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Cheif Operator revives idea for awesome AM air monitor

T

Tom Wells

Cheif was asking about a wideband air monitor in another thread. I've liked some Sonys, my 1936 Philcos, some car radios, etc,
all of which are fine with my 100mw, but all consumer gear presumes it won't get used in near-field AM reception.
No shielding, AC line not bypassed both sides to common, poor agc in high DC condition, bandwidth limitations,
and non-linearities of PLL detection makes it hard to find anything that sounds quite like the audio you're really putting out.

My favorites have always the cleanest. So he reminded of the air monitor used at WNWI at Valpo Tech.
This place was built by hi-fi nuts at a radio engineering school. Foot-thick doors, all the glass, Main studio, conference studio,
and production booth, Hand built huge bass traps and incredibly hi-fidelity monitor sound in the main studio.
The station was about 200 feet from the tower, and in one class or another, I learned that the air monitor was a diode at
the tower base, into to the monitor amp and that sent back to the studio.

Then I wondered why I'd never tried that here at home on my play pipsqueak 100mw.

Tapped the antenna out with 100 pf, one side that to a germanium diode, shunted with 100 ohms.
Connected mic input of laptop across the diode loaded by the hundred ohms.

Naturally, a real station wouldn't need no stinkin 100 puffs to couple it.

Whew! I'me just sorta blown away here with the headphones for the past hour.
The definition seems impossible, it's not just frequency response, it's the elimination of so many places
in receivers where non-linearities accumulate in the sound of every design.

It's like an AM audio macro lens. Is anybody out there in professional AM land doing it this way, is it, was it common?

It's far too cheap for you not to try this out.
This whole receiver is hanging in mid-air literally, allygator clipped into the parts hand twisted, then allygator clipped to
the end of audio cable into the laptop. And it's so shockingly clear, compared to the most expensive and finest AM tuners I have
encountered in my whole life, and I've been at it 45 years. No hum, no buzz, no frequency distortion from any "selectivity".

Every element sounds like there's no transmitter at all in the way. Which is what I was trying for, all along. :)
 
While I don't think I deserve credit, I will certainly accept it :) I'll have a listen to the files. Your concept is interesting--I know of no station doing this....
 
I don't know it first hand, but anecdotally I remember someone telling me that WWUN, Jackson, MS was using this back in the 70's. A diode was attached to the control room window screen, and provided the audio for the monitors. If Mr. Ingram checks in, he could probably shed some light on this. I have airchecks of the station from that era, and one odd feature I recall is how well the high frequency information like the cue burns on the records stood out! I had always assumed they were "off the board" airchecks, but they could have been "off the diode" airchecks.
 
Old AM modulation monitors were designed to be very wideband. No tuned circuits. Just a biased diode detector followed by an audio amplifier.
They sounded spectacular.
 
frankberry's right. Check the schematic from a GR 1931-A or B, or the Gates MO-2639 for example. And they DID sound spectacular.
 
What an odd coincidence that I found this thread today.
I was just testing the audio on my home built AM transmitter last night using this same method.

What I like to do is take a HIFI stereo amplifier that has a sensitive input. Phono input works best for a sniffer wire antenna, or you can just use an AUX input for a higher powered transmitter you want to monitor.
I simply take a large broadcast band coil or even an old BC band choke and connect one end to the diodes positive side (anode), the other end of the coil to the monitor amplifiers ground. Sniffer wire connects to the positive part of the diode also, the negative side (cathode) straight into the amplifiers input. You can place a .01 uf capacitor across the jacks input to keep RF out if needed.
Take the shielded sniffer wire and wrap it around your antenna output, close enough to produce audio, but loose enough not to really effect the resonance of your antenna.
Using quality headphones you will be amazed at how simple yet good of a quick and dirty monitor this makes.
Problem is even if you get her sounding good doesn't mean the neighbor down the block has a radio worth a crap.
I find even when modulation sounds perfect, it still sounds awful on some peoples radios. You can make AM sound good but will they hear it? ;)

Another piece of advice is to equalize the audio monitor to the NRSC deemphasis slope if you have a good graphic equalizer.
Use another graphic EQ to create the preemphasis slope for your transmitter. You may always want to keep it in your audio chain just like the big boys do.
This way most newer AM radios that apply the deemphasis will still sound good after you get your modulation sounding good - to your ears.

Here's a simple modulation monitor...
http://www.naturemagics.com/ham-radio/modulation-monitor.shtm
Take out the tuning capacitor for no tune operation, replace the headphones with a quality headphone amplifier.

