Cecil Moore wrote:
Ian White GM3SEK wrote:
What kind of wire, what kind of insulation, and how much gap between
turns? It affects the self-capacitance.

Unfortunately, the spare (75 ohm) coax that I
have to experiment with came free from the local
TV cable company and is unmarked. It is ~0.26"
in diameter with a ~0.04" outer insulation. All
conductors are aluminum. There is an aluminum foil
wrapped around the foam insulation under the aluminum
braid. The choke is fairly close wound with approximately
3.5 turns per inch.

OK, that's close enough to several alternatives such as RG58.

I'd like to try that on the VNA, but here in the homeland of
porridge, oatmeal boxes are rectangular!

Do you have 2 liter plastic pop bottles available
there? I'm going to re-do the experiment with one
of those as a coil form. There's an endless supply
of those coil forms from the day care center next
door.

Ours spell "litre" differently but I guess they're the same - 95mm or
3.75in diameter? If plastic bottles from the USA are a different size,
I'll go fetch one from our local beach... you'd be amazed at what floats
across.


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

On Fri, 27 Oct 2006 23:52:03 GMT, Cecil Moore wrote:

Owen Duffy wrote:
Those models also reveal the standing wave nature of the common mode
feedline current, and the futility of taking a current probe
measurement at a single location to infer any more than the current at
that specific location (if that was important).

In fact, the common-mode currents on the feedline
turn the feedline into a standing-wave antenna.
I suspect that's how an Isotron antenna works.

For what it's worth, the following is the method I used in developing the
W2DU current balun in 1981, that was published in QST, March 1983.

I wanted the balun to cover 80 thru 20m. I considered the worst case
situation would be on 80 m with the dipole cut to resonate at mid band, 3.75
MHz. Measured impedance of my dipole centered at 3.75 MHz yielded a terminal
impedance of 53 - j122 ohms at 3.50 MHz, for an impedance magnitude of 133 ohms
at 66.5 degrees.

We must consider that the center conductor of the coax feed line connects
to one half of the dipole, and the outer conductor to the other half of the
dipole. Therefore, the outer conductor sees only one half of the total terminal
impedance of the dipole, 66.5 ohms. I then considered that the choking impedance
of the balun should be no less that 10 times the half-dipole impedance over the
entire frequency range from 80 through 20m.

Fifty No. 73 beads satisfied that requirement with lots of margin to spare,
right down to the top end of the 160m band at 2.0 MHz. Reference to Fig 21-3,
Chapter 21 in Reflections shows the impedance to common mode current on the
coaxial feed line throughout the designated portion of the spectrum. Fig 21-3
can be downloaded from my web page at www.w2du.com by selecting Chapter 21 from
the menu.

Walt, W2DU

Ian White GM3SEK wrote:
Ours spell "litre" differently but I guess they're the same - 95mm or
3.75in diameter? If plastic bottles from the USA are a different size,

Our 2L bottles are very close to 4 inches in diameter
with about 6 inches of effective winding length. They
are very popular here for these kinds of chokes. I
just put 20 turns on one. The GDO says parallel resonance
is 14 MHz and series resonance is 28 MHz. 20 turns on
20m on a 2L pop bottle is what my rule-of-thumb says.
--
73, Cecil http://www.w5dxp.com