I have been looking at the MFJ-1622 antenna. Part of the instructions
for antenna set up, which you can find here....
http://www.mfjenterprises.com/man/pdf/MFJ-1622.pdf
say to roll up about 30 feet of coax into a 12" diameter coil. This
they label as a choke. This just seems like a "duct tape" solution to
me. Is there a better way to accomplish the same thing? Would a few
ferrite beads on the coax do the trick or do I just have no idea what I
am talking about?

P.S. please check out my new Ham Radio Repeater Database web site.
http://hrrdb.com


--
Chris W
KE5GIX

"Protect your digital freedom and privacy, eliminate DRM,
learn more at http://www.defectivebydesign.org/what_is_drm"

Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
One stop wish list for any gift,
from anywhere, for any occasion!
http://thewishzone.com

Chris W wrote:
I have been looking at the MFJ-1622 antenna. Part of the instructions
for antenna set up, which you can find here....
http://www.mfjenterprises.com/man/pdf/MFJ-1622.pdf
say to roll up about 30 feet of coax into a 12" diameter coil. This
they label as a choke. This just seems like a "duct tape" solution to
me. Is there a better way to accomplish the same thing? Would a few
ferrite beads on the coax do the trick or do I just have no idea what I
am talking about?

P.S. please check out my new Ham Radio Repeater Database web site.
http://hrrdb.com


Chris;

Your best choke is about 30 feet of coax rolled up into a 12" coil.

This is not a duct tape solution but one that has been used for
decades. It is the cheapest and easiest choke you can use. Others are
better but not easier.

Dave WD9BDZ

David G. Nagel wrote:
Chris W wrote:
I have been looking at the MFJ-1622 antenna. Part of the
instructions for antenna set up, which you can find here....
http://www.mfjenterprises.com/man/pdf/MFJ-1622.pdf
say to roll up about 30 feet of coax into a 12" diameter coil. This
they label as a choke. This just seems like a "duct tape" solution to
me. Is there a better way to accomplish the same thing? Would a few
ferrite beads on the coax do the trick or do I just have no idea what
I am talking about?
P.S. please check out my new Ham Radio Repeater Database web site.
http://hrrdb.com

Chris;

Your best choke is about 30 feet of coax rolled up into a 12" coil.

This is not a duct tape solution but one that has been used for
decades. It is the cheapest and easiest choke you can use. Others are
better but not easier.

Agreed.

The coil only *looks* crude - especially if you actually do use duct
tape. But looks can be deceptive.

With the right length of coax, number of turns and method of winding,
such a simple coil resonates with its own self-capacitance and is
unbeatable as a single-band choke. With about the same amount of care, a
slightly different coil can be selected to give an adequate common-mode
impedance across several bands. The design data has been in the ARRL
Antenna Handbook for many years, and there's more data about
solenoid-wound chokes on the web.

The really crude solution is to use just "a few ferrite beads" without
giving any thought to which ferrite beads, or how many, or why.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"David G. Nagel" wrote in message
...
Chris W wrote:
I have been looking at the MFJ-1622 antenna. Part of the instructions
for antenna set up, which you can find here....
http://www.mfjenterprises.com/man/pdf/MFJ-1622.pdf
say to roll up about 30 feet of coax into a 12" diameter coil. This they
label as a choke. This just seems like a "duct tape" solution to me. Is
there a better way to accomplish the same thing? Would a few ferrite
beads on the coax do the trick or do I just have no idea what I am
talking about?

P.S. please check out my new Ham Radio Repeater Database web site.
http://hrrdb.com


Chris;

Your best choke is about 30 feet of coax rolled up into a 12" coil.

This is not a duct tape solution but one that has been used for decades.
It is the cheapest and easiest choke you can use. Others are better but
not easier.

Dave WD9BDZ

I agree that the coiled up coax is a good way to go. As far as cheapest ,
with the price of good rg-8 size coax the cost is now about the same as the
ready made chokes with the beads on them. If you factor in the cost of a
ready made center insulator for a dipole the cost could be more. Also 30
feet of coax can be another heavy weight to support.

Chris W wrote:
I have been looking at the MFJ-1622 antenna. Part of the instructions
for antenna set up, which you can find here....
http://www.mfjenterprises.com/man/pdf/MFJ-1622.pdf
say to roll up about 30 feet of coax into a 12" diameter coil. This
they label as a choke. This just seems like a "duct tape" solution to
me. Is there a better way to accomplish the same thing? Would a few
ferrite beads on the coax do the trick or do I just have no idea what I
am talking about?

I have an EXCEL calculator which indicates that particular
coiled coax choke should have maximum effectiveness around
~8 MHz. So it probably does a reasonable job on 40m-20m.
Unfortunately, it will reach 1/2WL self resonance somewhere
around ~16 MHz and thus be essentially useless 17m-2m because
of self-capacitance shorting out the choking impedance. Anyone
who says a particular coiled coax choke will cover 40m-2m is
mistaken. However, the stress on the choke is reduced because
the MFJ antenna appears to be resonant, i.e. a low feedpoint
impedance.

