19 Feb 2015

Multiple RX responses on 160m?

I am not aware of this on other bands but on several stations I am giving WSPR reports to the same station at the same time but at several frequencies separated by 50Hz or 100Hz from the main response. I am not aware of this being an issue on other bands or an issue on TX.

Most recently I saw the issue with ON7KB and with G4BOO earlier. The issue seems to be with my end and not a TX fault.   It is not as if signals are that strong, for example the strongest from ON7KB was -13dB S/N.  As I have not seen this before and as I am a "transient" 160m visitor I am not keen to start pulling things apart.

Anyone else seen this?  I use a SignaLink USB to interface the FT817 to the PC. Maybe I have some RF getting into the heavy duty linear PSU  I am using and this only happens on 160m?

For example, I have never seen the issue on 630m (472kHz) WSPR with the same FT817, same PC interface via the same SignaLink USB.  Tonight I am not on 472kHz as I am using the kit on 160m WSPR.

I intend to stay on 160m overnight using the earth-electrode "antenna".


Anonymous said...

Have seen spurious signals with an Icom 7062 IIG, seems to be audio intermodulation, slight drop of levels very rapid drop of products.
However the very interesting bit is that Roger's earth electrodes work well on Topband.
This does indicate that people who cannot put wires up in the air CAN work Topband if they have ground access, a very interesting day's work- great stuff Roger!


Anonymous said...

One other point, WSPR seems to ignore the false weaker images. 160 does have a few constant carriers so intermods would just have the modulation of a WSPR signal but inverted images would not decode. Intermods between two WSPR signals have wider deviations and are visually obvious. Going to be odd order products to get inband signals.


Roger G3XBM said...

Thanks Alan. Best DX on TX was Spain last night.

Anonymous said...

Hi Roger,

I have the same problem on RX & TX.

It is a 50/100Hz voltage differential induced as current flows on the coax cable between the mains supply earth connected to the kit in my shack and the RF earth at the end of the feed coax.

Ideally the two earths should be properly bonded with a suitably rated cable, but the diameter of cable required to do this is often excessive when coax runs are long.

The best alternative is to connect coax inner and outer conductors at the far end via a suitable Low frequency blocking capacitors, say 0.1uF at 1KV, which will prevent current flow at 50/100Hz but will pass RF with hardly any loss.


Martin - G8JNJ

Anonymous said...

It is very probable that there is a mains derrived circulating current between two earth electrodes, it would be interesting to have an idea of the magnitude. In the past I have seen a few volts difference between rods that were around 100m apart. Assuming a loop R of -say 20 ohms at AF that could be 100mA through a ferrite core's windings which could cause magnetic modulation, that would be at twice the 50Hz rate. Obviously inserting a capacitor anywhere in the path between the rods will reduce the 50Hz current to a very low level, I would -like Martin-reach for a 100nF or so but failing a local lightning strike not worry about the voltage rating, this is a sensory system not a ground for a pylon.

If Roger uses the same config on 474KHz then the 100Hz sidebands should also have been apparent so something else is different.

I did have an interest in geophysical prospection some time back, a series C is good practice as even given identical electrodes direct potential differences due to differential chemistry can cause unexpected currents and corrosion of electrodes/ connections. The normal resistivity survey is performed with around 1mA at 1KHz from a constant current source into two "current" electrodes, the two potential electrodes indicating resistivity. It's always AC coupled!.

Still think that Roger's experimental results are very interesting.