Hello all,
I am bloody beginner in the radio area. Few days ago, a friend of mine pointed me to an article about possibility to receive weather satellite images with an RTL-SDR dongle. I was really hooked, I was not aware that something like that is possible. I have one dongle by chance, I used for DVB-T reception, and lucky enough, the dongle is one with the right chipset. So, I downloaded SDR# a tested the dongle with air traffic listening. For satellite reception tough, I do not have the right antenna, so I browsed internet and found several articles about QFH antennas. As I have a 3D printer, I designed my own construction of the antenna, the biggest benefit is (imho) that you can align the parts of the antenna very easily and that the parts can be fixed in place with hot glue gun, so the construction is very easy also for people without workshop (in fact, the only tools you need is solder iron, drill and hot glue gun). If you are interested in the design (still work in progress) check this link please. QFH antenna 137.5MHz by osvold - Thingiverse The antenna works, I was able to hear NOAA 15 yesterday, the antenna was positioned in dense urban area, where just part of the sky is visible, I was not able to decode images yet, because I am still in testing and learning phase.
Now, as beginner, I have lot of questions.
1) Coaxial impedance.
I know that the amateur radio equipment usually uses 50ohm components. I guess, that the RTL-SDR dongle uses 75ohm, because the intended usage is for DVB-T. So, it is right to use 75ohm coaxial cable for signal feed, please? (Somewhere, I did read that the loss between 75 and 50 is just 0.2dB so that should be anyway marginal)
2) Material used
I do not want to use copper tubing. My aim is to be able to create cheap antenna, which can be easily built. Work copper tubing is not easy, and the material is very expensive. So, I decided to use wire. In some guides, people used coaxial cables, in others aluminium wires, I used wires for connecting reproboxes. The disadvantage of this wire is, they do not hold shape, so the helical form of the antenna is not as it should be. So, my question is, what is the best and yet affordable material for QFH antenna, please?
3) Dimensions of the antenna
The antenna should be tuned for 137,5MHz. The dimensions are the biggest issue. I based my design on dimensions from those links: A QFH antenna for the weather satellite band and http://myweb.tiscali.co.uk/wxsatellite/cvl-qfh.pdf
So, according to this, the longer loop is 2443 mm, the shorter is 2216mm. Well, then, there is also a (partially) functional QFH calculator here: Quadrifilar helicoidal antenna - Javascript on-line calculator and when I enter the data from the 2 links above, I do get very different results :/.
Last but not least, there are the theoretical papers from Mr.R.W.Hollander Documenten and guess what, again, completely different dimensions. Now, as I have no tools to measure, how well my antenna works, and I would like to create version, which is really optimal, what dimensions are the right ones?
4) The actual length of the loops
How is this measured? In theory, there are 2 loops, but in fact, both those loops are interconnected, so 1 big loop exists instead. So, my question is, where are the dimensions measured? Do you take in account also the length of the connection at the top of the antenna or not? (in other words, the length of the long loop + length of the small loop is equal to the length of the long loop wire + length of the small loop wire + length of the connection?)
I will be gratefull for any tips, thanks in advance.
I am bloody beginner in the radio area. Few days ago, a friend of mine pointed me to an article about possibility to receive weather satellite images with an RTL-SDR dongle. I was really hooked, I was not aware that something like that is possible. I have one dongle by chance, I used for DVB-T reception, and lucky enough, the dongle is one with the right chipset. So, I downloaded SDR# a tested the dongle with air traffic listening. For satellite reception tough, I do not have the right antenna, so I browsed internet and found several articles about QFH antennas. As I have a 3D printer, I designed my own construction of the antenna, the biggest benefit is (imho) that you can align the parts of the antenna very easily and that the parts can be fixed in place with hot glue gun, so the construction is very easy also for people without workshop (in fact, the only tools you need is solder iron, drill and hot glue gun). If you are interested in the design (still work in progress) check this link please. QFH antenna 137.5MHz by osvold - Thingiverse The antenna works, I was able to hear NOAA 15 yesterday, the antenna was positioned in dense urban area, where just part of the sky is visible, I was not able to decode images yet, because I am still in testing and learning phase.
Now, as beginner, I have lot of questions.
1) Coaxial impedance.
I know that the amateur radio equipment usually uses 50ohm components. I guess, that the RTL-SDR dongle uses 75ohm, because the intended usage is for DVB-T. So, it is right to use 75ohm coaxial cable for signal feed, please? (Somewhere, I did read that the loss between 75 and 50 is just 0.2dB so that should be anyway marginal)
2) Material used
I do not want to use copper tubing. My aim is to be able to create cheap antenna, which can be easily built. Work copper tubing is not easy, and the material is very expensive. So, I decided to use wire. In some guides, people used coaxial cables, in others aluminium wires, I used wires for connecting reproboxes. The disadvantage of this wire is, they do not hold shape, so the helical form of the antenna is not as it should be. So, my question is, what is the best and yet affordable material for QFH antenna, please?
3) Dimensions of the antenna
The antenna should be tuned for 137,5MHz. The dimensions are the biggest issue. I based my design on dimensions from those links: A QFH antenna for the weather satellite band and http://myweb.tiscali.co.uk/wxsatellite/cvl-qfh.pdf
So, according to this, the longer loop is 2443 mm, the shorter is 2216mm. Well, then, there is also a (partially) functional QFH calculator here: Quadrifilar helicoidal antenna - Javascript on-line calculator and when I enter the data from the 2 links above, I do get very different results :/.
Last but not least, there are the theoretical papers from Mr.R.W.Hollander Documenten and guess what, again, completely different dimensions. Now, as I have no tools to measure, how well my antenna works, and I would like to create version, which is really optimal, what dimensions are the right ones?
4) The actual length of the loops
How is this measured? In theory, there are 2 loops, but in fact, both those loops are interconnected, so 1 big loop exists instead. So, my question is, where are the dimensions measured? Do you take in account also the length of the connection at the top of the antenna or not? (in other words, the length of the long loop + length of the small loop is equal to the length of the long loop wire + length of the small loop wire + length of the connection?)
I will be gratefull for any tips, thanks in advance.
