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,
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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

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

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

Ian White GM3SEK wrote:
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).

Please don't imply that I said anything about the total
length of wire - I didn't. What you say is true and I
never said otherwise. Well-designed coils can be modeled
as rough approximations to transmission lines.

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).

I didn't say exactly twice the frequency and I said it
was an approximation. The chokes at:

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

average close to double the frequency.

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.

No one would expect a bunched coil to be very well behaved.
Everything I have said applies to a coax choke wound on
some kind of coil form with some care given to its design.

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.

Again, no one would expect a bunched coil to be well behaved.

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

You keep saying that as if I said otherwise. I didn't. The
length of the wire is irrelevant to this discussion.

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 point is that there is a 1/4WL high impedance resonance
and a 1/2WL low impedance resonance that are roughly where
they should be. The 1/2WL low impedance resonance should
be avoided.

As shown above, "1/2wl self resonance" ceases to be a valid concept once
a length of wire is wound into a coil...

The 1/2WL self-resonance has little to do with the length
of wire. It is where the phase angle flips at a point of
low impedance. The 1/4WL self-resonance is where the phase
angle flips at a point of high impedance. The length of wire
is irrelevant, a moot point. I don't know why you brought
it up in the first place.
--
73, Cecil http://www.w5dxp.com

Ian White GM3SEK wrote:

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.


Ian and Cecil,

I think you both should disclose any ties you might have to the cable
industry. (hi)

I haven't been able to find out the proper placement of ferrite beads.
Are they placed ANYWHERE along the cable. Do you calculate the
transmission length for voltage peaks?

Thanks,

John
AB8O

"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.

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

Cecil Moore wrote:
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

Why worry about VHF though? Most people wouldn't want to use
their HF wire for VHF. Or at least, I wouldn't. I always use other
antennas for those bands. Seems to me, if you were to use two
chokes, one for 160-40, or 80-30, and one for 20-10 would be more
practical. A good choke for 20-10 "tribander", is 8 turns of RG8 wound
on a 6 inch form. There is no real need to wind in an orderly military
manner,
although some do. I often just throw them together liking rolling up a
rope,
and they work fine. I had to place a small choke on my mobile for 2m.
I kept having problems tuning a 5/8 wave 2m whip I made a few weeks
ago.
Tried trimming, adjusting the coil, etc. No joy. Came to the conclusion

common mode currents were causing me grief, and this is using a hole
mount drilled in the trunk lid. I usually don't have problems with
those..
I wound a small coil from rg-58, maybe 4-5 turns on a small appx 2 inch
dia,
and all my problems were gone.
A coax choke will need to be pretty small to work well on VHF.
I don't really see one made to work the upper HF bands as being very
good. So, in designing one for upper HF, I'd use 30 mhz as the upper
limit. I've got some info somewhere, maybe in the ARRL ant book that
gives appx dimensions for chokes at various frequencies. 30 ft of coax
wound on a 12 inch from would more likely suit the lower bands more
than
the upper HF bands. So I'd use a 8 turn, six inch dia choke for 20-10,
and whatever choke for the low bands. I'd leave VHF out of it, being
most
HF wire antennas are fairly lame for VHF. Heck, on my 706mk2g, using an
HF wire antenna is inviting lots of image problems for receive when
using
6 meters. 2 meters, I use ground planes, yagi's, etc.. I use a 3 el
yagi on
6m.
MK