Cushcraft 2 el 40m Yagi coil mod.
|Cushcraft XM-240 (402-CD) loading coil.
(with black shrink cover removed)
5.57" long coil, 68 turns of 12 ga. wire on a 3/4 " diameter fiberglass form.
Inductance : 15 uH
Calculated loss: 8 ohms. (see below)
|Software program showing the calculated loss of the
Cushcraft coil. Notice that 57 % of the loss is form loss because
of the poor properties of fiberglass.
Q=81, Loss= 8.226 Ohms.
Some have indicated that this coil has a measured Q of 200. This is surprising since Brian's program would require an AIR core...ie. NO loss to get the Q this high.
The plan at VE6WZ is to find an HP-4342A Q meter and measure it.
|New low-loss, high Q coil.
11.5 turns, 5" diameter, 5" long 3/16" Al tubing.
This complete unit weighs 12.1 oz.
The original CC coil weighs 11.4 oz.
The coil will be covered with electrical insulating enamel paint, and the fiberglass rod will be wrapped in vinyl electrical tape to protect it from UV light.
The hose clamps provide mechanical support and a stainless steel bolt aids electrical contact. Anti-oxidizing grease is applied to the joints.
This tubing is made from alloy 3003-O which has a resistivty of 3.649E-08 Ohm-m which is slightly better than structural Alloy 6061-T6 which is 4.066E-08. Copper is 1.724E-08 Ohm-m.
The 3/16" Aluminum tubing is available from the internet see Mcmaster-Carr.
|Brian's software program showing the calculated loss for
the new coil.
Q=767, Loss=.859 ohms
If copper tubing was used instead of Aluminum the Q would go up to 1200 with only .5 ohms loss. It was decided the extra .3 ohms of loss was not worth the extra weight from copper tubing.
|The finished coil.
This coil is not mechanically robust and would not be suitable for a commercial shipping or "plug and play" type application. These large open coils are also subject to significant inductance changes in wet and snowy/frosty weather.
The crank up tower at VE6WZ makes regular inspection, adjustment and maintenance easy.
|View of both the 40m and 80m coil mounted at VE6wz.|
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