I have long been intrigued by the magnetic loop antenna and have been reading widely on the Internet on the subject for some time. I also saw Tony VK3CAT’s AlexLoop and thought it would be fun to try and make my own as an interesting alternative antenna for SOTA. The independence from large squid poles and small antenna footprint that would be possible appealed too.
I wasn’t totally convinced on the small loop matching approach used in the AlexLoop for SOTA applications, so explored further and came across the design by DG2IAQ using capacitive matching.
Whilst this has it’s limitations, principally more ground loss due to the proximity of the capacitor and it’s E field to the ground and higher common mode current due to the high impedance feed point, the convenience of the set up made for a good trade off.
The key to a successful magnetic loop appears to be keeping the loop resistance as low as possible. In fixed loops, that means soldered joints everywhere and either a butterfly or split-stator capacitor to avoid lossy sliding contacts (or a vacuum variable capacitor).
I wanted to keep the antenna as light and compact as possible, so I selected a split stator receiving capacitor with a 6:1 reduction drive to make the tuning usable. These aren’t so easy to find these days. I obtained mine from MidnightScience. The capacitance range was nominally 23 – 182pF with the two sections in series but it actually went down to around 4pF which allowed a very wide tuning range.
The plate spacing on this capacitor is 0.01″. This in theory provides a voltage breakdown rating of around 1500v when the two sections of the capacitor are placed in series. This provides a potential power rating of up to 50w on 40m.
For the loop I used a 3m length of LDF4-50B 1/2″ Heliax that I bought at a hamfest a few years ago. This type is “super flexible” and can be coiled into quite a small package, but still has a solid copper outer. It was also fitted with high quality N connectors on each end. This is probably about as good as you can get while remaining backpackable.
A 3m loop length resonates with the chosen capacitor from just below 7MHz to about 28.300MHz which makes it usable on the bands from 40m to 10m.
To keep the loop resistance as low as possible, I used silver plated N jacks on the capacitor housing and connected these to the capacitor with copper strip soldered at both ends.
The 6:1 reduction drive for the tuning capacitor makes it possible to manage the extremely narrow bandwidth of the loop reasonably. I added a tuning dial marked with the band positions to help get the rough tuning right. This is attached to the reduction drive on the slow side.
Magnetic loops can be fed in a number of ways – coupling loops, gamma feed, toroidal coupling and capacitive coupling. I decided to give capacitive coupling a go following the ideas of DG2IAQ. This is also known as “Army Loop” or Paterson coupling. You simply feed the loop directly across half the capacitor with a series capacitor to match the predominantly inductive load. The series capacitance required varies by band, but is insensitive enough that a single value provides a good match across a given band.
Following DG2IAQ’s approach, I added 4 silver mica capacitors in series with a switch across each to bypass it. In this way I could select a wide range of capacitance values with different switch combinations. I could probably have managed with 3 switches, but 4 gives more flexibility.
The required capacitance to match the loop ranged from 33pF on 40m to 6pF at 10/12m. This resulted in my using measured capacitance values of 12.1, 20.8, 32.6 and 41pF with each value made from two capacitors in series (described later).
Initially I used 500V Silver Mica capacitors, but these proved problematic (see below).
The only other component needed is a feed line choke to keep RF off the feed line. This was fabricated with a FT140-43 toroid with 13 turns of RG-316 coax. This proved to be only just adequate given the high impedance feedpoint (see later measurements of common mode current).
I built the variable capacitor, matching capacitors, switches and feed line choke into a waterproof polycarbonate box with a BNC socket on the bottom to attach a feed line. This provides a neat installation with only the loop cable and the feed line as separate components.
I also built in a neon indicator to act as a rough tuning indicator. I found that this really only lit up above 10W of power, so in practise is of limited use. This simply has both connection tied to one side of the tuning capacitor and the RF field strength inside the bulb causes the neon gas to glow.
When it came to testing, I found that the matching capacitors worked quite well with an acceptably low SWR across the 40m through 10m bands (it only covers up to about 28.300MHz on 10m). This coverage range is greater than I had expected and makes this a very versatile antenna.
