Category: HF

Overview

After years and years of QRP experiments I finally decided to build myself a QRO magnetic loop.

My requirements for this new antenna:

• “Cheap” to make
• No special tooling needed
• 100+ Watt
• Quick to design and build

Is with this spirit that I based my project upon two existing ones:

• Ham Radio Loop Antenna by Eric Sorensen
• Controller for 3 Magnetic Loop Antennas With Endstop Switch by Jose Bascon EA7HVO

Eric’s design consists of a set of 3D printed supports that allow pole-mounting a Comet CVBA-500BC series capacitor and a NEMA 17 stepper motor with a 5:1 planetary reduction. One notable deficiency of Eric’s design is that it lacks proper housing of the capacitor/stepper assembly and – most importantly – it also lacks a viable control system.

That is where Jose’s controller came into play. Based around the venerable Arduino Uno board and a Pololu A4988 stepper driver hat, the controller is simple yet fully functional and battle tested by dozens of previous builders.

Loop Structure

In order to make the antenna construction quicker and simpler, I decided to avoid using copper pipes and replace them with LMR600 coaxial cable. In all honestly I didn’t even know LMR600 existed (I would have used LMR400) but then I was “inspired” by preciseRf’s HG3 QRO loop design and decided to use LMR600 instead.

LMR600 as a radiator coax is good and bad: good because it is larger and stiffer than LMR400, bad because crimping requires special clamps and special connectors. Fortunately I found a cheap pre-made, 10 foot N-to-N assembly on Amazon ($29) so I used that. As it turns out 10 foot is too long (should have been 8 or 9 instead) so I will have to figure out how to shorten that. I ended up purchasing a custom made 8 foot LMR600 equivalent (called “Bolton 600”) directly from its manufacturer Bolton Technical. The cable I received is actually 265 cm long (8.7 foot) and allows the antenna to resonate between 4 and 30MHz.

From Eric’s project I kept just 2 parts, which I had to customize to my specific needs (the capacitor holder had to be cut to fit my enclosure box, the motor holder as well because the motor somehow did not fit).

For the rest, the structure is built on 1-1/2″ schedule 40 PVC pipes. The feed loop is made out of LMR400 with a 3D printed splitter. The enclosure is a standard electric box I found on Amazon and the control cable is CAT5 Ethernet. Two RJ45 connectors are predisposed to carry both the loop control signals as well those for a future antenna rotator.

Capacitor

When it comes about Magnetic Loops, the Elephant in the Room (or in the box!) is the Capacitor. So let’s spend a few words about it.
Vacuum capacitors are expensive and hard to find and, you guessed it, mine is no exception.
The only chance you have to find one at a “reasonable” price is the aftermarket of used capacitors that were pulled from dismissed equipment. The main source is obviously Ebay but other reputable, specialized online sellers also exist. I got mine from mgs4u.com, a company I have known for at least a decade and that I trust. I willingly paid them $450 for an almost new CVBA-500BC/8 (the /8 variant can handle up to 8KV peak voltage) well knowing that other /5’s (the 5KV variant) were available at around half the price from international sellers on Ebay.

The million dollar question I bet you are asking now is: what kind of power can your antenna handle? Answer: I have no idea. All I know for sure is that it handles 100W without any problem, under any circumstances. By comparing it the HG3 loop, however, I guess that with an appropriate balun it might exceed 1500W SSB (I think – but I don’t know for sure – the H3G uses a 5KV capacitor). As I said this just hypothetical, and I believe my hypothesis will forever remain such because I doubt I will ever use more than 100W.

Given this is a QRO loop, I could not avoid adding a choke balun into the mix. I ended up choosing the one from Palomar Engineer that can take up to 500W, which is more than enough for me.

The project is still a work in progress and definitely requires some adjustments but, as of today, the antenna is up and running.

As it is to be expected from a loop of this size, it works very well above 30 meters and not so well below (a proper antenna for the low bands would require a bigger and/or multiple-spire radiator).

Future improvements

The improvements I am planning to make are:

• having the radiator resonate between 5.3 and 29.7 MHz (it currently goes from 3.8 to 28.3 MHz) DONE
• reimplement the controller to allow auto-tuning/remote operation DONE
• add an antenna rotor that shares the existing control cable

Cost

When finished I will publish more detailed build plans. For now, here is a preliminary parts list: