Why is this cheap "RTL-SDr Blog V3 Kit" antenna so good, and why is it so directional?

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ScubaJungle

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I'm just extremely curious as to why these antennas work so well. They manage to pull in signals that even my Diamond RH789 cant, and even compete with my Wilson 800mhz Yagi. I am not trying to "push" this antenna/brand, I am just perplexed by how it out-does almost all my other antennas, and why it behaves the way it does.
1. It is extremely directional. I have it on a tripod right in my living room, next to my desk, and I revolve the tripod around to pick up different systems.
- for example, one system, which I can barely get one bar of signal on my BCD436HP with the Diamond, and nothing at all with the stock rubber duck, reliably decodes it (especially in DSDPlus, which has near 100% decode rate).
2. It is extremely odd in its orientation preferences. Everything I can find online says that vertical or horizontal polarization are the two main orientations, but this antenna does noticeable better when put in the 1pm -> 7pm position. Picture below:

3. Other than wanting to learn about the reasons behind all of this, I'd also like to know if there is actual reasoning behind this. I've been looking all over for antennas that will pull in VHF/UHF better, but if this isnt just luck I am having with this antenna, Im considering buying 5 of them to keep at a static length for specific frequencies, instead of having to break out the tap measure every time I want to change frequencies/bands (at $12, thats pretty good).

This is the link to the kit and about it:


(my RH789 does well, dont get me wrong, I am extremely happy with it, but I'd like to increase my range for home/base listening)
 

Ubbe

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A dipole antenna with that ferrite choke on the coax will be a good antenna, compared to the others you mention.

Antennas always follows the saying that bigger is better. Moving antennas around and angle them differently to find the best signal are always a good idea. It will proabably be different positions and angles even in the same frequency band depending of the direction to the transmitter.

When adjusting element lenghts it's not always that the mathematical lenght are the best. You could try and push the telescopes together an inch and see what happens to the signal strenght. Easiest to do when the scanner are set to analog mode and monitoring a weak signal. Then push together again until you hear a degradation in signal and then start to pull the telescopes apart to make it longer. And continue doing that while the signal improves and until it starts to degrade. Then set the lenght to the middle of those two points where the signal starts to degrade.

/Ubbe
 

ScubaJungle

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A dipole antenna with that ferrite choke on the coax will be a good antenna, compared to the others you mention.

Antennas always follows the saying that bigger is better. Moving antennas around and angle them differently to find the best signal are always a good idea. It will proabably be different positions and angles even in the same frequency band depending of the direction to the transmitter.

When adjusting element lenghts it's not always that the mathematical lenght are the best. You could try and push the telescopes together an inch and see what happens to the signal strenght. Easiest to do when the scanner are set to analog mode and monitoring a weak signal. Then push together again until you hear a degradation in signal and then start to pull the telescopes apart to make it longer. And continue doing that while the signal improves and until it starts to degrade. Then set the lenght to the middle of those two points where the signal starts to degrade.

/Ubbe
I move it around a lot to find the sweet spots. It's somewhat annoying sometimes moving it around at different angles and lengths, but the result is worth it - which is why I think Im going to buy a few to keep at the sweet spots/angles/lengths, until I have access to better tools and room for bigger antennas.
Ill give that technique a shot, I've just been moving it around randomly until I get the best signal.

If I were to buy/make a different one with a better balun/coax/longer elements, would I see a lot of improvement, or do you think I should just go with the same one?
The balun piece of these is just a black plastic piece, the inner wire goes to one element and the outer coax goes to the other. Theres also a 50 ohm resistor connected to the inner coax wire. The elements are pretty flimsy, so I wonder if getting better ones would improve reception, or if the balun would make more difference?
 

Ubbe

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You wont see a lot of improvement, maybe a minor one if you use a proper transformer balun. Dipoles usually have a 75 ohm impedance so those kind of 75-50 ohm baluns are not as common as the 300-75 ohm ones and would be more expensive. You can look at MiniCircuits home page for those but the performance difference probably does not justify the cost. You can probably get a free sample if you ask them for a balun without connectors, the $5 cheap ones, if you say that you gonna use it in an antenna experiment.

If you have an old tv antenna balun, or a cheap seperate 300-75 balun, you could remove half of the turns on the 300 ohm side to the antenna to make it more suitable for an open ended dipole antenna.

The coax, it's probably RG174, are so short (6ft ?) that it shouldn't matter if you replace it with a higher quality coax.

It's still a good idea to have ferrite chokes on the coax to stop any signals that the coax shield picks up, as it works as an antenna and receives RF, to travel up to the antenna and enter into the coax and interfere with reception.

Is that 50 ohm resistor connecting the two telescope antennas together? That's not necessary and will only load the signal and it can be removed.

It doesn't matter to its performance if the antenna are flimsy and use cheap plastic details. If you get thicker elements it will widen the useable bandwidth. If you get tubes that are really wide, look at the Omni-X antenna, you could get a huge frequency range, but that's probably the only noticable thing you could do but will still give the same signal strenght as thinner elements tuned to a specific frequency.

