[SFDXA] Review: the “X-Phase QRM Eliminator” Victor Rosenthal, 4X6GP

[Bill]

From the CWops list. I know Vic through a close friend since I was a Novice in 1958 - Bill W2CQ 

This is a review about a smallChinese low price noise canceling unit. It works. 


> Noise reduction
>> Recently I posted my review of the cheap Chinese antenna noise canceler. Today I made a recording to show how effective it is, which is attached (along with a copy of the review if you didn't see it). I didn't take a lot of trouble with the recording; it's just a random signal. Maybe later I will take more time and find something that is actually buried in noise. But you can hear how it works: the first 6 seconds or so of the recording is with the device bypassed. Then it is on until about 20 seconds into the recording, when I turned it off again. 
>> -- 
>> 73,
>> Victor, 4X6GP
>> Rehovot, Israel
>> Formerly K2VCO
> 
Review: the “X-Phase QRM Eliminator”
Victor Rosenthal, 4X6GP
One of the biggest problems HF operators face today is the increasing noise level on the bands. Switching supplies, dimmers, and countless other electrical and electronic devices all generate noise. National and international standards for RFI are often ignored, and even when a device is compliant, it may radiate an unacceptable amount of noise for weak-signal reception in the vicinity.
How it works
One way to improve the signal/noise ratio at HF is to use an antenna phasing unit. The device is connected between the main antenna and the receiver, and to a separate noise antenna. It sums the signals from the two antennas, allowing for adjustment of the gain of each channel and the relative phase. The operator balances the gains and adjusts the phase to null undesired signals or noise. It works best when the s/n ratio on the noise antenna is much worse than that of the main antenna. A phasing unit can also be used to steer an array (for example, a pair of verticals) by changing the relative phase and amplitude (just like an AM BC station with a directional array). Of course, it is only usable on receive.
Commercial and homebrew devices
There are several devices on the market (here are some of them), with prices ranging from about US $210 to US $900. There are several versions that hams have built themselves. And then there is the Chinese “X-Phase QRM Eliminator,” which can be had for about US $46 (the price has gone up by a dollar since I got mine). In the past I have owned an MFJ-1026 and a DX Engineering NCC-1 (the $650 predecessor to the current $900 NCC-2), so I have some idea of what to expect.
My experience
I have had the device for about a week. I have connected it to my TS-890s between the RX IN and RX OUT connectors, which allows it to be in the receive path without exposing it to transmit RF. Some transceivers do not have this capability, which requires you to put it in the put it in the antenna circuit for both receive and transmit. In that case, there is a PTT-operated relay in the box which bypasses it on transmit. I haven’t tried this, and it raises questions about the suitability of the unit for fast QSK as well as its lifetime in such service.
I am using my small amplified loop antenna (Pixel Loop, now sold by DX Engineering) as the noise antenna. This is a relatively quiet antenna, which is not what you normally want in a noise antenna! But it is directional, and most of my noise comes from a tall building about 100m away; so I have found that it is effective when aimed at the building. I intend to experiment with other noise antennas in the future.
In order to use it, I select RX ANT on my transceiver, which puts it into the receive antenna path (If it is connected directly in line with the main antenna, it's possible to just turn the unit on or off, in which case it is bypassed). Then I fiddle with the three controls – mostly the GAIN 2 (noise antenna gain) and PHASE knobs for the least noise. The GAIN 1 (main antenna gain) seems to be less critical. Usually, when the noise is nulled, the signal strength is also reduced; but the signal-to-noise ratio is significantly improved. The TS-890s has a lot of surplus gain, so I compensate by turning up the RF gain slightly. Some comments that I’ve read say that these devices are difficult to adjust; but I’ve found that it’s quick and easy.
The device definitely works. It may make it possible for me to copy signals that are too weak to copy without it. Obviously it can be more or less useful depending on the number of noise sources and their relative locations. The noise antenna and its placement are extremely important, and may require experimentation to optimize performance.
Although I don’t have the MFJ or DXE units any more to compare to it directly, it seems to me that it works at least as well as the MFJ box. 
Device protection
Even if you do not connect the unit in line with the transmit antenna, you need to worry about the RF picked up by the noise antenna when you transmit. So in that case you might want to use the built-in PTT-operated relay, or some other method, to protect it and the receiver to which it is connected. In my case, my loop antenna already has such a relay in it, so I don’t need to do anything additional.
Quality and accessories
The unit seems sturdily constructed, in a solid little box with UHF connectors. The potentiometers feel very smooth. Whether they will hold up over time is as yet unknown. It comes with a pair of PL-259 to SMA connectors, apparently since they also sell an SDR receiver that uses SMAs. You also get a power cord for the 13.8v supply a plug for the PTT control circuit, and a small telescoping antenna with an SMA connector on it that could be used for a noise antenna. I would be wary of using this because of the possibility of RF overload or static electrical damage to the unit.
Ethical issues
The device looks externally like the WiMo QRM Eliminator which sells for US $210. I have no idea if it is a direct copy of the circuit. If I had known about the WiMo unit, I would have investigated that first. I’m not comfortable encouraging knockoffs of equipment that has been priced to include R&D expenses, as well as decent wages for employees.
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