RadioBanter - View Single Post - Hf Antenna Question

Cecil Moore · October 25th 06, 01:54 PM posted to rec.radio.amateur.antenna

wrote:
Ian White (GM3SEK), Cecil Moore (W5DXP), and probably others have
discused HF chokes made from flat vs. bunched coils of coaxial cable,
but I must have missed some posting(s), for I haven't recognized
anything that looks like a CONCLUSION.

Ian and I agree that the ARRL was overly optimistic when
they asserted that a coiled coax choke can cover all three
octaves of HF.

The requirements for a choke feeding an
all-HF-band, ladder-line fed dipole are much higher than
the requirements for a choke feeding a resonant fan dipole.

* Is "bunched" or "flat" better?

Bunching disadvantage: Lowers the maximum choking impedance
and decreases the effective bandwidth. (Bunching increases
the stray capacitance.)

Bunching advantage?: Lowers the parallel self-resonant frequency
so not as many turns are required for narrow-band operation.

* Are they basically single-band or wide-band or somewhere in-between?

Depends upon the minimum required choking impedance. A 2:1 to
3:1 frequency range might be a good rule of thumb, e.g. one
choke can cover a tribander's 2:1 frequency range.

* Are there "rules of thumb" to make them for hams with
* No test equipment?
* A grid-dip meter?
* A noise bridge?
* An antenna analyzer?

How about: Using a 2L pop bottle as the coil form, don't use
more turns than the number of meters in a wavelength, e.g. no
more than 10 turns on 10 meters? That would probably work for
20m-6m as a rule of thumb.

The MFJ-259B will measure impedance up to 650 ohms. One could
define a "bandwidth" based on that 650 ohm value and knowledge
of a typical impedance waveform between those two points. For
instance, my 8 turn, 5.35" diameter choke falls below 650 ohms
at 4.63 MHz and 28.1 MHz. It might be useful over a 3:1 frequency
range from 40m-15m.
--
73, Cecil http://www.w5dxp.com