[sdiy] Preventing Radio Ham Interference

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Tue Nov 29 14:21:51 CET 2016

> I have done the ferrite clamps and they seem to work on the hi-fi when
> added to the speaker and mains cables.

Get the ferrite clamps as close as possible to where the cables enter 
the amplifier.  Check the enclosure is earthed.

> 20W causes a noticeable buzz on the Event monitors in my music area
> even with the audio cables unplugged and ferrites on the mains cable.

Solder high-quality (low-k) 10nF ceramic disc capacitors directly across 
each diode in the bridge rectifier(s) of the power supply.  The buzz is 
caused by the varying capacitance of the diodes in the bridge modulating 
the RF as they pass through each mains cycle.  You can stop this by 
swamping the diode's reverse capacitance.  This is commonly done in 
equipment designed for the broadcast industry that is "rad hardened".

> Getting him to
> test it with a dummy load sounds a good idea

You should get no RFI problems as there *should* be no radiation.  Any 
RFI suggests leakage from the transmitter or the feed line.  Might be 
tricky getting the dummy load up into the previous location of the 
antenna though.

> although it should be
> noted that the same rig driving a different but omnidirectional
> antenna causes no problems.

Then it is likely that either:

1. The high forward gain of the beam antenna is causing the field 
strength to be much higher in your studio,


2. The new beam antenna isn't installed correctly.

 From what I can remember, you can just stick a omnidirectional 
quarter-wave whip antenna straight on the end of the coaxial feeder, and 
it will form a good match and radiate all of the power as intended 
provided it's element length is correctly adjusted:


However, you can't just stick a beam yagi antenna onto the end of a 
coaxial feeder because it requires a balanced drive:


Connecting it directly to the end of un-balanced coaxial cable results 
in reflected power and a large RF voltage can develop on the outside 
screen of the coaxial feeder line.  This can cause lots of problems as 
the coax feeder now becomes an antenna and radiates.  This is not meant 
to happen, and can couple RF into nearby metalwork, house wiring etc.  A 
balun (choke balun?) is supposed to be used at the antenna end of the 
coaxial cable to transform between un-balanced and balanced, in order to 
prevent RF coming back down the outside of the cable and wreaking havoc. 
  This might be built into his antenna, or it might not!  I think you can 
fit a ferrite sleeve to the feed-line at the antenna end, or even just 
coil up a few turns of the coax in this location to improvise a balun, 
but the ham guys could probably comment more about this?

> I will look into adding some capacitance across the bridge rectifier
> diodes of the modular power supply.

This is more for preventing buzz in amplifiers, mixers, etc. but it may 
help.  I'd go for screening the modular enclosure better, in particular 
paying attention to any seams in the metal-work, and filter the mains 
where it enters the enclosure.  A VCO may detune due to RF because of 
the non-linear response of the expo converter to RF, or because the 
saw-core comparator will trip early and shorten each oscillation period 
if there is RF present at it's inputs.

> However, that idea I had
> of buying an old shipping container and converting it to a home studio
> is starting to become more and more attractive.

Shipping containers are often used for improvised RF test chambers.  
Just filter the hell out of the mains where it enters the chamber.  Also 
pay careful attention to improving the electrical contact around the 
seams like the entry doors...  These leak RF badly unless there are 
multiple points of electrical contact at most a tenth of a wavelength 
(20cm) apart.  That's why real EMC chambers have those fancy copper 
finger contacts all the way around the door seal.

Who pays for this, whether you want to work inside a shipping container 
with no windows, and the acoustic treatment required for the inside are 
surely big downsides though.


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