Harmonic on Homebrew RF Transmitter

You're probably overloading the receiver front end and generating the harmonic in the receiver itself. The best way to check though is with a spectrum analyser - that will allow you to see the frequency and amplitude of any harmonics present in the tx output.

Rodney.

You should _always_ follow an RF amp with a lowpass filter before applying the results to an antenna. As others have mentioned it can be hard to see harmonics on your scope, and you should have your 3rd harmonic at least 40dB down for amateur radio.

You may want to cross-post on rec.radio.amateur.homebrew -- there's some pretty wise folks over there.

Thanks Joerg. I don't have a spectrum analyzer so will have to go the attenuator route. Thanks for the tips. By the way, what is the IIRC Atlas?

I don't see evidence of clipping in the waveform John. Overload of the receiver is a possibility I guess, but I don't have an antenna attached to the receiver. I believe I'll try moving the receiver several feet away and watch the response.

Any clipping at the output stage will generate some 3rd harmonic. But overloading the front end of the 21MHz receiver can also clip the waveform and generate the third harmonic, there. If filtering the transmitter has little effect on the receiver, then the later is probably the case.

Hello dgc,

IIRC Atlas did that w/o, just using a diode that was thermally mounted to the heat sink of the transistors.

Could be. Where are you on the S-meter scale? Make sure you don't run into protective diodes that would generate harmonics from a clean signal.

Definitely not. Even a harmonic content of 5% (which is a whole lot) is hardly discernible on a scope. You'd need a spectrum analyzer. A good receiver can be used as well. The trick is to establish a reliable path from dummy load via various attenuators into the receiver. I have those in the lab but you might have to make some. Don't go more than 10dB per attenuator stage. I think the ARRL handbook shows how to build them (and mind the power levels).

Don't try this with some cheap SW-receiver in a plastic enclosure. It's not going to work.

Good. Some folks would just fire it up and then wonder why the FCC guys showed up. I always rely on a good lowpass such as pi-filters.

Regards, Joerg

I have been developing a 100W tranmitter years ago, used a dummy antenna,was inside a bunker half buried. Next day sombody living in a village 10 miles away, told me reception was fine, but I should watch my language. Well.. so much for a dummy antenna. You will leak much more power from a set you are working on, then you might think,so dont be suprised to overload a nearby reciever,try shorting/shielding the reciever antenna input ,and see if you still have any 3rdharmonic, and what strength it has compared to the base frequency.

Clipping is taking things to an unecessary extreme. All that would be necessary for substantial harmornic distortion to occur would be for the amplifier to become slightly non-linear at the greater excursions of the sine wave. It doesn't have to literally clip, just become a little more rounded off on top than a sine wave would be, but to a smaller degree than you could possibly see on the scope.

Incidentally, a 40m wire dipole (or derived antenna) will operate fairly well on the 15 meter band, so any third harmonic you don't keep from the antenna will go world wide. Get the exciter operating in a legal part of the 15 meter band and you can use this to advantage.

The sine wave is squashed either on the negative or positive half-cycle - this will give considerable 3rd harmonic, but won't be obvious on a scope. The asymmetry is because of the transistor's nonlinear operating region.

Do you have any tuned circuits in there?

Good Luck! Rich

Hello dgc,

"IIRC" is newsgroups slang for "if I remember correctly". Atlas is a company that made one of the most clever shortwave tranceiver series. I think these were the Atlas 210 and Atlas 215. A long time ago. From a minimum of parts they wrestled a superb dynamic range.

Regards, Joerg

In article , dgc wrote: [...]

You can see 5%, or so, distortion on a scope so you can't say there are no harmonics there. If you construct a notch filter to notch away the carrier, you can do a lot better. Since your frequencies are low RF, you may even be able to use op-amps and stuff in your notch.

Also, there is "none" and then there's "none" or "none". You may have little enough harmonics that no-one will notice, you pass the CE/FCC or, for example, you GPS keeps working.

Many thanks for replies to my query. I learned something about Oscopes. I had always thought there would be a defined peak (second waveform) at any harmonic frequencies. I do see a "flattening" on the ascending voltage side at the top of the positive peaks . I will study up on and build a low pass filter.

Thanks again.