A bit of info on NRSC if you want to apply it...
http://part15.us/node/703
 
Tom Wells said:
Cheif was asking about a wideband air monitor in another thread. I've liked some Sonys, my 1936 Philcos, some car radios, etc,
all of which are fine with my 100mw, but all consumer gear presumes it won't get used in near-field AM reception.
No shielding, AC line not bypassed both sides to common, poor agc in high DC condition, bandwidth limitations,
and non-linearities of PLL detection makes it hard to find anything that sounds quite like the audio you're really putting out.

My favorites have always the cleanest. So he reminded of the air monitor used at WNWI at Valpo Tech.
This place was built by hi-fi nuts at a radio engineering school. Foot-thick doors, all the glass, Main studio, conference studio,
and production booth, Hand built huge bass traps and incredibly hi-fidelity monitor sound in the main studio.
The station was about 200 feet from the tower, and in one class or another, I learned that the air monitor was a diode at
the tower base, into to the monitor amp and that sent back to the studio.

Then I wondered why I'd never tried that here at home on my play pipsqueak 100mw.

Tapped the antenna out with 100 pf, one side that to a germanium diode, shunted with 100 ohms.
Connected mic input of laptop across the diode loaded by the hundred ohms.

Naturally, a real station wouldn't need no stinkin 100 puffs to couple it.

Whew! I'me just sorta blown away here with the headphones for the past hour.
The definition seems impossible, it's not just frequency response, it's the elimination of so many places
in receivers where non-linearities accumulate in the sound of every design.

It's like an AM audio macro lens. Is anybody out there in professional AM land doing it this way, is it, was it common?

It's far too cheap for you not to try this out.
This whole receiver is hanging in mid-air literally, allygator clipped into the parts hand twisted, then allygator clipped to
the end of audio cable into the laptop. And it's so shockingly clear, compared to the most expensive and finest AM tuners I have
encountered in my whole life, and I've been at it 45 years. No hum, no buzz, no frequency distortion from any "selectivity".

Every element sounds like there's no transmitter at all in the way. Which is what I was trying for, all along. :)
Click to expand...

Way back in 1971, when I was CE at WSSB in Durham, NC, our old RCA mod monitor stepped out for a smoke, and didn't come back. So I wound a few turns of # 20 solid wire around a Magic Marker , pulled the coil off the form, soldered a 1N34A and .05uF ceramic in series with one end, and soldered the cap to the center wire of a few feet of shielded cable, and the shield to the other end, and hung it inside the top of the cabinet of our BC-1G (away from the loading inductor, natch), and soldered the other end to the line going to the air mon input on the "Yard" in the control room. Every jock in the place was amazed at how good our air signal really sounded. Only problem was when I'd get in the car after work, and check into the local, (then new and cutting-edge) 2-meter repeater) with my 20 watt Motorola T-Power transceiver. Jocks thought someone had hijacked the signal. :).
 
You can use this same idea for a cheapo way to tune an FM transmitter for minimum AM noise. A sample loop in the transmission line, in series with the diode of your choice and a small blocking capacitor. The resulting output is the demodulated audio of the FM transmitter's AM noise. Tune the tx for a null or minimum audio. Be mindful that you don't get too carried away with tuning for minimum AM noise as you may tune the transmitter pretty far away from good efficiency.
 
This thread is too cool. I am also student from Valparaiso Tech and vividly remember the studios and equipment at WNWI. My brother and I kept that place on the air and sounding fantastic during its last days. The air monitor audio feed came off of the frequency monitor in the top right hand rack. I remember the studios to this day. Unfortunately, I didn't get much more than a couple pictures while I lived on campus, although I'd been there about 5 years. I didn't get to graduate because the place was literally falling down around us as we were trying to take classes. The building is gone now, but the memory of it all lives on. Now I am an IT administrator and engineer for a broadcasting company. WNWI was where I got my start engineering though. We finally got a CD player from Audio Junction at that time... man what a sound through that old tube console and Audamax/Volumax pair! Of course, when a channel would act up in the console, you'd just go in the back... wiggle the tube that wasn't glowing... and the music would fade back in. ::)

The frequency monitor came with the transmitter as a setup from the friendly RCA dealer (as Ed Hershman used to tell me). So it was whatever came standard with a BTA-500M transmitter. I remember it was a biased detector, because it had a bias pot on the back or the unit. I think that frequency monitor ran for 20 years between tube changes.
 
Good to run into another VTI alumni here. I wish I had lived on campus now, and wish I'd had enough money to keep the tech going. Congrats on making it into radio. I went elsewhere. I spent more time at WNWI as a boy in the early 70's than I did
while going to school there. I used to listen to the old long wave weather broadcasts in the teletype room, and rip copy off to bring into the studio...loved the super thick doors and the little 3-door vestibule between the studio, production studio and reception area. Wonder wonder happened to the scale model antenna and map. Wonder what ever happened to the RCA control console. Did you have to deal with the hornets on the top floor of Dodge Hall pestering you during final exams? :D

I wish I'd had time to volunteer at the station, but I had to work an evening job to be able to afford to attend, and also lived at home, about 12 miles away, so living at school was not viable. Never got to tip a beer at the Franklin House until
2008!

