How to model a receiving dipole antenna in spice - Page 1
Forum section: electronics / rf-microwave

author: Etesla
author_rank: Regular Contributor
timestamp: March 04, 2020, 06:42:03 am
content: Hello all,I'm trying to decide how to model a simple receiving half wavelength dipole antenna in ltspice for FM around 100 Mhz.Here's my best guess, please let me know if it's reasonable.I would model the antenna as a voltage source with a series resistance of around 50 ohms, with the voltage source referenced to the same ground as the rest of the receiving circuit.The signal the voltage source is generating would be an identical waveform to the transmitted signal (an fm waveform of consistent amplitude, center frequency 100 Mhz, 200khz of 'width'). The amplitude of the waveform would then be a function of the distance the receiving antenna is from the transmitter, and the power of the transmitter. I would get a ROUGH estimate for the amplitude by doing something like amplitudeReceivedSignal = sqrt((transmitterPower/r^2)*377).for a 1 watt transmitter at 100 meters, this means I would model my antenna as a .2V amplitude FM signal referenced to ground with 50 ohms series impedance, coupled to the rest of the receiving circuit through maybe a 1nF capacitor.Does this all sound alright/ reasonable? I am most worried about my hackjob at guessing the amplitude, and also that the 50 ohms should be a lot more ohms... Thank you in advance!

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 12, 2020, 12:32:12 pm
content: A half wavelength dipole doesn't present an impedance of 50 ohm. I believe it is something like 74+j43 ohm? I don't know if LTspice can do non-real resistances? Perhaps you can hack something together by adding a bit of inductance that gives you the j43 ohm in series with a 74 ohm resistor.Some pointers:A dipole is a differential structure. It could be that if you use some off-the-shelf dipole, it includes a matching network and balun, so it presents a single-ended 50 ohm impedance.In the former case, you can't really just hang one end to ground, you would want to model the differential nature properly and integrate a balun.Also, if you add a transmission line between your antenna and whatever it connects to, you should model this. The impact of the reflections at the detector will change as you increase transmission line length (rotating on the smithchart). You could potentially match your antenna to whatever impedance input you have using the right length/impedance transmission line.I think you also want to model your source as having twice the power. After all, half the power will be lost in the matching network, and this is not there in reality.With regards to the power: You should look at the Friss equation, it is pretty much the 'proper' solution to estimate input power based on antenna gain and distance (keep in mind that esp. at lower frequencies, it has limited accuracy because of the fresnell zone being obstructed by the big ball of dirt we live on - there are more accurate channel models out there, the ITU has a book on it).

author: k8byp
author_rank: Newbie
timestamp: March 13, 2020, 05:36:12 pm
content: There is no such thing as modeling an antenna in Spice.Spice requires parameter files. Since no one in the history of engineering that I can find through a historical search has ever come up with an exact electrical equivalent model for any antenna (including IEEE sources) then you have no parameter files to give to Spice to analyze with.Engineering papers sometimes use a simple parallel or series circuit, but that in no way represents the complex values of a real antenna. Thats a grossly over-simplified approach.The closest you can get is the feedpoint characteristics. The ideal dipole, half wave long, half wave above an ideal ground plane is 73 + j 48.Thats not a good dipole as its out of tune. The "73" needs to be "75" to match a 75 ohm Tline, and the j48 has to go away.IF youre looking for something to terminate a circuit in just to run the circuit, then it must be terminated in the resistance of the circuit feeding the antenna with no reactance. Thats called a "dummy load"And if youre looking for optimal, a recent IEEE APS paper stated that the optimal receive antenna feedpoint characteristic is NOT the same as the same antenna for a transmitter.