DESCLAIMER
– This writeup or any other articles I share are purely my practical observations/experiences
and do not claim authority on the subject. It is upto reader’s discretion to consider
its value as applicable in similar situations. For accurate, scientific and authoritative
expert knowledge, you are strongly encouraged to seek books, professionals and experts
in the subject.
Introduction:
Most of us one time or other observed RFI issues. There
could be various reasons due to which RFI is observed such as unfiltered SMPS, electrical
arching on the poles, harmonic generated by the amplifiers etc. There may be
considerable unwanted RF floating via Coax shield due to imbalance near the
antenna load feed point as well. Coax shield’s current issue is mostly solved
by common mode chokes. Whether in the form of ugly BalUn (length of coax wound
as a coil near the antenna) or ferrite beads placed on coax or Coax wound on
toroid etc. There is a wealth of information available on website nowadays and
can be easily accessed. Couple of excellent references are mentioned towards
the end of this article.
Intent of this article is to share another angle of RFI
within one’s shack, and my own experience in resolving the RFI for my SDRPlay receiver
interfaced with Kenwood TS590.
For sometime now, I have been using with SDRPlay
receiver along side with TS590 transceiver. I had done a simple mod
for TS590 to tap RX only output and feed it to SDR. Though it works very well
as panadaptor and additional receiver capability taking advantage of TS590’s internal
TRX switching, I had always found that panadaptor showing strong presence of RF
when transmitting on TS590. This was the
fact event when SDR was not connected to the RX port. Stray RF in the shack for
sure was causing some issues.
Setup:
- Kenwood TS590 with RX out mod
- BufferAmp from Clifton lab Z10000
- NKK dummy load
- SDRPlay RSP1 SDR receiver
- HDSDR software with Omnirig synchronization
Equipment layout for testing:
Following is how I figured out the root cause:
Step 1. Removed coax between TS590 RX Buffer amp out to
SDRPlay – No change observed
Step 2. Shorted SDR’s RX antenna input SMA port – No change
observed
Step 3. Placed SDR Play inside aluminum box – No change
observed
Step 4. Distanced USB cable of SDRplay from any other shack
cable – change observed as distance increased from computer and other equipments
Step 5. Wound SDR’s USB cable few times on Toroid of 43 mix.
Saw a noticeable change in SDR display. So stray RF is entering via USB cable
to SDR’s circuitry for sure. I continued with changing number of turns on the
toroid, changing the bands and changing the mixes etc.
Following are the further snapshots of various band observations
with actual snapshots from HDSDR.
Band
|
8 Turn Mix 31 (240-31)
|
8 Turn Mix 43
(240-43)
|
8turn each Mix
31 and Mix 43 in series
|
160m
|
 |
 |
 |
80m
| Not tested | ||
40m
|
 |
 |
 |
20m
|
 |
 |
 |
15m
|
 |
 |
 |
12m
|
 |
 |
Not tested
|
10m
|
 |
 |
 |
Conclusion:
Above pictures explain what cores were working for the
situation. I could not do very accurate test of audio signal as I was not using signal generators. As per my observation 31 Core and 43 behaved similarly at first sight. At lower bands, I could see tiny to some differences. However, as we moved to higher bands, 43 had slight edge. on 12 mtr and 10 mtr combination of 31 mix core and 43 mix core worked better. I didn't had 61 mix core at hand. I understand that they works better at those bands.
Many a times some of us constraint ourselves thinking that aspects
of the hobby are highly technical in nature. Though it is so, for the real seeker
and ham who wants to advance to next level, it cannot be a bottleneck. I
have not touched subject of measurement of complex impedance, phase, permeability,
power saturation etc. though I know a very little in the area, it is still a
bouncer for me and also not the intent. It is all about learning, applying it
to practice and sharing what worked!.
Hope this helps someone in similar situations.
73
DE VU2XE Kiran
