G as in Gary

It goes all the way back to middle school, and it's as simple as me and my friends coming up with dumb-sounding nicknames. Gip-Gip was really catchy and has somehow stuck with me since



Can't blame you, writing fast isn't my forte



I use the op-amp to bump the voltage up to where there's always (ideally) a carrier. This also prevents a negative voltage from reaching the comparator, so the whole thing doesn't blow up.

P.S. I'm going to try to explain the mod. scheme in another post, give me time...

Modulation is very simple in my opinion. The op-amp U2 steps the voltage up to ~3v and the audio signal modulates that voltage (with a gain of two). This signal gets fed into the comparator-based LC oscillator, which is in turn connected to the antenna.

I just noticed the frequency of the transmitter is affected by the rate of change in voltage. How mathematically I don't know but if I change the value of the bleedoff POT the frequency changes, due to the voltage supplied going down. The same effect can be observed when the audio wave changes the applied voltage, resulting in effectively FM.

Normally this would be undesired but this effect is unheard as far as I can tell, and brings the possibility for a dual AM/FM radio just by changing the frequency. The only obvious downside is it will interfere with nearby stations.

Just out of curiosity, what would be the best way to construct an antenna for my design?

Typically, it's just a length of wire strung out from the transmitter. To be legal, it must be no more than 10 feet long. And of course you'll need a matching network between the transmitter and antenna to get any significant amount of power radiated.

This is true if operating under the regulations in Part 15.219 which stipulate input power and antenna/ground system limitations in lieu of field strength readings for the AM broadcast band. If you are willing to accept somewhat lower but still useful range for most applications then the 15.209 rules might be more useful. Then there is no need to worry about transmitter power and antenna design as long as the signal doesn't get out more than about 200' or so as a rule of thumb. Also since the field strength allowed is a function of frequency (24000/f(kHz) µV/m measured at a distance of 30m for the AM band) the allowed field strength is higher at the low end of the AM band. IMO too many AM transmitter discussions center on maximum range which can come at the expense of audio quality if the antenna system ends up so sharply tuned for maximum range that the sidebands end up being cut off because of regulatory limitations on the antenna/ground length. I personally decided to throw out the 15.219 rules when I did my own design so I could focus on audio quality which to me is much more important than being able to get a 1/2 mile or more signal range but with potentially reduced audio quality. I just want to be able to listen to my own radios here, not broadcast to the whole neighborhood, so sound quality is #1 for me.

Using the FCC 15.209 formula gives an allowed field strength at 30m (about 100') as 16uV/m at 1500kHz, which would be a bit noisy but usable depending on the radio in question. I currently don't get anything close to that right now with the lousy antenna I am using since the signal fades into nothing before 100'. Still sounds strong and clear around the house, though. Something to think about when discussing AM transmitter designs and whether or not maximum signal range is the #1 priority over other considerations like signal quality. 15.219 is only an alternative to 15.209 rules for the AM band, so one could use either set of rules depending on the situation and what the design goals are.

do you pronounce Gip-Gip? Gip like "The Gipper"
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Let's help get it on the market for the "Gipper"!

Hi. There is another section of the rules that would suit you better maybe. Part15.221. It is similar to part15.209 but you feed the power line neutral which is the safe way to do it. You can have a higher bandwidth am signal and comply with the rules and your local radios will have a good signal to receive. If your radios are plug in then they can get a strong signal and those on batteries will get a good one from your wiring radiation and not go all over town unless the neutral does.
Here is a link to the rule.

https://www.ecfr.gov/cgi-bin/text-idx?S ... 1&rgn=div8

Bob

Regardless of which paragraph of the Part 15 rules that you decide to follow, electrically, your antenna will have a radiation resistance of a few milliohms in series with a large capacitive reactance of several kilohms. You'll need to cancel the capacitive reactance with an equal amount of series inductive reactance, and then match the milliohm radiation resistance to the output impedance of your transmitter.

Very interesting. Did not know that could be used by us looking to transmit to our antique radios.

I might have to build an LM-386 transmitter or one of the other designs a couple forum members are working on.

Would be easy enough to try it with my two tube part 15 broadcaster as I could just cap couple to the neutral terminal of the IEC jack I have on the transmitter.

Not sure if my current pi net antenna tuner would still work though.

Hi. My setup uses a cp15 coupler and it is set with capacitance to cancel the inductance of the power line here. You can neutral feed as I do with a separate ground or if you only want the signal in your own home it will couple across the line and the energy will stop at the next pole transformer. the lpb atu-30 is also a good coupler. These couplers have safety provisions so they can be connected to the line. As far as hum is concerned these couplers have filtering to block 60 HZ from back feeding the transmitter. Hum can have many other sources however. Your individual power line characteristics wil vary from mine or anyone elses so you have to adjust it to get a good match. Note you will need more than 100mw to set the swr using the built in meter but you can make a simple bridge and place it between transmitter and coupler. You can legally run higher transmitter power than 100mw as long as you follow the rule. A good test is a good portable radio outside and follow you power line and then back away from it and if the signal goes under the noise then you are in the ball park as far as level goes. So if the formula says at your frequency say 100 feet then at 100 feet away the signal disapears you are near the cutoff point.

I'm near finishing the V2 schematic and your thoughts on it would be greatly appreciated. I will post the theory of operation and my thoughts on your proposals in my next reply

Audioman, I Googled the CP15 coupler as I had no idea what it was. Looks to be an interesting device. Probably not cheap.

Here's what I used to tune 8-10 feet of wire at 1540 kHz. It comes off the output of the AD811 opamp, which has a low output impedance. The capacitors are variable and the coil is a ferrite solenoid. It vastly increases the signal strength over just a straight wire.

What's the impedance at the noninverting terminal connected to the antenna? Does that connection isolate enough not to need a buffer amp between it and the antenna if you are going to use an antenna tuner?

Hi. The cp15 I have was a buy from ebay. It was not expensive. The circuit of it is simple as it uses tapped toroids and capacitors with 4 switches to adjust it to acceptable swr. It presents a 50 ohm load to te transmitter. The lpb atu30 is similar in circuit. Before I bought mine I googled them and got the diagrams on them and was going to build one with standard coils so I could tap them even closer to a flat swr.

Welcome Gip-Gip to the forum! Let us know how well your transmitter works.

Finished the V2 transmitter today!



Sadly, I do not have any documentation regarding the transmitter (besides the schematic), since I've been spending far more time on other things. I'll probably work on that last

The bias current is only ~6uA, if that is what you're referring to

So to make an antenna tuner I just put a ~47nH adjustable inductor in series with C10?

That should be easy to try. I question whether that would do what you want because your oscillator performance probably relies on both legs of C9 and C10 being equal components. Because that oscillator is literally at the antenna, it might foul up the works if an antenna tuner is connected there without a buffer stage. This is why I was asking about the output impedance.

I don't see what it would hurt for you to try, though. Then try the configuration I posted above while leaving C10 as it is. If it doesn't work, it's not hard to add a buffer amp. Those ICs you are using come in packages of 2 amps, so you may have to plan for two chips.