Question about RF and Transmission

Hi,

So I built oscillators that produce near-triangle waves and I try to transmit them over a short distance of a few meters using several kinds of antennas. I use a half-wave and quarter wave antennas, although most of them are imperfect. I made 5 oscillators from 13MHz - 25MHz and they have an output voltege of about 600mV pk-pk to 1.2V pk-pk. I use an RG6 coax on the half wave antenna and RG58 on quarter wave antennas and the oscillators have an output impedance of around 50-80 ohms, measured by using a potentiometer as a voltage divider. Given all that, I connected the output of the oscillator to the feedpoint of the antenna and the ground of the oscillator to the ground of the antenna. The problem is this, sometimes, the receiver, also half-wave and quarter wave antennas, only get high values for the harmonics and low values for the fundamental frequencies and on different days, fundamental values are higher than harmonic values. Leaving the alligator clips on the output also serve as antennas and sometimes, I get higher values when the alligator clips are connected. The oscilloscope probes also provide higher values than that of antennas. My question is that, is this all normal, I know that RF is really unpredictable and i also considered the possibility that the signals produced travel through the AC mains and enter the scope through another AC outlet, but i tried removing the alligator clips, leaving the connecting wires connected to the oscillator output and the signal level on the receiver side drops down substantially to barely noticeable levels, so i assume that i must have been transmitting for it to behave like that. The system behaves differently on different days but seem to be consistent on same days (all tests done on the same day seem to follow a pattern). Another thing is that as I get farther, the fundamental frequency disappears and only harmonics can be seen. Is it supposed to behave like that? My professor said that RF is really unpredictable so i get interference from anywhere that could have affected my tests during some days that may or may not be present on other days. So, is this at all normal? for it to behave like that?

you need a buffer amp

As you are operating the two aerials close together, any reflection from metalwork will affect your results. For better results do your measurements outdoors and with the aerials at least 10 wavelengths apart.
Frank

patrickian01 said:

Hi,

So I built oscillators that produce near-triangle waves and I try to transmit them over a short distance of a few meters using several kinds of antennas. I use a half-wave and quarter wave antennas, although most of them are imperfect. I made 5 oscillators from 13MHz - 25MHz and they have an output voltege of about 600mV pk-pk to 1.2V pk-pk. I use an RG6 coax on the half wave antenna and RG58 on quarter wave antennas and the oscillators have an output impedance of around 50-80 ohms, measured by using a potentiometer as a voltage divider. Given all that, I connected the output of the oscillator to the feedpoint of the antenna and the ground of the oscillator to the ground of the antenna. The problem is this, sometimes, the receiver, also half-wave and quarter wave antennas, only get high values for the harmonics and low values for the fundamental frequencies and on different days, fundamental values are higher than harmonic values. Leaving the alligator clips on the output also serve as antennas and sometimes, I get higher values when the alligator clips are connected. The oscilloscope probes also provide higher values than that of antennas. My question is that, is this all normal, I know that RF is really unpredictable and i also considered the possibility that the signals produced travel through the AC mains and enter the scope through another AC outlet, but i tried removing the alligator clips, leaving the connecting wires connected to the oscillator output and the signal level on the receiver side drops down substantially to barely noticeable levels, so i assume that i must have been transmitting for it to behave like that. The system behaves differently on different days but seem to be consistent on same days (all tests done on the same day seem to follow a pattern). Another thing is that as I get farther, the fundamental frequency disappears and only harmonics can be seen. Is it supposed to behave like that? My professor said that RF is really unpredictable so i get interference from anywhere that could have affected my tests during some days that may or may not be present on other days. So, is this at all normal? for it to behave like that?

Click to expand...

From your extensive talk I feel you rather get confused and follow your professor who apparently never did experiments like you are trying to do.

To transmit a RF signal over a short or long distance, you need a good receiver first, then your oscillator. Then you can extend the range like Marconi and others did.

