Approximating a Lithium-polymer dielectric constant - Page 1
Forum section: electronics / rf-microwave

author: Koen
author_rank: Frequent Contributor
timestamp: June 12, 2016, 02:19:47 am
content: Hello,I would like to calculate something but the formula requires me to know the dielectric constant of a Lithium-polymer battery pack (cheap single cell Shenzhen type). I tried Google and a few material databases but found nothing. Would you have an idea ?Thank you,Koen

author: uncle_bob
author_rank: Unknown
timestamp: June 12, 2016, 02:25:39 am
content: HiWhat are you trying to calculate? Put another way, is this an RF or microwave sort of number or an audio number that you are after?Bob

author: Koen
author_rank: Frequent Contributor
timestamp: June 12, 2016, 02:32:51 am
content: I've got another weird idea. I'd like to try an RF patch antenna with a Lithium-polymer battery for dielectric instead of FR4 or ceramic. I've got an okay one running by trial and error earlier and would now like to compute improved dimensions. I'm using this "tutorial" :http://orbanmicrowave.com/wp-content/uploads/2014/12/Orban-Patch-Antennas-2009-rev.pdf (page 3) :QuoteThank you !

author: uncle_bob
author_rank: Unknown
timestamp: June 12, 2016, 02:53:51 am
content: Quote from: Koen on June 12, 2016, 02:32:51 amHiYou are talking about an organic so it's going to have all sorts of strange "fun" in it's dielectric characteristics vs frequency. Best guess: it's lossy as all get out at the frequencies you are talking about. Most organics are (think heating things in the microwave). There are exceptions (teflon) but they are all very good insulators.Bob

author: Koen
author_rank: Frequent Contributor
timestamp: June 12, 2016, 05:07:06 am
content: Do you expect it to be pure crap ? Or would it be impossible to reliably produce because of the battery variations ?It's for a GNSS antenna (1.57-1.62 GHz). I currently have a correct linear polarisation omni but wanted to give a circular polarisation directional a try to avoid multipath problems. First try without any thinking yielded good enough results to pursue experimentation. I'll prepare A/B tests today.I had something like the attachment in mind, with the battery between patch and ground plane because space is at a premium.Thank you !

author: uncle_bob
author_rank: Unknown
timestamp: June 12, 2016, 12:04:18 pm
content: Quote from: Koen on June 12, 2016, 05:07:06 amHiI would expect the antenna to have quite a bit of loss. The simple answer is to set up and measure the loss. The same network analyzer gear you will use to evaluate the antenna will let you measure a filter structure. From that you can get the numbers.The next layer to the problem is the fact that you are using the battery case as the back for your antenna. That's not the same as the battery it's self. The same issues apply (it's not microwave material).After that you need to come up with a good way to connect to the case to use it as a ground. Again, doing that in a microwave suitable way (without damaging the battery) ... not likely unless you are fabricating the battery from scratch.If the antenna is fabricated into the battery, you will need a fancy (and expensive) RF connector to connect to it.=====So, let's back up a bit:Your antenna connects between ground and the GNSS receiver's input. Both the ground and the antenna need to work at 1.5 GHz. It's a compact antenna. The ground is part of the antenna structure. To fabricate an antenna, you use pcb fabrication techniques. The most basic pcb these days is a two sided board. There is not a cost advantage anymore to making a single sided board. A two sided board has everything you need. There is no reason to get the battery case involved.Bob

author: Koen
author_rank: Frequent Contributor
timestamp: June 12, 2016, 12:28:52 pm
content: Thank you ! The antenna - battery - ground plane sandwich is a requirement, I can't change it. If I'm not mistaken, a bigger distance between the antenna and the ground plane should yield more bandwitdh too which is a good point for me. I made more tests today and will make a full try tomorrow. I'll let you know the results.

author: uncle_bob
author_rank: Unknown
timestamp: June 12, 2016, 12:37:08 pm
content: Quote from: Koen on June 12, 2016, 12:28:52 pmHiIf the entire battery is going between the antenna and the ground plane, you have your model wrong. The RF will not go through the battery at all. It will simply go around it on the outside case.If somehow the battery is not encased and is in the middle of the antenna, then you also have the electrolyte and electrodes both in the mix. That is guaranteed to be an utter mess at microwaves.Bob

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 12, 2016, 04:54:11 pm
content: The battery will act as a solid hunk of metal, wrapped in whatever Kapton is visible.It could be used as the patch itself, if the lead lengths can be controlled, and the pack dimensions happen to be right, and you can couple a little common mode RF onto the power connector.Tim