Here are some actual measurements.

http://www.k1ttt.net/technote/airbalun.html

In a nutshell, the coiled coax choke works well over a
frequency range of 2:1 (conservative) or 3:1 (liberal).
The beads in the above measurements work well over a
frequency range of 4:1. It's hard to beat stacked ferrite
toroids of differing materials as a foundation for an
excellent all-HF-band 1:1 choke-balun. I know of no
single balun that can cover 7 MHz to 148 MHz well.
1/2WL self-resonance defeats the choking action when
trying to cover 4+ octaves.

I recently used an MFJ-259B to measure the choking
impedance of 8 turns of RG-400 on a 5.35" form. The
choking impedance dropped below 650 ohms at 4.63 MHz
on the low end and 28.1 MHz on the high end. 650 ohms
is not enough choking impedance for all-HF-band ladder-
line fed dipoles. Assuming SWR ranging up to 18:1, which
is the case for my all-HF-band 130 ft. dipole, the choke-
balun may encounter as much as 8000 ohms looking into
the Z0=450 ohm ladder-line.
--
73, Cecil http://www.w5dxp.com

David G. Nagel wrote:
Your best choke is about 30 feet of coax rolled up into a 12" coil.

Seems to me, the best choke(s) would be two chokes, one designed
for 75m-15m and the other designed for 12m-2m. I cannot bring
myself to believe that a coiled up piece of coax could ever be
an effective choke over a 40:1 frequency range.
--
73, Cecil http://www.w5dxp.com

Ian White GM3SEK wrote:
With the right length of coax, number of turns and method of winding,
such a simple coil resonates with its own self-capacitance and is
unbeatable as a single-band choke.

Unfortunately, at approximately double that single-band
frequency, the choke is 1/2WL self-resonant and essentially
useless. Someone on QRZ.com quoted the 2006 ARRL Handbook
as saying the following: "A flat coil (like a coil of rope)
shows a broad resonance that easily covers three octaves,
making it reasonably effective over the entire HF range."

Such a coil is certainly NOT "reasonably effective over
the entire HF range" when used on a typical ladder-line
fed all-HF-band dipole. With a 50 ohm 75m dipole, the
SWR on 450 ohm ladder-line will be 9:1. Worst case, the
choke will see 9*450 = 4050 ohms. An effective choke of
five times that value would be 20K ohms or about 850 uH
of inductance. What do you reckon would be the 1/2WL self-
resonant frequency of an 850 uH coil of coax?
--
73, Cecil http://www.w5dxp.com

It's hard to beat stacked ferrite
toroids of differing materials as a foundation for an
excellent all-HF-band 1:1 choke-balun. I know of no
single balun that can cover 7 MHz to 148 MHz well.
1/2WL self-resonance defeats the choking action when
trying to cover 4+ octaves.


Can you direct me to more good information (as in NOT sales literature)
about ferrites and toroids as choke baluns?

The ARRL Handbook mentions the use of 1:1 baluns on p. 21.16
(Transmission Lines):
For 3.5 to 30 MHz, the recommended choke is 10 ft and 7 turns using
RG-8, 58, 59,8X and 213 (I don't think I've even heard of the last two)

W2DU has a 50 bead balun over RG-303.

As you may recall, I am using a homemade G5RV and I simply connected
RG-8 directly to the ladderline portion. I can see this connection
right out my window (2nd floor shack). I am unaware of any significant
problems as it is. Should I consider using one of these methods and
re-do my antenna?

Thanks for being one of the experts that doesn't talk down to the "rest
of us".

John
AB8O

jawod wrote:
As you may recall, I am using a homemade G5RV and I simply connected
RG-8 directly to the ladderline portion. I can see this connection
right out my window (2nd floor shack). I am unaware of any significant
problems as it is. Should I consider using one of these methods and
re-do my antenna?

Are you having problems with RF-in-the-shack? If not,
your feedline radiation may be doing you more good than
harm. I'm a purist. I install a very good balun/choke at
every BALanced to UNbalanced junction. Some people are
good and some people are lucky. If you have a current
maximum common-mode loop in the shack, you may not even
notice it. It's those voltage maximums that tend to bite.

Thanks for being one of the experts that doesn't talk down to the "rest
of us".

Heh-heh, I only talk down to the gurus for the purpose of
creating a crossed transactional analysis. :-)
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Ian White GM3SEK wrote:
With the right length of coax, number of turns and method of winding,
such a simple coil resonates with its own self-capacitance and is
unbeatable as a single-band choke.

Cecil makes several different points here.

Unfortunately, at approximately double that single-band
frequency, the choke is 1/2WL self-resonant and essentially
useless.

