More FM02 learning

[pattani]

I am new to this forum.
HELLO to all and thanks for the highly informative forum.
Trying to understand and build a low power RDS FM transmitter for lab testing. Suggestions please.

[RFBurns]

Did I do some bonehead mathematics when I arrived at 65.5" for the half-length dipole length? Maybe I should trim it a bit to get closer to 0.47 lambda to achieve the magical infinite impedance mentioned in one of the links provided by RFBurns?

Anything at this point, I'm out of ideas, and hate being stuck like this. Thanks.

Obviously you got more problems than an antenna issue. At 75 feet, even with a 6 inch piece of wire sticking out of the center conductor of the SMA connector at the TX, you should get that signal.

Now as to what caused this incredible reduction in range is anyone's guess..including me..since there is no way at the moment to check the output of that TX and the antenna system.

Don't worry I wont say the terrible 4 letters and word. And it's not a curse word either!!

We do not need nTh decimal place precision here to attain a signal @ 75 feet, not even 200 feet or 2 blocks. But just to toss my 2 centavos out here after spending some time reading all the complaints about loss of range and hum noises, and all the steps taken by many to resolve these problems, the problems seem more severe than most are considering, or there is something common to all these setups that are resulting in the same issues spanning hundreds of miles apart from one system to another.

Now you gotta ask yourself why would so many living in different parts of the nation, and world, all have such common problems repeating again and again?

The history record is not a good one for the FM02 in these applications (light shows). Perhaps with all the information available to date, something is bound to break this common ice burg everyone is running into.

Happy Holidays!!

RFB

[ultanium]

Does anyone have a contact at Vast, so that the modifications & upgrades provided in this thread, can be implemented in the FM02?

Tommy J

[ukewarrior]

Yes, I do.
As this fleshes out, I will send them one wish list.

Does anyone have a contact at Vast, so that the modifications & upgrades provided in this thread, can be implemented in the FM02?

Tommy J

[buymyemu]

Couple nights ago someone told me that my signal was noisy again. Stumbled on something new today. I have the audio feed from the headphone jack on the back of the PC. I have a 3 way splitter, one leg to fm02, one to a set of speakers. Speakers are disconnected for the show. A few days ago I connected a set of headphones to the 3rd leg of the splitter, so I could listen in. Tonight I had a radio playing about 50 feet away, turned down pretty low. Noticed there was a little static on the radio. Unplugged the headphones to use on a different computer, and the radio volume doubled or better and no static at all. Plugged back in and more static, volume dropped.

So... PC headphone output must not have much power and the headphones split off too much?

[RFBurns]

Couple nights ago someone told me that my signal was noisy again. Stumbled on something new today. I have the audio feed from the headphone jack on the back of the PC. I have a 3 way splitter, one leg to fm02, one to a set of speakers. Speakers are disconnected for the show. A few days ago I connected a set of headphones to the 3rd leg of the splitter, so I could listen in. Tonight I had a radio playing about 50 feet away, turned down pretty low. Noticed there was a little static on the radio. Unplugged the headphones to use on a different computer, and the radio volume doubled or better and no static at all. Plugged back in and more static, volume dropped.

So... PC headphone output must not have much power and the headphones split off too much?

The problem is too much loading on the output of the PC sound card. IE...too many things plugged into the single output jack...same thing as plugging in too many extension cords and extenders and outlet expander overloading the power outlet.

Since the audio is being attenuated by the over-loading, the AGC/Leveler circuits in the BH1415 chip are reacting and trying to "pump up the volume" when there is very little input audio to the FM02, and with the lack of 19Khz pilot tone adjustment in the FM02, it all adds up to a plethora of noise.

To resolve the audio loading issue, a distribution amplifier should be used. Any typical distribution amplifier like those used in home theater applications will work. A distribution amplifier isolates each load from the main source and allows for proper levels on all of the outputs.

The other noise eliminating tips for the 19Khz pilot level are in another thread.

Happy Holidays!

RFB

[IdunBenhad]

Hi:
buymyemu: your headphones are probably low impedance and loading the splitter/output of your pc.

ags0000: after all the time and effort you have put into finding your problem, I would say you are right, there is a problem with the FM02. The closeness of the antenna to the transmitter may or may not cause a problem This type of interference, called RF feedback, is unpredictable and will vary according to the installation and other factors, including high VSWR. Getting the antenna in the clear and farther away from the transmitter might help. Even if you were to use a feedline of 50 feet or so, you would lose approximately 1/4 to 1/2 the transmitter power. If everything else is operating properly, this loss should not hurt you, as 10 mw on a good antenna should give you more range than is needed. I can get 4 blocks with an iPod type transmitter that is supposed to only work in the car!

The mounting position of the antenna does make a difference. The antenna being close to other objects can raise the VSWR and can change the resonant frequency of the antenna. The two go hand in hand. That being said, even if your antenna showed a VSWR of 2 or 3 to 1, it would not be of great concern. While it is very desirable to have the VSWR as low as possible (many people work very hard to get it down) 2 or 3:1 VSWR readings are not a killer and I completely understand not wanting to invest in something that you may use only once and it gets put on the shelf. Maybe you could find a nearby ham radio operator that could help you out if you wanted to measure it.

