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 Post subject: 30 dB Gain Broadband Amp
PostPosted: Jul Mon 01, 2019 5:51 am 
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I've been playing with this broadband amp. It's supposed to have variable gain 0-30 dB depending on VCC 4-8 volts. Although its impedance is supposed to be 50 ohm, I'm testing it with 75 ohm TV cable with F59 adapters to fit the tiny soldered connectors.

I'm testing it with a university FM station at 91.3 about 60 miles away. Without the amp, I can hear the station with some noise using RG59 cable connected to a dipole I had previously cut to 90.5. The dipole is simple, just straight wires connected to an F59 connector.

Hooking up the amp works but does not relieve the noise unless I feed it with 5.6-5.8 volts. Any more or less voltage causes increased noise.

To reduce noise, I have to use a 5 turn coil in parallel with a Polyvaricap from an old FM radio, both connected to the low end of the dipole cable. The input into the amp is just 1" piece of wire, with that wire and amp placed within an inch of the coil, but not touching it in any way.

The longer the amp input wire is, the more noise it sends to the FM receiver, overloading it and splattering multiple local stations, like 94.7 so they are heard on 91.3. That's why the wire is short. It only works with my inductive and capacitive coupling described above. I've not found a way to directly couple the amp without overloading.

So it's definitely demonstrating noise in, noise out. My question is, how can I assure a quiet tuned signal into the amp so that it outputs a quiet tuned signal?


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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Mon 01, 2019 1:06 pm 
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Where do you have the Pre-Amp inserted in the coax. If you have it at the receiver end, then you are going to increase the signal and any noise. If the Pre-Amp is at the dipole, you should see an increase in signal strength and lower noise. Assuming that the Pre-Amp is a low nose design.


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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Mon 01, 2019 3:12 pm 
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Preamp is LNA. The coax from the dipole to amp input is 6-7 feet. With that short a run does it really make that much difference whether the amp is on the dipole? That would be kind of hard to mess with coils and such as the dipole is mounted just below the ceiling.

Also how what is the effect of a 75 ohm dipole connected to 50 ohm input impedance preamp?


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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Mon 01, 2019 4:15 pm 
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I'm wondering if there are self oscillation problems or if that unit is overloading the FM receiver (it amplifies from almost DC to light!) Impedances would be very hard to judge, since the antenna and receiver input are probably no where near being 75 ohms. Otherwise, the small mismatch should not be an issue. Put a scope on the Vcc line and see if you can see any parasitic oscillations.

Pete

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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Mon 01, 2019 4:37 pm 
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A dipole is a balance antenna, the feedpoint is also balanced.

Coax is an unbalanced feed line.

A small 1:1 balun can be made, (I do not know of a cheap commercial one) then coax used and reduced noise pickup from coax

Or, create a folded dipole, connect the ends together, characteristic impedance rises to 300 at resonance but is more broadband.

A 300 balanced to 75 unbalanced (coax) "F" connector (TV) balun is available almost everywhere. No more (less) noise pickup on the coax, antenna more forgiving of out of resonance signals.

Be sure balun is DC passing, some are not and may have an influence on the signals overall.

Option: Develop a DC coupler for the amp to power the amp via coax. Move the amp to the balun at the antenna.

Put the amp in a metal box, be sure power source is bypassed to keep noise out of DC.

IMHO now the dipole + amp can be moved much higher and further from noise, even rotated :shock:

Consider that the amp is vulnerable to RF pulse, nearby lightning, even if not powered. Something may have to be done to protect the amp...

