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 Post subject: Signal Booster for FM Generator ?
PostPosted: May Fri 13, 2022 4:54 pm 
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Location: Reno, NV
Hopefully this is the right section for this post. I have a Sound Technology 1000 and a 1100 signal conditioner, with a copy of the Ramsey dipole antenna, that use it as a little FM transmitter for around the shop . I need a little more power out of the generator, what would be the best way to increase the output for a little more range? I was thinking of using one of the Ebay RF amps, such as this one, but I am not sure if it would work or if there is a better amp or way of doing this. The output of the generator is 75 ohms.

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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Fri 13, 2022 7:48 pm 
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Location: Dallas, TX
If I understand correctly, what you are using the generator as a transmitter. It puts audio onto Radio Frequency using Frequency Modulation.
One problem using an amplifier with a transmitter is that if it is not intended to work at the correct power level it will distort.
The pictured amplifier does not indicate how much it amplifies and how large a signal it can handle.
That amplifier very well could be intended for much smaller signal levels.
Technically speaking broadcasting at a higher level than 100mW (20 dbm) would be illegal. However you should not need that much power anyway.

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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Fri 13, 2022 8:12 pm 
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It's better to improve the antenna than it is to raise the power.


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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Fri 13, 2022 9:02 pm 
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Where is the dipole antenna? I use redundant TV rabbit's ears antenna right behind my micropower transmitter with the shortest coax lead possible and I goes around 25yards outside the house.

IMHO that amplifier is to boost reception, not transmission.

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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Fri 13, 2022 9:23 pm 
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Location: Melbourne, Florida
Ewizard wrote:
.....I was thinking of using one of the Ebay RF amps, such as this one, but I am not sure if it would work or if there is a better amp or way of doing this. The output of the generator is 75 ohms.

Attachment:
RF Amp.jpg


When asking about something a link or some other way to get data will help those who are trying to help you.

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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Sat 14, 2022 2:49 am 
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Location: Long Island NY
The thing is, in order to spec an RF amplifier, you need to know two things in addition to the system impedances. One is called the first order intercept or compression point, or P1dB. This is the point where the amplifier is delivering as much gain and power as it can linearly. Any further increase in input level will not produce a corresponding increase in output level, so the amplifier is said to be going into compression. (P3dB is the third order point, where harmonic distortion begins. In a well designed amplifier it should be at least a few dB higher than P1). The other piece of information needed is the gain specification, usually given in dB. The way this works is, if a hypothetical amplifier has a P1dB of +2 dBm, most engineers would stay a dB or two below that to avoid compression. So the maximum output level that can be expected is 0 dBm. If the gain is 35 dB, then that means the input level should be -35 dBm for an output level of 0 dBm. (We're assuming the input and output impedances of the amp are the same).

Now look at the signal generator. If it is capable of producing 0 dBm on its own, the example amplifier would be a joke. You would have to turn the signal generator output down 35 dB to drive the amplifier linearly, to get you back to 0 dBm. In other words, the P1dB or P3dB points have to be higher than the signal generator output level, and the gain has to be sufficient that the generator can drive it, in order to realize any benefit. With RF amplifiers it's not just the gain, but also the power level at which that gain is achievable.

But I have to agree with the others above that you would be better off building a better antenna that is tuned to the frequency you want it on, rather than trying to amp-up the generator. There are many articles on VHF antennas in old ARRL publications so it should not be hard to find a 6-meter band antenna that could be scaled down to operate on the FM band, or a 2-meter antenna that can be scaled up. (A 6-M antenna would be too big to resonate on the FM band and a 2-M antenna would be too small). Many of the articles give the formulas to calculate the antenna dimensions for the frequency you want to use, so you'd just have to do a little math.

