Antenna tuning of a SDR

Thread Starter

Track99

Joined Jun 30, 2022
18
Hello my friends. I bought this software defined radio...https://www.amazon.com/gp/product/B011HVUEME/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

It is a RTL-SDR Blog V3 R820T2 RTL2832U 1PPM TCXO HF Bias Tee SMA Software Defined Radio with Dipole Antenna Kit.

I was also reading how every radio receiver need its antenna tuned to that particular radio frequency that it is receiving. This radio can tune from 500 kHz to 1.7 GHz.
Does it mean that I have to make the telescopic antenna shorter or longer in order to tune the antenna length of the telescopic antenna based on the frequency that I am listening to?
 
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Ya’akov

Joined Jan 27, 2019
6,266
TLDR; You need to describe what you intend to do with the dongle, that is what do you want to listen to, to decide on an antenna, there is no one size fits all solution. The more “general” you get the more the cost of decent performance increases so please do explain to us what you plan on listening to.

It is correct that a resonant antenna is critical to optimum performance. Your idea of a telescopic whip is sensible insofar as you can tune it but you really can’t have one antenna for that range that with it being bad at just about everything or very complicated. While your idea of tuning is a good one, the range and accuracy will be abysmal compared to the recieve bandwidth.

Instead of trying to accommodate such a ridiculously wide bandwidth you should have an antenna for what you want to receive. This might mean switching antennas if you are moving bands. There are some wide band antennas for situations where you really don’t know what you are going to try to listen to, but they are always going to be worse performances than purpose built antennas for the frequency of interest.

One design which is a good performer is the log periodic (LP) antenna. This is an array of antennas that are connected together in such a way that it creates an single antenna operating on multiple frequencies. It is a highly directional antenna which means you have to point it in the direction of the station of interest, this is a double edged sword.

1657875754464.jpeg
This one is quite expensive at more than $150 but covers 690-2700MHz

A related design is the slot antenna which has the advantage of being much more continuous in terms of resonance.* This one is designed for SDR and simlar applications. The slot antenna has an advantage over the LP in size and radiation pattern as well as flatness of response over its design range.

1657875797328.jpeg
This image shows the radiation pattern of the slot antenna which is highly directional.

The gain an LP antenna exhibits is due not to active amplifications—antennas are passive devices—but to the concentration of the received (or transmitted) signal to only a segment of the 3D space it is in. The more directional an antenna, the more it rejects off axis signals. This means if you are trying to discover signals you will have to rotate the antenna on each segment of the band according to the bandwidth you are receiving with the SDR dongle.

The most common LP is the ubiquitous but vanishing old school TV antenna. That had to operate over a wide range beause the spread of TV frequencies was very wide. But the advantage to the designer of the LP for TV what the frequencies were fixed and known. If you are trying to have broad tuning it’s much harder to optimize.

LPs and slot antennas are good, but none will have the full range of the RTL-SDR dongle. The truth is not even the RTL-SDR dongle has its own full range since performance varise considerably with frequency and it has trouble with the lower end of its theoretical range. You will need additional filtering if that’s what you want to do.

Another possible broadband design, which eliminates the downside of a directional antenna—while also losing its gain advantage—is the discone. This antenna consists of a set of radiators forming a flat disk with another set below it making a cone, hence the name. Some also have one long radiator on top for lower frequency operation.

1657876243202.jpeg

Discone antenna designed for 25MHz-1.3GGHz, note the long loaded radiator on the top.

The discone is a traditional choice for scanner operation since the bands of interest can extend from the divide between the HF and VHF bands up to end of the SHF band, about 30MHz to 1000MHz. They are relatively good performers an simple tough bulky. They are really designed for outdoor use.

You will also find various flexible antennas and one approach is to buy a set of antennas you can swap out depending on the band. When you get below about 50MHz these “rubber ducks”are going to be very poor performers. The largest of the telescopic antennas is a better choice.

You also need to minimize the length of the feedline which means keeping the antenna as close to the dongle as practical. This must be balanced against the real advantage of outdoor, high point mounting. If you do mount it remotely, even a few meters away, then the coax used as feedline is a critical component and it must be very low loss if you expect it to operate well at higher frequencies. The loss in the feedline can easily exceed more than half the power of the received signal. The rule of thumb is the bigger the coax the better but this must be checked against the loss specifications for the particular cable.

In the end it is very important to decide what you want to do with the dongle in order to select antenna(s). One thing is certain, whatever came bundled will be of limited use.

* I know there are reasons to consider the two designs distinct but this is a simplification or I would be here all day. If oyu are interested in either design from a theoretical or practical building perspective you will find them different and requiring separate efforts.

[EDIT: formatting cleanup and typo repair]
 

Ya’akov

Joined Jan 27, 2019
6,266
Tuning an antenna is more critical for transmitting than receiving.
That's correct, impedance matching on the output of a transmitter prevents overheating, for one thing.

But if you are dealing with relatively poor front end circuitry, and high frequencies where transmission line loss is significant, a resonant antenna can make a very noticeable difference.
 

nsaspook

Joined Aug 27, 2009
10,104
During my Navy Radioman days it was very rare to have a resonant antenna because we constantly switched frequencies due to daily conditions, natural interference and intentional (didn't care) jamming mainly from the USSR.
 

Ya’akov

Joined Jan 27, 2019
6,266
During my Navy Radioman days it was very rare to have a resonant antenna because we constantly switched frequencies due to daily conditions, natural interference and intentional (didn't care) jamming mainly from the USSR.
When I travelled o the East Coast by plane 4 times a week fo r a couple of years, one of the airports had, I think, a Reserve installation that included the biggest rotatable LPDA I have ever seen. It was clearly wideband for the entire HF spectrum. This one is something like it:

1657894454521.png
2-50MHz

 

nsaspook

Joined Aug 27, 2009
10,104
When I travelled o the East Coast by plane 4 times a week fo r a couple of years, one of the airports had, I think, a Reserve installation that included the biggest rotatable LPDA I have ever seen. It was clearly wideband for the entire HF spectrum. This one is something like it:

We had them on ships when I was in during the 70's. It's been replaced with satcom today.
1657894907392.png
https://www.wikiwand.com/en/USS_Blue_Ridge_(LCC-19)
At the time of her commissioning, Blue Ridge had the distinction of carrying the world's most sophisticated electronics suite, which was said to be some thirty percent larger than that of the aircraft carrier USS John F. Kennedy, which had been the most complex. Blue Ridge was armed with a "main battery" of computers, communications gear, and other electronic facilities to fulfill her mission as a command ship. An extremely refined communications system was also an integral part of the ship's radical new design. Through an automated patch panel and computer controlled switching matrix her crew could use any combination of communication equipment desired. The clean topside area is the result of careful design intended to minimize the ship's interference with her own communications system.[11] U.S. Navy long-range communications were heavily reliant on high frequency radio systems in the 1970s and have evolved to predominantly satellite communications in the 2000s. This is illustrated by the long wire antennas and the directional HF yagi or log-periodic antenna initially installed on Blue Ridge and later removed and replaced with a number of satellite communications antennas.
1657895246177.png
https://www.thedrive.com/the-war-zo...-navys-massive-blue-ridge-class-command-ships
 

ag-123

Joined Apr 28, 2017
259
I've an RTL-SDR, if they bundle a proper telecscopic antenna, pull out the antenna sufficiently long. That should improve reception.
I think accordingly, longer antenna works better than shorter ones. However, I've read about all that impedance matching etc.
That is normally *difficult* at least to calculate it, an idea is to try to have it at multiples of the quarter wavelength of the main frequency you are interested in. But even that won't assure an impedance match.

So I did something 'simplier', I looked at the 'water fall' spectrograph and try to extent or shorten the antenna and monitor the differences.
my guess is that it would show the best signals / amplitudes if it is impedance matched.

generally if reception is ok, you could leave the antenna alone at its extended length.
 
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Ya’akov

Joined Jan 27, 2019
6,266
I think accordingly, longer antenna works better than shorter ones.

That is normally *difficult* at least to calculate it, an idea is to try to have it at multiples of the quarter wavelength of the main frequency you are interested in. But even that won't assure an impedance match.

my guess is that it would show the best signals / amplitudes if it is impedance matched.
Longer antennas work better for longer wavelengths. This is not a general case. The general case is that antennas of the appropriate length work better. This means the antenna might need to be longer, or shorter, depending on the frequency 0f interest.

Impedance matching is not difficult if you aren't guessing at it. There are many antenna designs and there is a lot of literature on sizing each design.

To adjust the matching element of an antenna you can use the noise level of a receiver with a narrow filter on it, or if you have a waterfall display watching the peak if the antenna is resonant. If it isn't, that won't help because nothing will seem to change.

The best way to determine if the antenna is properly tuned is to get a NanoVNA and learn to use it for SWR measurements. They can do a lot more but they are more than worth their low price if you use it only for that.

They are particularly good for checking the little antennas supplied with WiFI/LoRa/2.4GHz/433MHz modules. Many of them are about as good as no antenna, of mislabeled since many of them use the same radome.
 