/Ubbe
 

slicerwizard

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A telescopic dipole antenna can perform very well because it's actually two antennas set up in a push-pull arrangement and they can be tuned for targeted frequencies, can have good capture area, can be arranged in a directional pattern and can do near-perfect RF power transfers between the air and the coax with little in the way of capacitive, reactive or resisitive mismatches.
 

Ubbe

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It would be interesting to set one leg of the dipole to 18 inches and the other to 48 inches....
If you detune one leg the antenna will be less efficient and loose signal strenght, it's a compromise and the end result will probably equal the other antennas he have. Offset antennas are a terrible solution if you are trying to receive weak signals. It's only supposed to be used to broaden the frequency range, to the cost of less signal strenght and getting strange directional patterns.

/Ubbe
 

ScubaJungle

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You wont see a lot of improvement, maybe a minor one if you use a proper transformer balun. Dipoles usually have a 75 ohm impedance so those kind of 75-50 ohm baluns are not as common as the 300-75 ohm ones and would be more expensive. You can look at MiniCircuits home page for those but the performance difference probably does not justify the cost. You can probably get a free sample if you ask them for a balun without connectors, the $5 cheap ones, if you say that you gonna use it in an antenna experiment.

If you have an old tv antenna balun, or a cheap seperate 300-75 balun, you could remove half of the turns on the 300 ohm side to the antenna to make it more suitable for an open ended dipole antenna.

The coax, it's probably RG174, are so short (6ft ?) that it shouldn't matter if you replace it with a higher quality coax.

It's still a good idea to have ferrite chokes on the coax to stop any signals that the coax shield picks up, as it works as an antenna and receives RF, to travel up to the antenna and enter into the coax and interfere with reception.

Is that 50 ohm resistor connecting the two telescope antennas together? That's not necessary and will only load the signal and it can be removed.

It doesn't matter to its performance if the antenna are flimsy and use cheap plastic details. If you get thicker elements it will widen the useable bandwidth. If you get tubes that are really wide, look at the Omni-X antenna, you could get a huge frequency range, but that's probably the only noticable thing you could do but will still give the same signal strenght as thinner elements tuned to a specific frequency.

/Ubbe
Awesome, thanks. If thats the case, Ill probably just order a few more of them since theyre only like $12.
I believe the resistor is connecting both elements, yes. Would it be better to remove it, or just leave it as is? Im not sure why they have it there, I think I read something on their website about it before though, as well as that you cant Tx with it because of it?

A telescopic dipole antenna can perform very well because it's actually two antennas set up in a push-pull arrangement and they can be tuned for targeted frequencies, can have good capture area, can be arranged in a directional pattern and can do near-perfect RF power transfers between the air and the coax with little in the way of capacitive, reactive or resisitive mismatches.
Thanks, I wouldnt have guessed it would work so well, but it makes sense being technically two antennas, and easily tunable.
It appears to be very similar to this antenna -
Homebrewed Off-Center Fed Dipole - The RadioReference Wiki
It would be interesting to set one leg of the dipole to 18 inches and the other to 48 inches.... turning it into an offset dipole. Might increase the frequency coverage range considerably without having to retune both elements every time for different bands.
If you detune one leg the antenna will be less efficient and loose signal strenght, it's a compromise and the end result will probably equal the other antennas he have. Offset antennas are a terrible solution if you are trying to receive weak signals. It's only supposed to be used to broaden the frequency range, to the cost of less signal strenght and getting strange directional patterns.

/Ubbe
Yes, I have tried this and the signals are weaker. Right now the best I've found is to find the length, and then play around with direction, and finally, angle. Vertical, with a slight angle seems to be the best orientation.
 

dem1

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The balun piece of these is just a black plastic piece, the inner wire goes to one element and the outer coax goes to the other. Theres also a 50 ohm resistor connected to the inner coax wire.
Is that 50 ohm resistor connecting the two telescope antennas together? That's not necessary and will only load the signal and it can be removed.
Double check the value of your resistor. I have the same antenna and it is 100K ohm on mine. This would serve to bleed any static charge (its intended purpose) without impacting performance. Ubbe is correct that 50 ohms would reduce signal to the receiver.

Those old enough to have used "rabbit ears" for analog TV reception will remember that every channel required different positioning and adjustment of the two antenna elements to maximize signal strength while minimizing interference and ghosting (multipath). Since no rule of thumb could account for all of the local variables, best reception was often non-intuitive and attained by trial and error. The RTL-SDR antenna is not all that different, especially when set up indoors.
 

ScubaJungle

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Double check the value of your resistor. I have the same antenna and it is 100K ohm on mine. This would serve to bleed any static charge (its intended purpose) without impacting performance. Ubbe is correct that 50 ohms would reduce signal to the receiver.

Those old enough to have used "rabbit ears" for analog TV reception will remember that every channel required different positioning and adjustment of the two antenna elements to maximize signal strength while minimizing interference and ghosting (multipath). Since no rule of thumb could account for all of the local variables, best reception was often non-intuitive and attained by trial and error. The RTL-SDR antenna is not all that different, especially when set up indoors.
Yep, I was mistaken, its 100k.
And yes, definitely lots of trial and error, but well worth it considering the results.
 
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