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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Sat 14, 2022 6:39 am 
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Actually you can run an FM amp at Psat. There is no envelope that can be distorted by amplifier compression. That unit has an output of -17 dBm so if the FCC allows 20 dBm you can use up to 37 dB of external amplification. Using the given gain you would have about 14 dBm output but the gain is given at 500 MHz. Also, 14 dBm is 3 dB over the P1dB point of 11 dBm, again at 500 MHz. No information about performance in the FM band is given. We would expect the numbers to rise at the lower frequency but we can't predict the output level with confidence. It could be close to or above 20 dBm.


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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Sat 14, 2022 12:18 pm 
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Location: Long Island NY
While it is true that FM is all about frequency variations and not amplitude ones, so minor amounts of compression won’t hurt things, once you pass the P3 point you have harmonic distortion and intermod which create new frequencies not in the original signal. Maybe not good for the outcome. FM final amplifiers in transmitters are linear.

The other thing to remember is when you drive an amplifier into compression it is usually at maximum power dissipation. Running things that way tends not to promote longest life and reliability.

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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Sat 14, 2022 12:22 pm 
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While experimenting with test gear and components is fine, if all you are wanting is a stereo FM transmitter for your home use, there are some much simpler and less expensive options that work very well.

I have been running this low power FM transmitter kit from Cana-kits for about 10 years, and they are excellent. I operate it from a second story bedroom, fed by audio streams from an internet radio. The range is sufficient to cover my whole house and 150 feet to my outbuilding.

It uses a simple 1/4 wave telescoping antenna.

https://www.canakit.com/fm-transmitters

https://www.canakit.com/hi-fi-stereo-fm-transmitter-kit-ck222b-uk222b-ck222-uk222.html

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Last edited by processhead on May Sat 14, 2022 12:37 pm, edited 1 time in total.

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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Sat 14, 2022 12:31 pm 
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Chris108 wrote:
The thing is, in order to spec an RF amplifier, you need to know two things in addition to the system impedances. One is called the first order intercept or compression point, or P1dB. This is the point where the amplifier is delivering as much gain and power as it can linearly. Any further increase in input level will not produce a corresponding increase in output level, so the amplifier is said to be going into compression. (P3dB is the third order point, where harmonic distortion begins. In a well designed amplifier it should be at least a few dB higher than P1). The other piece of information needed is the gain specification, usually given in dB. The way this works is, if a hypothetical amplifier has a P1dB of +2 dBm, most engineers would stay a dB or two below that to avoid compression. So the maximum output level that can be expected is 0 dBm. If the gain is 35 dB, then that means the input level should be -35 dBm for an output level of 0 dBm. (We're assuming the input and output impedances of the amp are the same).

Now look at the signal generator. If it is capable of producing 0 dBm on its own, the example amplifier would be a joke. You would have to turn the signal generator output down 35 dB to drive the amplifier linearly, to get you back to 0 dBm. In other words, the P1dB or P3dB points have to be higher than the signal generator output level, and the gain has to be sufficient that the generator can drive it, in order to realize any benefit. With RF amplifiers it's not just the gain, but also the power level at which that gain is achievable.

But I have to agree with the others above that you would be better off building a better antenna that is tuned to the frequency you want it on, rather than trying to amp-up the generator. There are many articles on VHF antennas in old ARRL publications so it should not be hard to find a 6-meter band antenna that could be scaled down to operate on the FM band, or a 2-meter antenna that can be scaled up. (A 6-M antenna would be too big to resonate on the FM band and a 2-M antenna would be too small). Many of the articles give the formulas to calculate the antenna dimensions for the frequency you want to use, so you'd just have to do a little math.

Definitely the antenna should be optimal before throwing more power at it. What I would
consider is a J pole for its simplicity and performance.
Do an internet search for: diy jpole antenna for fm broadcast band
There are lots of possibilities within the search results. If you don't want to build it yourself there are very economically priced J pole antennas available.

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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Sat 14, 2022 1:58 pm 
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Notimetolooz wrote:
Technically speaking broadcasting at a higher level than 100mW (20 dbm) would be illegal.