I tried it out in this configuration for a SOTA activation and was pleasantly surprised by its performance. I had good signal reports of similar level to those I had received earlier in the day from my trapped EFHW. I was driving the antenna with my 20W KN-Q7A SSB transceiver.
An interesting aspect of testing was the performance of the silver mica capacitors. I had thought that at 500V rating, they would be OK for this application and reasonably low loss. I found that at 30W continuous carrier, they heat up quite significantly and with this heating the capacitance value changes enough to de-tune the loop so that the SWR goes from about 1.2:1 to 6:1 within 15-20s! Clearly that’s not going to be good for any medium-high duty cycle mode.
Characterising the 47pF Silver Mica capacitor shows a equivalent series resistance at 7MHz around 1.5 ohms. At 30W, the current through the capacitor is approx 775mA resulting in a power dissipation of 900mW this is clearly enough to heat the capacitor significantly.
The second problem is the voltage across the capacitor. I had assumed that 500V rating of the capacitors would be more than adequate, but had not done the maths. Turns out that at 20W level, the peak voltage on the matching capacitor for the worst case band is 1080V on 17m! You really need 1kV rated capacitors even at only 20W power level for this type of matching.
The challenge was to find a low ESR, high voltage capacitor with a near zero temperature coefficient! Fortunately such capacitors exist. Advanced Technical Ceramics make some very nice porcelain SMD capacitors that fit this specification (ATC700B series high voltage variant). However obtaining them in small quantities is impossible at present. Thankfully a closely related type – the ATC800B series – is available from RF-Microwave in Italy in small quantities. This series are very low ESR, NPO, 500V SMD capacitors. By using two in series, you can obtain the required 1kV rating in a compact enough form for this application.
I measured the resultant performance of the antenna using my N2PK VNA and the plots across the bands are at the end of this article. The following table summarises the performance:
|Band||Frequency range||Q||SWR bandwidth [kHz]|
|40m||7.000 – 7.300||312||22|
|30m||10.100 – 10.150||272||37|
|20m||14.000 – 14.350||315||45|
|17m||18.068 – 18.168||299||61|
|15m||21.000 – 21.450||223||94|
|12m||24.890 – 24.990||173||144|
|10m||28.000 – 28.480||97||289|
As expected, the Q decreases with increasing frequency. Not sure I can explain why it dips on 30m though!
|Band||Maximum power (no arc)|
|40m||50W (no significant heating for 1 min CW tx)|
|30m||15W (occasional arcing)|
|10m||30W (limited by interference to transmitter)|
Again 30m performance appears anomalous.
The 30W limit on 10m was set by the interference I was getting to the transceiver when transmitting above this power level – the transmitter (an IC-7000) would switch off it’s power! I think this may be due to RF getting in to the transceiver control circuits via either direct radiation or common mode currents on the transmission line. The power supply was a battery for these tests and the whole transceiver and battery were isolated from ground.
To investigate common mode currents, I used a simple current transformer connected to a 50 ohm termination and RF voltmeter. I found the following currents on the outside of the transmission line using 20W transmit power (about 630mA RMS).
|Band||RF current on cable [mA rms]|
The currents are not excessive and are more than 20dB down on the actual transmitted current. The increasing common mode current with frequency is consistent with the lowering impedance of the common mode choke. The choke is optimised for lower frequencies (where I’ll mostly be using this antenna). A second choke with fewer turns in series with the first would likely reduce the common mode current on the higher frequencies, but is probably overkill for the application.
This antenna so far has met my expectations and is weight comparable to other SOTA antennas I have. Total weight: 2.44kg (my multi-band EFHW antenna weighs 1.91kg including the squid pole).
Tuning is fairly easy on the lower bands, but is quite challenging on 10m-15m due to the significant effects of hand capacitance at those frequencies. Even though the knob is separated with a nylon shaft from the tuning capacitor, the frequency shifts several tens of kHz when you move your hand away. Hence tuning is a bit tricky.
Now it’s time to take it into the field and try it out in comparison to a dipole and my EFHW!
Here are a couple of final photos of parts of the antenna for interest:
Here are the SWR plots for each band for reference – click on them to get a bigger image.