How to test a simple Dipole Antenna on Spectrum Analyser?

[Shubham Jaydeokar]

Hello Experts,
I have created this thread especially for clearing some of the important concepts of Antenna and Designing Issues.
I have a question for all of you, related to Dipole Antenna.
I have designed a Folded Dipole Antenna resonating at center Frequency of 145.800MHz.
This antenna is made up of simple Copper Wire. The complete antenna is now ready for testing but I have a small problem regarding testing this antenna.
The antenna needs to be connected to a Spectrum Analyser for checking its parameters.
What should be the intermediate devices that I should use for connecting antenna to the (SpectrumAnalyser)SA?
To be more specific, which Balun & connector should I use for this purpose?

 

[Baluncore]

Welcome to PF.
A folded dipole is usually designed to have an impedance of about 300 ohms. A standard TV BALUN will match that 300 ohm to 75 ohm. But a spectrum analyser, SA, usually has a 50 ohm input. You will need to find out what your SA input impedance is and if you have a 75:50 impedance converter.

Do you have a tracking generator with the SA? Or is there a nearby continuous signal source at 145.8 MHz?
Next, what do you want to test? Directional gain pattern? Antenna impedance versus frequency?

 

[Shubham Jaydeokar]

Welcome to PF.
A folded dipole is usually designed to have an impedance of about 300 ohms. A standard TV BALUN will match that 300 ohm to 75 ohm. But a spectrum analyzer, SA, usually has a 50-ohm input. You will need to find out what your SA input impedance is and if you have a 75:50 impedance converter.

Do you have a tracking generator with the SA? Or is there a nearby continuous signal source at 145.8 MHz?
Next, what do you want to test? Directional gain pattern? Antenna impedance versus frequency?

I really appreciate your efforts in helping me out with my problem.

So the first thing, I have a TV Balun in my warehouse and fortunately, it is of 300:75 Ω.
The SA is of the company named "Signet Auto." And yes, it is compatible for 50Ω.
About the Continuous Signal Source, I am designing this antenna for receiving Audio Signals sent from ISS (International Space Station). So, It is a kind of Continuous Signal at 145.800MHz.
And the last thing, I want to test my folded dipole with the many parameters viz. Resonant Freq., VSWR, Radiation Pattern, Power Levels while receiving the signals, etc.

As you said, I will require a simple Balun for the connection between an antenna and SA, Is there any sort of additional RC Network that I would require for impedance matching even after Balun?
And if that's the case, would you help me by providing any Design formats of it?

 

[berkeman]

Hello Experts,
I have created this thread especially for clearing some of the important concepts of Antenna and Designing Issues.
I have a question for all of you, related to Dipole Antenna.
I have designed a Folded Dipole Antenna resonating at center Frequency of 145.800MHz.
This antenna is made up of simple Copper Wire. The complete antenna is now ready for testing but I have a small problem regarding testing this antenna.
The antenna needs to be connected to a Spectrum Analyser for checking its parameters.
What should be the intermediate devices that I should use for connecting antenna to the (SpectrumAnalyser)SA?
To be more specific, which Balun & connector should I use for this purpose?

Since this frequency in in an Amateur HAM radio band (licensed), you need to be careful if you want to do any transmitting on that frequency. The astronauts who transmit from the ISS do have their HAM radio licenses:

https://www.issfanclub.com/node/33379

Are you a HAM? If so, you would just use another transceiver to send test transmissions (CW/Morse or voice, not a long continuous carrier with no modulation). If you do not have your license, or want to use continuous unmodulated signals, you will need to do that in a shielded room of some sort, to avoid causing unwanted interference in the licensed radio bands.

 

[Baluncore]

Is there any sort of additional RC Network that I would require for impedance matching even after Balun?

You will use low loss 75 ohm coaxial cable to get down from the balun to the SA. Then a resistive pad or transformer to convert 75 ohm coax to the 50 ohm SA input.

Search ebay for ' 75 ohm to 50 ohm converter ' and you will find several a solutions hopefully with the connectors you need.

You could simply make an attenuator pad from two resistors. It will have a loss of 5.7 dB.
Use a 43.3 ohm resistor (47R // 560), between the 75 ohm and 50 ohm cables.
Place an 86.6 ohm resistor (82R + 4R7) between the 50 ohm connector and ground.

You need a local continuous test signal.
An orbiting satellite will not do because it keeps moving and will be very weak.

Edited resistor values.

