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Making a portable wideband VHF antenna

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mateusbatera

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Hi there,

I need help in designing a single antenna to cover the whole band between 30MHz and 300MHz. I know that there are some types that would do the job, like log-periodic or spiral antennas, but the problem is that I am supposed to make it as portable as possible.

I believe that anything larger than 50cm will be too large, so I have a tough mission ahead... Can anyone help?
 

Re: Wideband VHF antenna

If you need antenna logo-periodic is a good choice, but size will be bigger
 

Re: Wideband VHF antenna

I found something interesting: https://www.wellbrook.uk.com/pdf/ALA1530S+.pdf
I'm not familiar with the negative impedance (non-foster) matching technique though. And I can't seem to find any useful lectures about it either. Any suggestion will be welcome.

PS: I just forgot to mention that the antenna is RX only.
 

Re: Wideband VHF antenna

mateusbatera said:
I found something interesting: https://www.wellbrook.uk.com/pdf/ALA1530S+.pdf
I'm not familiar with the negative impedance (non-foster) matching technique though. And I can't seem to find any useful lectures about it either. Any suggestion will be welcome.
PS: I just forgot to mention that the antenna is RX only.
Click to expand...

well that covers one HF band, magnetic loops are good, they work well have built and used one myself. They have VERY narrow tuning range ie... High Q even one built for 30 MHz is going to be at least a metre in diameter to be effective, and its not going to work on any other freq ...eg... 35, 40, 50MHz etc etc

Yout initial size restraints are very unrealistic considering you choice of freq range.
they would be ok if you were doing 300MHz to 1GHz. :)
"ye canne break the laws of physics" as Scotty would have said :)

As others have said ... a Log-periodic yagi is you only real choice for a wideband antenna with respectable gain.... A well built LPY will have ~ 6 dBd Gain on any given freq within its operating range. ie. that of a 3 ele yagi

cheers
Dave
 
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Re: Wideband VHF antenna

A magnetic antenna that's possibly fine for 30 MHz may be useless for 300 Mhz. The datasheet seems to focus on low intermodulation but doesn't give any noise data. So you're unable to calculate an effective sensitivity expressed in field strength numbers. But you didn't tell your application and I can't know if it matters.
 

Re: Wideband VHF antenna

FvM said:
A magnetic antenna that's possibly fine for 30 MHz may be useless for 300 Mhz. The datasheet seems to focus on low intermodulation but doesn't give any noise data. So you're unable to calculate an effective sensitivity expressed in field strength numbers. But you didn't tell your application and I can't know if it matters.
Click to expand...

I agree about the magnetic antenna being useless for 300 MHz. I just wanted to point out the so-called Negative impedance matching technique. I've been sweeping the web, and so far I figured out that you can eliminate the antenna's reactance in a wideband by means of a negative inductance or capacitance. Even though this is impossible to realize with passive elements, you can do it with op-amps (Negative impedance converter - Wikipedia, the free encyclopedia). I have no further info about this kind of network, but if it is possible maybe I can tune a small antenna to work in this band.

However this just guesswork. I'm kinda noob, so I'm not really sure about what is feasible or not. And BTW, the application for this antenna is solely to detect the presence of a signal in this band. I have no interest demodulate or extract any information. For instance, assume I want to be aware if someone is using a wireless device in a enviroment in which it's not allowed.
 

Re: Wideband VHF antenna

In my view, the said negative impedance matching technique is just one of many ways to design an electrical small, active wideband antenna. It's the complementary to well-known properties of electrical small passive antennas: They must be either high Q or lossy. You can design in with built-in losses and add an amplifier or cancel the resonance with simulated negative reactances. Which solution actually gives higher sensitivity must be analyzed for a specific design.
 
Re: Wideband VHF antenna

FvM said:
In my view, the said negative impedance matching technique is just one of many ways to design an electrical small, active wideband antenna. It's the complementary to well-known properties of electrical small passive antennas: They must be either high Q or lossy. You can design in with built-in losses and add an amplifier or cancel the resonance with simulated negative reactances. Which solution actually gives higher sensitivity must be analyzed for a specific design.
Click to expand...

That said, I beleive my best option is to design a lossy antenna and amplify the signal.
 