From your tests I would say the antenna is probably OK, but rechecking never hurt anything.

There is one more quick test you can make. Using a short piece of coax, 6" to 2' or so (length not critical at all) solder a flashlight bulb across it. The flashlight bulb can be most anything, but if you have a 1.5 volt bulb, this is better. Disconnect everything from your FM02 except the power and the "Dummy Load" (no inference intended), turn on the transmitter and listen to it on a good FM radio. You should have a very quiet carrier. One thing, be aware that even a battery powered FM receiver can add a bit of hum, etc. to the signal. This is internal noise and should only be heard if you turn the volume on the receiver way up. At this power level, the bulb will probably not glow.

There is another and better way to build a dummy load. If you can find a 50 or 72/75 ohm resistor, any wattage rating, solder that across the coax instead of the flashlight bulb. If you can't find a 50 ohm resistor, use two 100 ohm resistors in parallel. 51 ohms is a common value, but may not be obtainable at Radio Shack. If you want a 75 ohm load, that is also a "common value" but again, may not be stocked. Two 150 ohm resistors in parallel will give you 75 ohms. Either value of dummy load, 50 or 75 ohms is acceptable for this test.

The flashlight bulb dummy load is the crude way to go, but it works. It presents a load to the FM02 and makes a small antenna. While not purely resistive, it presents a load approximating the antenna to the FM02.

With the resistor type dummy loads, they present a purely resistive load to the transmitter, which is the best way to go. The resistors will radiate and act as a small antenna, also.

You will probably not get any range at all with the dummy loads, but that is not what we are trying to achieve. We are testing the transmitter itself to attempt to prove if it is your problem. Just as an aside, dummy loads have been known to transmit for a long way if they are not shielded. Doesn't ordinarily happen, but it can.

A lot of testing and setup can be done using the dummy load. Adjustment of the audio level before going to the antenna is one thing. If you can get it sounding good on the dummy, then there should be no difference when connecting to a good antenna.

One thing should be brought up here: I have seen it mentioned that the antenna can affect the sound quality. This is only true if the antenna is so bad that it is causing feedback in the transmitter or possibly the audio source. If the load (antenna) is so far off, then the transmitter can't handle it and can cause all sorts of strange things to happen. If most of the power is being reflected back into the transmitter, then this reflected power can get into the PLL and Audio circuitry, creating mayhem as it goes. This is why we try to get the VSWR as low as possible. Also, a bad antenna can drop the range considerably and because of the noise received along with the weak signal, make the audio appear to be distorted or low.

[RFBurns]

Without a VSWR meter how the heck are any of you going to know if your antenna is actually tuned correctly or has any other issue?

That is like saying the sun is out when the sky is dark and cloudy.

Just because you cut rods to a specific length, does not mean that the antenna is properly set up. Once mounted, nearby objects, environment conditions, type of coax used, length etc all play a role in tuning that antenna up right. So you need a VSWR meter to tell if it is actually working correctly or not.

But do not let me stand in the way of guessing progress!

Enjoy!


Happy Holidays!!

RFB

[ukewarrior]

Mr Burns,

Could you be any more caustic ?? !

Give it a rest, please.

Without a VSWR meter how the heck are any of you going to know if your antenna is actually tuned correctly or has any other issue?

That is like saying the sun is out when the sky is dark and cloudy.

Just because you cut rods to a specific length, does not mean that the antenna is properly set up. Once mounted, nearby objects, environment conditions, type of coax used, length etc all play a role in tuning that antenna up right. So you need a VSWR meter to tell if it is actually working correctly or not.

But do not let me stand in the way of guessing progress!

Enjoy!


Happy Holidays!!

RFB

[IdunBenhad]

Hi:

RFB
Without a VSWR meter how the heck are any of you going to know if your antenna is actually tuned correctly or has any other issue?

You don't! There is no argument there. But a simple dipole is easily checked without a meter. If the dimensions are correct, no shorts between the elements, decent coax and good mounting, one can be reasonably sure that the antenna will work. These checks, of course, will not tell the whole story about the antenna, but if it works and has decent range, then don't argue with success.

I built, tested and described the simple dipole without an SWR meter. It performs very well and if the instructions are followed, it should do likewise for other people. And by the way. I have 3 SWR bridges and have not felt the need to use one of them on this antenna, even out of curiosity. I have no doubt the SWR is not 1:1, but it performs and does what I want it to do. Note that the FM02, with its simple output circuitry, is tolerant of mismatches and while this is not the best way to run the transmitter, results tell the tale.

While a VSWR meter is highly desirable and will tell the user a great many things, there are more pressing issues than spending $50 on a meter that may be used only once or twice.

Maybe a group buy on a simple VSWR meter capable of the frequency? Maybe someone can design and present one to the group.

And, I'm putting it to rest. That's all I have to say about that!