Simple (maybe?) Questions from a Beginning listener. - Page 1
Forum section: electronics / rf-microwave

author: SirAlucard
author_rank: Regular Contributor
timestamp: August 06, 2019, 09:06:48 pm
content: So like the title says I only listen (so far) on the ham bands. Mostly it has sparked my curiosity at the time being, but I haven't been inclined to get my license so I can transmit. Mostly for privacy reasons. Anyways I've been searching around and while there is a HUGE influx of information regarding antennas and the like I've mostly found that all of this information was directly attached to transmitting on those bands. If I try to find information on just RX I don't really come up with much. So I've come here to ask a few questions.Firstly my setup, it's not much, but it is mine. All I'm using to listen is an RTL-SDRv3 connected up to my PC. I've listened plenty in the 70cm and 2m bands, altho mostly been in the 70cm. I've played around with air traffic tracking, and listened in on some pilots coming in to land. So I've picked up plenty of those signals. What I'd like to try to do is Listen in on HF bands.So My questions are, When it comes to listening does the grounded part of the antenna matter? Probably a stupid question, but my reasoning for this question is thinking of AM radios and a Short Wave radio that I have that can pick up signals with a single whip antenna. So it is making me wonder if I can just have a single random wire without an counterpoise ground wire like you would for a dipole.Secondly with that asked if I do have to have a counterpoise, do I also need to have a balun attached to the antenna to receive? I've been trying to further understand what a balun does and I think I understand the reasoning, however I can't seem to figure out how you figure what ratio you need 4:1, 9:1, how would you go about figuring that out?I'm sure I've got more question, but atm I can't think of them. Will post more when I think of them.Thanks for reading.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: August 06, 2019, 09:36:26 pm
content: Transmitting and receiving antennas have different objectives, particularly on HF.A good transmitting antenna maximises your signal in the desired direction and at the desired angle for the communication you want.A good receiving antenna maximises the signal to noise ratio and thus the clarity of the received signal.On VHF/UHF this normally means good height and gain to maximise signal.On HF receivers are normally quite sensitive and noise is a big problem. So you want an antenna that minimises noise.A random wire without counterpoise or balun will probably be OK.But it will pick up noise all along it. This is why a dipole or similar with the feedpoint up high will probably be better. Note though that the dipole is only good for one band if fed with coax. Though you could add multiple dipoles to the one feedpoint. Or you could use something with tuned feeders and an antenna coupler. But all this is less critical on receiving as receivers are often sensitive enough.Or you could try a directional loop and try to null out interference.

author: SirAlucard
author_rank: Regular Contributor
timestamp: August 06, 2019, 09:42:13 pm
content: Would a lowpass filter be able to filter out any of those noises above 30mhz?

author: radiolistener
author_rank: Super Contributor
timestamp: August 07, 2019, 12:36:19 am
content: Quote from: SirAlucard on August 06, 2019, 09:06:48 pmYes, it matters, because it affects your antenna efficiency. The same as for transmitter mode.Transmitting and receiving antenna works the same, the difference is just max power requirements for antenna components. Transmit antenna should be able to handle enough power with no damage. All other things works the same.If you use just a piece of wire it will works the same for both - transmitter and receiver. It's efficiency will be the same low in both cases.Balun and RF choke is required to cut off noises on outer braid of the coax cable from working antenna body and to match coax cable impedance with antenna impedance. Impedance match is required in order to get better efficiency of energy transmission between antenna and transmitter or receiver.You can measure antenna impedance with vector analyzer. For hobby purposes you can use this cheap one for example:https://www.aliexpress.com/item/33016805196.htmlYou're needs to measure your antenna impedance with no cable. By direct connection vector analyzer directly to antenna feed point. Also you can calibrate vector analyzer through some piece of cable and then measure antenna impedance through this piece of cable. But don't forgot to calibrate your VNA through this piece of cable first!When you know antenna impedance you can calculate what balun ratio you're needs exactly. For example, if your antenna has impedance 200 Ohm, then you're needs 1:4 balun (200/50 = 4). If your antenna has impedance 450 Ohm, then you're needs to use 1:9 balun (450 / 50 = 9). It's not a big problem if you will use 1:9 balun for 400 or 500 Ohm antenna. Just use balun which better fits your antenna impedance.For unbalanced antennas (for example non-symmetric, such as a piece of wire) it is even more critical to use RF-choke and balun. Because unbalanced antenna has high impedance and will use outer braid of cable as counterpoise.Quote from: SirAlucard on August 06, 2019, 09:42:13 pmYou cannot filter noises taken by outer braid of coax cable in your home due to bad matching and unbalanced antenna (for example just random piece of wire). There is no way to filter noise with filter if the noise is already got into cable. This is why it is important to properly match antenna with balun and use RF choke to cut off all noise sources from antenna.When you use transmitter at more than 30-40% power, it is also important to use properly matched antenna in order to avoid transmitter output damage due to power reflected back into transmitter from unmatched antenna. Since transmitter has it's power limit this limit will be exceeded by sum of forward and reflected RF power and it may damage your transmitter output.For receiver and low power transmitter mode you can use any piece of wire as antenna, but it's efficiency will be very-very low. In RX mode it will give you a lot of noise from your home environment and very low station level which will be flooded by noises. In TX mode it will heat your home environment. Instead of transmit and receive signal to/from ether, you will transmit and receive it into/from all metal things and electronics around antenna cable.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: August 07, 2019, 10:10:07 am
content: Just put as much wire in the air as you can, the higher and the more horizontal the better. Dont stress or over think it. You will hear more than the nothing you do now, from there you can make improvements based on your unique circumstances.Rob.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: August 07, 2019, 12:36:55 pm
content: Quote from: vk3yedotcom on August 06, 2019, 09:36:26 pmHow is this any different? An antenna should be a reciprocal component, and hence if it maximizes your ability to get signal in a certain direction, it also maximizes your sensitivity to signals in that direction. I don't see how these two differ? Or are you talking about directivity vs beam angle/side lobe level?

