On Sat, 2 Apr 2011 12:06:16 -0700 (PDT), Tom Horne
wrote:
Richard
I didn't ignore choking the feed line but I will readily confess that
I did not choke it twice. Starting immediately below the bottom of
the matching stub I followed the recommendation of the various authors
and wound a multi-turn coax balun with a six inch diameter coils of
coax. They call for ten turns if I recall correctly.
Hi Tom,
This sound like very common advice - so common that it begs
investigation because it is common advice for HF Choking, not 2M, and
certainly not 70cm. However, this common advice acknowledges the need
for choking.
You would be better served using ferrites (W2DU style BalUn/Choke),
or, if you really wish to stick with wound coax, then use a Grid Dip
Meter to test its resonance (which should reveal you can't serve both
bands). Using an antenna analyzer to do this will give you measurable
Z, and that may give you the data to see how well you are doing. You
may wind a lot of chokes to discover that the diameter is
extraordinarily huge (or so the same for turn count - one or the other
or both).
I was not aware
of the need for a second choke at one quarter wavelength away. Do you
have the energy to explain why that is necessary?
[Do I have the energy.... With all the junk cluttering this space
outside of this thread, I have plenty of energy.]
The antenna fields will try to excite the transmission line's common
mode (the shield of the coax that it sees). This will induce currents
that will become radiative (just like stacked elements in a vertical
multi-element radiator - sound familiar?). The use of chokes at
quarterwave intervals snubs these currents.
Sometimes more chokes are needed. I use a 20' long line with a bead
every four inches or so. The intent is to create a very long resistor
(very wide bandwidth) with very short leads (very high frequency)
between its distributed resistance.
How critical is the
length between the two chokes.
Not particularly, you would be well served to attempt to make it at
the interval of quarterwave at the highest frequency used (70cm).
This, then, would snub lower frequency (2M) common mode currents.
Do I use the middle of the two chokes
as my measuring points?
This sounds deceptively exact (and probably a consideration for those
enormous chokes of common advice vintage). No, such exactitude is
going to be lost at the 70cm scale anyway which will be perturbed by
other factors (have I talked about environment?).
Could I substitute a one to one current balun
built of ferrite beads? That would have a less intrusive appearance
and accumulate less ice in the winter.
If you check the link that I gave for Ed Fong's dual band j-pole;
available here
http://f1.grp.yahoofs.com/v1/
kL6QTZLk1DQjM_Cn3vuvnsLUIuEsvRHSqUZyX2mw294a7mYKk c
\FBIXXlRY_6QxreqNWVpn0b7Dogiw9LafU63W429yoO/DBJ2_port_art.pdf or just
look it up in the files section of the Yahoo reflector;
I would prefer not to add yet one more account registration to simply
view this, sorry. However, the narrative here should tell you if we
agree.
you will see
that he did test it in the field with fairly sophisticated
instrumentation. He uses a trapped radiator to obtain similar gain on
seventy centimeters as a simple J-pole without the trap gets on two
meters.
My background was working in the standards laboratory system of NBS
(my speciality was RF measurement to the highest accuracies). I know
that sophisticated instrumentation and quality results don't
necessarily track each other. My point was about environment, not
instrumentation anyway. You don't need $1000 meters, but you might
need $1,000,000 environment.
The description of Apple's echoless environmental chamber used to test
their iPod is a marvel of engineering, and cost a stack of dollars.
His work was published in QST and I didn't find any
authoritative repudiation, or even strong criticism, of his design.
The same could be said of publishing it in Playboy. QST needs content
to offer subscribers. Vanity articles satisfy that need. Think of
these articles as the introduction to a topic, not the final word.
Why would the presence of the trap in the lower half wave of the two
meter collinear half wave J-pole wreck the tuning on two meters.
The trap's tuning is heavily influenced by the geometry of the
elements because they are also part of the tuning. The trap disturbs
that symmetry. You have also introduced new L and C components that
further upsets the total circuit. These things are not super-critical
when you consider that moving a resonance (say) 145KHz out of 145MHz
is only a shift of 0.1%. That isn't super-critical either.
Or maybe it is.
I'm only trying to learn here.
Let me know if this helps.
73's
Richard Clark, KB7QHC