[DARARepeater] FW: [GCBG] Potential Build Idea

Sat May 4 02:42:26 EDT 2019

Some of you had expressed interest in the loop antenna.

I have combined 4 emails into one so it is a LONG read.

Jack’s email is at the end

George 

From: GCBG at groups.io [mailto:GCBG at groups.io] On Behalf Of Jack Purdum via Groups.Io
Sent: Monday, April 29, 2019 5:02 PM
To: GCBG at groups.io
Subject: [GCBG] Potential Build Idea

All:

Al and I have talked about building a controller for mag loop antennas. The target would be a 20M loop, although the controller would be designed to work all bands 160-6M. The capacitor would be tuned remotely using a stepper motor controlled by a microcontroller (probably an STM32 or ESP32) since the can be programmed in the Arduino IDE and cost about the same as a Nano (under $5), but are clocked up to 240MHz with 1Mb of flash. The antenna loop would require about $10 worth of copper tubing (5/8") and some support materials (to be determined). We would also need a tuning bender, which each member of the build would share...probably $5-10 each, depending on the number who sign up. This would then become an "asset" that members can use for other loops if they want. We think the 20M version would be sized such that it could be carried in a car trunk and set up "in the field" for portable use. BTW, the ESP32 has WIFI/Bluetooth built in and we're trying to think of ways to use that for the tuning. 

The purpose of this note is to determine if there are enough of you interested in doing the build. Al and I envision a PCB and "parts bag" for the controller and then doing the tubing construction at the build, with you showing up with support "parts" that we'll assemble then. My guess is the project will cost around $35-$40 for the 20M version. 

Please let me know if you interested.

Jack, W8TEE 

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  _____  

Part 2

Subsequent research shows that, because the coax shield is not a solid conductor, it induces resistance into the antenna that wouldn't be there otherwise, making it a little less efficient. Yes, copper tubing will be more expensive. Konrad and I split a 50' roll of 5/8" soft-drawn copper which costs $120. However, that's enough for 4 20M-40M loops, so we split the cost and I have enough to make 2 antennas. So figure about $30 for the copper per antenna. The stepper motor will probably be under $20, and the controller board probably another $20, so the total should be under $70. Compare that to the $300+ they charge for a QRP coax loop. The power output for the build will depend on the cap used. Yours will easily handle 100W. We're going to try an design it for max efficiency on 20M, but useful on 40M with the idea that it will fit in a car so it can be taken "into the field".

Jack

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Part 3

All:

We currently have 12 people signed up for the Mag Loop (ML) build, which Al and I agree is enough to warrant buying the tubing bender. We're asking each person to contribute $10 towards the bender, which will cost $180. It is a good piece of equipment that should give us years of service and will be available to the group for use after the build. We've had a few people contribute even though they are not, officially, part of the build...thank you for your support of the group!! 

Here's what Al and I envision: we will be developing the controller board for the ML, probably using the ESP32 WROVER-32. This is a $12 board, but has over 1Mb of flash and 350Kb of SRAM, all scooting along at 240MHz...15x faster than an Arduino Nano, yet still programmable in the Arduino IDE. It also has built-in WIFI and Bluetooth and we may be able to use that, but RFI might make it impossible. When the PCB board is done, we will have it produced and send it along with a bag of parts for you to assemble at home. (Al and I are meeting with Doug Hendricks, the principal of QRP Guys, in Dayton to see if he wants to handle the board. He currently sells our AA and JackAl boards. If he does, I'm going to suggest having the board built using SMD parts but excluding the ESP32, which Doug will allow us to sell to you at cost. We think this might get the board cost down to less than $10 with everything but the ESP32, encoder, and small TFT color display. The display we are experimenting with right now costs under $15. If we don't use the touch feature, the cost drops almost in half.)

Keep in mind you will need a variable capacitor that's big enough for the power level you expect to use and an appropriate stepper motor. Al has researched the cap requirements quite a bit and has determined that the air cap I mentioned before (eBay #152224455639 @ $63) can handle the 100 watt xcvr. If immersed in an oil bath, it can probably handle twice that power. The down side is the "bath tub" makes it less "field-friendly" for portable use due to spillage. A vacuum variable cap is more expensive, but wouldn't have to contend with any liquids (synthetic or mineral oil). If you're going to stick the ML in the attic, portability is less of an issue. Think about how you envision using the ML, as that will affect you cap needs and support structure. Konrad has already built a loop and has a beautiful gear-cap assembly. (Right now, he "hand tunes" it.) We'll pass along our experiences as we build prototypes.

When you have the PCB connected and mounted in a case of your choice and have gathered your support materials, we will do a build at Al's house, as he has a large driveway that will make tube-bending doable. Since we will be using his driveway, we can start building at O-dark thirty and work until they're done...no public building rules to contend with. If it rains, we will still have enough room in the garage to build the loops. Clearly, we want to do the build this summer while the weather is nice. We realize that people are often on vacation, so we may do two "assembly meetings" to accommodate such issues.

Al is buying the tubing bender within the next week or so, which will allow us to build prototypes. As such, it would be nice to collect the $10 "bender fee" as soon as possible so we can pay Al. For those who have a PayPal account, you can use my econjack at yahoo.com account for payments and I'll probably give him the money...

Al and I will make posts here as we develop the PCB.

Jack, W8TEE
Al, AC8GY 

Part 4

All:

Many of you have already paid $10 for the build--thank you!--so even if the rest don't pay, I said I would cover the balance. As a result, Al and I went to HF today and bought the tubing bender. It's heavy duty and weight just under a jazillion pounds. I'm pretty sure we could take a truck frame and make a loop out of it. Anyway, Al bought a 10'x1" length of hard-drawn copper tubing (i.e., not the soft refrigeration tubing), annealed it, and gave it a try:
cid:attach_0_159B45CDA84EE2E1_23593 at groups.io
It came out pretty good, especially considering he had no one there to help feed the tubing. The next step is for Al and I to start working on the controller board which will control the movement of the tuning capacitor via a stepper motor. As I said before, if we can get QRP Guys on board to sell it, we will likely pre-mount the components like we did for the JackAl board. If the parts count is low and the parts easily available, the board will likely be through-hole.

Anyway, thanks for your payments. It will be a valuable tool for the group.

Jack, W8TEE 

_._,_._,_

  _____  

jjpurdum=yahoo.com at groups.io

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