I am getting mixed reports on the optimum length of a dipole antenna.
I found full wavelength total length worked better than half wavelength total length.
QuoteI found full wavelength total length worked better than half wavelength total length.
Oh...."rabbit ears".
You should have opened with that
Forget calculating and "adjust antenna for best reception".
I found full wavelength total length worked better than half wavelength total length.
What I'm wondering is what the logic/physics here is.
What is the RTL-SDR input impedance?
Since these dongles are intended for TV, most dongles will have an input impedance of approximately 75 Ohms, although it is unlikely to be exactly 75 Ohms over the entire frequency range.
Remember that the mismatch loss when using 50 Ohm cabling on a 75 Ohm input will be very minimal at less than 0.177 dB.
The 75 Ohm impedance for the R820T can be checked on the datasheet which can be downloaded here.
However, newer dongles that come with SMA connectors will be 50 Ohms.
I'm not sure what dipole radiation resistance is...
I'm also not sure what's meant by feeder input.
I've looked up the input impedance of my radio:
Now I found a resource suggesting the length of the total dipole should be half the wavelength, each element a quarter of the wavelength
For my reference frequency let's do 117MHz, so 468/117MHz gives a wavelength of 4 feet.
So each element is 2 feet, or half the wavelength.
I should add what I'm doing. I bought this kit and am exploring the frequency spectrum.
I should add what I'm doing. I bought this kit and am exploring the frequency spectrum.
That kit may be ok for V/UHF and then, only if you go listening outside in some place far away from the urban "electrosmog cloud", willing to get serious you'd better consider putting up a better antenna and place it outside, be it on a balcony or a window if not over the roof, connect it with a run of RG6 (TV/Sat) coax cable and start listening
Well, at 0.6 Lambda height it's more than 7 dBi, now maybe you'll need to search for dBi
Well, at 0.6 Lambda height it's more than 7 dBi, now maybe you'll need to search for dBi
A dipole is not 7 dBi gain. It's about 2.15 dBi.
https://en.wikipedia.org/wiki/Dipole_antenna
Well, at 0.6 Lambda height it's more than 7 dBi, now maybe you'll need to search for dBi
A dipole is not 7 dBi gain. It's about 2.15 dBi.
https://en.wikipedia.org/wiki/Dipole_antenna
That's gain in free space.
The height above ground of a half-wave dipole will cause the gain to increase at some elevation angles, and decease at others. I don't have the numbers handy, but I believe that the 7dBi value is possible. Obviously the quality of the ground also is a factor.
I'm not sure what dipole radiation resistance is...
https://en.wikipedia.org/wiki/Radiation_resistance
Radiation resistance is that part of an antenna's feedpoint electrical resistance caused by the emission of radio waves from the antenna.
In other words, this is a part of antenna input resistance which leads to energy losses for EM radiation.
The antenna also has a part of resistance due to unwanted energy loss due to heating.
For receiver antenna you can view it as a signal source resistance.
I'm also not sure what's meant by feeder input.
Feeder input is a transmission line input end point which is used to put RF signal from source.
For receiver antenna this is transmission line endpoint which is used to connect to antenna.
If you're using coax cable as transmission line, it will be coax cable end which is connected to antenna.
I'm confused. "Transmission line input end point" - so the end of the antenna? Where the antenna connects to the receiver? That's where the resistance in a coax cable is put, at the endpoint?
Well, if you are only interested in SHF signals up to the GHz, that one may be a start, but then don't expect it to give you some decent reception down at UHF/VHF or HF, they just won't perform; if you're going to install an antenna up the roof, my suggestion is to start with a discone, most of them will offer acceptable performance from 25MHz up to about 2GHz
Willing to get down to HF/VHF with decent performance, here's my suggestion
Pick this small 9:1 BalUn https://www.amazon.com/Balun-One-Nine-v2-Barebones/dp/B07XJRTF94 and put it inside a plastic w[e]at[h]erproof junction box, then use some 16AWG insulated wire and lay it out over the roof (or inside attic if you have a wooden roof) forming an horizontal loop and connect the ends of the wire to the balun, cut the loop wire wire at the opposite side from the Balun and insert a resistor with a value around 500...600 Ohm, the latter will flatten the impedance curve allowing to obtain a good match over a wide bandwidth, such a terminated loop will also offer the advantage of being less "noisy" than other types of antennas, yet it will be able to set the SNR and allow you to pick up signals from MW up to around 50MHz at least
I'm confused. "Transmission line input end point" - so the end of the antenna? Where the antenna connects to the receiver? That's where the resistance in a coax cable is put, at the endpoint?
It's the location where the transmission line ends, and the antenna element(s) begin. In your case it's where Coax is connected to the 2 dipole elements.
I'm confused. "Transmission line input end point" - so the end of the antenna? Where the antenna connects to the receiver? That's where the resistance in a coax cable is put, at the endpoint?
It's the location where the transmission line ends, and the antenna element(s) begin. In your case it's where Coax is connected to the 2 dipole elements.
On the dinky included dipole antenna, they pulled the shielding and put it on one dipole as the negative, and then took the inside and merely connected it to the other dipole antenna.
There's a resistance here? I mean it must certainly be negligible?