Cecil Moore wrote:
Cecil Moore wrote:
I also have a GDO. If I get an open circuit dip, would
that be the parallel resonant frequency? If I get a
short circuit dip, would that be the series resonant
frequency?

I have a 20 turn choke wound on a 5.35" diameter
Quaker Oatmeal box at four turns per inch. I used
my Grid Dip Oscillator to determine the open-circuit
(parallel) self-resonance at 11 MHz. The short-circuit
(series) self-resonance is at 23 MHz.

This is a Knight Kit G-30 GDO with the original
6C4 still working. I built it from a kit in 1959.

What kind of wire, what kind of insulation, and how much gap between
turns? It affects the self-capacitance.

I'd like to try that on the VNA, but here in the homeland of porridge,
oatmeal boxes are rectangular! Also we'll soon have visitors arriving
for the rest of the weekend, so if anyone in the land of cylindrical
oatmeal boxes can get to it in the next couple of days, please go ahead.


--
73 from Ian GM3SEK

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.

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.
--
73, Cecil http://www.w5dxp.com

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

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