Troubleshooting a troublesome QTH
(Receiving HF in a RF polluted environment)

(c) 2001, Paolo Cravero, IK1ZYW

(September 2001)
I proposed him my BFO, to accompany his HF cheap receiver, but he immediately reported he could only hear

• an impulsive noise (300 Hz square wave)
• an FM broadcast image/splatter

(October 2001)
Then I introduced him to a fellow HAM (Angelo "QLD"), that showed him the HF universe and borrowed him a 2-conversion HF RX (LOWE SRX-30D). Fabio has also been instructed on how to build a dipole. He does it, installs it at home and... no changes: both problems are still present, all over the band, even with a more professional receiver.

First the impulsive noise could be tracked down: the power supply of low-consumpion fancy alogen lamps in the flat. With the lights turned off, no noise can be heard. Nowhere. In fact those devices use some fast square-wave switching and have no protection from tunnelling RF into the power line behind them. Nor from radiating into the air.
Fortunately neighbours don't use the same lightning system, so Fabio can control the impulsive noise.

(27 October 2001)
The remaining noise requires a visit at his QTH! From Fabio's descriptions it seems that he lives right under a bunch of FM broadcast transmitters!
His room faces the Torino hill, that is overcrowded with FM broadcast (BC) transmitters: I am sure they drop in very strong, since I live in a similar location. On the other hand, at my QTH, I don't have those interference problems! We have not tried using eachother's equipment in both QTHs (heavy and we live quite far apart, 40m traffic drive one-way).

My first though is that there is some local strong signal that mixes with FM broadcasts, and brings them down to HF. This theory is enforced by the fact that BCs location on HF varies, according to the time of the day, randomly. So we made some observations:

• located an FM broadcast image on HF (QRG1)
• located it on an FM receiver with digital reading (QRG2)
• computed QRG2-QRG1

(3 November 2001)
Some days pass, and I find an AC filter in my junk box. I prepare it with the correct plugs, and we insert in on the power cord of the LOWE RX. No changes. Still, we leave it on.

During the same visit to Fabio's QTH, I tune 28,605 MHz, where a broadcast image/splatter is always present. I happen to unplug the antenna from the LOWE and.... SURPRISE !! The BC is still there, weak, but audible! I also try to short-circuit the antenna input: still audible!
Putting everything together, we can draw some conclusions:

1. there is no 72 MHz coming through the power line (interference present with battery supply)
2. interference is generated within the receiver
   1. with the signal coming through the antenna
   2. with the signal directly entering into the RX case

2.1 Quite simple: we need a low-pass filter on the antenna input. The receiver covers 0-32 MHz, so the filter has to cut everything above 30-40 MHz. My junk-box has a 3-pole LC low-pass, with -3 dB cutoff frequency set to 60 MHz. I got it from Coilcraft (P3LP-606) and it has a characteristic impedance of 50 ohm, just what we need. From the datasheet, attenuation at 88-108 is better than -10 dB. I wire it and try it.
Little or no improvements can be heard on higher HF bands, but on 14 MHz and down the noise is dramatically reduced!!! Filter insertion loss is evident (0.3 dB from datasheet) but negligible comparing with the signal-to-noise ratio (SNR) improvement!
In order to give the reader an idea of the improvement, no signal could be heard on the broadcast band on 12 MHz without the filter. With the filter, the band sounds "normal" to my HAM ear.

2.2 This is harder to minimize. One possibility would be to ground the RX case. But Fabio lives on the 4th floor, and I doubt his ground path is effective (would be the heater pipe, anyway). But we found that relocating the receiver into the room helps reducing interference! This is probably due to the multipath propagation of interfering BCs into the room, that at some points cancel eachother. Well, in other points the signal becomes much stronger, though!

Unfortunately the LOWE receiver lacks of any schematic diagram. If the filter could be inserted right before the antenna preamplifier, or after the input tuned circuits, I bet also interference of type 2.2 would disappear. But I doubt Angelo, the LOWE owner, would let us play inside it...

Some considerations on the antenna.
The dipole mentioned above has soon been replaced with a 10m long thin wire, ran around the room or thrown down the balcony. Relocating the antenna through those points where multipath brings a constructive interference (paths are in-phase and sum eachother, not cancel), receiver's noise increases A LOT.
Antenna length has proven to have effect only on signal intensity, not on interfering noise level.

What I would like to try now, is the P7LP-156 Coilcraft filter that I also have in the junk box. It provides a -3 dB cutoff at 15 MHz, and more than 50 dB attenuation in the 88-108 band!

Bibliography

1. Coilcraft, "Low Pass LC Filter Modules", Document 118-1, rev. 4/9/01

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