Re: Wideband VHF antenna

So, let me see if I understood. Let's say I want to use a lambda/10 dipole in the frequency of 200 MHz, that is, a 15cm long dipole. It is known that the radiation resistance of a short dipole is really low, and the capacitive reactance is high. I also know that I can tune this dipole with a simple matching network, but in a SINGLE frequency. Now, I need to know what is the best way to match this dipole in the wideband. Since I am only receiving, there is no nee to worry about efficiency, right?
 

I think, the best method for a small wide band receive antenna is to connect a high impedance amplifier direct to the antenna. That's what most active antennas do as far as I know.
 

I don't understand how this works. I mean, if you connect an antenna to a high impedance load, the mismatch will result in reflection of most of power received. How exactly will this work?
 

Re: Wideband VHF antenna

mateusbatera said:
Since I am only receiving, there is no nee to worry about efficiency, right?
Click to expand...

Please tell me you are joking ??? !! I really hope you understand antenna fundamentals better than to make a statement like that ? :shock:

Dave
 
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Re: Wideband VHF antenna

davenn said:
Please tell me you are joking ??? !! I really hope you understand antenna fundamentals better than to make a statement like that ? :shock:

Dave
Click to expand...

Well, I'm sorry for not being an expert in the subject, like you are. But for this reason I was expecting to get some useful advices from people who understand better the concepts. Therefore, if you don't have any useful advice to share, there's no need to post such a hostile comment.

By the way, what I meant back there is: when the antenna is built to receive only, the most important parameter is SNR, so I can retrieve the information from the signal. If I design an electically small lossy antenna, I will sacrifice efficiency, but in the other hand I will get a lower Q (or a broader band). As long as the SNR is high enough, I can amplify the received signal to a reasonable level, even if the antenna is inefficient.

I may be wrong, but I will really apreciate if someone can help me get this straight.
 

Yes, for receiving if you have "unnecessary" high SNR margins, you can spend it on a less effective antenna in combination with a LNA with rather high input impedance (avoid unnecessary high impedance). It does cost performance in several plans. Higher noise in the active circuit due to the high input impedance. This construction will be more sensitive for ESD compared to a 50 Ohm well matched circuit. Adding a big coil to ground from antenna will reduce risk for destroying the amplifier. Alternatively can diodes be used instead of coil, which add capacitive load and in worst case can they add noise and inter modulation. Due to less antenna gain that must be compensated for => more gain in amplifier=> increased noise floor and increased risk for self oscillation. Wide band requirement does not make it simpler. For best result, try to make the antenna to not have too extreme high impedance at any frequencies and match the amplifier input to the antenna average impedance.
 
E Kafeman said:
Yes, for receiving if you have "unnecessary" high SNR margins, you can spend it on a less effective antenna in combination with a LNA with rather high input impedance (avoid unnecessary high impedance). It does cost performance in several plans. Higher noise in the active circuit due to the high input impedance. This construction will be more sensitive for ESD compared to a 50 Ohm well matched circuit. Adding a big coil to ground from antenna will reduce risk for destroying the amplifier. Alternatively can diodes be used instead of coil, which add capacitive load and in worst case can they add noise and inter modulation. Due to less antenna gain that must be compensated for => more gain in amplifier=> increased noise floor and increased risk for self oscillation. Wide band requirement does not make it simpler. For best result, try to make the antenna to not have too extreme high impedance at any frequencies and match the amplifier input to the antenna average impedance.
Click to expand...

Wow, this ain't no easy job! Anyway, thanks for the info!
 

To achieve max performance/size ratio can it be complicated to balance different types of cost in design of antenna and LNA, but a 50 cm monopole with serial bottom coil can be a good enough solution, depending on requirement of antenna efficiency.
 

That antenna is most likely according to my first proposal with an integrated LNA that have high input impedance at lower part of the frequency range. Due to this is it also likely that it have a higher noise level at these lower frequencies. NF for the LNA is specified as 3.5 dB at 500 MHz. Probably additional 10 dB noise or more at 30 kHz.
 

I am really curious about this antenna: **broken link removed**

I mean, it's not active, although it covers this huge bandwidth (20MHz - 3GHz), despite of its size. Will someone enlighten me about this? What's the secret?
 

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