Getting rid of QRM with diversity receive mode

It has been around for a long time, but is only used on a small scale in the ham radio world; diversity reception. This is a pity, because diversity reception not only offers the possibility to reduce QSB (fading). With diversity you can also eliminate QRM (manmade noise) very effectively. Before I explain how you can do that, some more explanation about this reception method.

What causes QSB (fading)?

A radio signal that is converted to audio in your receiver often consists of several components. Especially with radio waves that are reflected in the ionosphere, these can be many different components. They amplify or attenuate each other. That is what causes those signals to increase or decrease in strength. Known in our world as QSB (fading).

Two separate antennas to reduce QSB

A well-known QSB phenomenon is the rotation of polarization. If the vertical component of a radio wave is strong, you won’t benefit from it with a horizontally polarized antenna and vice versa. You can overcome this by receiving with both a horizontally and vertically polarized antenna. Then switch to the antenna that has the best reception at that moment.

Diversity through multiple antennas and receivers

Diversity reception is a method that has long been used on HF. Initially mostly for military communications. It is still used to improve reception of weak signals. Multiple antennas and multiple receivers are linked together. By shifting the phase of the radio signals of the different receivers and their connected antennas, adjusting the strengths and then combining them together, you get an optimal signal-to-noise ratio (SNR). Literally using diversity.

Diversity dual receiver back in the 50’s in use by the US Navy.

Amplify wanted and attenuate unwanted signals

You can use diversity in this way to enhance the reception of a radio signal. What was lost in the noise can be made intelligible with diversity. The ability to amplify signals can also be used the other way around: to attenuate certain signals. Attenuating signals makes me think of only one kind of signal: manmade noise or QRM!

Receivers with diversity on the market

On the market for ham radio equipment, the range of receivers with diversity option is limited. However, there are more and more of them. Receivers equipped with two tuners often offer this option. An excellent model receiver is the RSPduo from SDRPlay. This SDR receiver has a range of 0 to 1200 MHz and has two separate but identical tuners, each with it’s own antenna connector. So with two antennas and the SDRUno software, you can already get started.

SDRPlay RSPduo receiver with dual tuner.

Limited effect with the QRM Eliminator X-phase

Previously, I was using the QRM Eliminator X-phase. That was effective on certain types of QRM, but could not eliminate some sources of interference. So I started working with the RSPduo. Initially with my Cobweb combined with a Miniwhip or M1GEO Wellgood loop. That already worked pretty well.

Wellgood loop by M1GEO active antenna for use in HF diversity receiver for noise reduction.
The Wellgood loop v4 with 15mm aluminium strip used as loop material.
Mounted onto a T-shape pvc tube construction.

How to use diversity with the SDRuno software

In the SDRuno software, you switch to DIVERSITY mode. The software will launch the control box. When APPLY AUTO is lit up, the software will automatically combine the signals into finding the best signa with te least QSB.

But in our case, we want to attenuate the QRM. That needs the human hand! First, you turn off the APPLY AUTO mode. Then, by turning the pointer in the diversity clock around, you shift the signals from the two receivers in phase from each other. By sliding the pointer in and out you increase and decrease the signal from one receiver stronger and weaker relative to the other receiver. It takes some trial and error, but eventually you will be able to find the best settings and eliminate the nastiest QRM!

Two dedicated receive antennas

It’s not really convenient if the antenna you transmit on is also the antenna attached to your RSPduo. Then you need to put an expensive TX switch between antenna, SRSPduo and transmitter, such as the MFJ-1708B-SDR or SV1AFN’s TRS-460. It can also be simpler; just connect two antennas to the SDRduo.

In my case, I added a second Wellgood loop connecting two loops to the RSPduo. This turned out to be a golden touch, as I reduced even the most persistent QRM from 0 to 30 MHz to virtually zero. The QRM Eliminator X-phase has proved very effective for me in eliminating certain sources of interference. But the RSPduo in diversity mode is in a class of its own. Watch the Youtube video I recorded.

How much is that going to cost?

Now you may be wondering, how much money is all this going to cost. I put the three most common solutions side by side (price level April 2023):

System (commercially available)Total price
QRM Eliminator X-phase with Miniwhip as QRM antenna 200 euro
NCC-2 Receive Phasing System with Miniwhip as QRM antenna1330 euro
SDRPlay RSPduo with two M1GEO Wellgood loop* active antennas360 euro
This excludes coax which can be as simple as RG-58, RG-59 or RG-6.

*The Wellgood loop you have to partly assemble yourself, but that is not too difficult. Rather not build it yourself? Then the MLA-30+ Chinese loop is a fair alternative.

Realize that with the purchase of an SDRPlay RSPduo, you also get a pretty good SDR receiver. But whatever you do or choose, I wish you lots of QRM free DX!