Biasing the detector would eliminate the DC offset I see from the simple detector.
If I record the output, it's simple enough to correct while editing the wave file.

I can still hear Art H as "Eric Scott" playing the Bach Tocata and Fugue in D minor...really thundered...or Telstar by the Ventures, with that one super-deep note...their audio really held together and made WNWI sound like it had a lot more than 250 watts. How 'bout that giant room of junk behind the main studio!
 
Yes, I remember the model of the tower. We were sure proud of the coverage area. Someone bought the station (I think a company out of eastern Illinois, along the lines of Barrash Broadcasting) - they were broadcasting Serbian ethnic programming from Ill when they got the CP to move the station. The guys who bought the property tore down the building with no regard of the history they were destroying. We didn't have many classes in Dodge hall, although the Alumni Assn was renovating when I left and had moved the museum over there. Bryant Mytol had his industrial electronics service business in the west end of the basement.

I remember the doors and the three way vestibule very vividly - how the front office had been specially designed to make it look like the "out doors" with the molding on the windows and the window where you could peer in and see the transmitter, the green-grass colored carpet. I wish I had more money too at the time, I was just a poor boy looking for an education and a way to help out, but now that Ed and everyone has passed away, I suppose it's all just good memories now.

Just use an low leakage capacitor in series to couple it into the sound card to block the DC. Most AM transmitters are good down to about 10 cycles anyway, so I can't imagine a 10uf cap in series at 600 ohms is a big deal frequency response-wise. Most sound cards have some kind of DC blocking anyway, so initially, when the recording starts out, it may have some DC offset, but that will generally go away in the first couple seconds and the center line will drift back to zero... this is of course unless you have an esoteric card of some kind. Even the expensive Audio Science cards have DC blocking on their balanced inputs - I think generally 100uf at 35v.

I did an oldies show for about 5 years on Saturdays... and filled in where needed.
 
Jasonce66 said:
Yes, I remember the model of the tower. We were sure proud of the coverage area. Someone bought the station (I think a company out of eastern Illinois, along the lines of Barrash Broadcasting) - they were broadcasting Serbian ethnic programming from Ill when they got the CP to move the station. The guys who bought the property tore down the building with no regard of the history they were destroying. We didn't have many classes in Dodge hall, although the Alumni Assn was renovating when I left and had moved the museum over there. Bryant Mytol had his industrial electronics service business in the west end of the basement.

I remember the doors and the three way vestibule very vividly - how the front office had been specially designed to make it look like the "out doors" with the molding on the windows and the window where you could peer in and see the transmitter, the green-grass colored carpet. I wish I had more money too at the time, I was just a poor boy looking for an education and a way to help out, but now that Ed and everyone has passed away, I suppose it's all just good memories now.

Just use an low leakage capacitor in series to couple it into the sound card to block the DC. Most AM transmitters are good down to about 10 cycles anyway, so I can't imagine a 10uf cap in series at 600 ohms is a big deal frequency response-wise. Most sound cards have some kind of DC blocking anyway, so initially, when the recording starts out, it may have some DC offset, but that will generally go away in the first couple seconds and the center line will drift back to zero... this is of course unless you have an esoteric card of some kind. Even the expensive Audio Science cards have DC blocking on their balanced inputs - I think generally 100uf at 35v.

I did an oldies show for about 5 years on Saturdays... and filled in where needed.
Click to expand...


I very much enjoyed listening to your oldies show on Saturdays.

The DC offset isn't enough to be a problem, and I'm just going into the mic input on an HP laptop.
The offset is only about 10% and the DC coupling lets me get the full asymmetric thud of over 100% positive mod.
It doesn't drift down to zero as i it would if I were watching a blocking cap charge up....

I believe Steinmetz was built out of terra cotta made in Hobart, where I lived, so it hurts me see it gone in many ways.

Did you ever get to hook the WWV receiver up to the tower? I heard some stories about that.
 
WNWI and VTI need its own thread ;D!

I grew up in Hammond and remember hearing the station when it was on from Valpo.

Tom, Jason, do either of you have a recording of the WNWI Jingle....W-N-W-I, in Indiana (sung to the melody of "Back Home Again in Indiana")?
 
I think I must have a recording of that somewhere. I still have 99% of my cassettes from those days.

WNWI, in Indiana, In The Center of Your Dial.
 
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