establishing the characteristic of an antenna using 2 identical ones - Page 1
Forum section: electronics / rf-microwave

author: Simon
author_rank: Unknown
timestamp: October 03, 2025, 06:43:51 am
content: I have this theory. If I have two home made aerials that are identical and connect one to the test output of a spectrum analyser and the other to the input of the spectrum analyser I can work out the characteristics (losses) of the antenna design.Is that correct? I'm not talking cal lab grade but just a rough idea. As both aerials will have the same characteristic the losses (after the cables) will simply be divided by 2 as that's the advantage of dB.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: October 03, 2025, 12:30:32 pm
content: Well the measurement can of course be made, the question is what it will show, and what secret variables would affect it.But if we imagine the antennas two parallel monopoles, then it's basically just a stripline bandpass filter you're measuring.I guess you could do that for more complex antennas too, but I suspect the environment (room, people, metal) would affect the measurement quite a lot. Maybe a wooden table in a field would be good.In addition to this measurement, I would think measuring both antennas as transmitting antennas against the same wideband receiving antenna would be warranted, to verify that the two are actually identical.I like the idea of writing the standard antenna out of the equation though, but I think making actually identical antennas might be practically challenging.

author: Simon
author_rank: Unknown
timestamp: October 03, 2025, 03:11:29 pm
content: Well what I have is in fact two telescopic antenna's attached to a piece of plastic to form a dipole antenna.We have a small so called wideband antenna but it's near useless at low frequencies (sub 200 MHz). I found my homemade dipoles where much more sensitive.I wondered about the sort of aerial shape they actually use in EMC labs and yea, not too hard to make I guess but it would then need characterising, so I'd have to make 2 identical ones. Yes outdoor space is available.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: October 03, 2025, 04:45:01 pm
content: In that case I don't think it's the most optimal solution.It's a lot of of extra work for not very much information. I think A/B comparing an antenna to a simple half wave dipole will give a better picture.There exists 120 years of ham radio literature on characterising antennas with a minimum of gear, I'm sure everything that can be measured can be found there.

author: A.Z.
author_rank: Super Contributor
timestamp: October 03, 2025, 07:37:10 pm
content: Quote from: Simon on October 03, 2025, 06:43:51 amand what are you thinking you'll measure with such a setup ?keep in mind that reflections/refractions from nearby objects will play a role, also, the height of antennas and their reciprocal distance will be related to the wavelenght which depends from the frequency in use; before going on, ask yourself why such kind of thing is usually carried on in anechoic chambers

author: radiolistener
author_rank: Super Contributor
timestamp: October 04, 2025, 04:17:47 pm
content: Technically, yes – your idea is correct. If you use two identical antennas, one connected to the generator (or spectrum analyzer tracking output) and the other to the analyzer input, the measured result will include the combined effect of both antennas. Since the measurement is in dB, you can simply divide by two to estimate the performance of a single antenna.However, there are a few important caveats:1)Distance between antennas.They need to be separated by at least one wavelength (preferably more). Otherwise, what you measure is mostly near-field capacitive or inductive coupling, not the true radiating characteristics of the antennas.2)Orientation and reflections.Proper alignment and polarization are important, and you should try to minimize reflections from walls, furniture, and other objects, otherwise the readings will be strongly distorted.3)Instrumentation.A full spectrum analyzer is not strictly necessary for this kind of rough test. Any selective receiver or field-strength meter that can measure signal level in a narrow frequency band will do. A spectrum analyzer just makes it more convenient to scan across frequencies.4)Accuracy.This method can give you a rough estimate of efficiency and radiation pattern.It is also important to note that the two antennas will reflect signals toward each other, much like mirrors, and this effect must be taken into consideration as well.Taking all this into account, the most suitable way to perform such measurements is in an open field, where walls or nearby buildings will not interfere. Nevertheless, you still need to be aware of reflections from the ground, as they can affect the results.