Over the last few days I had the opportunity to borrow a 40m Transmitting Magnetic Loop Antenna with a big thanks from Steve – VK5SFA for the loan of his 40m loop so I could conduct some comparisons.
We were at the shack in PF95wu where I have the GAP Challenger Vertical and where I could put up my portable 40m Inverted V antenna.
The loop is physically quite small at only about 1.6m in diameter – and as you can see it was mounted on a tripod with the bottom only 1.8m off the ground, and as far away as my coax would allow from the shack – which was about 10 or 12 Metres.
The Gap Challenger, which has been put to use as the primary RX antenna for the KiwiSDR – was one of the comparison antenna’s used over the few days.
For the TX experiments I used the KX3 and HR-50 Amp and was transmitting at between 40 and 55 Watts of power.
Observations – the Loop
The bandwidth of the loop was quite pronounced – tune the loop, tune the RX across the band and it was pretty obvious – about 40khz wide.
TX – well, it was nowhere near 40Khz wide – more like about 12 or 13khz 1.5:1 VSWR bandwidth.
The first night, we had a bit of rain earlier and the ground was a little damp. We found it impossible to actually tune the loop – best we could get was about 4:1 VSWR on TX. The next night it was much better!
Tuning – well touchy is an understatement! The tuning is manually done via an insulated rod – and 1/4 of a turn was like 40khz! It was a case of tuning for maximum RX noise, then some careful fine-tweaking to get it there.
RX Bandwidth
The Loop was tuned to 7.140 and plugged into the SDR
Without changing any settings on the SDR, the Vertical was plugged back in.
IT is pretty obvious as to the RX bandwidth and how sharp it drops off away from the centre frequency.
Click on both Images to see full sized.
Usage
I made use of the Loop to talk during the day – local, single-hop contacts (VK2, VK3, VK4) – and when 40m is open, it is open – 59 contacts – no real difference between any antenna. DX – I did have a couple of contacts to the West-Coast US using the Loop – and got respectable reports back based on my 50W!
The Loop has 2 large lobes, and quite a deep, but fairly narrow null on each side. The null is about 50-60 degrees and 2 S-points – or probably around the 12db mark (give or take). We did have a little bit of local noise, and rotating the loop we observed the noise drop right off to S1-2 when nulling the noise.
Side by side, with the Vertical – RX was on par, and TX was much better – just because there is a slight directivity associated with the loop .
Side by side with the Inverted V – well this was much more interesting – The Inverted V was only 8m high with each end about 2m off the ground.
The Inverted V was 2-3 S-points noisier than the Loop, but also 1 S-point stronger on the signals. With an S6 noise and S9 Signal for the VK contacts, versus’s the loop, with an S3-4 noise and an S8 signal.
It clearly had at least 10db better signal to noise versus’s the Inverted V – and well, hands down beat the Vertical on TX!
The far stations reported slightly stronger signals on the Inverted V, but I don’t hold much faith in that as there was heavy QSB – I would call it a tie.
Conclusions
I would conclude that it is about on par with a dipole in typical performance – but has a considerably smaller footprint! I was actually not expecting that the performance to be as close to the Dipole/Inverted V as it actually was – remarkable considering that you only need a very small space to have the equivalent of a rotatable dipole!
For the Amateur who is space-challenged or has a lot of noise, then yes, a loop is certainly worth considering for the low bands.
As we have the Loop for a couple more days, we will set it up in a typical urban lot – where we simply cannot use 40m as it is always S9+ noise and make some observations as to how effective it is in a much noisier environment.
