AM TX Tuning

[flintstone mop]

My brain cells need refreshing............
Why do we tune a BC AM transmitter a few (20-30ma??) off of the dip on plate tuning??

And for the broadcast engineers in the FM world. Does plate tuning still affect the AM noise in the FM transmitter? We would always have to dink with the plate tuning of a Gates FM-3 and watch the AM noise in the Belar monitor for minimum AM noise.

And another question for AM B'cast types. Does the FCC allow an AM station to operate WITHOUT a modulation monitor?

A friend of the family is a sales manager for a small 1kw AMer and he thinks the engineering staff is being lazy and not adjusting the mod for 125% peaks. We looked at the station's mod in my shack with the R-390a and 'scope and it appeared to be a fully modulated signal. His concern was..."Can you tell if it's 125% peaks?" I explained that the 'scope presentation could not give me a calibrated view using the I.F. output from the R-390A. (he was getting a little frustrated, at this point) I said it looks like a fully modulated signal..............fer around $500 you can buy an FCC accepted mod monitor without the Belar price tag.

OK thanks

Fred

[KA1ZGC]

A friend of the family is a sales manager for a small 1kw AMer and he thinks the engineering staff is being lazy and not adjusting the mod for 125% peaks. We looked at the station's mod in my shack with the R-390a and 'scope and it appeared to be a fully modulated signal. His concern was..."Can you tell if it's 125% peaks?" I explained that the 'scope presentation could not give me a calibrated view using the I.F. output from the R-390A. (he was getting a little frustrated, at this point) I said it looks like a fully modulated signal..............fer around $500 you can buy an FCC accepted mod monitor without the Belar price tag.

 

Tell your friend the sales manager that his concern is managing the sales staff and making money for his employer.

There's nothing worse for a radio station than a non-engineer who gets wrapped around the axles over some perceived technical issue that must be addressed, dammit and won't rest until he gets his way (no matter how full of crap he might be).

Did you ask him what made him think the station was modulated less than 125% positive?

Did you ask him what effect he thought having merely 115% modulation would have on the listening populace?

Did you ask him what effect he thought that would have on his ability to sell airtime for the station?

Did you remind him that if he has to ask "but can you tell if it's 125% peaks" that he's probably splitting hairs that are imperceptible to the listener?

Did you ask him how much of his time (that is supposed to be spent making money for the station) was wasted trying to address a problem that it's not his job to worry about?

I've seen too many people in too many stations hired for purposes other than engineering (like sales or progamming) suddenly decide they're the authority on All Things Radio, get some bee in their bonnet over Optimod settings or modulation percentages (or something else equally stupid) and neglect their real jobs in order to persue this new pet crusade of theirs. 1 out of 3 results in the station losing sufficient revenue that the person gets canned, or the station as a whole suffers (in other words, real working people lose their jobs while these idiots keep theirs and continue focusing on everything but their own job).

Next time, tell him "yes, it's 125% modulated" and save their engineer (and indeed, their revenue) from yet another Radiophool Crusader.

--Thom
Kilowatt Amplifier One Zero Grid Current

[W2ZE]

Why do we tune a BC AM transmitter a few (20-30ma??) off of the dip on plate tuning??

In a class C amp, a few mils off resonance toward the minimum side of the tuning C is the point of maximum smoke (highest power output).

And for the broadcast engineers in the FM world. Does plate tuning still affect the AM noise in the FM transmitter? We would always have to dink with the plate tuning of a Gates FM-3 and watch the AM noise in the Belar monitor for minimum AM noise.

Yes, but loading the amp effects it more than plate tuning.

And another question for AM B'cast types. Does the FCC allow an AM station to operate WITHOUT a modulation monitor?

47CFR73.1400 Transmission system monitoring and control.
Too long to spell out the rule here, but while it doesn't say specifically, it does say you need to have the proper tools to do the job. An O'scope would work, but a mod monitor is alot easier, and its just good engineering practice.

Hope this helps.

Mike, W2ZE

[flintstone mop]

Thanks
I'll pass this stuff on to the "sales manager" And Thom, I agree a few more percent of peaks ain't gonna make much difference. Especially 'news-talk radio" He's from the 60's and still clings to the loudness wars of times past.

Fred

[John K5PRO]

GM down here at 1490 hasn't a mod monitor, and he asked me to help him out as he had a complaint from someone that he was overmodulated. I hauled my AM90 Harris box over, plus a scope, and found he was actually a little light on the levels, from his Optimod and DAX1 transmitter, so I raised it a bit. Now I really don't think the GM cared or not, just that he had some bug that it might be off, so we checked. Otherwise, i suggested he get an old mod monitor, for $200-300 on epay. He hasn't got the change for it right now....

[steve_qix]

A friend of the family is a sales manager for a small 1kw AMer and he thinks the engineering staff is being lazy and not adjusting the mod for 125% peaks. We looked at the station's mod in my shack with the R-390a and 'scope and it appeared to be a fully modulated signal. His concern was..."Can you tell if it's 125% peaks?" I explained that the 'scope presentation could not give me a calibrated view using the I.F. output from the R-390A. (he was getting a little frustrated, at this point) I said it looks like a fully modulated signal..............fer around $500 you can buy an FCC accepted mod monitor without the Belar price tag.

OK thanks

Fred

ModMonitor for under $500.00:  Your friend could have bought an R.E.A. Mod monitor (now being used at many broadcast stations) for $459.00, but that price - assuming R.E.A. does another run of units, is no longer in effect.  The new price, due to inflationary pressures will be higher.

The FCC no  longer types accepts, nor requires type acceptence of equipment such as modulation monitors.  So, no new monitors bear an FCC type number.  If you want an FCC accepted monitor, you have to buy an old one 

It appears to me, that the price of Belar and other mod monitors is very high, not due to the price of the parts (the parts certainly don't cost the thousands of dollars the units cost), but because they sell so few of them, with respect to the production and support costs, the price has to be high.  Using Belar or Inovonics pricing, the R.E.A. mod monitors should cost well over $1000, and that's discounted.

Anyway, a new software defined mod monitor is in the works    Pricing will be very attractive 

Regards,

Steve

[k4kyv]

My brain cells need refreshing............
Why do we tune a BC AM transmitter a few (20-30ma??) off of the dip on plate tuning??

This is particularly true if the transmitter uses fixed  capacitors and variable inductors in the tank circuit.  With variable capacitors and fixed inductors, the resonant point is exactly on the dip, if all the reactance has been tuned out of the load.  At least this is true with link coupled output circuits.  Pi-networks require re-dipping the final every time the loading is changed regardless, since the loading cap is effectively in series with the main plate tuning cap.

I recall years ago studying exactly why the resonance point is slightly off the dip with inductive tuning, and it made perfect sense, but I can't recall the details.  I would have to go back and dig the info out of the engineering books.  I recall something about vector addition and subtraction and rates of change of the inductive and capacitance reactances in the tuned circuit.  Maybe someone could refresh my memory on the topic.