It's a persistent ham myth that an RF choke has specially good
properties when the total length of wire is a quarter-wavelength, and
specially bad properties at twice that frequency. When the wire is wound
into any kind of coil, neither of those claims is true (except maybe by
some rare coincidence).

The choke acts essentially as a parallel-tuned circuit, with its
inductance tuned by its own self-capacitance. There will be a series
resonance at some higher frequency, but not at twice the
parallel-resonant frequency (except, again, perhaps by a rare
coincidence).

We don't have any performance data for the particular choke recommended
by MFJ (and I'll return to that later) but the ARRL Antenna Book does
have some measured data on two chokes, both made from 8 turns of RG213
wound into a coil of 6-5/8in diameter. The first choke is a bunched
flat coil, and the second is a solenoid. I took the time to import the
data (20th Edition, Table 3) into Excel and analyse it carefully.

The bunched choke has a sharp parallel resonance at about 6MHz, with a
maximum |Z| value of about 8500 ohms (could be higher because the data
are in 1MHz steps). The total winding length at this frequency is about
0.085 wavelengths - a very long way from a quarter-wave. At other
frequencies up to about 30MHz, the choke behaves like a classic
parallel-tuned circuit: the phase angle of Z is almost purely inductive
(+90deg) below the resonant frequency, and almost purely capacitive
(-90deg) above it.

There is NO series resonance at twice the parallel-resonant frequency -
that would be about 12MHz, and nothing at all "special" is happening
there. At 18MHz, where the total winding length is 0.25 wavelengths,
there is a very small wobble in the data, but nothing more.

The series resonance, where the phase angle flips from negative to
positive again, is at 31.5MHz, which is totally unrelated to any of the
other frequencies above. The winding length is 0.5 wavelengths at 35MHz
(where the data runs out) but again nothing "special" is happening
there.

Thus there is no evidence whatever for the myth of the "resonant length
of wire in a choke".

Turning now to the solenoid-wound choke, the different method of winding
has increased the parallel resonance of the same length of cable from
6MHz to 9MHz. This is consistent with simple L-C behaviour, and with the
solenoid having less distributed capacitance than the bunched winding.

Once again, this choke behaves almost entirely as a parallel-tuned
circuit. There are slightly larger wobbles in the data at the
frequencies where the total winding lengths are a quarter-wave and a
half-wave, but these "transmission-line" effects are still very minor,
and completely dominated by the simple L-C behaviour.

The other difference is that the solenoid-wound choke has a much higher
parallel-resonant impedance - almost 16,000 ohms, compared with 8500
ohms for the bunched choke. Because of its higher resonant frequency,
the solenoid choke would be useful (Z 1000 ohms) from 7MHz up to at
least 18MHz, covering at least four amateur bands, while the bunched
choke would only hit 7MHz.


Someone on QRZ.com quoted the 2006 ARRL Handbook
as saying the following: "A flat coil (like a coil of rope)
shows a broad resonance that easily covers three octaves,
making it reasonably effective over the entire HF range."

That's in my 2005 ARRL Handbook also... but the claim of a "broad
resonance" is not supported by the more detailed information in the ARRL
Antenna Book. On the contrary, the parallel resonance is rather sharp.
It would be fair to claim that a carefully proportioned coil balun with
a resonance around 10-14MHz can have a usefully high impedance as low as
3.5MHz and as high as 30MHz... but the impedance won't be spectacular at
either end of that range, almost certainly less than 500 ohms. So I'd
agree that those claims need to be revisited.


Such a coil is certainly NOT "reasonably effective over
the entire HF range" when used on a typical ladder-line
fed all-HF-band dipole. With a 50 ohm 75m dipole, the
SWR on 450 ohm ladder-line will be 9:1. Worst case, the
choke will see 9*450 = 4050 ohms. An effective choke of
five times that value would be 20K ohms or about 850 uH
of inductance.
What do you reckon would be the 1/2WL self-
resonant frequency of an 850 uH coil of coax?

As shown above, "1/2wl self resonance" ceases to be a valid concept once
a length of wire is wound into a coil.... but I do see the point you're
getting at: such a large inductor would have too much self-capacitance
to be workable solution. If you were absolutely determined to tackle
this extreme problem head-on, the best choke balun would be one that
exploits its self-capacitance to give a parallel resonance on the
operating frequency. However, a far better solution to this problem
would be to avoid the extreme impedance by changing the feedline length.

As for the choke recommended for the MFJ-1622, that comes out at 12
turns of 9in diameter, which seems to be aimed at that antenna's lowest
operating bands. On the higher bands, the choke will be largely
ineffective because it's too big.

For a more broadband solution based on coiled coax, I'd agree with
Cecil's suggestion of cascading a large coil for the lower bands with a
smaller coil optimized for the higher bands. It would also be possible
to cascade a large coil with a small ferrite choke.




--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek