MHz) and very nearly
as good at higher frequencies. Of course choking impedance is not the
only design criteria – we should worry about power handling and how
smooth the response is over the entire frequency range of interest.
I
found the excellent charts produced by Steve Hunt G3TXQ(SK) to be a
good starting point – he did a lot of choke measurements. My
studies took me to excellent documents by Jim Brown(K9YC) and some
really useful calculators and spreadsheets by Owen Duffy(VK1OD). You
should study their websites for further information.
Here
we see that 17 turns on a single FT240-43 should give 4k to 8k of
choking from topband to low down in the 10m band. But it is only
resistive from 4.5 MHz to 12 MHz (the thin black line shows this, but
we really need to know values)
On
the face of it the type #31 seems not as good as the #43, 12
turns on a single
core
FT240-31 gives
2k to 4k of choking over all bands. More subtly, and more importantly
is the black
line
– the choke appears as a resistance of a large portion of the
spectrum – the #31 is very broadly resonant and less prone to a
peaky resonance, this makes it better behaved if presented with a
nasty range of antenna reactances. In fact after further study I now
do not use the charts above as we really need to know the resistive
values (the details of the black lines above). The reactance, and
overall impedance are less important, since you may be unlucky enough
to combine a high choking reactance with an antenna which has the
same reactance but of the opposite sign. In which case the choke does
nothing. You really need resistance in a choke. I give my own graphs
and tables of resistance at the end of this article. I will add them
to my blog after this article is published.
Many
hams have used the charts above and ended up with 12 turns of coax
wound on an FT240-43 – a useful choke, certainly from 80m to 10m.
However a closer inspection of the datasheets and the writings of
others show that the relatively new type 31 (promoted since 2006) is
actually better below 10MHz, and nearly as good from 10 to 30MHz.
Again, for well behaved antennas either will do. For antennas well
off resonance and presenting reactive loads then the 31 may be
better.
Manufacturers
do not really produce products to suit radio ham bands – they are
in the business of producing ferrites for EMI suppression and also
for transformers and inductances for filters. Fair-Rite make ferrites
and Amidon distribute them. Also worth knowing is that for some
reason Amidon and others rebadge the ferrites so what Amidon calls an
FT240-31 has a Fair-Rite part number of 5931003801.
Here are what the sellers of these toroids actually recommend.
1Amidon(the
distributor) is quoting a different range from Fair-Rite the
manufacturer
2
These
figures are from Palomar Engineering, rest from Fair-rite and Amidon
sites
Not a bit of wonder we can have variable performance from 3 to 30MHz! A lot of what is currently done is because originally we only had type #43 – it was easy to get. So custom and practice was to use #43 for 3MHz to 30MHz. It would be better to use type #31 for chokes from 1 to 10MHz and #43 for chokes from 10MHz to 30MHz. If you are buying fresh pick #31. Jim Brown says “a single #31 choke is good over a 8:1 frequency range while a #43 choke is good over a 4 to 1 frequency range”.
Here
is another chart from Fair-Rite showing its recommendations for
chokes
For
ordinary inductors or transformers different rules apply, sometimes
we pick material for a transformer based on its complex (sic)
magnetic properties rather than its recommended frequency range; this
is because it is hard to make transformers with a very wide bandwidth
and choosing the number of turns and tweaking the transformers
becomes an art not a science. This article is only about chokes.
Jim
Brown (K9YC) has an excellent set of documents describing choke
design – some of it relating to choking RF out of audio cables but
he also covers RF choke design fairly rigorously. He sometimes
suggest using 5 toroids at a time – which is a bit expensive, Ian
GM3SEK has produced designs that are cheaper to make using type 31
tubes rather than toroids. I have FT240 toroids so that is what I
will use. I will buy some of the tubes that Ian recommends (in
http://www.ifwtech.co.uk/g3sek/in-prac/)
as well as some clip on ferrites the next time I go shopping for
Ferrites.
Toroids
are given values in their datasheet for Al and ui – the Al constant
is used for a quick and dirty calculation of inductance. However this
is of limited use with ferrites as the value is only accurate at very
low frequencies. Real ferrites have a reactance that may start out
inductive but at some stage it actually becomes capacitive! And it
has considerable resistance as we will see later. The initial
permeability ui is just that; an initial value measured at a
unrealistically low frequency– real ferrites have a permeability
that varies with frequency, and it is not even just a simple number,
it is best considered as a complex number, it has two parts µ’ and
the µ’’.
