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Pre-Wound Coil Form

scwis's picture
July 15, 2008 - 03:15 — scwis

Hard to find Pre-Wound Coil Form.


Measures 2-1/2" Wide x 1-1/4" Dia. Inductance=350µH.



Nice size for loading a 3 meter antenna - just unwind to get the uH you need. More

My rough calculations indicate that with a 9'8" long 1" diameter copper pipe, 350 uH would tune you down to around 1400 KHz. If you use 1500, 1600 or 1700 you might try unwinding 10, 17 or 24 turns to get a match.

At $3.95 each these will be hard to resist.

Comments

August 4, 2008 - 08:11 — Ermi Roos
Ermi Roos's picture

Buy it! It's a bargain.

I wish the ad had more details about this coil so that I would have been able to duplicate it exactly. I would have liked to know the wire size, the number of turns, and the coil form material. Using the Wheeler formula for inductance, I expected that the wire size is # 28, but I don't have this wire size. I have # 26 and #30. To get a rough idea of how well this coil would work, I close-wound #26 wire over 2 1/2 inches of an ordinary 1 1/4 " diameter amber polypropylene pill bottle made for dispensing prescription medicine. Luckily, the pill bottle is just long enough for a 2 1/2 " winding.

At 1570 kHz, the inductance measured 341.5 uH, and the Q was 302. This corresponds to an equivalent series resistance of the coil of about 11 ohms. I did not expect the Q to be that high. Because the coil is close-wound, the proximity effect should be high. Also, the wire that was used is enamel magnet wire rather than Litz wire. I expected the Q to be no higher than 150.

Since the Q was measured under very favorable conditions, the actual Q when using a real transmitter and a real antenna and ground system may be lower than what I measured.

I intend to use this coil as a baseline loading coil for finding out how much radiated power a legal Part 15 AM system can have. I found a way to make rough field strength measurements, and this will enable me to approximate radiated power. I will use this small coil as the first loading coil I will test in my system.

Note added 8/04/08:

Winding another coil the same as the one above, but using 54/44 Litz wire instead of #26 enamel magnet wire, gave a Q of 532. This is a very high Q, even when using Litz wire, for such a small coil.

August 20, 2008 - 05:22 — Ermi Roos
Ermi Roos's picture

I changed my mind

When I connected the magnet-wire coil described in my previous post to a transmitter and a dummy load consisting of a 30 pF capacitor and a 22.8 ohm resistor in series, the in-circuit Q of the coil was only 115, compared with my measurement of of the Q of the isolated coil (302). The in-circuit Q is just too low for good performance. I would not now recommend the use of this this coil with an actual transmitter, because the coil loss would be too high.

Installing the coil wound with 54/44 Litz wire in the transmitter circuit, the in-circuit Q was 342. This is smaller than the Q of 532 measured with the coil in isolation, but the Q is high enough to produce acceptable performance.

The Q of a coil is not an intrinsic property of the coil itself, but depends upon the method of measurement. This is explained in detail in the following thread initiated by Radio Joe:

http://www.part15.us/node/1535

The use of Litz wire alone does not necessarily increase the Q of a coil. Using 26/44 Litz wire (The first number of the Litz wire description refers to the number of strands of # 44 wire) results in an in-circuit Q of only 93, which is even less than when using #26 magnet wire. A higher number of strands in the Litz wire is likely to produce higher Q.

August 20, 2008 - 06:22 — PhilB

changed my mind

Your measurements bring up a more general problem with loading coils. Specifically, the ideally very low ground resistance of a base-loaded antenna system places a very high demand for the least possible loading coil loss. Unloaded coil Q is a measure of loss, but the loaded Q is what we have to deal with in reality.

Instead if little coils with small wire, we should really be discussing big coils, like 12" diameter or more, space wound with fat wire or tubing. But, I guess the reality of practical antennas limits us to something less than optimum.

Phil B

August 20, 2008 - 21:38 — Ermi Roos
Ermi Roos's picture

Loaded Q

The loaded Q of the measurements in my last post were quite a bit lower than the in-circuit Q measurements that I reported. My loaded Q measurements were between about 49 and 59. The loaded Q is due to all of the losses of the antenna circuit, and the in-circuit Q measurements were due to the losses in the coil alone.

The surroundings of a coil when it is connected to a circuit affect the in-circuit Q. For example, the presence of metal near the coil reduces the Q.

November 7, 2008 - 15:09 — wdcx
wdcx's picture

Another Coil Source

http://www.mfjenterprises.com/Product.php?productid=404-0009

—

Dade City Radio AM1610, Part 15
John - WA4JM
Owner-Operator-Chief Engineer-Program Manager
http://wdcx.webs.com

September 7, 2009 - 20:02 — hanahbr

prewound coils into stator slots

A device for placing prewound coils into stator slots comprises an annular series of fingers housed in respective longitudinal grooves of the outer surface of a hollow cylindrical finger holder.

The fingers project beyond one end of the holder for placement thereon of prewound coils. A stripper member axially movable through the stator is provided on its periphery with a series of longitudinal grooves in which the portion of each finger which projects beyond the holder is received.

Some fingers, and preferably at least one or both fingers of each couple of adjacent fingers between which a prewound coil is interposed is slidably mounted both within said stripper member and within said holder

toroidal transformer

September 12, 2009 - 01:45 — radio8z
radio8z's picture

Coil Winder

hanahbr,

Your post reads as if it is a patent claim. Is it?

I am familiar with this method of winding coils for motors (the company I once worked for used the "Globe" winders and I designed the pneumatic controls for them) but I don't see any application to radio frequency coils. Care to elaborate?

Neil