 

[Averagesupernova]

I would say with a spectrum analyzer at your disposal a good tool to have would be an impedance bridge and a signal source. An impedance bridge works like the well known wheatstone bridge. You have a reference port, the output port, a generator port and of course the unknown port. When the unknown ports impedance matches the reference ports impedance the bridge is balanced and there will be no output on the output port. You can use a low level signal on the generator port, or a wideband noise source, or of course the tracking generator that was mentioned in post #2.
-
http://www.radiomuseum.org/r/palomar_noise_bridge_r_x.html
The device in the above link will not cover the frequency you want but it should give you a pretty good idea of what you can do.
-
Here is a better link:
http://palomar-engineers.com/tech-support/downloads
-
You can download an instruction manual.

 

[davenn]

And the last thing, I want to test my folded dipole with the many parameters viz. Resonant Freq., VSWR, Radiation Pattern, Power Levels while receiving the signals, etc.

A network analyser does these things --- Resonant Freq., VSWR, not a spectrum analyser

you can use a spec an to check received signal level ... but you can use your ham receiver for that too
you ham transceiver on 2m band can also be used for VSWR with an appropriate meter in line and you can do radiation pattern tests
in an open field away from all obstacles

Dae

 

[Shubham Jaydeokar]

Since this frequency in in an Amateur HAM radio band (licensed), you need to be careful if you want to do any transmitting on that frequency. The astronauts who transmit from the ISS do have their HAM radio licenses:

https://www.issfanclub.com/node/33379

Are you a HAM? If so, you would just use another transceiver to send test transmissions (CW/Morse or voice, not a long continuous carrier with no modulation). If you do not have your license, or want to use continuous unmodulated signals, you will need to do that in a shielded room of some sort, to avoid causing unwanted interference in the licensed radio bands.

Thanks' @berkeman for your help.

The answer to your first question. No, I don't have any such kind of license. I am not a Ham.

But the thing is, I am not going to transmit anything over this channel. I am only focusing on using my antenna for reception purpose. The Signals transmitted from ISS are Highly Coded with FM Modulation. So the main aim of doing this experiment is to receive the FM Signals transmitted by ISS.

The last thing, I won't be able to arrange this setup in a Shielded room or something. I am simply going to root the antenna in the center of my college's Playground. That will not completely eliminate the external interferences but this would at least help in reducing the unwanted noise.

I really appreciate your response for my question.
That is really helpful.

 

[Shubham Jaydeokar]

You will use low loss 75 ohm coaxial cable to get down from the balun to the SA. Then a resistive pad or transformer to convert 75 ohm coax to the 50 ohm SA input.

Search ebay for ' 75 ohm to 50 ohm converter ' and you will find several a solutions hopefully with the connectors you need.

You could simply make an attenuator pad from two resistors. It will have a loss of 5.7 dB.
Use a 43.3 ohm resistor (47R // 560), between the 75 ohm and 50 ohm cables.
Place an 86.6 ohm resistor (82R + 4R7) between the 50 ohm connector and ground.

You need a local continuous test signal.
An orbiting satellite will not do because it keeps moving and will be very weak.

Edited resistor values.

That was really informative.

I think the use of a resistive circuit would be most convenient for my experiment. I would think about this topic and will try to make it.
I would definitely ask you if there are any queries regarding the resistive circuit.

Thank's @Baluncore for your contribution.

 

[Shubham Jaydeokar]

I would say with a spectrum analyzer at your disposal a good tool to have would be an impedance bridge and a signal source. An impedance bridge works like the well known wheatstone bridge. You have a reference port, the output port, a generator port and of course the unknown port. When the unknown ports impedance matches the reference ports impedance the bridge is balanced and there will be no output on the output port. You can use a low level signal on the generator port, or a wideband noise source, or of course the tracking generator that was mentioned in post #2.
-
http://www.radiomuseum.org/r/palomar_noise_bridge_r_x.html
The device in the above link will not cover the frequency you want but it should give you a pretty good idea of what you can do.
-
Here is a better link:
http://palomar-engineers.com/tech-support/downloads
-
You can download an instruction manual.

The use of an Impedance Bridge seems to be more challenging for me. But as you've given me the links to study more about them, I would definitely give it a shot. The balancing of a Wheatstone bridge was one of my academics practical that I had performed. The use of such resistive bridge would require a great study and more calculations(specifically for my application).

@Averagesupernova your idea of Impedance Bridge is worth giving a shot. I would run few simulations of such a network after getting a clear idea about balancing a resistance sensitive network.

Thank's for your help, @Averagesupernova.

 

[Shubham Jaydeokar]

A network analyser does these things --- Resonant Freq., VSWR, not a spectrum analyser

you can use a spec an to check received signal level ... but you can use your ham receiver for that too
you ham transceiver on 2m band can also be used for VSWR with an appropriate meter in line and you can do radiation pattern tests
in an open field away from all obstacles

Dae

Thank's @davenn for correcting me with the use of a Network Analyser instead of a Spectrum Analyser.

Unfortunately, I don't think my institute would be able to provide me a network analyzer for my project. But since i want to measure those parameters that can only be tested on a network analyzer, I would now have to make an arrangement for it.

Thank's @davenn for helping me out.
I really appreciate your help.

 

[berkeman]

Do you have a local HAM radio club? Even though you don't have your HAM license, often HAM clubs are happy to help with projects like this. Our local club (in Northern California in the US) has a couple of HAMs that have built their own receiving rigs for listening to the ISS transmissions.

 

[davenn]

But the thing is, I am not going to transmit anything over this channel. I am only focusing on using my antenna for reception purpose. The Signals transmitted from ISS are Highly Coded with FM Modulation. So the main aim of doing this experiment is to receive the FM Signals transmitted by ISS.

then the accuracy of the measurements of the antenna are a non-issue
as long as you have made sure that you have the antenna length cut reasonably close to your required frequency
and your coax connections at the radio and antenna end are well done, you won't have a problem
... Radiation pattern, VSWR are also non issues for a dipole receive antenna for this situation they are more important for a transmitting antenna

a folded dipole cut for 146.0 MHz is going to work well on any freq from at least 140 to 150 MHz
that is to say, it's going to have a wide bandwidthSo, after all that, if you have made this antenna ... show us a photo or two
tell us the tip to tip measurements. and the gap measurement where the coax goes on
show a close up of the coax termination point

Also, others earlier on commented about impedance matching, this isn't much of an issue either
use 50 Ohm coax and a 4:1 BALUN ... I did one on this forum for some one else recently will see if I can find itDave

 

[davenn]

OK here it is ...

and some info to go with it

again for this BALUN, the loop is 1/2 wavelength at the freq of use
Ohhh I must state too, to save confusion, the lengths mentioned ... the 1/4 sleeve and the 1/2 wave loop ... these are an electrical 1/4 and 1/2 wave for the coax used. If a freespace 1/4 and 1/2 wave are used, the lengths will be too long

An example
freespace 1/2 wavelength at the 144MHz Amateur band calc ...
1 wavelength in metres = 300 / freq (in MHz) = 300 / 144 = 2.08 metres
1/2 wave/length = 2.08 / 2 = 1.04 metres
Electrical 1/2 wave = 1.04 x the Vf ( velocity factor of the cable)
1/4 inch diameter RG58U has a Vf of about 0.66 therefore
1.04 * 0.66 = 0.6864 metres (68.64 centimetres)
Dave

 

[Averagesupernova]

A simple dipole flat-top antenna has a feed impedance of around 73 ohms if memory serves me correctly. Attaching to this point with 50 ohm coax is a better match than just about anything else you can do here other than connecting it to actual 75 ohm coax. I would not agree that a 4:1 balun is a good idea. If you are fussy about matching balanced to unbalanced then a 1:1 balun would be appropriate. If one wants to learn about antenna matching a good idea might be to use a half wave length of conductor that is not split in the middle. Feed it with a coaxial feedline with a gamma match. With an impedance bridge and spectrum analyzer there is plenty that can be learned. The gamma capacitor required at this frequency can consist of a short length of coax with the shield and the center conductor serving as each a plate in the capacitor. There are plenty of references on-line to show the type and length of coax for this type of match. I don't know if they are still published but I always thought the W1FB books were pretty good beginner antenna design books. The ARRL has also published antenna books as thick as a couple of telephone books. I really wish I had more time to do some hamming. This discussion brings back some very fond memories.
-
http://www.arrl.org/shop/Antennas/
-
Here is a place to start. I didn't see the copies of Doug DeMaw (W1FB) antenna books that I have but it looks like there is plenty to choose from.
-
The below link shows a gamma match in a different type of antenna than a flat-top dipole but it uses a gamma match so there is a bit of knowledge to be gained other than reading what some guy on the internet said on a message board. (Me). Haha
-
https://en.wikipedia.org/wiki/Halo_antenna

 

[davenn]

I would not agree that a 4:1 balun is a good idea.

please don't take what I said out of context

a 4:1 balun is for his folded dipole so it is a great idea for that purpose

a straight dipole doesn't need a 4:1 balun

 

[Baluncore]

Antenna is a folded dipole. Zo = 300 ohm.
Then 4:1 BALUN to 75 ohm low-loss TV coax.
For testing the minimum resistive loss L-pad from 75 to 50 ohms, can be used to prevent line length sensitivity during testing, but I see little point in doing much testing or with having close to 6 dB loss in the receive cable from ISS.