YMMV

Chas

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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Tue 02, 2019 6:16 pm 
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@Macrohenry, several major problems with what you are trying to do.
1) You don't use a 30dB 0.1-2000 MHz broadband amplifier as an antenna amplifier. You need a low noise tuned amplifier for your specific frequency/band.
2) Changing the supply voltage would NEVER change the gain of such amplifier from 0 to 30 dB.
3) To get any decent performance from any low noise antenna amplifier you have to match the amplifier input impedance to the coax/antenna impedance and your amplifier must be shielded.
4) you cannot use any random length of wire for your dipole antenna and expect it to match to your coax and amplifier input impedance.
5) The impedance of a 1/2 wave dipole is 50Ω and for it to tune at 91.3MHz it has to be ~1.64 meters long and the coax should be RG58 which has an impedance of 50Ω as well. Also, as some other mentioned, a dipole is a balanced antenna, your coax and amplifier are unbalanced, you would need a matching transformer to feed the coax.


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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Wed 03, 2019 5:05 am 
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Thanks for the tips. This is the type of info I'm looking for. Can you explain the following?

upsss wrote:

2) Changing the supply voltage would NEVER change the gain of such amplifier from 0 to 30 dB.


From data sheet, voltage measurements are mine. Contrary to the data sheet, my unit starts passing signal at 4 V and doesn't really kick in until 5.7 V or so.


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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Wed 03, 2019 5:38 am 
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Can you provide a link of the amplifier datasheet including its schematic.


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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Wed 03, 2019 3:42 pm 
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Data Sheet https://www.google.com/url?sa=t&rct=j&q ... _7EFmebjtM

Schematic is in the data sheet. Very few parts.


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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Wed 03, 2019 6:18 pm 
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The MMIC device on your board is designed to operate as a very broadband fixed high gain amplifier with a fixed Vcc of max 7V. If you powered it @ 9.5V, it is probably damaged now. Regardless, this amplifier board AS IS, isn't designed to work as an antenna amplifier and it will not do what you are attempting to do.


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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Wed 03, 2019 7:30 pm 
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upsss wrote:
If you powered it @ 9.5V, it is probably damaged now. .


I'm not giving up yet. Max current I tested it at was 28 mA. Specs say max should be <=50mA, so I think I'm good. It still works, just not like I want it to, so I'll try the other suggestions, including a folded dipole with matching transformer. I've learned never to completely trust the internet, but there are a lot of reviews demonstrating success using this device as an antenna amplifier. And it does work for me that way, albeit with noise I think I can filter out.


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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Wed 03, 2019 7:46 pm 
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I'm following this post with some interest so keep trying.

Since most front ends already have an amplifier, it can only amplify from the new antenna amplifier with no discernible noise degradation. If you can find a way to do that, I will be very interested.

Antennas themselves can provide considerable signal gain over noise by way of RDF, which is the Receiving Directivity Factor calculated from the maximum Azimuth Gain – Avg Gain. A greater than 10 dB gain over the noise can be achieved this way.

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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Wed 03, 2019 9:56 pm 
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You might try a trap between the antenna and the broadband amp , also a a grounded metal in-closer may help . A band filter that would only let the 88 to 108 MHZ , or a tune filter would be best

There is a lot of LTE broadband noise now days , with the amp the front end of the FM receiver may be overloaded with out of band signal .

I take it that an outdoor beam antenna is out of the question .

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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Thu 04, 2019 1:06 am 
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Macrohenry wrote:
I'm not giving up yet. Max current I tested it at was 28 mA. Specs say max should be <=50mA, so I think I'm good. It still works, just not like I want it to, so I'll try the other suggestions, including a folded dipole with matching transformer. I've learned never to completely trust the internet, but there are a lot of reviews demonstrating success using this device as an antenna amplifier. And it does work for me that way, albeit with noise I think I can filter out.

You say that you don't "completely trust the internet" but then you say "there are a lot of reviews demonstrating success using this device as an antenna amplifier". Yes, they must have a lot of success amplifying by 30dB every signal AND noise generated in their neighborhood in the spectrum from 10kHz to 1GHz and feeding it directly to their receiver input and that is exactly what this device was designed for. If that is success then I don't know what is a failure. I don't think there is any worse device to use as an antenna amplifier than this device, good Luck with it.


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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Thu 04, 2019 4:44 am 
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upsss wrote:
Macrohenry wrote:
I'm not giving up yet. Max current I tested it at was 28 mA. Specs say max should be <=50mA, so I think I'm good. It still works, just not like I want it to, so I'll try the other suggestions, including a folded dipole with matching transformer. I've learned never to completely trust the internet, but there are a lot of reviews demonstrating success using this device as an antenna amplifier. And it does work for me that way, albeit with noise I think I can filter out.

You say that you don't "completely trust the internet" but then you say "there are a lot of reviews demonstrating success using this device as an antenna amplifier". Yes, they must have a lot of success amplifying by 30dB every signal AND noise generated in their neighborhood in the spectrum from 10kHz to 1GHz and feeding it directly to their receiver input and that is exactly what this device was designed for. If that is success then I don't know what is a failure. I don't think there is any worse device to use as an antenna amplifier than this device, good Luck with it.


Correct. I don't completely trust the internet, but there's a reason for the old phrase "where there's smoke, there's fire." Sometimes there is fire. I've not failed enough on this project not to give it the benefit of the doubt. I've also learned that professionals can be wrong in their assessments (don't get me started with anecdotes), so it's pretty easy for me to keep a healthy skepticism of even professional opinions unless something can be proven pretty crystal clear. I appreciate the helpful suggestions and the wishing luck, and if I get to the point of success or diminishing returns, I"ll post anything of interest.


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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Thu 04, 2019 10:42 am 
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I think that there is a difference between a head end amplifier for FM and
30 dB line amplifier.

A head end end amplifier, connected right at antenna is going to deliver a power
gain, to overcome a coax loss to deliver a usable signal to the 30 dB line amplifier.


TV systems can be designed to reference, a usable signal measured in dBmv. or dBJ
J for Jerrold, makers of the first system. Bottom line 1000 micro volts to the TV set , or so,
at 75 ohms.

Back to the head end amplifier. Your station needs to deliver a working field strength
at e.g. 90 mHz. It wont 60 miles away.

When you say you hear the station, it is the heavy lifting the radio is doing,
responding to the faint signal by employing very high gain amplification, in the
Fm tuner pack. That 30 dB broadband chip, cant even 'hear' the femptowatt
levels your tuner can. But it sure can amplify very well a signal within its
operational sensitivity range.

Just opinion, am I on the right track?

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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Thu 04, 2019 4:50 pm 
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Here's the spec sheet: https://www.google.com/url?sa=t&rct=j&q ... _7EFmebjtM

It states max input power (+13dBm) but not minimum input power. It says "They are designed for narrow or wide bandwidth commercial applications that require high gain and low noise IF or RF amplification." If it's not designed for antenna amplification, i wish it would at least give a hint.


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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Thu 04, 2019 6:26 pm 
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I gave you more than a hint that what you are trying to acomplish with your existing VERY broadband HIGH gain amp is totally wrong. You don't need an EE to tell you that, ANY Ham Radio with some very basic experience can tell you that which I am both with over 40 years of experience. Your device is a great Distribution Amp (if it is not damaged by now when you applied Vcc voltage way over its limits), however its the worst device you can pick as an antenna amp.

Why don't you look at some FM car antenna amplifier which are designed specifically for your purpose. https://www.google.com/search?client=firefox-b-1-d&biw=1293&bih=949&tbm=isch&sa=1&ei=c5UdXdTeIcmU0gK0jZOQBA&q=FM+car+antenna+amplifier&oq=FM+car+antenna+amplifier&gs_l=img.12..35i39.2764366.2765755..2769198...0.0..0.101.393.1j3......0....1..gws-wiz-img.2C89PbImjIA

The most important parameter in picking any antenna amp is its Noise Figure and not its Gain. As a matter of fact, you want to use the minimum gain possible for the best reception of your desired frequency. The problem with too much gain of ANY antenna amp is that strong stations will overload your receiver and defeat the purpose of your antenna amp. The ideal antenna amp will have a low noise figure with an adjustable gain control and be tuneable. Yes I know, it may be too much to ask from a low cost antenna amp. Again, good luck and Happy 4th!