Papabravo

Joined Feb 24, 2006
18,978
I've an RTL-SDR, if they bundle a proper telecscopic antenna, pull out the antenna sufficiently long. That should improve reception.
I think accordingly, longer antenna works better than shorter ones. However, I've read about all that impedance matching etc.
That is normally *difficult* at least to calculate it, an idea is to try to have it at multiples of the quarter wavelength of the main frequency you are interested in. But even that won't assure an impedance match.

So I did something 'simplier', I looked at the 'water fall' spectrograph and try to extent or shorten the antenna and monitor the differences.
my guess is that it would show the best signals / amplitudes if it is impedance matched.
That is not likely to be an effective strategy since changing propagation conditions may give you a false picture of what is going on. The pro way to do this is to measure the Return Loss or SWR of the antenna and feedline as you make adjustments. That way it will be unaffected by propagation conditions.
 

Papabravo

Joined Feb 24, 2006
18,978
Brings back memories. Shortwave seemed a lot more interesting back then.
That is saying a great deal. As a young teen in the 1958-1962 era, I learned a great deal about the world by listening to Radio Netherlands, BBC, DW, and Radio Brazzaville. I spent hours trying to make better long wire antennas for my Hallicrafters All American "five" receiver
 
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Ya’akov

Joined Jan 27, 2019
6,266
Brings back memories. Shortwave seemed a lot more interesting back then.
It was. Many fewer national world service programs on the air now along with many more sources for exotic news.

I used to love utes (not Aussie sport utility vehicles, utility radio stations). Ship-to-shore, commercial radiotelephone stations for remote operation, numbers stations, and mysterious stations transmitting tones and voice intermittently.

And, of course "SKYKING SKYKING DO NOT ANSWER / DELTA GOLF ROMEO LIMA 6 / STANDBY / I SAY AGAIN DELTA GOLF ROMEO LIMA 6 / TIME 16 / AUTHENTICATION KILO WHISKEY INDIA / ANDREWS OUT"
 

ag-123

Joined Apr 28, 2017
259
That is not likely to be an effective strategy since changing propagation conditions may give you a false picture of what is going on. The pro way to do this is to measure the Return Loss or SWR of the antenna and feedline as you make adjustments. That way it will be unaffected by propagation conditions.
I'm gradually learning those things. As I don't have a fast oscilloscope, one of those things that I tend to struggle with is how to measure things e.g. frequencies / amplitudes at those VHF (e.g. 100 Mhz) or higher frequencies.
I've been wanting to build a VSWR thingy but has been procrastinating.
The frequency generator is inspired by the use of Si5351
https://hackaday.io/project/161761/logs?sort=oldest
https://gist.github.com/NT7S/3acfd9e00e60b1bfd0f6
https://www.adafruit.com/product/2045
https://cdn-shop.adafruit.com/datasheets/Si5351.pdf
https://en.wikipedia.org/wiki/SWR_meter

btw if anyone needs a 'spectrum analyzer', RTL-SDR is a 'crude' one, it is fun thing to play with and I think it works better than just a toy.
 
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Papabravo

Joined Feb 24, 2006
18,978
I'm gradually learning those things. As I don't have a fast oscilloscope, one of those things that I tend to struggle with is how to measure things e.g. frequencies / amplitudes at those VHF (e.g. 100 Mhz) or higher frequencies.
I've been wanting to build a VSWR thingy but has been procrastinating.
The frequency generator is inspired by the use of Si5351
https://hackaday.io/project/161761/logs?sort=oldest
https://gist.github.com/NT7S/3acfd9e00e60b1bfd0f6
https://www.adafruit.com/product/2045
https://cdn-shop.adafruit.com/datasheets/Si5351.pdf
https://en.wikipedia.org/wiki/SWR_meter

btw if anyone needs a 'spectrum analyzer', RTL-SDR is a 'crude' one, it is fun thing to play with and I think it works better than just a toy.
I recommend the nanoVNA. They are dirt cheap and very useful.
 

nsaspook

Joined Aug 27, 2009
10,104
..
And, of course "SKYKING SKYKING DO NOT ANSWER / DELTA GOLF ROMEO LIMA 6 / STANDBY / I SAY AGAIN DELTA GOLF ROMEO LIMA 6 / TIME 16 / AUTHENTICATION KILO WHISKEY INDIA / ANDREWS OUT"
You heard us. :D

We liked to use magnetic loop antennas on the receive side coupled to extremely good receivers.

https://www.usantennaproducts.com/antennas/usap-aperiodic-loop-antenna/

Here's a picture of one example where I once worked.
https://www.google.com/maps/place/2...e1af5ef0f4d8a30f!8m2!3d24.5721944!4d-81.67425
 
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