Cecil Moore wrote:
Ian White GM3SEK wrote:
In the chokes we're looking at, the low-impedance series resonances
of which Cecil complains do not occur below 30MHz. Those resonances
exist, but not on the HF frequencies where the chokes would actually
be used.
But, Ian, what about 75m and 40m?
Those resonances above 26 MHz are transmission line effects.
The largest coil is only 12 turns, not nearly enough for 75m
operation. The largest 12 turn coil has a choking impedance
of only 100 ohms on 75m. Scale the choke to 75m and 40-50 turns
are probably required.
Sorry for the delay in replying, but I just did something completely
radical for this type of discussion: hauled out some cable, made some
chokes, and measured them.
These chokes were intended to be like the ones recommended in the ARRL
Handbook for 3.5MHz and for 7MHzz, both using 22ft of RG213. What I had
was actually 21ft, and this length was wound into a flat coil having
various numbers of turns. The choking impedances and phase angles were
measured on an N2PK Vector Network Analyser.
Ten turns made a choke that had 1000 ohms impedance from 3.4MHz to
6.4MHz, falling to about 650 ohms by 7.3MHz. This makes a respectable
two-band choke, if you judge it by the criterion of needing about 500
ohms minimum impedance, but it's a stretch to cover both bands from edge
to edge (9 turns was probably a slightly better compromise). Moreover
the maximum impedance of 17k is "wasted" between the amateur bands at
4.6MHz.
That confirms our shared suspicion that the Handbook's claim of "a
broad resonance that easily (sic) covers three octaves" is
over-optimistic. Where the coiled-cable choke really excels is as a
single-band device, exploiting its extremely high peak impedance at
parallel resonance.
Without needing to change the overall length of cable, the peak
resonance can be shifted to different frequencies by increasing or
decreasing the number of turns, although I couldn't persuade 21ft below
4.6MHz (10 turns) because the bending radius was becoming too small for
RG213.
However, it's perfectly clear that Cecil's estimate of "40-50 turns"
needed for a 75m choke is way off.
That would certainly lower the 1/2WL
resonant frequencies into the HF ham bands.
The 12 turn coil's peak impedance is around 15 MHz and its
1/2WL low impedance response is around 31 MHz. If we scale
the peak impedance value to 4 MHz by increasing the turns,
the 1/2WL low impedance response would be around 8 MHz. Such
a choke would be useless above 10 MHz.
Bottom Line: If enough turns are used to achieve a high choking
impedance on 75m, the 1/2WL resonant transmission line effects
will occur in the middle of HF making it useless on the higher
HF frequencies.
Right conclusion about the choke being useless a long way above its peak
resonance - but not the right reason for it. The true reason is the very
simple property of a parallel LC circuit (the inductance of the coil,
resonated by its own self-capacitance) which means that far above
resonance its parallel impedance drops to a very low value. That's what
makes it useless at those frequencies.
The 4.6MHz choke does have series resonances at about 23MHz 33MHz ,but
those cannot be said to affect the performance of the choke in any
practical way, because the choke doesn't have any usable performance at
these frequencies anyway.
Within the practical working frequency range of all of these
coiled-coax chokes, the performance can be accurately described as
that of a simple parallel tuned LC circuit, which displays no
transmission-line behaviour whatever.
This is obviously a false statement proved by your own graphs.
If there were no transmission line effects, the phase graphs
would all converge on -90 degrees. All of the phase bumps above
26 MHz are transmission line effects. If the chokes had their
maximum impedance at 4 MHz, those 1/2WL transmission line effects
would probably be in the range of 10 MHz rendering them useless
on 30m-10m.
Cecil keeps resolutely ignoring the reservation: "within the practical
working range of the choke".
Within the frequency range where the choke has a practically useful
value of impedance, there are no - repeat NO - signs of transmission
line effects. Its behaviour is purely LC, its own inductance resonating
with its self-capacitance.
Cecil complains that
One might say it is misbehaving and is a very poor design.
That sounds to me like the complaint of someone who has a pet theory
to hammer, and is disappointed when he can't find a nail.
Actually, I was just quoting this web page:
http://www.k1ttt.net/technote/airbalun.html
"- Don't bunch the turns together. Wind them as a single
layer on a form. Bunching the turns kills the choking effect
at higher frequencies." I agree with WA2SRQ that the bunched
coil choke is a poor design. Do you think that WA2SRQ has a
pet theory?
I think WA2SRQ's advice is one-sided, because it fails to recognize the
benefit of bunching the cable together - namely that it moves the whole
resonance curve downward (due to the increased self-capacitance) without
needing to increase the length of cable. For someone who wants a
low-band choke, that is a Very Good Thing. The poorer performance at
higher frequencies is the obvious natural tradeoff for having moved the
whole resonance curve downward - but that doesn't make it a "poor
design".
I think that's it, really. The graphs themselves say the rest.
They certainly do. Everything above 26 MHz is obviously transmission
line effects. If the chokes were designed for the lower part
of the HF spectrum, the 1/2WL low impedance points, caused by
the transmission line effects, would be in the middle of the HF
spectrum.
Scale the high impedance points for 75m and the low impedance
bumps due to transmission line effects will occur in the
middle of HF.
Hands-on measurements prove otherwise - the spurious resonances stay
pretty much where they were, above 20MHz.
As stated above, I measured a choke which is resonant at 4.6MHz. It has
over 1000 ohms impedance at 3.5MHz, rising to over 2k at 4MHz, so it
would be a very good performer on both 75m and 80m.
But above the main parallel resonance there are no bumps in the
impedance or ripples in the phase response at any frequency below 20MHz
- none.
It is ridiculous for Cecil to describe such a choke as "misbehaving and
a very poor design". On the contrary, bunching the turns to increase the
self-capacitance has proved a very good way of moving the main resonance
downward to make an effective single-band choke for 75/80m, while
leaving all the unwanted resonances parked above 20MHz.
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek