Dielectric properties of air core inductor coil materials

ABS is very lossy at RF. Its dissipation factor at 1 MHz is .02. PVC is a little better at around .006, but is still not very good. PP, which is at .0003, is used for plastic prescription containers, which are good for small loading coils.

Thanks Ermi ...

For future reference, where did you find those values? I could find plenty of other data on ABS, PVC, PP, PE, PS , acrylic, etc. but literally nothing for RF loss. Wouldn't Polystyrene work well too?

I'm trying to think of where I might find tubes available with sizes I want, mainly diameter and wall thickness, made from other plastics besides PVC.

BTW, I used a 4" Schedule 20 white PVC drain pipe, 1/8" wall thickness for the coil that got me out of the enclosed business park and could be heard at the Library, maybe 3000'. I attribute the good inductance with less loss to the thinner wall, larger diameter, and heavier wire gauge. But it lacks strength in an inline configuration. I'm trying to figure out how to reinforce it without disturbing the inductance field.

Because there are so many plastics, no one site covers all of them. I search "electrical properties of plastics" and look for dissipation factor. Usually, dissipation factor is measured at 1 MHz, but sometimes it is measured at other frequencies. For example, a particularly good plastic, like PE, is sometimes measured at 10 MHz.

Polystyrene has a very low dissipation factor. I have used it for loading coils.

Hi Ken,

I think there is some confusion and possibly a touch of urban legend going around concerning the effect of various plastic choices for coil forms. The dielectric constant and dissipation factor specifications for various plastics are very important for capacitor design, but not for coil design. As long as the coil form material is non-magnetic and is a reasonably poor conductor (plastic, wood, cardboard, etc.) there will be very minimal effect on the coil loss.

A magnetic core coil (ferrite, iron powder) has a "core loss" factor which robs energy to heat the core.

The major contributing factors to losses in non-magnetic core coils are the AC resistance of the wire at the operating frequency and the winding capacitance, which is a function of wire spacing.

Phil B

Hi Phil,

Thanks, I understand. At these wee power levels I'd just like to avoid any unnecessary loss.

Close-wound coils seem to be the order of the day ... but I notice some commercial 'true' air coils with plastic spacers are large dia. wire or tube and have quite a bit of spacing. However 65 turns of close-wound 18 awg insulated stranded wire wound on 4" Sch 20 PVC drain pipe seems to have good characteristics at the upper end of the AM BCB.

I wish I knew more about why. Heck, I got over 500 ft. with the system inside my studio and no RF ground. I didn't expect it to get out of the business park, but it did. Much static in the town with all the Christmas lights (there was a decoration/display competition among local businesses), but I could still get signal in few spots. I'm amazed!

I hope it works as well from my boat. I want to lower the frequency from 1710 to 1650 (nearly as quiet here). Means a little more antenna length (I can make it up to 1.5 ft. longer) though.

Hi Phil,

Thanks, I understand. At these wee power levels I'd just like to avoid any unnecessary loss.

Close-wound coils seem to be the order of the day ... but I notice some commercial 'true' air coils with plastic spacers are large dia. wire or tube and have quite a bit of spacing. However 65 turns of close-wound 18 awg insulated stranded wire wound on 4" Sch 20 PVC drain pipe seems to have good characteristics at the upper end of the AM BCB.

I wish I knew more about why. Heck, I got over 500 ft. with the system inside my studio and no RF ground. I didn't expect it to get out of the business park, but it did. Much static in the town with all the Christmas lights (there was a decoration/display competition among local businesses), but I could still get signal in few spots. I'm amazed!

I hope it works as well from my boat. I want to lower the frequency from 1710 to 1650 (nearly as quiet here). Means a little more antenna length (I can make it up to 1.5 ft. longer) though.

For the highest Part 15 AM system efficiency, the loading coil should have as high a Q as possible. The RF fields of an air-core coil penetrate into the axis of the coil, and using a lossy dielectric for the coil form reduces the Q. If the antenna system has even a minimal metallic ground screen, the loading coil loss usually exceeds the ground loss; and therefore the loading coil is the most important component for establishing the antenna system efficiency.

Another thread in this Forum deals with the various causes of loading coil loss in considerable detail:

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

"The RF fields of an air-core coil penetrate into the axis of the coil, and using a lossy dielectric for the coil form reduces the Q."

This statement isn't supported by theory. The field inside a coil is magnetic. There might be some small component of electric field, but it is ignored by all the equations for solenoid coils that I've seen. Dielectric constant is a function of an Electric field, so dielectric losses in the plastic would be very small because the electric field is very small.