That is incorrect. Part 15 of the FCC Rules and Regulations limits unlicensed AM transmitters to a maximum power supply input of 100 mW to the final RF amplifier (Section 15.209). Unlicensed FM transmitters are limited to a radiated field of 250 microvolts/meter at 3 meters from the antenna (Section 15.239).

Dale H. Cook, Retired AM/FM/TV Chief Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Sat 14, 2022 2:01 pm 
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khutch wrote:
... so if the FCC allows 20 dBm ...

It does not - see 47CFR Section 15.239.

Dale H. Cook, Retired AM/FM/TV Chief Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Sat 14, 2022 2:23 pm 
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Four years ago I bought a slightly lower power version of this transmitter for use around the house. I have a range from it (unobstructed line of sight) of almost a kilometre when I use it with it's supplied whip antenna and on 100mw power setting. On the 500mw setting it goes about three times that distance. I use it on the 100mw setting 99% of the time because I don't need any more distance most of the time. We use it to broadcast internet feeds around our property and I am told that quite a few neighbours tune in to see what we're listening to.

At this price I couldn't afford the time to jury-rig something - I had better things to do.

Attachment:
FM-Transmitter.jpg
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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Sat 14, 2022 3:37 pm 
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Location: Melbourne, Florida
The way to find out if something like the one pictured by the OP works the way wanted is to try it.

RRM


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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Sat 14, 2022 4:33 pm 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 4123
Location: Dallas, TX
Dale H. Cook wrote:
Notimetolooz wrote:
Technically speaking broadcasting at a higher level than 100mW (20 dbm) would be illegal.

That is incorrect. Part 15 of the FCC Rules and Regulations limits unlicensed AM transmitters to a maximum power supply input of 100 mW to the final RF amplifier (Section 15.209). Unlicensed FM transmitters are limited to a radiated field of 250 microvolts/meter at 3 meters from the antenna (Section 15.239).

Dale H. Cook, Retired AM/FM/TV Chief Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html

You are of course technically correct.
I intended to give an approximate limit to the antenna input. He is unlikely to have the calibrated equipment to measure the field strength from the antenna.
In practical terms unless he interferes with the reception of the neighbors' radio, TV or services like police, fire or aviation the FCC is unlikely to investigate. If an added amplifier distorts he may interfere with those other services since they use the frequency bands where the distortion generated harmonics would be. That would be something very important since he could get good FM radio reception with overdriving an amplifier but be unaware that he was interfering with other bands. (Until a knock come on his door.)

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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Sat 14, 2022 5:32 pm 
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Posts: 63
Location: Reno, NV
Thanks for all the replies. The generator is very high quality and has a very linear modulator and will put out a little over 30000 uv. The antenna is mounted inside a commerical building up high and have it trimmed to mid band (98mhz). Here is a picture of the generator/signal conditioner,

Attachment:
1000A.jpg
1000A.jpg [ 797.78 KiB | Viewed 305 times ]


This is the antenna that I am using, a copy of the Ramsey tm100, that Mike Yancy did.
https://www.amazon.com/Ramsey-TM100-Tru ... B0002OERGC

The amplifier that I showed before is this one, it shows a gain of 31db
https://www.ebay.com/itm/123910494389

As I mentioned before, I need to increase the range a little more than what I have right now. I can decrease the output into the booster/amp very easily with the calibrated output control from the generator.


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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Sat 14, 2022 11:02 pm 
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John Bartley wrote:
Four years ago I bought a slightly lower power version of this transmitter for use around the house.

The pictured transmitter is illegal for unlicensed use in the US. The lower powered version is likely also illegal for unlicensed use in the US. There are hordes of illegal (i.e., without FCC certification) transmitters sold through the internet. Despite insufficient staff the FCC Enforcement Bureau is looking for people using those illegal transmitters. They generally do so in two instances. One is where there has been a complaint to the Commission about a specific transmitter installation. The other is when EB is in a city or town for unrelated business and has some time available to look for illegal unlicensed transmitters.

