Passive AM Monitor Antenna

At the place of my former employment, there is an issue with AM reception. The building is full of old, electrically noisy fluorescent light balasts, computers, mercury vapor parking lot lights, and every other electrical noise generator under the sun.  The second issue is that one of the EAS monitor assignments for two FM class B stations is WABC in NYC.  Under normal conditions, WABC puts a fine signal into the area.  Listening to it is not problem at my house, in the car and what not.  However, at the studio the station is audible, but terribly noisy.  Every time one of those FM stations  ran a required monthly EAS test originated from WABC, it was full of static and just sounded bad on the air.

The state EAS folks were inflexible as to the monitoring assignment.  “WABC is the PEP station for NY.  You should have plenty of signal from WABC at your location,” said they.

At one time, the studio had an active loop antenna (LP-1A) from Belar, which worked, but also seemed to amplify the noise.  I decided that the best thing to do was go big and ditch the preamp.  I made a diamond shaped receiving loop on two pieces of two by four by eight foot lumber.  I wound four turns of #14 stranded wire around this frame and made a 4:1 balun to feed the unbalanced 75 ohm RG-6 coax.

That cured the noise problems and for eight years, WABC sounded pretty good on the EAS monitor.

Fast forward to about a week ago.  The roof at the studio building was being redone and all the monitor antennas had to be removed from the roof.  The homemade loop was not in good shape.  The balun box was full of water, the lumber was cracking and falling apart, the insulation was degraded by UV exposure, etc.  My boss asked, “how much to make a new one?”  So I said something like forty dollars and a couple of hours.  He then said, “Make it so we don’t have to ever make another one.”

Music to my ears.  I started by checking my assumptions.  I made a model and ran NEC to see what the electrical characteristics for that size loop were on 770 KHz.   It came out better than I thought, about 1 ohm resistance and 282 ohms inductive reactance.  Fooling around a little more showed that roughly 1.3 uH inductance and 720 pF capacitance in a L network would bring this inline for a 50 ohm feed point.  Since this is a receive only antenna, that is not a prime consideration.  I am more concerned with noise reduction and maintaining at least the bi-directional quality of a loop antenna.

NEC 2 model AM receive loop
NEC 2 model AM receive loop

Then, I decided to get fancy.  What if the capacitance was put on the end of the loop to ground instead of the feed point.  That, in effect should make the loop directional off of the unterminated side.  Driving the feed point with a 9:1 balun would also bring up the inductance on the feed point.  Finally, grounding the whole thing with a separate ground lead might also get rid of some noise.

The final configuration looks something like this, which is essentially a top loaded vertical:

Low noise AM loop antenna
Low noise AM loop antenna

Now to build it.

Once again, I felt that a non-conductive support was needed, so I used two by four by eight foot lumber, but this time I painted them with oil based paint.  The side length worked out to be 5.7 feet per side, or 23 feet per turn for a total of 92 feet of wire.

I purchased 100 feet of PV (photovoltaic) wire (Alpha wire PV-1400), which is UV, heat, moisture resistant and designed to last for 30 years in outdoor, exposed environments.

For the balun box, I used a metal outdoor electrical box with a metal cover.  I put a ground wire jumper between the box cover and the ground common to maintain shielding.  I used a water tight bushing to feed the antenna wires and the ground wire into the box.  I drilled a 3/8 hole for a type F chassis connector.  Everything was given a little extra water proofing with some silicone based (RTV) sealant on all threaded junctions.

The spreaders for the wire windings are UV resistant 1 inch PVC conduit.  I drilled four holes, three inches apart in each spreader to run the loop wires through.

The balun is 7 trifiler turn of 24 AWG copper wire on an FT-43-102 toroid core.  Trifiler means three wires twisted together before winding the toroid core.

I used all stainless steal screws and mounting hardware.

The loop is terminated with a 500 pF, 500 volt ceramic capacitor to ground.  Once in place, I am going to experiment with this by jumping it out of the circuit to see what effect it has on noise and signal strength.  I may also try replacing it with a 200 ohm resistor and or a 1000 pF capacitor.