So best to use something like a 75 ohm to 50 ohm transformer with an SDR receiver so you can watch the doppler shift on the computer screen. I believe that the ISS is operating on 145.8 MHz as a digital packet radio these days. A cheap SDR should be able to handle that demodulation and display with free software.

Attached is a picture from the web that shows a 78 to 50 ohm "UNUN" which will need to use a couple of turns of five-filar twisted wire on a UHF ferrite core, (pinched from a TV BALUN). The example attached was for HF.

 

[Averagesupernova]

My bad. Forgot the op was using folded dipole.

 

[Shubham Jaydeokar]

then the accuracy of the measurements of the antenna are a non-issue
as long as you have made sure that you have the antenna length cut reasonably close to your required frequency
and your coax connections at the radio and antenna end are well done, you won't have a problem
... Radiation pattern, VSWR are also non issues for a dipole receive antenna for this situation they are more important for a transmitting antenna

a folded dipole cut for 146.0 MHz is going to work well on any freq from at least 140 to 150 MHz
that is to say, it's going to have a wide bandwidthSo, after all that, if you have made this antenna ... show us a photo or two
tell us the tip to tip measurements. and the gap measurement where the coax goes on
show a close up of the coax termination point

Also, others earlier on commented about impedance matching, this isn't much of an issue either
use 50 Ohm coax and a 4:1 BALUN ... I did one on this forum for some one else recently will see if I can find itDave

I would send some pics of the antenna that i have made, in my next message of this thread.
And regarding the calculations for the exact dimensions of this antenna, I would also send you the details about it.
Thanks for helping me out.

 

[Shubham Jaydeokar]

A simple dipole flat-top antenna has a feed impedance of around 73 ohms if memory serves me correctly. Attaching to this point with 50 ohm coax is a better match than just about anything else you can do here other than connecting it to actual 75 ohm coax. I would not agree that a 4:1 balun is a good idea. If you are fussy about matching balanced to unbalanced then a 1:1 balun would be appropriate. If one wants to learn about antenna matching a good idea might be to use a half wave length of conductor that is not split in the middle. Feed it with a coaxial feedline with a gamma match. With an impedance bridge and spectrum analyzer there is plenty that can be learned. The gamma capacitor required at this frequency can consist of a short length of coax with the shield and the center conductor serving as each a plate in the capacitor. There are plenty of references on-line to show the type and length of coax for this type of match. I don't know if they are still published but I always thought the W1FB books were pretty good beginner antenna design books. The ARRL has also published antenna books as thick as a couple of telephone books. I really wish I had more time to do some hamming. This discussion brings back some very fond memories.
-
http://www.arrl.org/shop/Antennas/
-
Here is a place to start. I didn't see the copies of Doug DeMaw (W1FB) antenna books that I have but it looks like there is plenty to choose from.
-
The below link shows a gamma match in a different type of antenna than a flat-top dipole but it uses a gamma match so there is a bit of knowledge to be gained other than reading what some guy on the internet said on a message board. (Me). Haha
-
https://en.wikipedia.org/wiki/Halo_antenna

Thanks For the additional Help.

 

[Shubham Jaydeokar]

please don't take what I said out of context

a 4:1 balun is for his folded dipole so it is a great idea for that purpose

a straight dipole doesn't need a 4:1 balun

It is also important to consider the type of dipole antenna that I've made.
It's a Folded Dipole Antenna.
So, I guess 4:1 Balun is ok to go with.

Correct me if I'm not on the right track.

But, I appreciate your help @Averagesupernova
And thanks @davenn

 

[Shubham Jaydeokar]

Antenna is a folded dipole. Zo = 300 ohm.
Then 4:1 BALUN to 75 ohm low-loss TV coax.
For testing the minimum resistive loss L-pad from 75 to 50 ohms, can be used to prevent line length sensitivity during testing, but I see little point in doing much testing or with having close to 6 dB loss in the receive cable from ISS.

So best to use something like a 75 ohm to 50 ohm transformer with an SDR receiver so you can watch the doppler shift on the computer screen. I believe that the ISS is operating on 145.8 MHz as a digital packet radio these days. A cheap SDR should be able to handle that demodulation and display with free software.

Attached is a picture from the web that shows a 78 to 50 ohm "UNUN" which will need to use a couple of turns of five-filar twisted wire on a UHF ferrite core, (pinched from a TV BALUN). The example attached was for HF.

Yes.
The combination of a simple SDR and a computer software can get me done with the experiments' aim.
When this problem regarding a balun and a connector is solved, then I'm going to turn on this topic(FM Demodulation of the received signals using an electronic circuit).