author: borjam
author_rank: Unknown
timestamp: August 07, 2019, 01:08:16 pm
content: Some specialized receiving antennas, such as active broadband loops or the T2FD give excellent S/N ratio on reception. However, broadband active loops are useless for transmission and the T2FD is lossy.If you can give a try to a Wellbrook loop or maybe the new MFJ 1886. Or, if you have room for it, build a T2FD which is much cheaper.

author: radiolistener
author_rank: Super Contributor
timestamp: August 07, 2019, 03:58:12 pm
content: Quote from: borjam on August 07, 2019, 01:08:16 pmthey are useless because they using high gain low noise amplifier. You can attach high gain power amplifier with the same gain and it will works the same. But there are a lot of troubles to make high gain power amplifiers for the same gain. That's the problemSo, there is no difference for antenna to transmit or to receive. The difference is in amplifier, because it is more easy to make RX amplifer with high gain, than make high power TX amplifier with the same gain.

author: borjam
author_rank: Unknown
timestamp: August 07, 2019, 04:52:32 pm
content: Quote from: radiolistener on August 07, 2019, 03:58:12 pmBeware, an untuned loop with a preamplifier is not the same as an antenna with a preamplifier. Both are entirely different beasts

author: radiolistener
author_rank: Super Contributor
timestamp: August 07, 2019, 05:52:01 pm
content: Quote from: borjam on August 07, 2019, 04:52:32 pmYes, but antenna part works with the same efficiency for transmit and for receive mode. It doesn't matter if antenna tuned or not. Antenna tune and impedance match is critical for transmission line and amplifier, but not for antenna. Any piece of wire (even a household pin) can be used as TX or RX antenna. The problem here is just to feed it with RF energy or get RF energy from it

author: ogden
author_rank: Super Contributor
timestamp: August 07, 2019, 06:05:24 pm
content: http://dl1dbc.net/SAQ/miniwhip.htmlWorldsmost popular web SDR receiveruses such antenna.

author: borjam
author_rank: Unknown
timestamp: August 07, 2019, 08:04:56 pm
content: Quote from: radiolistener on August 07, 2019, 05:52:01 pmThe difference is the unbelievable S/N ratio achieved by some reception loops like those made by Wellbrook.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: August 15, 2019, 09:33:27 am
content: Quote from: TheUnnamedNewbie on August 07, 2019, 12:36:55 pmHow is this any different? An antenna should be a reciprocal component, and hence if it maximizes your ability to get signal in a certain direction, it also maximizes your sensitivity to signals in that direction. I don't see how these two differ? Or are you talking about directivity vs beam angle/side lobe level?The best transmit antenna radiates maximum signal.The best receive antenna provides clearest reception.Consider a receiver with a 1uV sensitivity and the following antenna scenarios:Antenna A: Signal 50uV, Noise 25uV. (this happens to be a very good antenna on transmit)Antenna B: Signal 10uV. Noise 2uV. (happens to be poor on transmit)Even though Antenna B picks up less of the signal the reception will be better on it as the signal will stand out far more out of the noise.

author: radiolistener
author_rank: Super Contributor
timestamp: August 15, 2019, 10:02:05 am
content: Quote from: vk3yedotcom on August 15, 2019, 09:33:27 amin your example antenna A has 5 times better RX efficiency than antenna B (50 uV vs 10 uV). This is why antenna B is poor on transmit, because it also poor on receive. But antenna B doesn't have noise source in it's near field region and this is why it has better SNR.Probably antenna B in your example has much smaller size, so it is less effective for RX and TX. But since it has small dimension, it also has smaller near field region. So, noise source may not fit in it's near field region and will not be amplified by Q times, where Q is antenna Q factor.In your example antenna A is just flooded with noise source in it's near field region, but it doesn't means that it has less RX efficiency than antenna B.Just remove noise source away from antenna A near field region and it will have Signal 50 uV, Noise 10 uV. Which is much better than antenna B.