How to make a SW amplifier work with a telescopic antenna radio?

[SparkyChem]

I've found this circuit here:

AM-FM-SW Active Antenna

Active Antenna AM-FM-SW, MPF102, mpf102, like a 30 feet wire antenna

www.learningelectronics.net

The diagram and idea came from the late Tony Van Roon who made this as an active antenna booster for AM/FM/SW bands.

The circuit seems to be an amplifier but I don't know exactly the logic behind it. However my question is on how to make it work in a telescopic antenna radio?.

I mean the intended use is on a unit which lacks of a jack as it is presented in the schematic. Can just one end I mean the one which is going in the other end of the 470 pF capacitor be clamped onto the antenna of the radio?. I mean without connecting it to ground?.

I have spotted this video on youtube from a guy who made this for a transistor radio but it isn't show very clearly if had he plugged one end to the antenna (seems to be the tunning capacitor of the radio) and the other end to the ground of the radio.

This part is confusing. Can someone help me out with this?.

The thing is I intend to use it in a Sangean SG-622 receiver and I don't know if it would work.

Is it really necessary to use ground for the antenna on the radio for this circuit to work?. Or can it be left floating?.

One comment in youtube indicated that the telescopic antennas in many radios are used for FM tunning while AM uses another antenna in the radio and the latter must be connected to this amplifier to make it work. But honestly I have no idea.

Can someone help me?.

 

[BradtheRad]

According to the article the circuit is comparable to 20-30 feet of wire antenna. (The drawing has its antenna labelled as 18 inches.)

The traditional long wire hung in a tree greatly improved reception in my shortwave radio. (At the time I don't think I ran a ground wire.)

An amplified antenna can be a big help provided it doesn't pick up major EM noise. It depends on proximity to lamp dimmers and computerized equipment often close by in a building.
On the other hand the long wire is more likely to be outdoors where most of its length picks up a greater proportion of desired broadcast, even if the near end is close to sources of EM noise.

 

[Audioguru]

If you live in a city then the booster will be overloaded by many nearby radio stations.

 

[SparkyChem]

According to the article the circuit is comparable to 20-30 feet of wire antenna. (The drawing has its antenna labelled as 18 inches.)

The traditional long wire hung in a tree greatly improved reception in my shortwave radio. (At the time I don't think I ran a ground wire.)

An amplified antenna can be a big help provided it doesn't pick up major EM noise. It depends on proximity to lamp dimmers and computerized equipment often close by in a building.
On the other hand the long wire is more likely to be outdoors where most of its length picks up a greater proportion of desired broadcast, even if the near end is close to sources of EM noise.

To be honest I feel my question is still unanswered. Can such circuit be used in a telescopic antenna yes or not?. I can use an aligator clip to connect the antenna to the output of the capacitor labeled

This guy used an FM active antenna circuit for his UV-3R Baofeng receiver.

Around second 37 he shows that, when tied to the telescopic antenna of a radio, the GND from the circuit is left floating, but because he used such circuit in a receiver which has an output with a GND access he could plug both connections in the BNC plug of the receiver.

Therefore, is it okay to leave GND as no connection or floating?. Or is it necessary to connect this to the GND of the receiver?

Can somebody please answer this question?.

I wish I could use a traditional long wire antenna in the outdoors, but I don't have access to it. My room has a window but I can't hang a wire outside as it is not practical, thus I though that I could use this circuit to improve reception.

I'm not near to dimmers which I can tell, but there can be EM noise. I could try to accomodate some thin wire inside my room but I don't think this would help much as it seems that the intended purpose of such antennas is to be placed outdoors.

In the video which I have mentioned above the quality of the audio and reception doesn't seem to be of much improvement.

Anyways should it be okay to leave GND unconnected from J1? and just connect to the antenna the other end of the 470 pF?, will it be safe?. I appreciate that this question could be answered.

If you live in a city then the booster will be overloaded by many nearby radio stations.

I really wished that my initial question would had been answered. Yes I understand the considerations which you are mentioning such as picking up close stations, but would it be just fine or have the same results is the GND from J1 is left unconnected? and just connecting the other end of the 470pF capacitor?.

 

[Audioguru]

If you do not connect the ground of the booster to the radio's ground and earth ground then the booster will not amplify much.
I think when all are connected then the output of the untuned booster will be a max-mash of many signals overloading the radio.
I do not know how many powerful stations are near you to overload the booster and radio. The booster is simple and cheap so try it.

 

[betwixt]

The answers overlook the fundamental point of how antennas work. They convert EM waves into a voltage that the receiver can use.
An optimal antenna has to be the correct length for the EM waves to resonate along its length and be the correct impedance it its feed point to efficiently convert the signal so the receiver can use it.

A telescopic antenna can work efficiently but only at the frequency it's extended length resonates at. The idea behind the 'active antenna' is that instead of making the antenna match the receiver, an intermediate stage is added. The amplifier has a high input impedance which to some degree lessens the need for the telescopic bit to be the right length and has a low output impedance so the signal is better coupled to the receiver. It may also have some amplification which can in some instances help.

The drawback to the telescopic antenna and active antenna circuit is it is poor at distinguishing the signal you want from all the other signals around it. Radio station signals typically produce a few microvolts at the antenna (1uV = one millionth of a volt) but other sources of signals may be hundreds or thousands of times stronger. An antenna alone will reject signals well away from their resonant frequency but being able to cover a much wider frequency range, the active antenna circuit may still pick them up and cause overloading, either in the amplifier itself or at the receiver. The typical effect is the receiver picks up lots of interference and stations at once, regardless of where you tune to.

So the answer to your question is: Yes it WILL work but have to be aware of its limitations and not knowing the strength of other signals in your vicinity we can't predict what may happen.

Brian.

 

[WA9VEZ]

Don't try to equate AM-HF antennas with FM (above 88 mhz) antennas. At AM frequencies, the background noise (even in a rural environment) is very high compared to frequencies above 30 mhz. A good AM receiver will have a sensitivity of 50 micro/volts per meter while a good FM receiver (entertainment) will work with 2 uV. For VHF communications receiver, I have measured, a .03 uV signal to open the squelch (ancient Motrac with a perma-K filter).

In the example video, it is HF (under 30 mhz) where voltage probe antennas work. Voltage probe antenna depend on a very high input impedance (think a few pfd in shunt with a couple of meg ohms) and the inherent capacitance of the small conductive rod/wire. A radio signal of 10 microvolts per meter would average out as half the potential at the center of a 1M conductor. You can use a high input impedance amplifier (or possibly a NIC [negative impedance converter]) to buffer the high impedance RF signal on the rod/wire to the receiver's input impedance.

At frequencies above 75 mhz, antennas are much smaller and do not capture much signal power (as well as ambient "noise") as 1/(f^2). Because of that, signal power matching of receiver and antenna becomes a significant issue. However, an optimum RF receiver preamplifier is not "power matched" on its input but is matched for best signal to internal (thermal) noise performance.

And for reception, the whole idea of "grounding" goes astray. For AM and HF, the problem is one of finding a "ground" that is "clean" and not connected to a "noise" source. On the transmitting side of antennas, life is easier, but Bode-Fano theorem shows power matching bandwidth limitation.

One of the important issues is that with RF you need to think about energy and spherical radiation. Forget about the volts and amps. All communications are done as work functions. It takes power to communicate. Example: watts will never make a signaling flag move, but joules will. On reception, everything is watts of signal power to watts of noise power. A corollary is that you can trade off signaling time for signal energy (Claude Shannon).

Jim

 

[SparkyChem]

If you do not connect the ground of the booster to the radio's ground and earth ground then the booster will not amplify much.
I think when all are connected then the output of the untuned booster will be a max-mash of many signals overloading the radio.
I do not know how many powerful stations are near you to overload the booster and radio. The booster is simple and cheap so try it.

I dind't know that. But from your answer it seems that I require to connect the radio's ground with the earth ground and the circuit ground.

Am I getting you right with this?. I mean, if I were to obtain better results, should this be the way to go?.

My receiver, which I mentioned above, does have a plastic case and since it is portable it doesn't seem to have a metallic chasis ground, hence I believe it is a floating one. Although I could gain access to the device's ground by attaching a wire from a screw terminal from an adaoter connected to the stereo plug or a similar attached to the DC jack (from the center negative pin), and those can be externally connected to the Earth.

However I can't reach a nearby earth ground. The best I can try is to connect it to a screw which is on my window which is embedded in my wall which is from concrete. Needless to say if it would "enhance" or improve the gain of this booster.

One reason which does let me feel worried is about the output of the C2, will the voltage on that end will be of any danger when connected to the antenna whip?. I mean I don't want to ruin the internal circuitry. To avoid this, I made a simulation in Proteus but the results which I'm getting makes me some confused. I mean if an input from the antenna which I am feeding it with 1 microvolt gives me an output on C2 of about 8.8V. What could be wrong there?.

Just wondering. It could be that I am not doing the right intepretation or maybe the simulation is not right?.

The answers overlook the fundamental point of how antennas work. They convert EM waves into a voltage that the receiver can use.
An optimal antenna has to be the correct length for the EM waves to resonate along its length and be the correct impedance it its feed point to efficiently convert the signal so the receiver can use it.

A telescopic antenna can work efficiently but only at the frequency it's extended length resonates at. The idea behind the 'active antenna' is that instead of making the antenna match the receiver, an intermediate stage is added. The amplifier has a high input impedance which to some degree lessens the need for the telescopic bit to be the right length and has a low output impedance so the signal is better coupled to the receiver. It may also have some amplification which can in some instances help.

The drawback to the telescopic antenna and active antenna circuit is it is poor at distinguishing the signal you want from all the other signals around it. Radio station signals typically produce a few microvolts at the antenna (1uV = one millionth of a volt) but other sources of signals may be hundreds or thousands of times stronger. An antenna alone will reject signals well away from their resonant frequency but being able to cover a much wider frequency range, the active antenna circuit may still pick them up and cause overloading, either in the amplifier itself or at the receiver. The typical effect is the receiver picks up lots of interference and stations at once, regardless of where you tune to.

So the answer to your question is: Yes it WILL work but have to be aware of its limitations and not knowing the strength of other signals in your vicinity we can't predict what may happen.

Brian.

There are some concepts which I am not very familiar but I get the idea. I mean the antenna will resonate at the frquency of the carrier wave this will be transformed into electricity at the voltage you mentioned.

Thus to compensate the length we use an amplifier, right?. (Sorry as I'm writting this I am not very savvy with the terminology). To understand the second part of your explanation I had to review basics on amplification theory so yes it makes sense that the input impedance has to be the greatest as possible so that the output is approximately equal to what the input signal is, and the output of the amplifier to have the least impedance so that this signal across a load is almost as equal from that of the amplifier. I'm not sure if this is the logic when you used the word coupled.

It is interesting that you mentioned about the poor distinction between the signals. You said that radio signals will generate few uV but others hundreds times stronger. Does this means that will it be "unsafe" to use in an environment with noise, i.e Wifi signals or other sorts?. Will those generate voltage spikes greater than microvolts?

Does it exist a way to minimize the overloading or perhaps to add an auxiliary circuit to tune a desired frequency?.

It seems that I can try this out, but to be honest, if I were to use my receiver's ground. The only way which I could access perhaps would be by connecting the ground of that active antena to the center negative barrel of the DC plug of the radio, or maybe by connecting it to the sleeve terminal of an external screw stereo plug which the radio has. As sleeve goes to ground. This latter part I'm not very sure. Although you mentioned that this might not be necessary as the circuit itself would work without tying it to the receiver's GND.

One peculiar aspect which I've also noticed is that what voltage will it be generated at the output of the capacitor?. Will it be okay to feel the radio with an amplified signal?. I used Proteus to do a simulation by applying a small signal on C1 and measured the voltage on the output on C2 to see what would happen.

In the attached picture it shows that the voltage which Proteus gives is around 8.8V, which makes me to worry as, wouldn't it be too much?. Or could it be something wrong with the simulation?.

It would be of much help if maybe you can clear this doubt of mine. I mean would it be safe to feed this voltage on the radio's antenna?.

I found a more elaborate circuit also from Tony, which appears here:

Active antenna to pull in those hard to get signals, 1-30 MHz

Active antenna (AA-8) to pull in those hard to get signals, 1-30 MHz at 1 to 20dB.

www.learningelectronics.net

I believe the logic is somewhat the same, but if you look nearby the end of that article the author hints that J1 output the exterior ring is connected to the chassis and not to the device's ground. I believe that this might be some confusing of what was intended to say, if this logic was also used in the first circuit.

I mean, does it imply that the GND of the circuit is a connection between GND of the circuit and the chassis ground of the device and the GND of the device (negative terminal of the device).

Maybe you can help me with this confusion and take a look at the screenshot of my simulation?.

--- Updated May 10, 2021 ---

Don't try to equate AM-HF antennas with FM (above 88 mhz) antennas. At AM frequencies, the background noise (even in a rural environment) is very high compared to frequencies above 30 mhz. A good AM receiver will have a sensitivity of 50 micro/volts per meter while a good FM receiver (entertainment) will work with 2 uV. For VHF communications receiver, I have measured, a .03 uV signal to open the squelch (ancient Motrac with a perma-K filter). ...

Gee I wonder what sort of device you used for doing all those measurements, mind to share?

The rest of your answer is abit too encyclopedical for me to understand but I think I get your idea. But I am not very sure about what you meant with clean grounding. Does it mean to find a ground clean from any noise?. How can you identify such ground?. I mean how can you tell this ground doesn't have any noise around?. Just by looking the surroundings or what?.

I don't know much about spherical radiation, but I belive you're talking about the radiation pattern, is this what you meant?. You haven't mentioned specifics regarding which sort of circuit could do for this purpose of increasing the power of the signal. Although you mentioned that a high input impedance amplifier can be used. It would had been nice if you pointed some examples.

 

[Audioguru]

The booster simulation does not have a load (your radio input) so the output capacitor has nothing to discharge the DC on it from the MPF102 Jfet drain wire.
Of course the ground of the booster circuit must be connected to the radio circuit's ground.
The booster will have a higher output level if the ground of the booster circuit is connected to an earth ground.
All half-decent radio circuits have a tuned LC at the input to select the weak signal you want but not pass other overload signals. Maybe you could make an LC at the input of the booster.
If your radio is old then it might use PNP transistors with a positive ground.

 

[BradtheRad]

However I can't reach a nearby earth ground.

Metal plumbing often goes to earth ground. Example, faucets, radiators, baseboards. You only need wire and alligator clips.

However it won't work if any plastic pipe intervenes (very likely compared to decades ago). To test continuity, hook up an AC voltmeter between live house voltage and your plumbing.

Avoid allowing substantial current to go through your plumbing, in case you have lines carrying gas or fuel oil.

 

[betwixt]

Two points, based on what I wrote earlier:

1. add a resistor from the antenna to ground (the left side of C2 in your schematic) the value isn't critical and I would suggest between 10K and 100K. It will not change the signal but it will stop static charges building up on the antenna that could damage C2 and then the FET.

2. The FET is to present a high impedance at the base of the antenna but also a low impedance to the receiver. For that, you need to place the load resistor in the source pin, not the drain. You will not get any voltage gain, but that isn't the objective. The receiver will no longer appear as almost a short circuit to the antenna so the signal it picks up will produce a higher voltage. Use a resistor rather than an inductor, or use both in series but not an inductor alone or you will risk instability.

You absolutely MUST join the FET circuit to the receiver ground. The best place to do it is at the receiver tuning capacitor, the further away from there, even by a few cm the poorer it will work. If you are connecting the output directly to the receiver input tuned circuit you should drop the output capacitor value substantially. I would suggest 10pF maximum or the receiver tuning will be 'pulled' by the additional wiring. Receivers of that type ("Superhets") have several tuned circuits that track each other as you turn the tuning knob so if you de-tune one you will probably lose performance instead of improving it.

Brian.