The assembly was pretty easy, although time consuming.  My four year old son helped me paint the wood and string the wires through the spreaders.

I soldered all wire connections with 5% silver bearing solder.

When the whole thing was assembled, I tested it out with my Drake R8 receiver.  It performs much as expected, low noise, directional away from the terminated wire loop.  It does not appear to be too narrow banded either, as the stations on the high end of the dial were also received with good signal strength.

Next was loading it on the pickup truck, driving it in and mounting it on the studio building.  I got some funny looks from my fellow travelers, then again, I usually do.

For the ground, I purchased an eight foot copper clad grounding rod and pounded it into the ground at the corner of the building.  This area is always wet as it is the lowest area around the building and all the gutters drain there.  This is not be best RF ground, but for the purposes of this antenna, it should work fine.  I used about 28 feet of left over #12 stranded wire from the ground rod up to the balun box and connected it to the common ground point inside the box.

The frame itself is mounted on a standard wall mount antenna pole.  Stainless steel clamps hold the wood frame to the pole.

Once it was installed, I used my Kenwood R-2000 receiver to find the best mounting azimuth and locked everything down.  I also put a toroid on the RG-6 coax coming up from the rack room to keep any shield noise from getting into the antenna.

AM receive loop PVC wire spacers
AM receive loop PVC wire spacers
AM receive loop wood frame
AM receive loop wood frame
AM receive loop balun transformer
AM receive loop balun transformer

The tuning capacitor is in there too, behind one of the loop wires.

AM loop antenna installed on roof
AM loop antenna installed on roof

Antenna installed.  I did try substituting the 500 pF capacitor with a 220 resistor.  The signal strength came up somewhat, but the noise increased more, therefore the capacitor is a good termination for this antenna.

With this antenna, the signal from WABC is nice and clean and sound good on the FM station when a monthly EAS test is retransmitted.

What is 200 KHz divided by 400 KHz?

The standard FM channel in the United States, as defined by the FCC is 200 KHz (See CFR 47  73.201).  The occupied bandwidth of an FM IBOC signal, as created by Ibiquity, Inc, is 400 KHz.  See below picture:

HD radio trace on FSH3 Spectrum Analyzer
HD radio trace on FSH3 Spectrum Analyzer

A picture is worth a thousand words. Engineering types will understand this without explanation. For non-engineering types, here is your thousand words (or so):

On the left hand side of the screen is the signal strength scale.  Each vertical division is 10 dB.  This is not absolute signal strength, it is referenced to -20 dBm.  However, it gives a good relative signal strength for both the analog carrier and the IBOC carriers.  The analog carrier is centered on the screen, it slopes upward like a steep mountain, peaking at -50 dBm relative.  The IBOC carriers are on either side of the analog carrier, they are flat, approximately 75 KHz wide and peak approximately 20 dBm below the analog carrier (-20 dBc).  For some reason, likely the bandwidth and/or impedance match between the antenna, high level combiner and the two transmitters, the left IBOC carrier is actually peaking around -14 dBc.

The span, as noted on bottom right hand side of the screen is 500 Khz.  Each horizontal division is 50 KHz.  The entire span of the measurable signal is eight horizontal divisions, thus 400 KHz.

As noted above, the allocated channel bandwidth is 200 KHz, thus this station is exceeding it’s allocated bandwidth by 100%.  This is allowed under CFR 73.404, which is titled “Interim hybrid IBOC DAB operation.”

IBOC proponents will make the argument the FM radios work on something called “The capture effect,” which is to say that if two signals are on or close to the same frequency, only the stronger signal will be demodulated.  Thus, the IBOC carriers have no effect on the adjacent channels that they are interfering with so long as the adjacent signal is stronger than the IBOC carrier.  The argument is further carried forward by assuming that with a stations protected contour (60 dBu in most cases), the IBOC carrier will never exceed that analog carrier.

That is not necessarily true, especially in areas where terrain (and buildings, underpasses, unintentional directionality in transmitting antenna, etc) can attenuate signals close in causing the IBOC signal to become equal to or stronger than the adjacent analog signal.  This effect causes picket fencing.  Lower powered FM stations; class A, LPFM, etc, are especially vulnerable to this effect.