Thanks for your help.

 

[davenn]

The last thing, I won't be able to arrange this setup in a Shielded room or something. I am simply going to root the antenna in the center of my college's Playground. That will not completely eliminate the external interferences but this would at least help in reducing the unwanted noise.

I hope you were not thinking about running a long run of coax back into the building ?
that will cause a major loss of signal ... you don't want any more than around 10 metres of RG58 ( 1/4 inch 50 Ohm) cable
and up to 20m of RG213 (1/2 inch 50Ohm) before the losses become unacceptable
see if you can find somewhere to mount the antenna just outside the window of the room

I would send some pics of the antenna that i have made, in my next message of this thread.

look forward to seeing your construction

Dave

 

[Shubham Jaydeokar]

I would send some pics of the antenna that i have made, in my next message of this thread.
And regarding the calculations for the exact dimensions of this antenna, I would also send you the details about it.
Thanks for helping me out.

These are the calculations for antenna:-

Freq= 145.800MHz.
λ=2.057Meters.
Length of dipole= (λ/2) = 102.85 cm.
Length of each side from feed point centre = (λ/4) = 51.42 cm.

 

[davenn]

These are the calculations for antenna:-

Freq= 145.800MHz.
λ=2.057 Metres.
Length of dipole= (λ/2) = 102.85 cm.
Length of each side from feed point centre = (λ/4) = 51.42 cm.

you forgot to take into account the velocity factor, this gives the electrical wavelength
your measurements are for a free space wavelength

the real antennal dimensions will be smaller

so let's start again

Freq= 145.800MHz.
λ=2.057Metres. good so far
Length of dipole= (λ/2) = 102.85 cm.
you need to multiply the 102.85 by the Vf (Velocity factor), in this case ( for open wire or aluminium tube ) around 0.95
if the wire or tube is insulated with plastic/Teflon etc the Vf drops even further

so 102.85 x 0.95 = λ/2 = 97.71 cm = electrical λ/2

Length of each side from feed point centre = (λ/4) =97.71 / 4 = 48.85 cmDave

 

[Baluncore]

I would use a velocity factor of 97% for bare wire. For 145.8MHz, λ = 299.8 / 145.8 = 2.056 m
The length of the wire you bend the folded dipole from will be; λ * vf = 2.056m * 0.97 = 1.995 m.

Next, the helical antenna on a satellite will appear as a vertical polarised wave at the Earth's surface. So you will need to mount the folded dipole with one end up, the other down. That will also be omni-directional and so receive both the north and south diagonal passes of the ISS.

Use 75 ohm TV coaxial cable, (with low-loss foam dielectric), not 50 ohm solid dielectric. If you need to match to a 50 ohm instrument then use a resistive L-pad.
There are many small USB interfaced SDRs available for about $10 that will do the digital processing and demodulation. They have 75 ohm inputs and are based on the RTL2832U + R820T DVB-T tuner chips.

 

[davenn]

I would use a velocity factor of 97% for bare wire. For 145.8MHz, λ = 299.8 / 145.8 = 2.056 m
The length of the wire you bend the folded dipole from will be; λ * vf = 2.056m * 0.97 = 1.995 m.

95 vs 97 or something close isn't really going to make much difference
as I said wayyyyyy back the folded dipole is going to have a very broad bandwidth anyway

same for the coax 50 or 75 Ohm ... the difference in practice is also not worth worrying about

 

[Baluncore]

95 vs 97 or something close isn't really going to make much difference

I spent a lot of time measuring the velocity factor on supposedly bare wire. For copper it is between 97% and 98% depending on the state of surface oxide or acetate formation. 95% is a better value for aluminium wire or tube dipoles as often used on Yagis.

as I said wayyyyyy back the folded dipole is going to have a very broad bandwidth anyway

I am not interested in the bandwidth, only in minimising reactance at the centre frequency prior to the balun. That is why it is the wire length used to make the folded dipole that is important, not the internal dimensions of a box that might hold the folded dipole.

same for the coax 50 or 75 Ohm ... the difference in practice is also not worth worrying about

I would not worry about using 50 ohm cable because, for a receive application where both the balun and receiver were 75 ohm impedance, I would use 75 ohm cable.
If 75 ohm TV cable is used it will have lower loss than a low SWR 50 ohm cable such as RG58. 75 ohm will match both the balun and the receiver. It will not have a comb of nulls at different frequencies dependent on line length.

It is time the folded dipole went to work on a SDR, first looking at the local VHF activity, then listening for the ISS doppler shift and packet data.