A receiver for the HF band like 13 or 25 MHz is a "crystal set". YOu need an LC resonant circuit to which you connect a detector diode, then headphones or better, an audio amplifier with a speaker. MOdify an old (new) AM/FM radio, connect your detector to its volume control potentiometer.
Then you should MODULATE your HF oscillator, best with an audio signal you can hear, 400 Hz to 1-2 kHz. If you set up your modulated transmitter and nearby your receiver, you should easily hear the sound being transmitted across a room and possibly far away.
For a longer range, add antennas. For your frequency like 13 or 25 MHz, use a whip on each side, half-meter long is OK. Investigate the way how to connect it best to the transmitter output and receiver input. This procedure is called impedance matching. With a good matching you can push more power to the transmit antenna and get more from your receiver.

Find the ARRL Radio Amateurs' Handbook, there you will find answers to your questions. After some experiments you may get the hands-on experience you need to better understand the RF and learn how predictable it really is.

Good luck!

the receivers i use are quarter wave antennas and a half wave antenna, with a metal screen as the ground plane. they are 1 ft long and the ground plane is 1.3ft x 1.3ft. They are also connected to loading coils 0.25 ft from the feedpoint and are connected to RG6 for the half wave (75 ohms) and RG58 for the quarter wave (50 oms) coax cables. the half wave is 0.5m from end to end with no loading coils. the measured approximate impedance of the oscillators are around 50-80 ohms, which means there could be little losses due to impedance matching. the antenna impedances are around 50 for the quarter wave and 75 for the half wave, could be a little more or less due to imperfections. the receiver antenna is connected to an RC High-pass filter tuned to 10MHz. the oscillator probes on the receiver are connected to the output of the RC filter and set to see the FFT of the signal. The level of the signal transmitted by looking at the FFT says that it is around -30dB and the signal received 14ft away is around -60 - -70dB. by removing pieces of the transmitter, such as the antenna and alligator clips, leaving connecting wires hanging, the signal level on the receiver drops to around -90 - -110dB. this would probably indicate that i am somewhat transmitting. the reason why the probes must have been better was because it is properly grounded and has an internal matching circuit inside. another thing i found out though was that the oscilloscope had a different settings on other days (oops!) which would make the signals look not uniform but when i inspected the excel file of the FFT, I found out that they were somewhat uniform, with 4 - 5dB variations.

what i meant by what i said, about RF being unpredictable, is that on some occasions, there are frequencies present on some times, due to unknown sources that could no longer be there the next time. You really wouldn't know how much stray signals are present at a given place on a given time. just to clear things up.

i wouldn't be testing using modulation because the purpose of my experiment is to prove that carrier signals are being transmitted without any sort of audible data on it. thank you for the replies. i would highly appreciate any more insights from you guys.

I have to ask.... why are you trying to transmit a triangular waveform, when a sinewave would be what is normal for a carrier ?

Dave

because it is the closest to a sine wave that the oscillator could produce. The one I used are oscillators made up of inverters and crystals. It naturally would have provided a near square wave output but I adjusted the feedback a little and the closest to a square wave that I could get was a triangle wave with around 3-4 harmonics. Whenever I transmit my signals though, I would also see some of the harmonics in the receiver.

since it would only take around 15 minutes to build up 5 lowpass filters, can I assume you do not know much about electronics? Your entire post makes little sense

my suggestion would be to split up your creation of your signal from your amplification/transmission.

Transmitter: Oscillator ---> Buffer Amplifier ---> Power Amp (if needed) ---> harmonic filter (ladder of low pass filters) ---> antenna.

The reason i don't have filters for the harmonics on my transmitters is because i don't really care if the transmission has harmonics as long as i receive the fundamental.

From your posts it seems like the harmonics are the largest part of the signal so I would worry about them if I were you. There is nothing unpredictable about RF but there are several unpredictable things in your test rig!

Firstly, as pointed out, get rid of those harmonics. A triangle wave is not suitable for radio transmission and furthermore, if you are loading an 'inverter and crystals' oscillator (I assume these are digital inverters) with a reactive load, your actual frequency could be anything. The mode of oscillation would be difficult to predict. In real life applications, the first step is to generate a clean single (as near sine wave as possible) frequency, then to filter it to further reduce any harmonic content and finally to match it to the antenna. The filtering and matching will inevitably require 'buffer' (linear amplification) stages to both increase the signal level and isolate the stages.

Brian.