Further, even in areas where the analog carrier is stronger than the IBOC carrier, the noise floor has been moved from -100 dBm or so to -70 dBm, which is a 1,000 times greater.  To assume that raising the noise floor by 1,000 times will have no effect is, as they used to say in the Navy, making an ASS out of U and ME.  Mostly you, in this case.  This effects the receiver by making it less sensitive, it will also add noise to the demodulated signal as the elevated noise floor will show up as background hiss.  Even further still, higher IBOC carrier levels, as authorized by the FCC in January of 2010 can interfere with the stations own analog carrier.

According to the both Ibiquity and the FCC, which stated in the Notice of Proposed rule making, the reason for interim IBOC operations are:

iBiquity’s IBOC DAB technology provides for enhanced sound fidelity, improved reception, and new data services. IBOC is a method of transmitting near-CD quality audio signals to radio receivers along with new data services such as station, song and artist identification, stock and news information, as well as local traffic and weather bulletins. This technology allows broadcasters to use their current radio spectrum to transmit AM and FM analog signals simultaneously with new higher quality digital signals. These digital signals eliminate the static, hiss, pops, and fades associated with the current analog radio system. IBOC was designed to bring the benefits of digital audio broadcasting to analog radio while preventing interference to the host analog station and stations on the same channel and adjacent channels. IBOC technology makes use of the existing AM and FM bands (In-Band) by adding digital carriers to a radio station‘s analog signal, allowing broadcasters to transmit digitally on their existing channel assignments (On-Channel) iBiquity IBOC technology will also allow for radios to be ”backward and forward” compatible, allowing them to receive traditional analog broadcasts from stations that have yet to convert and digital broadcasts from stations that have converted. Current analog radios will continue to receive the analog portions of the broadcast.

Few if any of those goals have been met.  As far as the forward/backward compatible thing, it just isn’t so unless a person actually owns an HD Radio.  As noted in previous posts, few consumers have seen fit to purchase an HD Radio, nor have car manufacture’s taken to installing them en mass in new cars, so there is no forward compatibility.  Instead, we have FM radio stations interfere with each other and themselves in an attempt to “modernize” the audio broadcasting business.  This is a bigger problem for small, community radio stations that can neither afford to install the expensive, proprietary HD Radio system, or broadcast quality receivable signals with an adjacent HD Radio signal raising the noise floor by 1,000 times or more.

I can think of no other greater threat to free over the air broadcasting than HD Radio and the degradation of AM and FM services that comes with it.  The consumer has shown that they don’t care.  If given the choice between free over the air broadcasting that has mediocre programming and is full of interference, and some type of paid internet streaming service that sounds reasonable with good programming, they’ll go for the latter.

In short, some cobbed together digital modulation scheme is the last thing that radio needs right now.

Crown D75 monitor amp goes terminal

Happened the other day, took out the monitor speakers too.  I am not sure how this happened, but the production director reported that the speakers began making very loud squeal.  Somebody finally thought to turn off the amp using the conveniently located on/off switch on the front panel.

Crown D75 audio board burned open resistor
Crown D75 audio board burned open resistor

The two watt resistor is burned open.  Also, this got so hot it burned a hole in the circuit board below it.  Truth be told, I think this amp was about 25 years old and due to be replaced when the new studios were built out.

I’ve seen these Crown amplifiers self destruct in the past.

AM IBOC turn offs?

I have received an e-mail from occasional reader John, who comments that many of the Windy City AM’s have turned their buzz saws off. I note myself today, the same can be said for many of the NYC AM’s.  WABC has had their’s IBOC turned off for quite some time.  The latest to turn off is WNYC on 820 KHz.  Several people have noted the loss of noise on their signal this morning.

According to Ibiquity’s own website, only six AM stations in the NYC market are currently using IBOC.

What does this mean?

Could it be that management is finally realizing that the cure is worse than the disease?  The disease being alleged poor audio quality, and the cure being IBOC itself.