Measuring Urban HF Noise with a Loop

For the last few years I’ve been interested in the problem of urban HF noise. As a first step I’d like to characterise the problem by taking a few measurements of noise at my home, around my suburb, and a few other locations. So I need a small, portable antenna. As I’m interested in high noise levels on HF, it doesn’t have to be very efficient.

On a “quiet” day – I see 7MHz noise from my dipole at S5 on my IC7200. I think the S meter is adding up all the noise in the IF bandwidth of the radio. Hooking up my spec-an and referring this to 1 Hz bandwidth, I measure the average noise at about -120 dBm/Hz which is 54dB above thermal (-174dBm/Hz). So even a low gain antenna will receive plenty of noise without hitting the noise floor of the receiver. Ideally I’d like to see -135 dBm/Hz, which is S0 on my IC7200.

Small Loop Antenna

I decided to build a loop, as they don’t need a ground plane, and are relatively insensitive to low permeability surrounding objects (like me holding it) [2].

Using the equations from [2] I decided to use 3 turns of RG58 (about 3m) with a 0.3m diameter. The braid is the conductor, I don’t use the inner at all.

My loop is fed using a toroidal transformer [5]. I like the idea of using a transformer to ensure good balance. All three turns of the loop coax pass through the toroid (the primary), and the secondary is a bunch of turns I wind myself. Using the equations on [2], I estimated the radiation resistance Rr at just 0.006 ohms. To match this to 50 ohms means a lot of turns on the matching transformer.

I tried 50 and 100 turns but for some reason the return loss was still really poor. Also it didn’t seem to be receiving anything, and touching antenna or capacitor terminals had no effect. On a whim I reduced the number of turns and the return loss started to improve, I could see a noticeable dip of a few dB as I moved the tuning capacitor. If I touched the capacitor or the loop it would de-tune, indicating it really was starting to become an antenna!

Turns out that for small loops, the resistive loss Rl is often greater than Rr. I confirmed this by connecting the loop in series with a fixed 100pF capacitor across the spec-an input. When driven by the tracking generator I can see a dip at resonance. By measuring the bandwidth I could estimate the Q and hence the loss resistance Rl of about 2.8 ohms, much greater than Rr of 0.006 ohms.

At about 15 turns I obtained a return loss of better than 10dB – good enough. However it’s really just a match to Rl, consequently the antenna is quite inefficient, I estimate a gain of 10*log10(D*Rr/Rl) = -25dB (D=1.5, the directivity of a small loop). But that still not bad for a 30cm loop on 40m, and good enough for my mission.

The ratio of 15 turns secondary, 3 turns primary (15/3)^2 would transform 2.8 ohm into around 70 ohms, and lead to an expected return loss of 15dB, about what I’m seeing.

EMI receiver

I decided to use my FT817 as a portable receiver to log noise values as I wander about with the loop. So I added a 26dB pre-amp [3] to compensate for the negative gain of the loop and to boost the Rx level high enough to move the S-meter. The pre-amp is increasing the signal and noise together, so unlike a regular pre-amp the receiver will not be more sensitive. Just louder.

I placed the loop on a 1m tripod at the front of my yard, away from the house as that seemed to reduce the noise a bit. Using distant SSB signals, I compared the dipole to the loop over a period of a few days. I used a RTL-SDR SDR running gqrx on the 40m band, with a 7MHz bandpass filter to prevent overload. At times the levels were quite close, at other times the loop down about 10dB. Sometimes the SNR was better on the loop, other times the dipole. It was hard to get exact measurements. I guess there are differences in pattern, polarisation, and the signals received at the slightly different sites, and the power of SSB bounces around.

However it’s fair to say the loop (with it’s pre-amp) is roughly as sensitive as the dipole. If the loop detects a signal (such as noise) the dipole will also detect the same signal at roughly the same level. So the loop is a useful antenna for portable EMI measurement.

By connecting my spec-an in parallel with the loop antenna, I created a calibration curve of noise density against the S meter reading:

My FT817 S-meter seems to have a very narrow range, and is wildly different from the IC7200 S meter. However if I can measure less than S1 on the FT817 (blue, attenuator off), it looks like I will hit my target of -135dBm/Hz.

Blowing up my FT817

During some portable measurements I blew up my FT817! The fix and fault (shorted filter EMI choke and blown internal battery fuse) were exactly as described in [4]. I managed to repair the Rx but the Tx power is still low at 200mW, it’s almost like it’s folding back due to high SWR (although the SWR into the dummy load is Ok). So I’ll need to think about that fault some more. I’ve added external fuses to both the positive and negative FT817 supply leads now, as well as the Lithium battery I use for external power.

Noise measurements

I hopped on my bike and logged the noise at various sites around my suburb. They all bent the FT817 needle with the attenuator off (blue), so I used the “attenuator on” curve (red). The actual measurements are the red circles in the figure above.

So it’s not just my house, this seems to be pervasive problem in my neighbourhood. The best case was walking out into the middle of a large sports ground. I guess that makes sense, as the EMI source must be around the perimeter, and we get some inverse square law fall off. Take that far enough and we have portable operation from a country site – putting some distance between us and the cloud of urban noise.

Discussion and Conclusions

I’ve been reading about antennas this year, and can recommend “Antenna Physics” by Robert Zavrel [1]. The technical level was just right for me – between typical Ham texts and deeply mathematical tomes. This project complemented my reading, and give me some practical experience in the concepts of loss resistance (Rl) and radiation resistance (Rr).

It was pretty cool to see the return loss improve as I adjusted the matching network, and understand “why”. I really enjoy the learning experiences with Ham Radio.

I now have a portable, sorta-calibrated EMI receiver, that I can use to measure the noise levels at various sites, and compare it to my target. As a bonus the loop is directional, so I can DF noise. The range of the FT817 S-meter is a bit limited, but it’s calibrated well enough to tell me if the noise level is similar to my house or getting near my target level of -135 dBm/Hz. Nowhere in my suburb is close, and the noise is not localised to my house.

I’m quite surprised a 30cm loop can receive 7 MHz signals at all, in a quiet location it works just fine as a receive antenna.

The occasional improvement in SNR with the loop is worth looking into. Maybe it’s the location away from the house. Or the lack of near B-field noise, compared to the near E-field noise that the dipole will respond too. It’s difficult to put my dipole in that location (or move it at all), as it’s so big and there are power lines nearby. The compact size of the loop is very useful for repositioning.

Time is a factor, my noise level varies over the day as various noise sources come and go. Over the course of the day I can see at least three different noise sources, for example (i) high energy clicks 20ms apart that seem to be coming from power lines (ii) some sort of (lighting?) noise from a neighbour in the evening, plus (iii) some other noise with a 30us period. Urban HF noise is a complex problem.

Reading Further

[1] Antenna Physics – an Introduction
[2] Antenna Theory Loop Antenna Page
[3] Experimental methods in RF Design, Ch2, Fig 2.34, Class A amplifier
[4] FT817 short circuit power supply repair – the exact fault I induced in my FT-817. This post was very helpful in fixing it!
[5] VK6CS Magnetic Loop Antenna Matching