The
datasheets for the Ferrites show the µ’ and the µ’’ values –
these represent the reactive and resistive (losses) values if we are
making chokes. They actually chart the complex permeability but this
will approximate to reactance (µ’) and resistance (µ’’) –
related to the losses in the material. Losses are good in a choke
So
we can anticipate the resistive portions being quite low up to 3MHz
and dropping again after 30MHz For comparison here is the #31 graph
Note
these have slightly different scales. – at 2MHz the type 31 has 4
times better resistance choking.
I
wanted chokes specifically for a new End Fed Half Wave antenna which
covers all bands from 80 to 10m but needs good common mode chokes. I
resolved to design something with as much resistance as possible.
Current thinking (since 2006) has been to try for 5,000 Ohms. It used
to be thought 500 Ohms was enough, to choke off the more obvious RF
currents. However 5000 Ohms helps reduce receiver noise.
Rather
than use Steve’s charts I wanted to calculate my own. I needed
tables of µ’ and the µ’’ to build up my spreadsheet. There
are tables on the Fair-Rite website at 1MHz intervals which I
imported into Excel’s polynominal curvefitting. This allowed me to
get a table for the hambands. I then found that Owen Duffy had beat
me to it and used more sophisticated cubic spline curve fitting, he
had also re-measured some of the values so I decided to use his µ’
and the µ’’ values. Thanks Owen.
The
formula for R and X depends on the initial ui and the µ’’ and µ’
values at the frequency (f) of interest. You also need the number of
turns (N) and the published Al value for your toroid. Ferrite Al
values are only given +/- 20% so the graphs and tables below are only
roughly indicative. The formulas for Resistance and reactance are;
Rs =
(µ'' /µi
) * 2πf
N2
Al
Xs =
(µ' /µi
) * 2πf
N2
Al
There
is one other wrinkle. Real chokes suffer from stray capacitance
which alters these equations. I copied an idea of Owen Duffy’s
website and re-calculated the resistances with a stray capacitance of
1.3pF, I will be experimenting with different values when I try
measuring some chokes.
here
are tables of µ’’ and µ’ – from
https://www.owenduffy.net/calc/toroid4.htm
You
can find the manufacturer’s data at
(there
is a link to a . CSV file halfway down the page )
Using
the table of µ’’ and µ’ above, I built a spreadsheet and from
it produced this graph and table. The spreadsheet on my blog
calculates X and Z as well (but those values are irrelevant –
Resistance is what matters, at least in demanding applications
Above
is a table of resistance for differing turns of RG58 wound on a
FT240-31 , I like the look of 15 turns, for 80m to 20m.
Here
is the data for type 43;
Below is a diagram of a choke
-
this one has 8 turns including a crossover winding known as a Reisert
Turn.
This
allows the input and output to be on opposite sides of the choke and
allows more turns. It may reduce capacitance slightly though this is
very hard to measure. It seems to make no difference but is easier to
wind
In Summary
Current
thinking is that common mode chokes need high resistance, high
impedance (reactance) is not a reliable way of choking badly behaving
antennas.
A
single choke will not cover all bands. An air-spaced choke may sort
of work on one band.
A
lot of commercially available chokes are old fashion designs and poor
performers in certain situations. High Resistance chokes always
perform well.
The
tables and graphs above are new and should be useful to anyone
wanting to make a common mode choke. – at least from FT240 types of
ferrite. The spreadsheet on my blog can be used for smaller
ferrites, although getting RG58 on them is tricky.
I
will attempt to measure my chokes in a future article – or try my
blog http://MI5AFL.Blogspot.com.
Note, measuring high impedance chokes is actually very difficult –
I have already fallen foul of my scope probe’s built in capacitance
and had bother with heavyweight manipulations of S11 and S21 data in
my VNA. (and getting bad data from the internet - http://toroids.info has the wrong Al value for a FT240-31, http://amidoncorp.com has the right value.
My
study of this area has depended on the work of others; in particular
- Owen Duffy VK1OD at http://owenduffy.net
- Jim Brown, K9YC (http://k9yc.com/publish.htm
- Steve Hunt, G3TXQ (SK) (http://www.karinya.net/g3txq/chokes/ )
- Steve wrote an excellent article for Radcom Plus in May 2015 – I wish I had read it early in my research!
- Chuck Counselman, W1HIS, (http://www.yccc.org/Articles/W1HIS/ )
- Ian White,GM3SEK (http://www.ifwtech.co.uk/g3sek/in-prac/ ) & also (https://gm3sek.com/ )
- Rick DJ0IP (https://www.dj0ip.de/rf-cmc-chokes/ )
If you go to the "pages" tab of this blog (http://mi5afl.blogspot.com/p/blog-page_15.html )you will get a link to various files I hae written - the txt of this article as a pdf and three spreadsheets, Ask me if you want me to explain anything.
(mailto: MI5AFL@ARRL.Net )
