What happens if I hook up a loudspeaker as antenna?

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JEL
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Joined: Wed Feb 07, 2018 2:08 am

What happens if I hook up a loudspeaker as antenna?

Post by JEL » Wed Feb 07, 2018 3:17 am

What happens if I hook up a loud-speaker as antenna?
(Not just the wire, but the actual speaker)

I am trying to get as much reception in low-frequencies as possible, so understand I need a very long wire (Longer than what is normally possible to lay out indoors)

So, I have a loud-speaker, which is attached to a long cable (About 15 meter, or 50 feet)

So if I attach the cable to the SDR... what'll happen?

I'm worried about the speaker acting as a microphone and inducing noise or, potentially, frying the SDR.

But I'm wondering if the wire that is coiled around the magnet, inside the speaker, will act as a long-wire and perhaps have some reception-benefits?



I already experimented with one of these speaker-wires (Without the speaker attached) and tried one option where I simply attached the lead-wire to the SDR antenna lead-in (So basically a long single-wire)

I laid out the cable as far as I could. I short-circuited the lead to the ground at the far end (The speaker-cable has 2 wires, obviously, and I didn't split them for my first experiment but let them sit together in the cable as they normally do when used with a loud-speaker), but didn't connect the ground to the SDR antenna ground-in.

I suppose you could think if it, visually, as a monopole antenna folded back on its halfway-point. So 15 meter from the receiver to its end, and then a u-turn and 15 meter back to the receiver.

I'm in Denmark and had fair reception on 160m and 80m bands from southern Europe with this.
When I attached the ground-wire to the receiver (Basically turning the antenna into a loop, rather than a monopole), reception was reduced significantly.



I then did a second experiment, where I took a similar speaker-cable (Except it was 'only' 5 meter, 16 feet, long) and separated the 2 wires into 2 single wires (Each 5 meter long) and laid them out as a horizontal dipole (One wire going in one direction, the other wire going the opposite direction)

This time I connected one wire to the SDR antenna lead-in, and the other wire to the SDR antenna ground-in.
I had about the same reception in the 160m and 80m bands (It was done the next day, at about the same time, so DX conditions was probably fairly even. But of course weather can change quickly, so I'm not drawing any real conclusions here), but a lot more in the 10 to 15 meter band, which I guess makes good sense, considering the antenna-length here)
Actually I had fairly good activity all the way from 10 meter to 160 meter.
I tried disconnecting the wire going to the SDR antenna ground-in, which made reception worse as expected.



So what I get from these first experiments:

A monopole long-wire works.
A dipole works.
A loop does NOT work.




So, back to the question:

If I leave the speaker sitting on the cable, rather than cutting the wire off, I get the extra meter of cable that sits in the coil around the magnet... although it will be coiled and therefore not making the antenna any longer relative to the air-wave.

If I remove the diaphragm... I shouldn't be inducing any low-frequency voltage, so I assume that would be safe for the SDR... (Not really sure though)

But would it benefit reception or likely make it worse?

And IF I leave the speaker on (Since its molded in plastic, so not easy to take apart to remove the diaphragm), CAN that cause problems for the SDR? (Maybe creating some type of electrical current the SDR electronics wouldn't appreciate of, so to speak)



The goal is simple; get as good indoor-reception on low frequencies as possible, without risking the SDR.

Any tips, tricks, warnings or advice welcome :)
Thanks.
jacob.

mikerobertsn
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Joined: Thu Feb 08, 2018 12:00 am

Re: What happens if I hook up a loudspeaker as antenna?

Post by mikerobertsn » Thu Feb 08, 2018 12:03 am

Why would you think a speaker would be any good at all?
If you are thinking its because of speakers can deal with low frequencies, it won't since antennas are dictated by wavelengths and the speed of sound is different to light.

JEL
Posts: 127
Joined: Wed Feb 07, 2018 2:08 am

Re: What happens if I hook up a loudspeaker as antenna?

Post by JEL » Thu Feb 08, 2018 3:43 am

mikerobertsn wrote:
Thu Feb 08, 2018 12:03 am
Why would you think a speaker would be any good at all?
If you are thinking its because of speakers can deal with low frequencies, it won't since antennas are dictated by wavelengths and the speed of sound is different to light.
It is not the speaker itself I think has any benefit, but the coiled wire around the magnet.

I was wondering if it might basically be an antenna somewhat similar to these: http://www.creative-science.org.uk/MSF3.html

The reason I was asking if I could let the speaker diaphragm remain, is that the speaker is built into, or molded rather, a plastic-cabinet that I would prefer not to have to try to take apart (It's a surround speaker (from a broken system) that is not really designed for self-service)

rtlsdrblog
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Re: What happens if I hook up a loudspeaker as antenna?

Post by rtlsdrblog » Thu Feb 08, 2018 10:59 am

The extra coil probably won't do much, but I don't see any harm in trying as long as the speaker is powered off.

JEL
Posts: 127
Joined: Wed Feb 07, 2018 2:08 am

Re: What happens if I hook up a loudspeaker as antenna?

Post by JEL » Thu Feb 08, 2018 3:03 pm

rtlsdrblog wrote:
Thu Feb 08, 2018 10:59 am
The extra coil probably won't do much, but I don't see any harm in trying as long as the speaker is powered off.
I will give it a go one of the next days and return with the results :)
(Experimenting with a horizontal 10 meter dipole at the moment)

The speaker is passive (So it will operate like a dynamic microphone in the setup I intend), so my own theory is that it will not create any problematic voltage or current, unless perhaps if some loud noise happens in the room where it is located (I don't really know how much energy such a speaker can output when being used as a microphone. It's a rear-surround speaker from a small 50-watt surround set, so the diaphragm is not very big... 2-3 inches perhaps)

I did a test where I used the soundcard as source for SDR#, and input a microphone signal (A real microphone)
Whenever the mic picked up sound; it pumped up the dB level on the spectrum (Which is no surprise of course)
Obviously a loud-speaker has more diaphragm-movement than a mic, so can produce more power going into the SDR-antenna. But at the same time, the loud-speaker diaphragm is much heavier than the mic's, so shouldn't be as easily moved.

I'm crossing my fingers and hoping the FM-stations from the nearby radio-mast is still the strongest signal-source I will run into :)

JEL
Posts: 127
Joined: Wed Feb 07, 2018 2:08 am

Re: What happens if I hook up a loudspeaker as antenna?

Post by JEL » Fri Feb 09, 2018 7:10 am

Experiment done.

No harm happened, but the dipole I mentioned outperforms the 'speaker-antenna' (Just barely)

The dipole is a 10 meter (2 poles each 5 meter long) laid across the floor from wall to wall (It's a small 1st floor apartment with a basement below, so not quite at the ground. I'd say my floor is about 1 meter above street-level. And of course, since it's an urban area, there's buildings all around boxing me in)

The speaker was set up about 7-8 meter away, so not quite as long as the dipole.

The dipole feeds both inputs on the SDR-antenna (Lead and ground), while the speaker-antenna only feeds the lead-in (When also feeding the ground, the signal faded almost completely away (Same result as with the other loop-antenna I tried and mentioned in the 1st post))

There was zero impact, that I could tell, from the speaker itself into the received signal. I had thought the diaphragm would induce some noise when it picked up sound, but even when tapping or scratching directly on the speaker-cabinet I couldn't see anything happen on the spectrum (SDR#-spectrum was showing the area from 0 Hz and up to about 120,000 Hz, and I was especially paying attention to 1,000 Hz, expecting the speaker to be most effective there. But nothing at all came through)
I was actually surprised by that.

I compared reception of both antennas at a few different spots from 0 Hz up to about 2,000 kHz.
The dipole consistently had a few dB more signal (I don't think the actual SNR was much different between the 2, but the whole spectrum seemed to lift up a few dB at all spots with the dipole)

So, I suppose the monopole long-wire antenna, folded at the middle (I mentioned the details of that layout in the 1st post, but just to repeat; lead-out, ground-in), and this speaker-antenna (Which technically is the same layout as the folded monopole, except for the 2 wires not being directly short-circuited at the antenna far-end, but rather taking a few turns around a magnet before heading back in) are pretty much equally good/bad.

Thanks to both of you who replied :)

HighSNR
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Re: What happens if I hook up a loudspeaker as antenna?

Post by HighSNR » Fri Feb 09, 2018 3:02 pm

your speaker coil is really just acting as a large inductor. At RF frequencies this will have a very high impedance and basically just look like an open circuit. i.e very little current will flow into it.

With your shorted ends two wires will couple inductively and should largely have the same current flowing in them. Shorting the far end will make very little difference. Unless the wires are further apart It wont act as a loop. You would probably be better shorting them at the near end.

JEL
Posts: 127
Joined: Wed Feb 07, 2018 2:08 am

Re: What happens if I hook up a loudspeaker as antenna?

Post by JEL » Sat Feb 10, 2018 3:55 am

HighSNR wrote:
Fri Feb 09, 2018 3:02 pm
your speaker coil is really just acting as a large inductor. At RF frequencies this will have a very high impedance and basically just look like an open circuit. i.e very little current will flow into it.

With your shorted ends two wires will couple inductively and should largely have the same current flowing in them. Shorting the far end will make very little difference. Unless the wires are further apart It wont act as a loop. You would probably be better shorting them at the near end.
Thanks.
I need much more education :)

Your comment led me onto this post: https://electronics.stackexchange.com/q ... od-antenna

Which, after reading it, tells me I need more education :)

And more experimentation also.

HighSNR
Posts: 40
Joined: Mon Sep 18, 2017 8:14 am

Re: What happens if I hook up a loudspeaker as antenna?

Post by HighSNR » Sun Feb 11, 2018 8:00 am

I really like this video for giving an easily understood and importantly accurate guide to how antennas work.

https://youtu.be/T-SbBlNgUTU

After that the http://www.antenna-theory.com/m/index.php website is a really good source of information.

Then, if you want to play and experiment 4nec2 is an easy to use and free EM solver. I used this to quickly test out your speaker wire experiments, but did not have time to post the results.

JEL
Posts: 127
Joined: Wed Feb 07, 2018 2:08 am

Re: What happens if I hook up a loudspeaker as antenna?

Post by JEL » Sun Feb 11, 2018 2:14 pm

HighSNR wrote:
Sun Feb 11, 2018 8:00 am
I really like this video for giving an easily understood and importantly accurate guide to how antennas work.

https://youtu.be/T-SbBlNgUTU

After that the http://www.antenna-theory.com/m/index.php website is a really good source of information.

Then, if you want to play and experiment 4nec2 is an easy to use and free EM solver. I used this to quickly test out your speaker wire experiments, but did not have time to post the results.
Thank you very much :)

So far I've been using radios and professionally made antennas (As opposed to home-made DIY antennas), since I began playing with CB-radios in the 80s, but only just recently did I begin to play with SDRs and their much wider frequency-ranges (And thus need for more different antenna-setups)

That's how I fell over the concept of wire-antennas, just recently, and began googling... and found that there is a lot to antennas :)

I've just never given it much thought before. I just plugged some antenna in to whatever device I was using and never asked any further questions.

I knew basic concepts; get it high, a 5/8 with radials is somehow better than a 1/2 (In CB-land the 5/8 was always considered at step-up from a 1/2 wave), have the least amount of SWR when broadcasting. Things like that.

But now I'm beginning to understand what the radials are for (Other than looking cool :) ) and how the antenna interacts with the ground-plane. Vertical vs horizontal polarity.
I'm learning more every day :)

I also found out that what I call a loop-antenna in the first post here, was in fact acting like a feed-line (The 2 wires was still running together, so canceled each other out, which explains why I lost signal almost down to nothing when shorting them. Rather than a loop-antenna I simply had a feed-line and not an antenna)

So I feel like I'm slowly, step by step, beginning to get a better grasp on this whole subject (While still having much more to learn of course. Impedance is still somewhat of an oddity to me, but one step at a time)

So the links you give are very much appreciated :)

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