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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Fri 05, 2019 8:09 pm 
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upsss wrote:
I gave you more than a hint that what you are trying to acomplish with your existing VERY broadband HIGH gain amp is totally wrong. You don't need an EE to tell you that, ANY Ham Radio with some very basic experience can tell you that which I am both with over 40 years of experience.

Thanks for the elaboration and pointing me toward the FM amps. I did some googling. There are tons of vehicle FM boosters on the net. Some are bogus, so on a lighter note, enjoy this review/expose: http://www.net4truthusa.com/JeepRestora ... ooster.htm

For me, the most significant excerpt is this: "This amp is cheap, less than $10.00. "For a few bucks, I figured "what the hell? It might work", and without knowing precisely what it was made of, out of curiosity, I ordered one."

I could say those same words! They sum up why I bought the amp in the OP: Curiosity. Does it work? Can I make it work? What would it take to get it to work? What can I learn from this?

I'm not an engineer, I'm a real estate professional. Any electronics education I've gained has been 100% self-taught. There are certainly holes in my electronics education. Some here have poked fun at that or castigated me for not knowing things they know. That doesn't keep me from craving messing with electronics.

I've loved electronics since the fifth grade. I learned too late why I flunked differential equations, that my form of dyslexia limits some needed capability to do math. So I develop circuits by "cut and try." Each of us has our own style. This is mine and I own it. Like the bumblebee who doesn't know he can't fly, I've gotten things to work that others have poo-pooed. More often than not, I get the results I want.

For example, before the internet I built a Tesla coil that uses a neon sign transformer to throw four-foot power arcs. Then then internet came along. The WWW schooled me that my Tesla coil wouldn't work well because it's tall and skinny instead of short and fat and has only a neon sign transformer and that my rotary spark gap is inefficient because it's not synchronous. Now what? I can choose to believe the authorities and not my own eyes or I can take with a grain of salt stuff I read on internet, including opinions of folks who are more qualified than I.

I feel my self-educated approach gets validated when others on this forum have used devices I've developed. it's enormously gratifying to learn that these circuits are worthwhile to others, for example:

$12 frequency synthesizer http://www.tompolk.com//radios/frequenc ... synth.html
Simple automatic volume control http://www.tompolk.com/hobbies/automaticvolumeleveler/avc.html
Most powerful one-transistor radio http://www.tompolk.com/radios/macrohenr ... ydyne.html
Simple 1W RF generator http://www.tompolk.com/testequipment/sinewave.html
And my tuning eye substitute proof of concept https://www.youtube.com/watch?v=4kEOK6r ... e=youtu.be

The problem I'm trying to solve with this $10.00 amplifier is NOT first and foremost to increase an FM signal. That's only a platform for the first test. The problem is simply trying to understand if this device is useful, and if so in what ways. If it is not an antenna amplifier, I'll find out for myself why, and my experience will be in accord with the admonitions against using it that way. In context of the OP, I'm playing with it. I value all input and I don't mind being told something won't work. And when I am told that, what I hope to learn is information WHY it won't work. Thank all of you who have responded for giving me that info.


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 Post subject: Re: 30 dB Gain Broadband Amp
PostPosted: Jul Fri 05, 2019 10:13 pm 
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An approach that could be used to evaluate your amp using TV equipment might be as follows:

With a signal lever meter, (SLM) ,measure the 75 ohm output of the amplifier . Set the signal level meter to a channel around 100 mHz.

With an alignment generator , set the 75 ohm output voltage to some value, e.g. 1000 microvolts.
on the same channel that the SLM is set to.

Now connect an attenuator box, like the type that came with the Heath post marker generator ,
in the line to the input of the amplifier.

With this setup, you can measure how many microvolts input is needed to produce a desired
reading on the SLM. Use an online program to convert dB to microvolts at 75 ohms.

Old vacuum tube SLMs are available, probably at low cost. They have a channel selector
from 2 - 13. And they were made to measure the strength of incoming TV signals from
aerials. Heath post marker generators pop up from time to time.
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de
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