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

Lee STRAHAN k7tjr at msn.com
Mon Sep 30 13:08:15 EDT 2019

Hi Matt,
   Thanks for the additional information. Even though you are filtering the harmonics from the source transmitter the SDR if overloaded will generate its own level of harmonics. Please be very careful if you use a spectrum analyzer and do NOT connect it to your antenna. There will possibly be enough energy on the RX antenna to burn out the front end of the spectrum analyzer. No need to ask me how I know this suffice to say building high level signal generators put out enough signal level to burn out the old HP spectrum analyzers. At the most a 4 inch piece of wire (maybe more length if needed) feeding into the analyzer with a close transmitter will yield plenty of signal for display. The best advice I can offer is to use a 20 dB pad on the input of the analyzer and leave it there. Unless of course the analyzer you use is rated for high levels of RF. Start with small signals and work your way up slowly paying close attention to the fundamental frequency level and not the harmonics for the maximum to connect to the analyzer.
Good luck and be careful.
Lee  K7TJR  OR

-----Original Message-----
From: RFI <rfi-bounces at contesting.com> On Behalf Of Matt NQ6N
Sent: Monday, September 30, 2019 9:27 AM
To: JW <jwin95 at yahoo.com>
Cc: rfi at contesting.com
Subject: Re: [RFI] tips for finding the source of broadband mixing products

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 weaker.

- 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 info.

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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