[RFI] tips for finding the source of broadband mixing products

Matt NQ6N matt at nq6n.com
Mon Sep 30 12:26:46 EDT 2019

I'll add a bit more information.  Thanks much for the suggestions so far:

- The rig is a Flex 6600 SDR transmitting 1 Watt.  There is a Morgan
Manufacturing 40m bandpass filter (I measured it as having 28 db of
attenuation at 14 MHz) between the rig and amp (which is in standby) and
also a VA6AM high power 40m bandpass filter (air coil version with claimed
performance of -70 db at 14 MHz) after the amplifier.

- The TX antenna is an inverted vee in the back yard, and the RX antenna is
an inverted L in the front yard.  So only about 50' of physical separation
of omnidirectional antennas, but lots of BPF filtering and low TX power.

- Completely disconnecting the RX antenna weakens the second harmonic
signal quite a lot and makes the note sound like a pure sine wave, but it
could be that I am not hearing the IMD products when the signal is so much

- I had a typo in my original message. It was the second harmonic. I'd just
been listening to the third harmonic when I hastily typed my message, and
didn't notice before hitting send.

- I'll be onsite today with a spectrum analyzer, so any tips on things to
measure would be helpful.  Just wanted to update the thread with the above

Matt NQ6N

On Mon, Sep 30, 2019 at 6:35 AM JW via RFI <rfi at contesting.com> wrote:

>  re: "If that happens, there are errors in design and/or construction."
> 50 or 60 dB down?
> Pshaw.
> de AA5CT
> PS 50 dB down can STILL be quite receivable WHEN the rig is operated into
> a
> dummy load. It's WHY operators often think their equipment is faulty IF
> they
> don't sample the RF coming out the antenna port but rather just 'listen'
> with
> another rig nearby connected to an antenna or just a piece of wire.
> One of the rigs we demonstrated this on was an old Collins tube-type radio
> too ...
> These 'things' are easily demonstrable, Jim Brown. Surprised you have
> _not_
> encountered it.
>      On Sunday, September 29, 2019, 10:28:53 PM GMT-5, Jim Brown <
> jim at audiosystemsgroup.com> wrote:
>  On 9/29/2019 7:48 PM, JW via RFI wrote:
> > How are you determining all this - you do realize, in close proximity to
> > a transmitting rig, much (albeit low-level) RF comes straight out the
> > power leads
> If that happens, there are errors in design and/or construction.
> and can 'modulate'/be modulated by the power supply
> > energy that also escapes, including simple rectifier supplies using only
> > diodes?
> Ingress/egress is a linear function, and depends strongly on details of
> both design and construction. Nearly all modern equipment fails to
> terminate cable shields and power green wires properly. They SHOULD go
> the the shielding enclosure (chassis), but they nearly always go first
> to the circuit board, THEN eventually find the chassis after wandering
> around return circuitry for a while. This equipment flaw, first
> discovered by a ham working in pro audio, is called "The Pin One
> Problem," because the designated shield contact of the connector
> commonly used for balanced audio circuits is Pin 1.
> The method in which equipment is built usually makes it impractical to
> correct these design errors, so the best fix is a serious common mode
> choke on the cable(s) involved. And because the ingress/egress is via
> the green wire or the cable shield, conventional line filters are
> useless UNLESS they are internal, and with their shielding enclosure
> bonding the green wire to the equipment shielding enclosure! They treat
> only the differential voltage and current between phase and neutral,
> phase and ground, and neutral and ground.
> The only effect of signal strength is on the strength of the mixing
> products.
> >
> > We demonstrated this at Heathkit several different ways, including using
> > a spectrum analyzer to 'sniff' the stray RF coming back out via the
> radio's
> > power cable WHICH in turn was modulated and showed 120 Hz sidebands ...
> So you added an AC line filter with its shielded enclosure bonded to the
> chassis, right? THAT would work.
> 73, Jim K9YC
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