Dale H. Cook, Retired AM/FM/TV Chief Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Sun 15, 2022 3:52 am 
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Well, 30,000 uV into 50 ohms gives -17 dBm. The eekBay amplifier pictured has 31 dB of gain at 500 MHz, but no specs at lower frequencies. We'll take a leap of faith and assume it's the same at 98 MHz. The P1 point is given as +11 dBm. So if you give it a 1-dB margin (i.e. run it at +10 dB output) then your input level would be +10 - 31 = -21 dBm. Easily achievable by turning the signal generator output down to 17,800 uV.

But I still think the antenna is the bigger part of the problem. Looking at the link given it appears to be a dipole in a piece of PVC pipe. But if you observe, they show it mounted at the top of an antenna tower on an outrigger with nothing above or below it. Dipoles don't like to be surrounded by metal surfaces, it throws their radiation patterns off quite a lot. Now if the antenna is inside a commercial building with lots of steel right above it and right below it in the floors and roof deck, I'd bet the pattern is a real mess. If so, amping it will only make a bigger mess.

Note that it is incumbent on the OP to do a field strength reading and confirm that he is staying within the 250 uV/M at 3 meters spec for unlicensed operation as prescribed by the FCC. Depending on transmission line loss and antenna efficiency it is possible that spec will be exceeded which could result in fines and equipment confiscation if there's ever an enforcement action.

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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Sun 15, 2022 4:21 am 
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Location: Dallas, TX
Ewizard wrote:
The antenna is mounted inside a commerical building up high and have it trimmed to mid band (98mhz).
This is the antenna that I am using, a copy of the Ramsey tm100, that Mike Yancy did.
https://www.amazon.com/Ramsey-TM100-Tru ... B0002OERGC

I'm a bit curious about it being in a "commercial building", I thought this was for home use.
If you are broadcasting on a school campus or something similar then this would be a different situation.

Is the antenna mounted horizontally?
FM antennas are usually used in a horizontal polarization.

What height is the antenna mounted?
If it is mounted 200 ft up and has a range of 150 ft you may not get much signal on the ground floor.

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 Post subject: Re: Signal Booster for FM Generator ?
PostPosted: May Mon 16, 2022 1:34 pm 
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I have one of these very inexpensive 50KC-2GC amplifier modules, which I have been using as a quite versatile adjunct to the TinySA spectrum analyser and the NanoVNA portable vector network analyser; and since I have recommended it on the ARF for those purposes, I thought I would take a few minutes and try a simple test.

I run the amplifier from three AAA-size 10440 rechargeable lithium cells, in a triple AAA battery holder that is attached to the housing.

I set up the TinySA in Low Output mode, as a signal generator, on 100MC with wide-band FM modulation at 1KC, and the output level at -21dB. I connected the amplifier to the Low Output, and attached to the output of the amplifier a small whip aerial that has an SMA connector (this whip is 19 inches in length, somewhat longer than the one that comes with the TinySA).

I picked up the signal with a 1961 Nordmende Transita K, a high-quality vintage portable transistor radio. With the “transmitter” deep inside the house, I was able to receive the signal in the road, at a considerable distance from the house. Increasing the output of the TinySA significantly raised the signal strength and reach.

Thus, although these broadband general purpose amplifiers are not actually intended as RF power amplifiers for transmission purposes, it does appear that such a unit would do what the OP desires. Similarly inexpensive modules are available, which are in fact power type amplifiers.

I did not make a measurement of field strength at the aerial. I do have an Anritsu ML521 VHF field strength meter, as well as some Holaday broadband isotropic field strength meters, but they are a bit difficult to get to at the moment. Realistically, operation at these levels is exceedingly unlikely to attract unwanted attention.


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