It would help us if you can cite a reference to the effect of dielectric material in the core on coil loss?

It's pretty obvious that there is a substantial electric field inside an RF coil. There is a voltage gradient along the axis of a coil because of self-induction. A voltage gradient impiles an electic field. In high-voltage coils, such as Tesla coils, dielectric breakdown of the coil form can occur. This dielectric breadown would be caused by very high electric fields.

Magnetic fields can also cause dielectric loss in coil forms. If the loss is caused by conduction, the magnetic fields can induce the flow of electric current that causes the dielectric to heat up. If the loss is due to polarization, it is the electric field that causes the heating of the dielectric due to the rapid rotation of atoms and molecules by the RF field..

Paragraph 18 of Chapter 2 of Terman's "Radio Engineers' Handbook" (1943) is called "Losses in Air-cored Coils at Radio Frequencies," and specifically warns against the use of materials such as cardboard for RF coil forms.

I love it when a couple of my favorite posters go at it. We all learn something. I remember that an early 20's QST had an article to build an entry level 5 watt CW transmitter. Used a UV-202 or #10 tube. One of the things they went on about was the R.E.L.( Radio Engineering Labs) coil which used 4 glass rod insulators/supports with air winding in between. They used to make a very big deal about low loss coils and I always wondered if it was mostly hype because I have built a lot of successful transmitters over the years using many types of coil forms. Modern rigs seem to use genuine crap for coil forms and they work fine. Probably apples to oranges with the different impedance levels or ??? used. The real question in my mind is could an air wound loading coil increase the range by any measurable amount in my part 15 rig over the PVC wound coil I now use?? Dave

A PVC-wound loading coil is considered to be essentially an air-core coil isn't it? i.e., the loss is much less than a ferrite-core coil. There is more air than material across the diameter of the coil.

Ideally, an air-core coil has no supporting material other than itself. However, accurately winding 60-100 turns of tubing, including precision spacing, which would support itself is not a project I'm willing to undertake. It would end up too large and unwieldly in any case. But using a plastic support and wire I can wind myself which has the least loss and most Q at frequency certainly falls within my limits, so it's a compromise that works better, yet it's a goal I can actually accomplish.

Ermi,

I don’t have the Terman book, so I am at a disadvantage here. Can you find the following information in your book? This quote is from the HAMCALC coil design software. I’m not advocating the software, but I just want to know if the information is in the Terman book.

“Coil form losses are not included because the forms used with single layer coils have little effect upon losses provided the dielectrics involved do not absorb moisture. (Terman – p. 74)”

Assuming this is a correct quote or interpretation from the Terman book, then I respond to your comment about cardboard coil forms with a resounding yes! However, I don’t think anyone would put a cardboard loading coil outside without giving it a few coats of varnish first! Otherwise, it will last until the next rain.

It has been stated on the web that PVC absorbs moisture over time. I don’t know if this is true or not (possibly more urban legend), but it will degrade under UV over time (we all know that from our crappy PVC patio furniture). So, the standard recommendation is to coat the PVC coil with UV protectant exterior varnish.

As for the concept of an electric field across a solenoid loading coil, I would say that it is minimal to insignificant. The RF voltage from end-to-end for a typical loading coil at 1.6 MHz is approximately 200 – 300 Vrms. The end-to-end distance is large for all reasonable geometries of the coil. I would say that for a good coil, the distance is at least 4 inches. That’s a WHOLE LOT more than the distance between plates in a capacitor.

The effect of atomic level magnetic dipoles in dielectrics is way down on the scale. I don’t think this is a measurable factor for any common plastics that we may use for coil forms.

Tesla coils have a unique problem with dielectrics (a.k.a insulators). When the RF voltage across the coil is in the range of hundreds of thousand of volts or more, electric field losses or spark-over can occur if the coil form material isn’t good enough. That’s not a concern for Part 15 transmitters. It would be a problem if someone tried to feed 1000 watts into a 3 meter antenna, but then it becomes a Tesla coil instead of a transmitter.

The R.E.L. coil hype you mentioned reminds me of the present-day audiophoolery seen on the web.

A real air-wound coil won't increase your range vs. a PVC coil. The best way to increase part 15 range is your GOUND system. Try 32 30 ft. ground radials. You will be pleasantly surprised if you are still able to function after all the work!

What advantage would it have to use a large glass jar as a coil form?