Archive for September, 2019

WKCW and AM Broadcast RFI

Friday, September 6th, 2019

I have three HF antennas at my QTH… a 40-meter OCF dipole, an 80-meter OCF dipole, and a 160-meter horizonal full-wave loop.  As a newly-licensed ham a few years ago, the 40-meter dipole was my first antenna and (thankfully!) performed largely like the books suggested a dipole ought to work.

Two months in, though, I hung the 80-meter dipole.  That antenna was not just longer than the first one, it was higher.  No surprise, it was (and is) a better antenna pretty much everywhere.

But it brought with it a strange curiosity… my MFJ 259C antenna analyzer rendered inconsistent numbers in some cases, and flat-out crazy ones in others.  Wonky enough that I wrote MFJ about what I was seeing.  They suggested I send the unit back to them so they could take a look at it.

But, then, on a hunch I took a set of readings from that first, 40-meter dipole… and those numbers were both in the range I expected as well as consistent with the spreadsheet of readings I made when I first hung the antenna.

More than a little confused, I put my 259C back in its box and put it on the shelf, fairly convinced it was an unreliable piece of gear.  (You can’t go long in the ham radio hobby without hearing all the disparaging remarks about MFJ – much of it justified – and so such conclusions aren’t terribly surprising).

Hint:  Those biases we too often wrap ourselves in rarely help us towards the truth!

Fast forward another couple of months and a much awaited box from Elecraft showed up.  You know those moments of anticipation when you first hook up a new rig, imagining all those people and all those places it might connect you with?  Yeah.  You can imagine my surprise when I first hooked up that glistening, new radio to my coax switch and slipped the headphones over my ears, only to find the bands were alive with… the sound of music!  Classic, 60’s and 70’s rock music to be precise.

Turns out the Elecraft KX3 has an astonishingly capable receiver.  The epiphany began to unfold.

On a hunch, I pulled out a little-used transistor radio and with the headphones to the KX3 still over my ears, punched up the AM broadcast band.  In a couple of seconds I had my answer.

WKCW.  1420 AM.  The radio station – or, at least its broadcast antenna – that I had driven past a million times.  Because it’s less than a mile from my house.

The good news is that a little dab of attenuation was all that was needed to extinguish Neil Diamond and The Beatles and Diana Ross whenever I wanted to operate HF.  But there was a larger story going on there.

A Sark-110 and, later, a Rig Expert AA-600 joined that unjustly-maligned MFJ 259C in the shack.  WKCW’s 22,000-watt daytime signal might easily be hidden simply by turning to the attenuation control on my rigs.  But the miniscule signals those antenna analyzers depended upon to suss out their data were completely overwhelmed.  A long conversation and a series of tests with Melchor Varela, EA4FRB – the Spanish designer of the Sark-110 – confirmed as much.

Elecraft’s panadapter for the KX3 – and, later, the bandscope on my Kenwood TS-890S – gave a visual reference to what was going on.  Without attenuation the displays light up with yellow and white RF energy, pulsing with the peculiar, strobe-like beat I’ve come to jokingly call “The Marching Band.”

The bottom line is that one simply cannot make accurate antenna measurements of physically long or tall antennas – the irony being that the better the antenna is, the more it is affected – at my QTH while WKCW is broadcasting its daytime signal.  You have to wait until darkness, when their signal drops to 60 watts.

It begs several interesting questions, including the degree to which even “reasonable” and expected SWR readings – like on that not-too-high 40-meter dipole of mine – might not actually reflect the true nature of things.

And although the mixing products of that commercial AM signal can easily be dialed out with an Attenuator button, that’s kind of like throwing the baby out with the bath water.  You lose more than just WKCW.

My survey of the literature found a lot of superficial references to “Broadcast AM interference,” mostly under the larger rubric of RFI in general, but not much in-depth dissection.  In particular, there wasn’t much on the actual mechanism of how AM interference does its thing.

Visit DX Engineering or Ham Radio Outlet or Palomar Engineers or even widely accepted subject matter experts like Jim Brown, K9YC, and you’re left with the inference that it’s a common mode current problem.  That the bouncing beat of Jefferson Airplane is riding down the outside shield of your coax.

A bunch of expensive Mix-75 ferrites will quickly disprove that notion, however.  WKCW is intent on taking a much more direct route into your shack.

Short of throwing your antenna up inside a gargantuan Faraday Cage – I think John, KX40, might be the only one around who might be able to do that – I don’t see there ever being a solution to the conventional put-a-miniscule-signal-out-on-the-wire-and-read-what-comes-back antenna analyzer problem.  But lighting up your antenna with 100 watts of RF and reading that with a Vector Network Analyzer gets around that nicely.

Or just wait until it’s dark.

As for sharing your receiver’s front end with all that wonderful classic rock… here’s a high-pass filter that works a treat…

Yeah, it’s pricey.  But it’s got a very sharp roll-off between 1.7 and 1.8 MHz, so if you’re wanting to work top band, you can.  And other than the very, very bottom of 160 meters, insertion loss at 0.1 dB is hardly noticeable. It’s limited to 200 watts, but if you put it between your rig and your amplifier, that’s not a problem.

It makes a profound difference.

I’d love to hear how others may have dealt with AM broadcast RFI in general, and WKCW in particular.  Especially if you’ve had any experience with wave traps, as they’ll probably be my next area to explore.

73, Jeff K4EI